Symposium MS03 : Mechanics of Nanocomposites and Hybrid Materials

Cu/Nb metallic multilayers with individual layer thicknesses in the range 7 nm to 63 nm were manufactured by accumulative roll bonding (ARB). Micropillars of square cross section of 5 x 5 µm²and an aspect ratio 2-3 were milled with a focused ion beam. The mechanical properties of the Cu/Nb multilayers were determined by means of in situmicropillar compression tests within a scanning electron microscope. Test were carried out at different strain rates (10^-2 to 10^-4 s^-1) and temperatures (25C to 400C) and the yield strength, strain rate sensitivity and activation volume were determined from these tests for each multilayer as a function of temperature. In addition, the deformation and fracture mechanisms were ascertained from in situ observations during deformation and from transmission electron microscopy analysis of foils extracted from the deformed micropillars.

The strain rate sensitivity of all the multilayers at ambient temperature was in the range 0.015 s^-1to 0.008 s^-1and decreased slightly as the layer thickness increased. Mechanical tests at 400C showed a large increase in the strain rate sensitivity, which ranged from 0.16 s^-1for a layer thickness of 7 nm to 0.05 s^-1 for a layer thickness of 64 nm. Surprisingly, the strain rate sensitivity of the nanoscale multilayers with layer thickness below 34 nm dropped to 0 at 200C while increased slightly with respect the ambient temperature value in the multilayer with 64 nm layer thickness [1]. In addition, serrations were observed in the stress-strain curves of the multilayers with small layer thickness tested at 200C. From these experimental observations, it was hypothesized that the unexpected reduction in the strain rate sensitivity of the multilayers with small layer thickness at 200C was due to dynamic strain ageing of the Nb layers due to the presence of oxygen in solid solution, following the results in the literature for dynamic strain ageing of Nb at this temperature [2]. The presence of oxygen in the Nb layers was confirmed by energy-dispersive X-ray microanalysis and atom probe tomography. The evidences for dynamic strain ageing of Cu/Nb nanoscale multilayers were discussed to the light of the experimental evidences and to the deformation mechanism map proposed for this system.

[1] J. Snel, M. A. Monclús, M. Castillo-Rodríguez, N. Mara, I. J. Beyerlein, J. LLorca, J. M. Molina-Aldareguía. JOM, 69, 2214-2226, 2017.
[2] C. Szkopiak, Acta Metallurgica 16, 381-391, 1968.

Metallic nanofoams are a unique class of materials possessing low density, large surface area, excellent attributes and energy absorption ability, making them good candidates as future radiation shielding materials. Tungsten is ideally suited as the base material for such a foam, as it is commonly used as a shielding material in nuclear facilities, medical diagnosis systems and a number of other circumstances in order to protect personnel and sensitive equipment from radiation. Therefore, it is of great value and scientific interest to tailor such a novel nanoporous tungsten, which combines the beneficial properties of tungsten with the positive attributes of nanoporous foams and has a great potential to satisfy the need for high performance materials that can endure harsh environments.

In this work, nanoporous tungsten was created on a bulk scale through a unique technique route involving the severe plastic deformation followed by reverse selective dissolution. Scanning electron microscopy and high-resolution transmission electron microscopy were utilized to characterize the microstructural evolution and analyze how the etching solutions affect the resulting nanoporous structures. The mechanical properties, which are an important consideration during many practical applications, were investigated by employing depth-sensing nanoindentation and other small-scale testing in situ in the SEM. Based on this, the elemental plasticity mechanisms governing the mechanical behavior were elucidated.

This work for the first time provides an innovative and adaptive approach to create nanoporous tungsten on a bulk scale. The developed reverse phase dissolution method is generally applicable and can be transferred to other refractory metals in the future. The promising mechanical results of nanoporous tungsten will serve as foundation for forthcoming related scientific studies and engineering applications.

The fracture behavior of high-temperature co-sputtered Cu/Mo nanocomposites were investigated in this study through in situ bending test of pre-notched microbeams in SEM. The as-synthesized nanocomposites present hierarchical heterogeneous architectures that are composed of matrices of bicontinuous interpenetrating Cu/Mo with nanoscale ligaments and sub-micron scale Cu-rich islands containing Mo-rich nanoparticles. From previous nanopillar compression tests, the nanocomposites possess high strength over 2 GPa and good deformability without shear band formation. Our micro-scale bending test further shows that the hierarchical nanocomposite has higher toughness compared to Cu/Mo nano-multilayers and monolithic Cu-Mo nanocomposite, where only interpenetrating Cu/Mo with nano-scale ligaments presents. The Cu islands were observed to neck and bridge the crack surfaces, as well as blunt the crack tip, deterring crack propagation. As a result, crack formation in multiple directions was observed after the initial crack propagation during the bending test. From this study, our hierarchical nanocomposite presents a successful approach to break through the strength-toughness trade-off dilemma.

Composite materials bring together the structures and properties of two or more materials to create improved functionality. The chemistries, microstructures, and topologies of the phases dictate their individual constitutive response, while those characteristics combined with the properties of the associated interfaces dictate the composite behavior. Exploiting size effects and unconventional structural geometry is a clever approach to achieving novel materials that go beyond functions predicted by conventional models of composite response. In the case of nanoporous gold (NPG) - based composites, the strong size effects found in nanoscale gold structures lead to a strongly tailorable strength of the nanoporous gold phase, while the high surface area allows actuation and sensing functions when submerged in electrolyte. When infiltrated or coated with a polymer, the boundary conditions governing the behavior of the nanostructured gold are modified.
In this presentation we consider two NPG-based nanocomposites: NPG fully infiltrated with epoxy to improve strength and ductility, and PPy coated NPG which enhances the electro-chemo-mechanical coupling in an electrolytic environment. Using various micromechanical testing methods and the quantification of the 3D network structure, the novel functional response has been investigated. It was found that the dependence of the strength on the mean ligament diameter is much weaker in the NPG/epoxy composites compared to pure NPG, while no size effect in the nanoporous epoxy phase was found. The failure of the composite presented as in-plane “horse-shoe" shape delaminations between the gold and epoxy phases, which serves to dissipate energy while maintaining its load bearing capacity. In the case of NPG submersed in NaF electrolyte, electro-chemo-mechanical coupling resulted in a considerable increase of strength of pillars when surface adsorption occurred, i.e. under positive applied potential, and the stress response to potential jumps was found to be fast and reversible. Coating NPG with an electrically conductive polymer, PPy, allows additional strengthening and stiffening of the NPG while also exploiting the actuation and sensing capabilities of the high specific surface area NPG network structure under potential in the NaF electrolyte.
Results will be discussed in terms of the individual constitutive relations, topological characteristics, and interfacial chemistry and strength, and perspectives for further exploitation of this class of nanocomposites will be given.

High-pressure and high-temperature (HPHT) treatment can make fine and unique texture in materials. We have realized HPHT treatments on the Mg₈₅Zn₆Y₉ alloy and then quenched and decompressed to ambient pressure. The alloy have a 18R-type long period stacking ordered structure synchronized with chemical concentration at ambient pressure, however it transform to duplex structure consist of “soft” hcp (α-Mg) phase and “hard” fcc-based superlattice (D0₃) phase at high-pressure condition (Matsushita, et al., Mater. trans. 56, 910, 2015, Mater. Let. 155, 11, 2015). The high-pressure phase can be recovered at ambient pressure. The recovered alloys after being treated at HPHT condition have ultra-fine and unique solidification pattern.
The alloy recovered after being treated at 10 GPa and 973 K have a lamella structure alternating layers of hcp and D0₃ phases with the layer thickness of 80~100 nm. On the other hand, the alloy recovered after being treated at 10 GPa and 1273 K takes spherulite with the diameter of 5~15μm. The term“spherulite” suggests a nearly spherical polycrystalline solidification pattern, which is rarely observed in solidification metals. As far as our knowledge, this is the first case the alloy takes spherulite of the volume percent above 80 %. The spherulite also contains duplex phase consist of hcp and D0₃. In the core part of spherulite, small grain considered as D0₃ in size below 50 nm distributed Mg matrix (Matsushita, et. al. JALCOM, 784. 1284, 2019). Further fine column shape grains consist of D0₃ and hcp phases were spread from radially from the core. Similar unique spherical microstructure can be observed in the alloy treated at 5 GPa/1273 K and 15 GPa/1273 K.
The compression yield stress of the alloy consisted from spherulite structure is over 700 MPa, which much higher than that of lamella structure and the other magnesium alloys with similar composition made by severe plastic deformations. These results show the HPHT treatments and spherulite structure have great potential to develop the alloy with high strength.

We present the results of a combined experimental and atomistic simulation study of the mechanical properties of a Co- Mo bicontinuous nanocomposites with varying ligament sizes. Simulations of bicontinuous nanocomposites under compression using atomistic molecular dynamics are compared to experimental results on in situ SEM and TEM microscale compression tests on co-sputtered Cu-Mo. The simulations allow the elucidation of the defect nucleation, glide and storage phenomena within individual Cu and Mo phases and Cu/Mo interfaces. The simulation results are also compared with the predictions of a dislocation theory based analytical model of glide of individual dislocations in nanoscale metallic phases confined by interphase boundaries that are strong barriers to slip transmission. The integrated simulation-experimental study shows that the bicontinuous intertwined morphology is effective in suppressing localized shear bands and promoting homogeneous distribution of plastic flow in metallic nanocomposites.

Pure metal nanofoams in the form of interconnected networks of ligaments have shown strong potential over the last few years in areas of catalysts, batteries, and optics. However, they are often fragile and difficult to integrate into engineering applications. In order to mechanically strengthen them two new types of foams are studied in this work: alloyed and nanolayered. These materials will operate at the macroscale but they will maintain a nano or atomistic ordering requiring a multiscale approach for the study of their properties. For that purpose, in this work, we combined molecular dynamics and finite elements to study the mechanical behavior of these metallic structures made of copper and nickel and compared them to their pure metal counterparts. Furthermore, manufactured nanofoams were tested using nanoindentation and the findings are used to compare and validate the simulation results.

Structural transition in Molybdenum has been reported to occur under large stresses in thin nanowires and tip of a crack. Here, we use Molecular Dynamics (MD) simulations to demonstrate that a uniform 3-step structural transformation of Molybdenum atoms can occur in Mo/Cu bicontinuous intertwined materials during tensile loading. The Mo atoms first transit from a <001>-oriented bcc structure to a <001>-oriented fcc structure via Bain transformation. The <001>-oriented fcc phase then transforms to a <110>-oriented bcc via Pitsh transformation. This novel homogenous transformation results into a stress-strain curve containing two elastic and two plastic regimes as well as enhanced plasticity with few dislocation slips and no twinning. Our results reveal that the driving force for such phase transformation is the high interfacial stress in the bicontinuous intertwined structure. This study suggests new strategies for improving the ductility of ultra-strong nanocomposite metals.

The field of micro and nanomechanical testing has expanded dramatically. Many sample geometries and testing techniques have been developed to investigate a variety of mechanical material properties. Nanoindentation contact experiments with pyramid shaped indenters have led the way. The properties that one can determine using contact mechanics has expanded as well. In this talk we will focus on high speed indentation. This technique requires that dynamic mechanical effects must be considered to correctly to perform the experiments. The instruments must be carefully designed and characterized so that the dynamic effects of the instrument and sample on the combined results obtained can be separated and quantitatively understood. In addition, the spacing of the contact experiments must be sufficiently large to avoid interactions between the indentations. We will examine the question of how close they can be in detail. The results indicate that the indentations can be safely spaced twice as close as has been suggested in the literature. Once these experiments are performed several analysis techniques help make the data useful. In particular, data clustering algorithms can be used to separate phases in multiphase materials. We will show the results of such analysis on the technologically important example of bond layers in thermal barrier coatings on turbine blades.

Structural nanomaterials and nanocomposites have excellent mechanical performance, which can be tuned through both structural architecture and material size effects. However, the behavior of individual component normally sized at 5 - 50 nanometer in the structural nanomaterials is key important, has not been explored before. This talk will be the in-situ mechanics for studying the mechanical behavior at atomistic scale for 5 to 50 nanometer-sized pillars (so-called lattice pillar) of metallic crystals. The experimental molecular dynamics with the in-situ high resolution transmission electron microscope is going to open a new approach to directly characterize atomic-scaled deformation with in-situ mechanics for such small sized pillar. I will cover the stress-strain behavior, high stress induced lattice disturbance, dislocation dipole nucleation and competition between slip and twinning in the deformation process, which potentially can be related to individual component of structural nanomaterials such as nano-porous and nano-lattice materials.

Recent experiments show that metallic nanocrystalline materials can be rejuvenated under the stimuli of femtosecond laser and the system’s hardness can be effectively tuned. However, a mechanistic understanding on such phenomenon remains unclear. Here we investigate the responses of a group of <100> symmetric tilt grain boundaries (STGBs) in a Cu modeling system under the driving of fast thermal cycling. A universal hysteresis behavior is observed during such process, and the GB is driven to a higher energy state after the pulsive stimulus. By employing an enhanced data-mining algorithm to analyze the annealing behavior of GBs at various thermal conditions, we are able to construct a high-resolution pixel map for the GB’s energetic evolution and show that it can be divided into an ageing regime and a rejuvenating regime over the broad energy—temperature parameter space. We further demonstrate that the origin of ageing/rejuvenating is attributed to the energy imbalance along with the elementary hopping processes in the GB’s underlying potential energy landscape (PEL). The hereby established picture then allows us to develop a self-consistent kinetic equation to describe the GB’s energetic and mechanical responses to external stimuli, which can qualitatively explain the recent femtosecond laser experiment. The present study thus enables a deeper mechanistic understanding towards the non-equilibrium evolution of nanocrystalline materials.

Origami inspired self-assembly of three-dimensional (3D) micro- and nanostructures shows diverse applications across a broad range of areas, such as metamaterial and plasmonic devices, electronics, and biomedical devices. Significant development of the assembly technique has been achieved based on various stimuli responsive materials. However, precise bidirectional motion control with a desired sequence remains a challenge due to the limited capability of delivering localized stimuli, resulting in constrained functions of origami. In this work, we report a route to fully mimic the functions of origami including bidirectional curving, folding, stretching, and even knitting. The mechanism involves electron irradiation triggered in situ crystallization in amorphous material, which results in volume shrinkage, generating sufficient stress for assembly. The scanning electron microscopy (SEM) offers capability of real time monitoring and fine focus, which enables precise stimuli delivery, leading to programmable assembly. Prototypes including nano-machines, nanoscale test platforms, and advanced optical devices have been demonstrated using this strategy, showing the possibility for innovation of next generation devices and smart materials.

Metal-ceramic nanocomposites exhibit exceptional mechanical properties with combinations of high strength, toughness, and hardness not achievable in monolithic metals or ceramics, which make them valuable for applications in fields, such as the aerospace and automotive industries. It has been shown that the interface between components in a nanocomposite plays an important role in controlling these properties. Here we demonstrate the preparation and mechanical properties of nanostructured tungsten–silicon-carbide composite materials, which contain a ductile metal component and a brittle ceramic component interspersed at the submicron scale in periodic fashion. In these nanocomposites, the ratio of interfacial area to volume is systematically controlled through templating and nanocasting methods, permitting a systematic investigation of the brittle-to-ductile transition through nanoindentation and micropillar compression tests.

The material synthesis is enabled through colloidal crystal templating of three-dimensionally ordered macroporous (3DOM) materials as host structures to create nanocomposites with high interfacial area. 3DOM materials are porous solids that feature a highly ordered, fully interconnected, porous structure with pore sizes in the 200–500 nm range and struts that are tens of nanometers in diameter. The macropores can be infiltrated with a second phase in a nanocasting process to form periodic composite structures. The component ratio is controlled by the choice of the host and guest components and through processing, and the interface-to-volume ratio through the pore size. Here we investigate 3DOM W and 3DOM SiC materials infiltrated with SiC and W guests, respectively. Silicon carbide is a material with high hardness, high thermal conductivity, and high mechanical strength/oxidation resistance. However, it suffers from unwanted brittleness at elevated temperature. Introduction of tungsten can introduce the ductility of a metallic phase to modulate the composite strength and strain hardening behavior. Moreover, compared to other metals, tungsten has only a small mismatch in coefficient of thermal expansion with silicon carbide. Because tungsten and silicon carbide can react with each other to form tungsten carbide or tungsten silicide phases at different temperatures, we found that various interfacial compositions could be achieved by controlling the synthesis temperature. Nanoindentation and micropillar compression testing reveal enhanced strength, as well as significant deformation prior to fracture.

Creating materials with a combination of high strength, substantial deformability and ductility, large elastic limit, and low density represents a long-standing challenge in materials science because these properties are typically mutually exclusive. Using a combination of two-photon lithography and high-temperature pyrolysis, we demonstrate micro-sized pyrolytic carbon with a tensile strength of 1.60+/-0.55 GPa, a compressive strength approaching the theoretical limit of ~13.7 GPa, a substantial elastic limit of 20–30%, and a low density of ~1.4 g cm⁻³. This corresponds to a specific compressive strength of 9.79 GPa cm³ g⁻¹, a value that surpasses that of nearly all existing structural materials. Glassy carbon samples with dimensions below 2.3 μm exhibit rubber-like behavior and sustain a compressive strain of ~50% without catastrophic failure; larger ones exhibit brittle fracture at a strain of ~20%. Large scale
atomistic simulations reveal that this combination of beneficial mechanical properties is enabled by local deformation of 1 nm-long curled graphene fragments within the pyrolytic carbon microstructure, the interactions among neighboring fragments, and the presence of covalent carbon–carbon bonds.
Building on these individual carbon nano-building blocks, we developed a process to create nano-architected glassy carbon that attains specific strength of up to one to three orders of magnitude above that of existing micro- and nanoarchitected materials. Experiments and simulations demonstrate that for densities higher than 0.95 g/cm3 the nanolattices become insensitive to fabrication-induced defects, allowing them to attain nearly theoretical strength of the constituent material.
A key challenge in studying mechanics of nano-architected materials is their lack of a proper separation-of-scales between unit cell size and sample dimensions, which precludes proper impact testing when the impact time scale allows elastic waves to propagate information from the free boundaries back to the projectile. Another difficulty in dynamic testing of nanomaterials is the lack of reliable methods to achieve consistent loading conditions while capturing proper temporal and length scales.
We developed a method to design, fabricate, and test nano-architected materials at supersonic impact speeds of up to 1 km/s that address these challenges by employing Laser Induced Particle Impact Test (LIPIT). In this method, a pulsed laser accelerates a microscale impactor to velocities <1.2 km/s, and the impact is captured using frames from a high-speed camera within a microscope, allowing for nanosecond temporal and micrometer spatial resolution. We probed different impact regimes by selecting spherical SiO₂ impactors of different diameters while varying impact velocity from 50 to 1,100 m/s. Post-mortem analysis of impacted samples via confocal and electron microscopy indicates that nanolattices with 26% relative density can withstand impact speeds that exceed 700 m/s, either absorbing or rebounding the projectile.
The combination of high specific strength, low density, impact resistance,and extensive deformability before failure lends such nano-architected pyrolitic carbon to being a particularly promising candidate for applications under harsh thermomechanical environments.

Carbon nanostructures represent a growing area of research due to their excellent mechanical, electrical and thermal properties. Research studying their synthesis has rapidly populated academic journals, but these procedures are difficult to implement in large-scale manufacturing due to their complexity and cost. In contrast, electrocharging assisted bulk processing of a novel class of materials, termed “covetics,” presents a practical option for macroscale production of nanocarbon-metal composites. Notably, this process facilitates incorporation of carbon on the order of a couple weight percent in metals where carbon solubility is normally in the low ppm range. A 40% increase in tensile strength and 40% increase in electrical conductivity has been measured in Al covetics; however, there is minimal understanding of the structure-process-property relationship and consequently there is high variability in measured properties among trials. To investigate the relationship between fabrication conditions and properties of the composite material, the parent alloy, Al 1350, is melted in a graphite crucible nestled within induction coils. Following the addition of activated carbon precursor, a DC current is applied to the molten mixture via a graphite electrode to initiate the electrocharging assisted conversion process. Raman spectroscopy is used to evaluate the structure and degree of crystallinity of carbon incorporated into the Al lattice via the Tuinstra-Koenig relation for graphitic carbon. XPS is used to measure the amount of carbon incorporated and determine the sp²/sp³ ratio. XRD is used to identify the formation of any secondary phases or carbides. Following structural characterization, four-point probe testing is performed to measure the bulk electrical conductivity of the covetic material. Atomic force microscopy techniques are used to measure the local electrical and mechanical behavior of the covetic by Kelvin probe force microscopy and nanoindentation, respectively. Results to date have found that the activated carbon precursor is converted to sp² hybridized graphitic carbon with an increase in crystallite size as measured by Tuinstra-Koenig. From XRD and XPS, it has been determined that there is no measurable formation of carbide phases despite a couple weight percent of incorporated carbon. The electrical conductivity of the covetic material has also been shown to increase. The local electromechanical behavior of the covetic measured by AFM techniques will be used to develop a fundamental understanding of the improved properties measured in the composite and used to inform adjustments to the fabrication process accordingly.

Funded by DOE-EERE grant DE-EE0008313. Sponsored by the Army Research Laboratory under Cooperative Agreement W911NF-19-2-0291.

Nanolattices exhibit wide range of unique properties and attract a growing interest in many scientific fields. The architected hierarchical structure allows tailoring the nanolattice properties to obtain objects of relatively large dimensions with nano-size features. In our work we fabricated a core-shell polymer-platinum nanolattice by combining several fabrication techniques. The polymeric nanolattice was designed by CAD, and fabricated by a two photon lithography technique to achieve the required architecture. The lattices were then coated with thin film of platinum using atomic layer deposition (ALD) method. The unique interaction of the gaseous ALD precursor with the polymer surface resulted in a porous film with nano grains, providing a hierarchy ranging from the micrometer-size overall dimensions down to the microstructural features of a few nanometers. The microstructure evolution (i.e grain size, pore size and defects) caused by annealing at elevated temperature was investigated, and correlated to the mechanical behavior of the lattices during in-situ compression inside a scanning electron microscope.

A practical route to exploiting graphene’s supreme properties for a variety of applications is to incorporate graphene layers in composite materials. Harnessing the high stiffness, intrinsic strength as well transport properties of graphene in its composites requires the combination of high-quality graphene having low defect density, and the precise control of the interfacial interactions between the graphene and the matrix. These requirements equally fold for polymer and metal matrices, and enable the use of graphene in applications ranging from tough thin films for flexible electronics to the design of advanced aerospace structures. In this study, to address the synthesis and control of these composites and their mechanical properties at the nano to the microscales, we propose a model system of ultrathin metal films coated with graphene monolayer via chemical vapor deposition (CVD) to study as-grown graphene’s contributions in graphene-metal nanocomposites.

However, several factors need to be considered to assure the stability of ultrathin catalysts at the elevated temperatures (1000-1100 C) for high quality graphene synthesis. We develop a highly dynamic CVD synthesis route (less than two minutes) to achieve high quality graphene monolayer growth on ultrathin (100-150 nm) Pd films while avoiding solid-state dewetting which easily takes place at the high temperatures. We study how the competition between temperature-driven segregation and precipitation of carbon atoms governs the graphene’s nucleation and growth kinetics on ultrathin Pd films. The result of the dynamic recipe is repeatable growth of graphene monolayers with over 2.5-fold faster graphene growth rate than that in typical Cu catalyzed synthesis, as well as very low defect density as confirmed in Raman spectroscopy.

Precise mechanical characterization of as-grown ultrathin graphene-Pd films is carried using micro bridge Nano indentation. CVD grown graphene-metal composite thin films exhibit unusual increase in the elastic modulus, strength and toughness. For example, there is 35 % and 57 % increases in the Young’s modulus and tensile strength in graphene-Pd nanocomposites compared to those for a bare Pd film having a thickness of 66.4 nm. Notably, this enhancement is beyond the rule of mixtures limits, and exhibits scale effects, where the composite modulus increase varies with the thickness, and is highest for the thinnest metal thicknesses. This usual elastic modulus enhancement is attributed to the significant change in the surface stress of ultrathin films after coating with graphene. Raman spectroscopy and electron imaging of graphene-Pd surface reconstructions confirm the high interfacial stresses due to the combination of the lattice mismatch between the graphene the underlying metals and the kinetics of CVD synthesis. We also observed significant increase in toughness and qualitatively different modes of crack propagation owing to the addition of the high stiffness graphene shield on the metal surface during synthesis. The findings of this study promote graphene-based thin film composites for flexible electronic devices, and enable fundamental studies of exploiting strain engineering at the graphene-metal interface for electronics, chemistry and mechanics.

It is reported that negative or no strain rate dependency of the flow stress of metallic glasses appears at high strain rate or low temperature condition [1][2]. We reveal that this curious strain rate dependency is strongly related to the rate of glass relaxation process from a rejuvenated glass state induced by activation of shear transformation. We use a constitutive model which estimates average residence time of thermal activation process and calculate the flow stress of metallic glass at certain strain rate and temperature [3]. In the model, we assume relaxation process recovers the activation energy of shear transformation of rejuvenated glass state and the relaxation is expressed as stretched or compressed exponential form. Interestingly, we find the negative or no strain rate dependency appears when the timescale of the activation of shear transformation approaches the characteristic timescale of glass relaxation process. Especially, the negative strain rate dependency only appears with the compressed exponential relaxation. The compressed exponential relaxation is always observed in the material with gel or colloid-like structure, and in this decade, it is observed in metallic glass too [4].
[1]J. Lu, et al., Acta Mater. 51 (2003)
[2]A. Dubach, et al., Phil. Mag. Lett. 87 (2007)
[3]F. Yue, et al., Phys. Rev. Lett. 109 (2012)
[4]B. ruta, et al., Phys. Rev. Lett. 109(2012)

Understanding the mechanical behavior of metal thin films at elevated temperature is essential for the design of reliable devices, which often operate above room temperature. So, from the past, several studies have been performed on the high-temperature behavior of freestanding metal thin films to understand their intrinsic properties. However, in real applications, there are very few cases where the thin films are placed as freestanding. Also, the mechanical behavior of metal thin films can be changed when they are layered with another thin materials due to the existence of interface. Therefore, understanding the performances of thin films with passivation layers at elevated temperature is important, but studies have been limited due to difficulties associated with sample handling, oxidation, temperature uniformity, and fabrication of multi-layers with passivation layers such as dielectric or 2D materials.

In this poster, we introduce an apparatus to perform micro-tensile tests at elevated temperatures inside a scanning electron microscope. The apparatus has a stroke of 250μm with a displacement resolution of 10nm and a load resolution of 9.7μN. Measurements at elevated temperatures are performed through use of two silicon-based micromachined heaters that support the sample. Each heater consists of a tungsten heating element that also serves as a temperature gauge. To demonstrate the testing capabilities, tensile tests were performed on submicron-thick freestanding metal thin films with and without the passivation layers from room temperature to elevated temperature. Ultra-thin dielectric (Si₃N₄) and 2D layers (e.g. MoS₂, graphene) were chosen as passivation layers, which are mainly used in electronic devices with metal thin films. Stress-strain curves show a significant decrease in yield strength and initial slope for the samples tested at elevated temperature, which we attributed to diffusion-facilitated grain boundary sliding and dislocation climb. However, when passivation layer is added on the film, the decrease of yield strength and necking of the metal thin films are expected to be inhibited and strain hardening may become significant due to dislocation pileups at the interface.

Taking microelectromechanical systems (MEMS)-based devices such as sensor, actuator, micropower generator into the harsh environments (temperature greater than 200 ^oC, power level above 100W) would greatly expand the applications ranging from miniature internal combustion engines, to high frequency switching, microheaters and high temperature sensors. However, the dominant structural material used in conventional MEMS devices are silicon, which can not be used in thermal environment due to its loss of mechanical properties at elevated temperatures. Therefore, the development of metallic MEMS alloys that possess enhanced mechanical and functional properties at intermediate to high temperature range, requisite dimensional stability, and the ability to be shaped on the microscale would provide the wider design space for novel MEMS applications. Metals and alloys are especially attractive in MEMS applications that require high density, electrical and thermal conductivity, strength, ductility and toughness.
Electrodeposited nanocrystalline LIGA Ni offers a route for microfabrication of metallic parts with high aspect ratios and have good room temperature yield strength, but temperatures as low as 200^oC can result in grain growth and a significant loss of strength. Alloying elements with limited diffusivity can strongly suppress grain growth, in particular, Ni-W thin films prepared by magnetron sputtering are known to exhibit thermally stable nanocrystalline microstructures. Adding another solute element Molybdenum to the system can effectively improve dimensional stability by lowering the coefficient of thermal expansion over long temperature range while retaining mechanical integrity.
Here we report the mechanical behavior of sputter deposited Nickel (Ni)-Molybdenum (Mo)-Tungsten (W) ternary alloy thin films at various temperatures. The microstructure and chemical composition of deposited materials are characterized using transmission electron microscopy (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The as-deposited films go down as single-phase solid solutions and possess high nanoindentation hardness of approximately 9 GPa. Custom built micro-tensile tester which operates inside the SEM is used to perform comprehensive in-situ thermomechanical study on Ni-Mo-W thin. First, the in-plane mechanical response of the as-deposited and heat-treated freestanding Ni-Mo-W thin films are investigated at room temperature. Then the mechanical properties of films annealed at 400^oC are explored at elevated temperatures by Joule heating of two micromachined tungsten heater that support the tensile specimen.

This study provides that relationship between distributed thermal elastoplastic stresses near the edge of the interface and practical tensile strength of ceramics/metal joint system with arbitrary wedge angles. Interface wedge angles were defined as a configuration angle between interface and free surfaces of both materials. The wedge angles on ceramics and metal were described φ₁ and φ₂ respectively. The wedge angles of the metal side φ₂ were set over the range in 20°< φ₂ ≤180°, as kept maintaining one of the ceramic side φ₁ = 90°.
Thermal elastoplastic FEM was carried out to clarify the distribution of the thermal residual stress near the edge of the interface each wedge condition using ceramics to metal joint FEM models bonded at high temperature ranges. The stress singularity was confirmed near the edge of the interface on the free surfaces of ceramic side each FEM model. The gradient of stress distribution was evaluated and compared in FEM elastoplastic analysis.
The numerical results were compared with the experimental results, which showed effects of the wedge angle on the practical tensile strength of silicon nitride to nickel joints specimens bonded at the same as numerical temperature conditions. Changing the wedge angle from a specific wedge angle improves the tensile bonding strength, since the residual stress was decreased by changing the wedge angle. This result corresponds with the FEM results as increasing φ₂ from the right angle decreases the concentration intensity of the residual stress.
The dependency of the optimum interface condition on the bonding temperature was discussed based on both results of numerical and experimental in high bonding temperature ranges.

In general, surface coating technology includes painting & organic coating, eletro-& electroless deposition, physical vapour deposition, chemical vapour deposition and thermal spraying et al. Electro-deposition and electroless deposition are widely used in industry. Electro-deposition, is a kind of process that the metal ions in the electrolyte are reduced into metal with external electric assistance, and deposited on the surface of the substrate for forming a compact metal or alloy coating. Electroless deposition is a kind of process that the metal ions in the solution are reduced into metal under a strong and suitable reducing agent without external electric assistance, and deposited on the surface with catalytic active sites of the substrate for forming a compact metal coating.
As a new type of two-dimensional (2-D) carbon nanomaterial, graphene (Gr) is of many excellent properties, such as high strength, ductility, thermal conductivity, wear resistance, extremely large specific surface area, remarkable chemical inertness unless exposed to harsh reaction conditions, etc., and shows great application potential as an excellent reinforced phase in the field of composite materials. In recent years, our group engages in preparation of the metal or alloy - Gr coatings via electro-deposition or electroless deposition for high surface performances, and explores their industrial applications.
1. Zn-Gr composite coating via pulse reverse electro- deposition.
In this work, the hydrophilic graphene oxide (GO), instead of the hydrophobic Gr, was chosen and added into the electrolyte bath. Due to the reduction of GO into Gr during deposition, a Zn-Gr composite coating was prepared successfully by using a pulse reverse electro-deposition method on the iron substrate. The experimental results reveal that: 1) The coationg exhibits a strong adhesion with the substrate; 2) The Zn crystalline and morphologies change greatly due to the co-deposition of Gr, i.e., the main preferred orientations of (112) and (101) crystal planes in pure Zn coating gradually transform into the (103), (102) and (110) planes in the Zn-Gr composite coating. 3) There is an optimal GO adding amount in a range 0.3–0.5g/L in the electrolyte, at which the Zn-Gr coating exhibits the highest corrosion resistance and corrosion current density just be a hundredth comparing with that of pure Zn coating in simulated seawater environment.
2. Alloy (Zn–Ni)-Gr composite coating via pulse reverse electro-deposition.
The alloy (Zn–Ni)-Gr composite coating was prepared by using pulsed-reverse electrodeposition. It was found that: 1) the coating grain size became smaller after GO added into the electrolyte; 2) the presence of Gr could effectively inhibited the corrosion of the substrate; 3) comparing to regular Zn-Ni coating, the (Zn–Ni)-Gr coating's microhardness and elastic modulus were increased.
3. Cu-Gr composite coating via electroless deposition
In this work, the Cu-Gr composite coating was prepared successfully by using electroless plating with sodium hypophosphite (NaH₂PO₂) as the reducing agent. The experimental results reveal that: 1) Gr exhibits a significant effect on the microstructure of the coating, for example, the grain size of the composite coating varies from the concentration of the added GO, and the optimal value shows the smallest grain size and the highest compactness. 2) At the optimal GO concentration, the Cu-Gr coating also is of the highest mechanical properties. 3) Compared with regular pure Cu coating, the corrosion resistance of the Cu-Gr coating has a significant improvement, i.e. the E_corr, i_corr, corrosion rate and inhibition efficiency are also optimized at the optimal GO concentration. It is expected that the Cu-Gr coating will have broad application prospects in the fields of electronics industry, marine engineering and military industry.

Electrodeposition is a widely applied method in fabrication of metal-based components in micro-electrical-mechanical systems (MEMS) devices. As an example, gold material, which has a high mass density of 19.3 x 10³ kg / m³ at 298K, is utilized as the proof mass in a MEMS accelerometer since performance of a MEMS accelerometer is highly related to overall mass of the proof mass. However, the relatively weak mechanical strength of gold limits the structure stability and reliability when used in movable micro-structures. Enhancing the mechanical strength is an important task to ensure high structure stability of movable structures in novel MEMS.
Co-deposition of metal matrix-oxide nanoparticles (NPs) composites is a simple and effective method to improve mechanical properties of the material. The NPs can be oxides such as Al₂O₃, TiO₂ and Cr₂O₃, carbides like SiC and TiC or graphitic materials like carbon nanofiber, carbon nanotube, and graphene oxide. A wide range of metals includes zinc, nickel, copper, nickel and silver has been used as matrix of the composite. In this study, TiO₂ NPs are introduced into the gold electrodeposition bath to enhance mechanical strengths of electrodeposited gold-based materials by forming TiO₂ NPs-reinforced gold matrix composites. Effects of the strengthening provided by the TiO₂ NPs are evaluated by micro Vickers hardness test and micro-compression test.
The Au-TiO₂ composite films were deposited on 1 cm² area copper plates. A common two electrode system was use for the electrodeposition with the copper plates as the working electrode, a platinum plate as the counter electrode, and a commercial non-cyanide sulfite-based gold bath containing various amounts of TiO₂ NP powder (AEROXIDE® TiO₂ P 25, Evonik) as the electrolyte. Temperature of the electrolyte was kept at 40°C. All the electrodeposition was carry out through galvanostatic method with a current density of 5 mA/cm² for 1.5 hours. A pure gold film was also deposited with the same electrodeposition parameters but without TiO₂ NP powder in the electrolyte for comparison. Composition and crystal structure of the composite films were characterized by energy dispersive X-ray spectrometry (EMAX, Horiba) equipped in an scanning electron microscope (SEM, S-4300SE, Hitachi) and an X-ray diffractometer (XRD, Ultima IV, Rigaku). Surface morphologies of the as-deposited films were observed with the SEM. Hardness of the films were evaluated by micro Vickers hardness measurement (HMV-G 20S, Shimadzu). Micro-compression tests were conducted using micro-pillars fabricated by focus ion beam (FIB, FB-2100, Hitachi) from the composites films.
Surface morphologies of the Au-TiO₂ composite films were found to be composed of particles through the observation of secondary electron images. A decrease in particle size was observed with an increase in content of TiO₂ NPs in the film from 1.45 to 2.72 wt%. XRD spectrum of electroplated gold, TiO₂ NPs, and Au-TiO₂ composite films were measured to confirm the crystal structure of composite films. The primary characteristic peak of P 25 TiO₂ NPs at 2θ = 25.1° (corresponding to characteristic peak of (101) plane, anatase phase) was also observed in spectrum of Au-TiO₂ composite films, which confirm the existence of TiO₂ NPs in the deposits. Broaden characteristic peaks of the Au-TiO₂ composite film indicated grain refinement caused by co-deposition with the TiO₂ NPs. Improvement of the mechanical properties was verified from the micro Vickers hardness measurement and micro-compression tests. The Vickers hardness increased from 135 HV (pure gold) to 207 HV (2.72 wt% TiO₂) and the Yield strength increase from 0.44 GPa (pure gold) to 0.84 GPa (2.72 wt% TiO₂). The results demonstrated the significant enhancement of mechanical strength by introducing TiO₂ NPs into the gold matrix.

Novel nanocarbon-metal composites called “covetics” have the potential to significantly improve upon the mechanical and electrical properties of established metals and alloys, including Al, Cu and Ag. When a stir-cast Al melt containing a C source is subjected to electric current above a threshold value, carbon atoms are ionized, causing nanoscale graphitic ribbons and chains to form and become incorporated within the resulting Al lattice. Greater understanding of the atomic-scale behavior of covetics is essential to improve desirable properties of Al alloys, and by extension their performance in aerospace, structural, and power transmission applications.

This work explores the relationship between the structure of nanocarbon-Al composites and their local mechanical and electrical properties. We characterize nanocarbon-Al interfaces in covetics using Raman spectroscopy, conductive AFM and nano-indentation, and TEM. AFM modulus mapping reveals areas of locally stiffer microstructure. All characterization is performed within a few µm of a chosen region of interest as guided by fiducial marks milled using a dual focused ion beam (FIB)/SEM. The FIB is also used to prepare TEM specimens from the region of interest for analysis of the dislocation structure. Dislocation densities on the order of 10¹⁰ cm^-2 are observed along with sub-grain misorientations of 2–3 degrees. Through close inspection of features identified by surface characterization techniques, we glean information about the role of nanocarbon phases on the mechanical and electrical performance of covetics.

Composition analysis is obtained by electron energy loss spectroscopy (EELS) in TEM as well as X-ray photoelectron spectroscopy (XPS). Raman and EELS spectrum images analyzed by nonlinear fitting and machine learning techniques reveal increased ordering of nanocrystalline carbon embedded within the Al lattice. The degree of ordering is found to depend on the applied electric current, with greater exposure corresponding to greater graphitic order as indicated by Raman G and D peak intensities and shifts. Further evidence of sp² carbon bonding and Al-C bonding is seen by EELS and XPS. Preliminary results indicate directionality in the microstructure potentially coordinated with applied current direction, which will be investigated for correlation with highly ordered graphitic features. The relationship between nanocarbon incorporation and grain growth behavior will also be explored.

This work is supported by the U. S. Department of Energy under Award No. DE-EE0008313.

Ductile cast iron (DCI) is becoming an increasingly important engineering material due to its remarkable strength, ductility, wear resistance and low production cost. However, the industrial heavy-loading components, like gears and bearings, require a highly improved wear resistance of DCIs. A new austempering process is developed in a commercial unalloyed DCI, which comprises the following heat treatment cycle: austenitizing at 900 degrees Celsius for 30min, rapidly quenching into a patented water-based liquid at 60 degrees Celsius isothermally holding at 190 degrees Celsius (slightly lower than the starting temperature of martensite formation of DCI matrix) for 8h and air cooling. A multiphase matrix consisting of lenticular prior martensite (PM), bainitic ferrite (BF), retained austenite (RA) and a nano-scaled super bainite ((BF+RA)nano) is produced. It is shown from the pin-on-ring wear test that a super wear resistance under the load range from 20 to 500N during dry sliding is mainly due to the synergistic strengthening between multiple phases and nodular graphite. In the initial stage of wear, the main mechanism is adhesive due to the strip-off and breaking effect of graphite. In the later stage, the more martensite can be induced from (BF+RA)nano and RA through a TRIP effect with increasing the normal load, which is attributed to the higher wear resistance.
The result shows a reasonable value of wear performance, which recommends the designed nanostructured multiphase DCI for more extensive application prospects.

Heterogeneous metallic materials with multimodal grain size distribution of constituent grains and phases show significant improvements in ductility at yield strength comparable to their nanocrystalline counterparts. However, the fundamental mechanisms for the increased plasticity were not well understood. Model system heterogeneous hypereutectic Al-20Si with µm-size Al coarse grains (CGs) embedded in nm-size Al-Si eutectic ultrafine grains (CGs) was fabricated by laser surface remelting (LSR). In this report, the mechanical properties of the LSR heterogeneous Al-20Si were studied. Hardness testing and micropillar compression reveal different mechanical properties for as-cast Al-12Si eutectic and LSR heterogeneous Al-20Si. In addition, the hardness increases as the fraction of nanoscale Al-Si eutectic UFGs increase. in-situ SEM and in-situ TEM further reveal the deformation behavior of micropillars and nanopillars. The different mechanical properties and in-situ observations of LSR heterogeneous Al-20Si may provide an explanation to the enhanced plasticity.

Silicon is a main semiconductor material used for fabricating different devices and systems including but not limited to integrated curcuits, microelectromechanical systems and various sensors. Physical properties of silicon are very well-studied and scientists have almost exhausted the possibilities of changing these properties by traditional methods based on implantation and diffusion of appropriate chemical elements in silicon. In this regard, there is a great interest in finding new methods for creating silicon-based composites exhibiting properties that silicon does not have. Such materials have been identified as a new solution for a significant expansion of the functional abilities of silicon.
This work is aimed at developing a simple and efficient method for the synthesis of nanocomposite materials that can be easily integrated with silicon wafers [1]. Our approach consists in formation of pore channels in a silicon wafer followed by deposition of different metals, which impart new functional properties to silicon. Research of mechanical properties of nanocomposites based on porous silicon is very important because these new materials (consisting of silicon skeleton and metals) are subjected to different chemical and thermal treatments at extrime conditions during fabrication of endpoint device or system.
Ferromagnetic metals (Ni, Co and Fe) were introduced into the pore channels to create new silicon-based materials demonstrating unique magnetic properties. To provide the luminescence, rare-earth metals and ZnO were deposited in porous silicon. Nanoparticles of noble metals were formed on the surface of silicon skeleton to ensure plasmonic properties.
Mechanical and structural characteristics of nanocomposite materials based on porous silicon and metals were investigated together with their magnetic and optical features. The most outstanding results are presented and discussed in this work.
[1] E. Chubenko, S. Redko, A. Dolgiy, H. Bandarenka, V. Bondarenko. Composite and Hybrid Materials Based on Porous Silicon. Chapter 9. In: Porous Silicon: From Formation to Applications. Ed. G. Korotchenkov. CRC Press, Taylor and Francis Group, 2016. P. 141-162.

Sn-Bi alloy provides a low melting, non-hazardous and feasible alternate to the conventional Sn-Pb solder alloys for circuit packaging and interconnects. In this work, Sn-Bi (Bi 27 wt.%) coating with varying concentrations of graphene oxide (GO) was electrodeposited over mild steel substrate. Microstructure-corrosion property correlation in SnBi-GO composite coatings was then investigated as a function of GO content. All the coatings exhibited compact and crack-free morphology with GO sheets embedded within the matrix. XRD analysis of SnBi-GO coatings revealed the formation of a two-phase matrix containing Sn-rich and Bi-rich phases. Tafel polarization measurements and electrochemical impedance spectroscopy (EIS) test results revealed that the corrosion rate of the coatings was sensitive to the amount of GO in the coating. The corrosion rate decreased with initial additions of GO to reach a minimum before increasing for higher GO additions. This indicated towards the presence of an optimum with respect to the amount of GO for achieving highest corrosion resistance performance. Microstructural characterization conducted using the electron back scatter diffraction (EBSD) technique correlated the observed corrosion behaviour with the relative fraction of the low and high angle grain boundaries present within the coating matrix. It was observed that the coating with highest corrosion resistance performance contained maximum fraction of low energy low angle grain boundaries. Transmission electron microscopy based analysis revealed that the addition of GO led to enrichment of Sn rich phase and dissolution of more Sn in Bi phase. TEM, EBSD and zeta potential studies were used to correlate the fraction of LAGBs with the amount and distribution of Bi in the matrix and the corrosion resistance performance of the SnBi-GO coatings.

Gold materials are commonly applied in electronic devices because of the superior corrosion resistance, electric conductivity, and chemical stability [1]. Recently, micro-electro-mechanical systems (MEMS) inertial sensors utilizing gold materials in the main components are reported to have 1000 times higher sensitivity than conventional MEMS inertial sensors [2,3], which is mostly because of the high mass density. For application of gold-based materials in next generation MEMS devices, further strengthening is needed to ensure high structure stability. Regarding strengthening of gold materials, grain boundary strengthening and solid solution strengthening mechanisms can be applied simultaneously by alloy electrodeposition process [4], which the yield stress reached 1.15 GPa when the grain size was refined to 5.3 nm with the copper concentration at 12.3 wt%. On the other hand, Young’s modulus of the high strength Au-Cu alloys is also needed in design of MEMS components, such as the micro-scale spring in MEMS inertial sensors. In this study, Young’s moduli of micro-cantilevers composed of the Au-Cu alloy are measured by a non-destructive resonance frequency method [5].
The Au-Cu alloy micro-cantilevers were fabricated by electrodeposition and lithography. The Ti adhesion layer and the Au seed layer were deposited by sputtering, and the layer thicknesses were both at 100 nm. The Au-Cu electrolyte used in this work was a commercially available electrolyte provided by MATEX Co. Japan, which contained 17.3 g/L of X₃Au(SO₃)₂ (X = Na, K), 1.26 g/L of CuSO₄, and EDTA as the additive with pH at 7.5. The electrodeposition was carried out at 50 °C, and the current density was varied from 0.5 to 2 mA/cm². A piece of Pt plate was used as the anode. Design-length of the micro-cantilever was varied from 50 ~ 1000 μm, and design-width of the micro-cantilever was ranged from 5 ~ 20 μm. Thickness of the Au-Cu alloy layer was from 2.6 ~ 4.0 μm. Composition of the films was characterized by energy dispersive x-ray equipped in a scanning electron microscope.
Young’s moduli of the Au-Cu alloy micro-cantilevers were calculated from resonance frequencies of the micro-cantilevers. The resonance frequencies were experimentally obtained as shown in the following. First, a voltage pulse (amplitude: 10V, pulse width: 100μs) was applied between the cantilever and a fixed electrode to initiate free vibration mode. Next, a laser doppler vibrometer was used to measure displacements of tip of the cantilever. Finally, the resonant frequency was obtained from a FFT (fast Fourier transform) analyzer.

Surface conditions of Au-Cu alloy micro-cantilevers produced at a current density ranged from 0.5 to 2 mA/cm² were all uniform and smooth. The Cu content increased from 1.40 to 4.03 wt% as the current density increased from 0.5 to 2 mA/cm², which is because standard reduction potential of Cu is more negative than that of Au, and an increase in the current density leads to a more negative applied potential. Young’s modulus of a micro-cantilever composed of 98.3 % Au with the length at 700 μm, the width 20 μm, and the thickness at 3.95 μm was 80.9 GPa. The value is close to that of pure gold (79 GPa) and much lower than that of pure copper (117 GPa). On the other hand, no obvious trend was observed when the width changed from 10 to 20.

References
[1] M. Merhej, D. Drouin, B. Salem, T. Baron, S. Ecoffey, Microelectron. Eng. 177 (2017) 41-45.
[2] D. Yamane, T. Konishi, T. Matsushima, K. Machida, H. Toshiyoshi, K. Masu, Appl. Phys. Lett. 104 (2014) 074102.
[3] K. Machida, T. Konishi, D. Yamane, H. Toshiyoshi, K. Masu, ECS Trans. 61 (2014) 21-39.
[4] H. Tang, C.Y. Chen, M. Yoshiba, T. Nagoshi, T.F.M. Chang, D. Yamane, K. Machida, K. Masu, M. Sone, J. Electrochem. Soc. 164 (2017) D244-D247.
[5] C.W. Baek, Y.K. Kim, Y. Ahn, Y.H. Kim, Sens. Actuators, A 117, 2005, pp. 17-27.

In this work, we have reported a very simple and facile route for the fabrication of multifunctional metal surface by simple hot water treatment (HWT). Micro-nano hierarchical structures were successfully fabricated on aluminium (Al) following a mechano-thermal synthesis. A self-assembled monolayer of low surface energy material 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FOETS) was also coated on the processed samples using a vapour deposition technique. The samples fabricated with process conditions exhibited a vivid range of wettability ranging from near 0° to 165°. Apart from high static and dynamic contact angles, the processed Al surface also showed extremely low contact angle hysteresis (CAH). The developed surfaces also showed extreme chemical stability while retaining the de-wetting behaviour as assessed by the immersion tests performed with salt water. The obtained results are explained on the basis of the ability of the processed surfaces to entrapped air resulting in the formation of the Cassie state. This entrapped air work as a protective layer and lowers the contact area with the corrosive media. These surfaces are thermally stable to a wide range of temperature from -25° to 200°C and exhibit excellent self-cleaning property. Our work introduces a simple strategy to fabricate multifunctional aluminium surface on a large scale which can be easily extended to other materials.

Heterostructured metals have attracted increasing interest because of their unique capability to overcome the strength-ductility tradeoff typically observed in engineering materials. Here, a new strategy for synthesizing heterostructured Zn-Mg hybrids is proposed using quasi-constrained high-pressure torsion (HPT) under an applied pressure of 6 GPa and a rotational speed of 1 rpm up to 30 turns, followed by a post-deformation annealing (PDA) treatment at 200 °C for 1h. Experimental results indicate a transition from a relatively homogenous nanograined structure of ~ 100-200 nm with an even distribution of Mg₂Zn₁₁ and MgZn₂ nanoscale intermetallic precipitates after 30 turns HPT, to a heterogeneous microstructure consisting of a bimodal grain size distribution with the nanosized intermetallics segregated in Mg-rich grains upon subsequent PDA. The microstructural evolution during HPT and PDA was characterized through SEM, XRD and TEM/STEM. The resulting micro-mechanical behavior was investigated using Vickers hardness and nanoindentation testing, and it was concluded that PDA led to a simultaneous increase of hardness and strain rate sensitivity. Close inspection of the strain hardening capability revealed that Zn-Mg HPT-processed hybrids followed a three-regime behavior during plastic deformation. This mechanical response is suggested due to the activation of multiple strengthening mechanisms during HPT and subsequent PDA, including grain refinement, in-situ precipitation, and back-stress strengthening associated with geometrically necessary dislocations. Moreover, the synergistic effect of PDA in this strategy can be further extended to the fabrication of other metal hybrids, paving the way to the design of novel materials with attractive properties for task-specific applications. In particular, the understanding gained on heterostructured Zn-Mg hybrids is intended to aid the development of Zn-based materials able to satisfy the demanding mechanical requirements of absorbable medical implant devices.

Deformation mechanisms governing the strength of nanostructured metallic multilayers have been studied extensively for various applications. In general, size effect is the most effective way to tailor the mechanical strength of multilayers. Here we report that three Cu/Co multilayer systems with identical layer thickness but different types of layer interfaces exhibit drastically different mechanical behavior. In situ micropillar compression tests inside a scanning electron microscope show that coherent FCC (100) and (110) Cu/Co multilayer systems have low yield strength of about 600 MPa, and prominent shear instability. In contrast, the incoherent Cu/ HCP Co multilayers show much greater yield strength, exceeding 2.4 GPa, and significant plasticity manifested by a cap on the deformed pillar. Molecular dynamics simulations reveal an unexpected interplay between pre-existing twin boundaries in Cu, stacking faults in HCP Co, and incoherent layer interfaces, which leads to partial dislocation dominated high strength, and outstanding plasticity. This study provides fresh insights for the design of strong, deformable nanocomposites by using a defect network consisting of twin boundaries, stacking faults and layer interfaces.

Biology has evolved to recruit metal ions from surroundings to produce structural materials and enhance their mechanical properties in remarkably efficient manners. One such way is to reinforce the soft matrix with the strong metal-binding interaction utilizing metal-affinitive organic ligands (e.g., mussel holdfast, marine worm jaws) and another is to go through the biomineralization of the recruited metal ions to produce stiff bioinorganic components (e.g., chiton teeth, nacre, bones). Herein, using metal-binding chemistry inspired by mussels holdfast, we showcase the control and combination of both pathways - the macromolecular crosslinking and mineralization for reinforcing soft matter mechanics and for producing high-value organic-inorganic nanocomposite materials towards potential hard tissue engineering application. First, we introduce the highly efficient mechanical reinforcement of hydrogel network via in situ mineralization of the metals (with very small amount: < 0.1 vol %) localized at the monodisperse metal-coordinate crosslinking sites. As such, the viscoelastic behavior of the hydrogel could be dramatically tuned from liquid-like to solid-like with increased modulus and strength. Furthermore, we demonstrate that this metal-binding mechanism can work universally as a mineralization controller for inducing amorphous or metastable calcium carbonates and their effective transition to hydroxyapatite (i.e., bone minerals) nanocrystals. The ability of bioinspired metal-binding chemistry controlling the mineralization thus suggests the efficient material design/processing strategy for reinforcing materials and for applying to the practical biomedical technology.

Downsizing trend of Multi-Layer Ceramic Capacitor (MLCC) required the structure of alternating stacking layers of dielectric and electrode in MLCC to be as thin as possible to microscale. For a thinner layer, smaller nanoparticles for dielectric and Ni paste for electrode are used. One of the major issue caused by the smaller particle size is the enlarged sintering behavior difference between the ceramic and metal layer of MLCC during the high-temperature sintering process in the manufacturing process. Eventually, the difference in sintering behavior between the layers causes fracture of the device. Therefore, suggesting a Ni- alloy material for the electrode layer for sintering process delay and in-situ tool to analyze the sintering behavior of thin film is in demand. In this study, Ni alloy films were fabricated and the sintering behaviors of Ni nanoparticles in thin films were quantitatively analyzed by optical laser stress analysis.
First, several metal precursors were selected using the phase diagram and surface energy to delay the sintering. The Ni paste and metal precursors were mixed and fabricated to thin films by spin coating. Then, the fabricated films were heated in the closed chamber. During this process, in situ stress curve versus temperature graph was obtained using the optical laser system. The inflection points for Ni films were analyzed thoroughly. The microstructure analysis was carried out for the film samples quenched above and below the inflection point temperature. Therefore, specific inflection point resulted to be significantly related to the sintering process. In addition, retardation of sintering behavior was observed from the microstructure analysis of Ni alloy films. Lastly, for a deeper understanding of sintering behavior, crystal structure analysis for Ni alloy films was performed to identify delayed sintering mechanism.
This study suggests an in-situ quantitative analysis method for sintering behavior and new material to reduce sintering behavior gap between the ceramic and metal layer of MLCC.

Metallic glasses are very promising structural materials and their mechanical properties as well as deformation mechanism have been extensively investigated from both simulations and experiments. However, a complete understanding of the strain rate effect on mechanical response of metallic glasses is absent given the disparity in time scales between classical molecular dynamics simulations (MD) and experiments. Using accelerated MD simulations, we aim to, for the first time, report the mechanical properties of metallic glass nanowires ranging from 10⁸ s^-1 (classical MD simulations scale) to 10^-2 s^-1 (experimental scale). Specifically, embrittlement and reduction of failure strength is observed as the applied strain rate reduces, accompanied by a transition from shear fracture to cleavage. We suggest that the reduction of ductility in metallic glass nanowires as the applied strain rate decreases is due to the gradual relaxation of liquid-like structure. Finally, the metallic glasses quenched from more fragile liquids will be subjected to slow-rate mechanical tests, where the correlation between strain rate sensitivity of ductility and fragility will be established.

Metallic glasses are promising structural materials owing to high strength, excellent wear and corrosion resistance. Brittleness of bulk metallic glass is the main drawback researchers have been trying to improve over the past decade. So far, compositing ductile crystalline phases is the major effective means of ductility enhancement in brittle metallic glass, which not only limits the materials selection for composites design but usually results in substantial decrease of strength in metallic glasses. Herein, using molecular dynamics simulations, we incorporated two brittle metallic phases with different stiffness and surprisingly obtained strong, ductile and work hardenable composites. Compared to monolithic metallic glasses (failure strain is ~7%), significantly enhanced ductility (over 80% in terms of failure strain) has been achieved for brittle-brittle composites. To understand the toughening mechanism, we have identified two figures of merits to characterize the flexibility of the strong glassy filament, and the propensity for crack deflection, respectively. Furthermore, we have constructed a ductility map using the above two figures of merits, which agrees with our MD simulation of over five hundred different composite designs. The composite design principles delineated here is likely to be applicable to other material systems beyond metallic glass system.

Composites are a large and versatile group, incorporating various materials such as metals, ceramics and polymers in multiple scales from the macro to the nano. As such, composites can be tailored to highly specialized applications such as biomed, sensors, energy storage and aerospace. Such tailoring requires understanding of the different mechanisms dominant at different length-scales, as well as correlating the composite morphology and said mechanisms – correlation that can work both ways. This correlation calls for means of observation at the desired scale in real-time stimulation of the material – be it mechanical, electrical or other types of stimulation.
In this talk, the need for real time imaging of mechanical tests will be explained using various length-scale case studies. These case studies will include digital image correlation (DIC) used to analyze standard tensile tests, micro-mechanics under scanning electron microscope (SEM), and nano-mechanics under transmission electron microscope (TEM). Discussion will include the additional data acquired from real-time imaging at such scales and its importance, as well as possible future applications for other nanoscale materials. The methods presented in these case-studies can therefore provide deeper understanding of composite materials behavior, which can be used to improve planning and designing of composites as multifunctional materials.

Nanostructured composites, which have been successfully used in high field magnets, reach their most desirable strength level when their interface spacing is below 100 nanometers. The usual fabrication methods for these composites are severe plastic deformation, which refines interface spacing and create anisotropy. The fabrication may also introduce shear-bands into the microstructure. Investigating both anisotropy and shear-bands helps to understand the deformation mechanisms of these composites under external force. The goal of our research is to relate microstructural anisotropy to the mechanical and physical properties of these composites in various geometries. Our current work is shedding new light on the detailed correlation between microstructure and properties. This presentation describes microstructure in different orientations, with emphasis on the impact of anisotropy and shear-band on properties.
This work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida

Focused electron beam induced deposition (FEBID) is a direct-write technique where an electron beam dissociates surface bound precursor molecules at the focal point of the electron beam. FEBID has become a more viable 3D nanoprinting tool due to the sophisticated applications shown recently, such as FEBID simulation [1] that predicts and controls the growth of 3D nanostructures by modeling the dynamic interaction of the electron beam and the precursor molecules, and its accompanying 3D CAD program (3BID) [2]. The ability to fabricate functional deposits such as plasmonic 3D nanostructures [3] and, scaffolds for 3D magnetic nanowires [4] with high degrees of freedom and precision using 3D FEBID show it is a promising additive nanomanufacturing tool. Unfortunately, distortions such as deflections in overhang 3D nanowires where linear growth is prescribed limit the reproduction of 3D FEBID designs. The 3BID program offers a tool to correct the distortions, but these are empirical. A pattern generating algorithm [5] released recently adjusts for proximity effects and compensates for height-dependent precursor coverage. The results show quite good congruence between the prescribed and deposited 3D nanostructures. Here, however, we quantify the mechanism that causes these distortions, paving the way to a more quantitative correction strategy.

We will show a combination of experiments, models, and simulations that reveal beam-induced heating as the culprit that constrains the replication of 3D FEBID designs. The electron beam facilitating deposition during nanowire growth also causes Joule heating at the beam interaction region (BIR). The BIR temperature increases gradually during growth as nanowire elongation impedes heat flow. Heat transfer from extended surfaces is used to model the heat flow from the BIR, through the nanowire to the substrate. The temperature gradient in the deposit gives rise to an opposing precursor concentration gradient resulting in a deflected nanowire where otherwise a linear nanowire was specified. This vital role beam-induced heating plays in determining the final deposit shape will be discussed.

References:

[1] J.D. Fowlkes, R. Winkler, B.B. Lewis, M.G. Stanford, H. Plank and P.D. Rack: Simulation-Guided 3D Nanomanufacturing via Focused Electron Beam Induced Deposition. Acs Nano 10, 6163 (2016).
[2] J.D. Fowlkes, R. Winkler, B.B. Lewis, A. Fernández-Pacheco, L. Skoric, D. Sanz-Hernández, M.G. Stanford, E. Mutunga, P.D. Rack and H. Plank: High-Fidelity 3D-Nanoprinting via Focused Electron Beams: Computer-Aided Design (3BID). ACS Applied Nano Materials 1, 1028 (2018).
[3] R. Winkler, F.P. Schrnidt, U. Haselinann, J.D. Fowlkes, B.B. Lewis, G. Kothleitner, P.D. Rack and H. Plank: Direct-Write 3D Nanoprinting of Plasmonic Structures. Acs Applied Materials & Interfaces 9, 8233 (2017).
[4] D. Sanz-Hernandez, R.F. Hamans, J. Osterrieth, J.W. Liao, L. Skoric, J.D. Fowlkes, P.D. Rack, A. Lippert, S.F. Lee, R. Lavrijsen and A. Fernandez-Pacheco: Fabrication of Scaffold-Based 3D Magnetic Nanowires for Domain Wall Applications. Nanomaterials (Basel, Switzerland) 8 (2018).
[5] L. Keller and M. Huth: Pattern generation for direct-write three-dimensional nanoscale structures via focused electron beam induced deposition. Beilstein Journal of Nanotechnology 9, 2581 (2018)

Laser-based powder bed fusion processes like the selective laser melting (SLM) is one of the additive manufacturing processes used to produce 3-D metal parts by selective melting of powders dictated by CAD data. Because of the high degree of freedom given by processing through additive manufacturing, it is possible to build parts with extremely complex geometries that would otherwise be difficult or impossible to produce using conventional manufacturing processes. However, until now, only conventional alloys like the AlSi10Mg, 316L, Ti6Al4V, etc. that either is developed for cast or wrought processes have been used for fabrication. Some of the alloys work well for the additive manufacturing process like the Al12Si, AlSi10Mg, because they have good fluidity and are readily weldable. Nevertheless, most of the materials fabricated by SLM show superior mechanical properties than their case counterparts. Even though superior mechanical properties were recorded, there are reports showing a pre-mature failure of the materials (i.e.) the material fail before they achieve their maximum strength. Some of the reasons behind these premature failures will be discussed in detail.

The demands on modern materials are highly challenging and partially contradictory. For example, materials should be strong like metals but chemically inert like soft low surface energy polymers. Another example is the need for extreme lightweight but fail-proof and high temperature stable materials. Furthermore, “intelligent” stimulus responsive materials and adaptive structures are requested. Those conflicts can be overcome by effectively combining disparate materials in composites that allow a fusing of the traditional material classes like ceramics, polymers and metals. For that purpose, in this presentation two basic concepts utilized in the functional nanomaterials group are introduced that might be employed to overcome such challenges. One is based on micromechanical interlocking at the interface, the other on interpenetrating composites with micro framework materials. In both cases, the chemistry and chemical compatibility of the materials play a negligible role, the selection of the constituents can be done by focussing exclusively on the material properties. This allows a targeted tuning of the micro-structure and enables the realization of atypical material properties like flexibility in ceramics. Furthermore, self reporting and actuating concepts can be added to create materials that replace complex micro-systems. Examples will be given including mechanical interlocking as universal material connection technology [1,2], a versatile template framework material [3], aero materials [4, 5], as well as sensor [6], and self reporting concepts [7].

[1] X. Jin et al., “Joining the Un-Joinable: Adhesion Between Low Surface Energy Polymers Using Tetrapodal ZnO Linkers", Adv. Mater. 25 (2012), 1342
[2] M. Baytekin-Gerngross, et al., “Making metal surfaces strong, resistant, and multifunctional by nanoscale-sculpturing" Nanoscale Horizons 1, (2016), 467
[3] Y.K. Mishra, R. Adelung "ZnO tetrapod materials for functional applications" Materials Today, 21 (2018) , 631
[4] R. Meija, Nanomechanics of individual aerographite tetrapods, Nature Comm. 8 (2017), 14982
[5] F. Schuett et. al., “Hierarchical self-entangled carbon nanotube tube networks”, Nature Comm. 8 (2017), 1215
[6] I. Paulowicz, Adv. Electronic Mater. 25 (2015), 134
[7] X. Jin, A Novel Concept for Self-Reporting Materials: Stress Sensitive Photoluminescence in ZnO Tetrapod Filled Elastomers, Adv. Mater. 25, 134 (2013), 134

In the Earth’s upper crust, dense ceramic materials can be formed at mild pressures and nearly ambient temperature. This represents a tremendous reduction in energy used for densification compared to technical ceramic processing, in which temperatures above 1’000 °C are needed to produce dense structures. Inspired by this geological phenomenon, we demonstrated a new method called room-temperature cold sintering. In this process, small amounts of water and pressures up to 500 MPa are utilized to produce calcium carbonate compacts reaching 85 % relative density within only 30 min. While the densification process in geological carbonates is presumably based on dissolution – precipitation of the inorganic material and the diffusion through the thin water film present on the surface of the particles, the mechanisms underlying the densification of synthetic carbonate nanoparticles remain unknown. From the several compositions that have been proven to be compatible with cold sintering, carbonates are one of the very few classes of material that can be densified at room temperature. To better understand this unique process, we developed a methodology to probe the time-dependent structure of these carbonates at multiple length scales during water-assisted densification. We combined the observation of in-situ compaction behavior of calcium carbonate particles with high-energy X-Ray tomography to small-scale creep experiment using instrumented indentation. The synchrotron tomography experiments allowed us to directly observe the deformation of the particles in real-time and thus discriminate between mechanisms involving deformation or simple rearrangement of the particles. For a quantitative analysis, we used digital volume correlation to calculate local deformations and strains within the sample. Indentation was utilized to further probe the mechanical behavior of the sample from the micron to the nanometric length scale, allowing us to pin point the influence of the water on the room-temperature sintering process. Combined the tomography and indentation experiments provide a powerful multiscale temporal platform to shed light on the main factors that drives the room-temperature sintering of nanopowders under pressure.

Martensitic transformations, which give rise to the interesting properties of Shape Memory Alloys (SMA), have long been studied in metals and alloys, but understanding and emergence of their presence in metal oxides have been limited. Known for their transformation toughening mechanism, chemical composition of zirconia-based ceramics can be altered to increase the interphase compatibility between its high temperature phase (austenite) and low temperature phase (martensite), which suggests possible emergence of Shape Memory Effects (SME) in these ceramics. We synthesized samples with weight percentage x of ZrHfO₂ ranging from 0.6 to 0.9 by pressing and sintering oxide powders. Transmission Electron Microscopy (TEM) and Electron Backscattering Diffraction (EBSD) characterization of these synthesized ceramics reveal finely twinned microstructure within single 5µm grains, with no visible defects in the composition x=0.735, which aligns with theoretical prediction. We performed uniaxial compression experiments on single-crystalline cylindrical pillars with diameters of 200nm to 2µm that were carved out of individual 5um average grains using Focused Ion Beam (FIB). These nanomechanical experiments revealed that the stress-strain response was elastic plastic with favored deformation mechanism through martensite rearrangement, and that the yield strengths follow a smaller is stronger power-law relationship, similar to the ubiquitously observed size effects in metals that deform by crystallographic slip. This prominent size effect can give insight into the role of deformation twinning in ceramic plasticity. We further demonstrate the one-way shape memory effect by heating the deformed samples to above their phase transformation temperatures to recover their original shape and reveal a strong dependence of shape recovery on chemical composition.

A flexible hard coating material displaying scratch resistance and flexibility was developed through the design of organic-inorganic hybrid coatings using alkoxysilyl-functionalized polyrotaxane cross-linkers (PRX_Si1). PRX_Si1 has a unique structure like a molecular necklace that can form organic-inorganic cross-linking points and can provide large molecular movement. Hybrid cross-linking points and dynamic molecular motion have been assumed to simultaneously increase scratch resistance and flexibility. To confirm this hypothesis, the crystalline structure and mechanical properties of PRX_Si1-based hard coating materials were analyzed by transmission electron microscopy, small angle X-ray diffraction, tensile, pencil hardness and scratch tests. Finally, the hard coating material based on PRX_Si1 was able to form homogeneously dispersed nanoscale siloxane crystalline domains, and the deformation at the breakpoint was three times higher than the commercial hard coating material so that even after 5,000 folding test runs, Did not occur. In addition, this material has very high pencil hardness (9H) and scratch resistance.

Dealloying derived nanoporous gold (NPG), a 3D bicontinuous network of nanoscaled ligaments and pores with large surface area, exhibits a plethora of mechanical and functional properties that can be precisely controlled by electric signals in electrolyte environments. Among them, actuation [1], switchable elastic modulus [2], Poisson’s ratio [3] and strength [4,5] are of special interest to understand interfacial phenomena at the nanoscale and to design novel materials with an adaptable mechanical behavior.
Recently, the actuation performance of NPG has been significantly improved by decorating the gold ligaments with a conducting polymer polypyrrole (PPy) film [6]. The enhanced length change of the gold skeleton was promoted by actuation of PPy upon ion exchanges in the polymer layer with surrounding electrolyte. The elastic modulus behavior of the NPG-PPy hybrid, which is also crucial for the functionality, has not been studied yet. In this work, a uniform PPy film with various thicknesses was formed on mm-size NPG by electropolymerization without blocking the original nanopores. The elastic moduli of the resulting NPG-PPy nanocomposites were monitored in situ during potential cycling in aqueous electrolytes in a dynamic mechanical analyzer (DMA). Remarkably, the in situ DMA experiments reveal a non-monotonic modulus response as a function of the potential during charging-discharging processes, as opposed to a nearly linear variation of the macroscopic length. Moreover, the modulus variation amplitude is enlarged for the thicker PPy films. In this contribution, we will discuss the origin of the stiffness behavior and elucidate a role of the Au/PPy interface in the unusual elastic modulus response of the hybrid material.
[1] H.J. Jin, J. Weissmüller, Adv. Eng. Mater., 2010, 12:714-723.
[2] N. Mameka, J. Markmann, H.J. Jin, J. Weissmüller, Acta Mat., 2014, 76: 272-280.
[3] L. Lührs, B. Zandersons, N. Huber, J. Weissmüller, Nano Lett., 2017, 17: 6258-6266.
[4] H.J. Jin, J. Weissmüller, Science, 2011, 322:1179-1182.
[5] N. Mameka, J. Markmann, J. Weissmüller, Nat. Comm., 2017, 8: 1976.
[6] K. Wang, C. Stenner, J. Weissmüller, Sens. Actuator. B. Chem., 2017, 248: 622-629.

Two-dimensional materials in mono- or few-layer forms have been used in the design of novel sensors, filtration membranes, and wearable electronic devices. In their multilayer form, nanocomposites are engineered to exhibit unprecedented combinations of specific stiffness and strength. However, the intrinsic brittle behavior of these atomically thin 2D materials, and their associated risk for catastrophic failure, has thus far precluded their adoption in practical applications requiring superior mechanical properties.

In the area of nanocomposites (structural materials), lightweight 2D materials are addressing high demand for safer and more energy efficient transportation systems, e.g., in automotive and aeronautical applications. Similarly, the next generation of protective body armor has inspired the search for lightweight materials with exceptional piercing resistance. Composites based on two-dimensional materials appear to be excellent candidates to address these societal needs. Unfortunately, the presence of “architectural defects” such as voids and wrinkles found in previously produced 2D materials, in thin film form, resulted in somewhat limited mechanical performance. This has precluded fundamental understanding of their full potential and their robust implementation in the aforementioned engineering applications.

A strategy to gain fundamental understanding of the potential of 2D materials, in their few layer or structural material forms, is to investigate 2D material specimens with tailored chemistries and controlled architectures in the form of a few layers. Here we present a combined experimental and computational approach, based on graphene oxide (GO) and poly-vinyl alcohol (PVA), as a model material system, to investigate surface chemistry effects and pathways for improved ductility and fracture of 2D materials. We will show that a larger-scale “extrinsic” toughening mechanism can be incorporated into GO monolayers through surface modification by an ultrathin strongly interacting polymer layer.

In this presentation, we will discuss nanoscale experiments that demonstrate such a crack-bridging and toughening behavior for GO-derived materials. We will show that by incorporating an atomically thin layer of a hydrogen bond-forming polymer, onto the surface of a GO nanosheet, the toughness of the resulting GO-Polymer nanolaminates is increased up to 300% that of GO monolayers, making them highly attractive as scalable building blocks for the next generation of engineered materials. Multiscale characterization reveals the presence of a hydrogen-bonding network on the surface of GO that can be exploited to reinforce its mechanical integrity via polymer adhesion to the oxidized domains in GO. This pairing results in a synergistic toughening mechanism in which the polymer chains effectively bridge a developing crack and allow the nanolaminate to exhibit superior piercing resistance, as verified by combined nanomechanical experimental and computational studies. As cracks develop, clusters of hydrogen bonds between GO and the polymer chains break and reform, which permits the polymer to act as a self-healing load-bearing element. Such a deformation mechanism provides key insight not only for the design of 2D materials-based nanocomposites but also for the engineering of more reliable sensors, filtration membranes, and wearable electronics devices.

As a newly discovered and large family of materials, which shown unique properties for many applications from electrochemical energy storage to sensing, and biomedical applications, two-dimensional (2D) transition metal carbides (MXenes) can be produced by etching of A layers from their MAX layered counterparts. The etching yields highly conductive and hydrophilic multi-layered MXene particles that can be intercalated with ions and other species, which is ideal for electrodes, energy storage and harvesting devices. However, the mechanical behavior of these multi-layered MXene layers has not been characterized, although it is critical for device design and reliability. In this work, we report the results from in-situ nanoindentation experiments conducted on two MXenes: Ti₃C₂T_x and Ti₂CT_x along both, in-plane and out-of-plane directions. Significant anisotropic behaviors were observed associated with various failure mechanisms. Molecular dynamics (MD) modeling was performed to analyze the tension and compression behavior of intact and defected mono-layer Ti₃C₂ and Ti₂C. Analytical models for in- and out-of-plane nanoindentation are presented and compared with experimental data, revealing that the overall mechanism is determined by elastic and plastic properties of MXenes, as well as their accordion structure. Materials properties are extracted from the analytical model based on experimental and MD results.

Similar to conventional carbon and glass fibre reinforced polymer composites, high temperature curing is mostly inevitable in the fabrication of polymer nanocomposites reinforced with carbon nanotubes. This paper presents a numerical approach to predict the thermal residual stresses in polymer nanocomposites reinforced with a periodic array of wavy carbon nanotubes. A novel unit cell model is established to accurately account for the waviness of the nanotubes. Periodic boundary conditions are rigorously determined for the three dimensional unit cell with a pair of curved surfaces. To validate the model, effective elastic constants of composites with compliant matrices are evaluated using the finite element method, and the results are compared with those in literature. Notably, orthotropic elastic constants are correctly predicted for the unit cell configuration which has only two planes of symmetry manifested explicitly in the model. By employing material properties of the two constituents, the thermal residual stresses induced by high temperature curing and cooling-down are examined for an epoxy/wavy-nanotube composite. In particular, temperature-dependent material properties are used for the matrix to better capture its response within the wide temperature range associated with the curing and cooling-down process. Numerical results show that the waviness has a significant influence on the distribution of the local residual stresses. Finally, it is demonstrated that the curing and cooling-down process tends to increase the waviness of the nanotube.

Nanofiber (NF) arrays, such as aligned carbon nanotubes, with exceptional intrinsic and scale-dependent properties, along with their easy tunability, have enabled applications in diverse areas including aerospace, energy, and biomedicine. Here, we use photolithography-based patterning and mechanical buckling to form densified aligned NF arrays, which exhibit wavelike folding buckling shapes. Such hierarchical shapes are known to introduce multiscale and mixed-mode reinforcement mechanisms in nanocomposites, leading to composite strength and toughness enhancement. In this study, we introduce the buckling-densified aligned NF arrays into the resin-rich ply-ply interface of laminated carbon microfiber composites as a through-thickness reinforcement. Short-beam shear (SBS) and double-edge notched tension (DENT) tests are performed to assess the effect of buckling-densified aligned NF array reinforcement. The aligned NF-reinforced composites are found to have a 7% increase in SBS strength and a remarkable 25% increase in the in-plane DENT strength. Scanning electron microscope imaging and lab-based high-resolution micro-computed tomography scans of post mortem specimens reveal that the buckling-densified aligned NF arrays suppress delamination and drive the crack into the more desirable intralaminar region. The findings demonstrate great potential for harnessing instabilities to fabricate high-volume-fraction and shape-tunable NF arrays, opening avenues for uses that extend beyond composite mechanical reinforcement, such as supercapacitors, thermomechanical devices, and sensors. Future work includes developing a model that can fully predict the buckling response of aligned NF arrays as a function of pattern size, NF elastic properties, and NF/substrate adhesion, as well as conducting 4D in situ mechanical testing of the DENT configuration by using synchrotron-radiation computed-tomography to reveal the 3D progressive damage state.

Organic-inorganic aerogels have extensively been studied to overcome the inherent poor mechanical strength (friability) of their inorganic counterparts which necessitate supercritical drying for their preparation processes. In order for the low-density wet gels to be evaporatively dried without fracture under ambient pressure conditions, the network should exhibit large reversible deformations, i.e. shrinkage and re-expansion, while enduring the stresses exerted by capillary force during the solvent removal process. So far, the authors' group has been attempting to prepare such organic-inorganic hybrid networks with a broad spectrum of mechanical properties utilizing appropriate combinations of precursors, additives, solvents and catalysts. Approaches can be classified into three categories; (1) Methyl-modified silsesquioxane (MSQ) using tri-functional methyltrialkoxysilanes as presursors, (2) Organo-bridged siloxane networks from bis-type di- or tri-functional alkoxysilane precursors, and (3) Double-crosslinked networks by hydrolysis-polycondensation of oligomeric precursors containing pre-polymerized hydrocarbon chains connecting Si atoms in the monomeric precursor. For each system, preparation conditions have been specified for the product to give low density (0.1-0.2 g cm^-3) and trasparency/translucency to visible light.
(1) Supercritically dried MSQ exhibited reversible deformation against uniaxial compression up to 80 % of the original height, suggesting the network is recoverable against isotropic deformations. The slow evaporation of nonpolar solvents such as hexane enabled one to remove the solvent under ambient pressure without causing serious cracks and fractures. A size recovery from ca. 70 % compressed (shrunk) state by the spring-back gave highly transparent, low-density xerogels in a monolithic form [1,2].
(2) Introduction of relatively short hydrocarbon chains between Si atoms resulted in the improvement of flexural strength evaluated by three-point bending tests without sacrificing other properties such as low-density and visible-light transparency. The decreased functionality of siloxane bridges further increased the range of flexural deformation reflecting in part the relaxation behavior of polymeric networks [3].
(3) Integration of longer organic crosslinks into di- or tri-functional siloxane resulted in the highest degree of dutility while minimizing brittle nature of inorganic aerogels. The ambient-dried gels could be easily cut into shapes with blade and was stable against the resorption of organic solvent into their hydrophobic pores.
These unique mechanical behavior of hybrid aerogels are under analysis in more details using an electron microscope to characterize the morphological features of the network in relation to the chemical interaction among precursors, additives and solvents [4].

References
[1] K. Kanamori, M. Aizawa, K. Nakanishi, T. Hanada, New transparent methylsilsesquioxane aerogels and xerogels with improved mechanical properties, Adv. Mater., 19, 1589-1593 (2007).
[2] G. Hayase, K. Kanamori, A. Maeno, H. Kaji, K. Nakanishi, Dynamic spring-back behavior in evaporative drying of polymethylsilsesquioxane monolithic gels for low-density transparent thermal superinsulators, J. Non-Cryst. Solids 434, 115-119 (2016).
[3] (Review) T. Shimizu, K. Kanamori, K. Nakanishi, Silicone-based organic-inorganic hybrid aerogels and xerogels, Chem. Eur. J. 23, 5176-5187 (2017).
[4] G. Zu, T. Shimizu, K. Kanamori, Y. Zhu, A. Maeno, H. Kaji, J. Shen, K. Nakanishi, Transparent, superflexible doubly cross-linked polyvinylpolymethylsiloxane aerogel superinsulators via ambient pressure drying, ACS Nano, 12, 521-532 (2018).

Carbon fiber reinforced polymer composites are widely used in aerospace applications because of their high strength and stiffness properties. One of the disadvantages of using carbon fiber reinforced composites is lack of strength and stiffness in the transverse direction and that makes them vulnerable to out of plane loading situations like bird impact, lightning strikes, etc. To alleviate this problem, typically aircraft structures are designed with a sacrificial layer of fiberglass composite in conjunction with carbon fiber composites. Even though this type of hybridization has many advantages, still much research is needed to understand the interlaminar failure of glass/carbon interface under out of plane loadings. The present study investigates the effects of mechanical loading on glass/carbon hybrid nanoengineered fiber reinforced composites. Hybrid 9 layers composite laminates with a single layer of glass fiber lamina on the outer surface and 8 layers of carbon fibers lamina were fabricated using aerospace grade epoxy and vacuum assisted resin transfer molding (VARTM) technique. In-plane mechanical properties such as tensile, flexural, impact and interlaminar shear strength (ILSS) characteristics were investigated. In addition, to improve interlaminar properties a layer of electrospun glass and carbon nanofibers are being used between glass fiber lamina and carbon fiber laminates.

Nano-architected ceramic metamaterials, such as polymer-derived pyrolytic carbons (PyCs), have great potential for next generation lightweight structural materials due to their high mass-specific strength and stiffness. In particular, PyC matrices are most desirable for low density structural applications when they are reinforced with high strength nanowire arrays, such as aligned carbon nanotubes (A-CNTs) at high volume fractions (V_f), which addresses brittleness, enhances hardness, and allows tailoring of multifunctional properties. While modeling indicates that CNT-PyC nanocomposites at V_f ∼ 30% could show > 300% enhancements in hardness when compared to the neat PyC matrix, performing similarly to diamond and cubic boron nitride for superhard lightweight structures, it is unknown how CNT confinement at these packing densities influences the structural evolution of the PyC matrix. Therefore, a study of these effects on the formation, self-organization, and geometry evolution of the graphitic crystallites that comprise the PyC matrix of the nanocomposites is necessary. In this report, we synthesize CNT-PyC nanocomposites via the pyrolysis of CNT-phenol formaldehyde precursors to create superhard and lightweight aligned CNT-carbon matrix nanocomposites. In this study, the influence of CNT proximity interactions on the atomistic, nanoscale, and mesoscale structural evolution of PyC is determined experimentally as a function of CNT V_f (up to 30%) using scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy to quantify the CNT-PyC matrix morphology, graphitic crystallite geometry, bonding character, and chemical structure evolution, respectively. This work identifies how high CNT confinement leads to modification of the graphitic nature of the PyC matrix, and it shows that the governing CNT-PyC interactions exist at the < 10 nm scale, thereby becoming increasingly significant as the inter-CNT spacing decreases below this value at V_f ≥ 20%. Using this approach, these results provide new insights to explain the predicted CNT-PyC hardness enhancements at high V_f and enable the design and manufacture of next-generation superhard lightweight nanocomposites.

The direct synthesis of carbon nanotube (CNT) on substrate surfaces and interfaces has been of great interest for enabling performance improvements of electronics, electrochemical energy systems, and advanced multifunctional composites among others. However, the extent of applications that may benefit from CNTs have been limited by the operational range of temperatures and substrates for traditional transition metal catalysts (Fe, Ni, and Co). Here, a new element has been discovered to catalyze CNT growth at temperatures below 400 °C using a thermal chemical vapor deposition technique: sodium. As the first alkali metal reported to catalyze CNTs, sodium enables direct and low temperature CNT growth across a wide array of substrates, including alumina fibers, polymer-coated fibers, silicon wafers, silicon nitride, titanium, and others. Aqueous solutions of common sodium-based compounds such as sodium hydroxide, sodium bicarbonate, sodium chloride, and sodium carbonate are all demonstrated to result in CNT growth on carbon fabrics using a single dipcoat step prior to CVD. Ex situ scanning electron microscopy and transmission electron microscopy analyses with energy dispersive x-ray spectroscopy as well as x-ray photoelectron spectroscopy performed on catalyst particles prior to growth and CNTs after growth confirm the identity of sodium as the active element for CNT catalysts. In addition, unlike transitional metal catalysts, Na exhibits a vanishing catalyst phenomenon in which catalyst-free CNTs can be formed during synthesis for applications that are sensitive to residual metal impurities, thus avoiding the need for post processing steps that may employ the use of harsh chemicals.

Moreover, as an exemplary application of Na-catalyzed growth, CNTs are circumferentially grown on carbon fibers (CFs) to form “fuzzy” CFs that can be laminated into hierarchical nanoengineered polymer composites with the potential for interlaminar reinforcements and electrical conductivity increases. Previous efforts required harsh surface roughening techniques of carbon fibers to enable Fe catalyst adhesion and high temperature growths typically on the order of 800 °C, resulting in carbon fiber strength losses as much as 60%. Here, we demonstrate that carbon fibers dipcoated in NaOH solutions and subjected to CVD at 480 °C resulted in conformal CNT growth and retained microfiber strength as assessed through single fiber tests. These fuzzy CF fabrics were then infused with epoxy resin to form unidirectional fuzzy carbon fiber reinforced plastic (UD fuzzy CFRP) laminates that were trimmed into short-beam shear specimens for interlaminar shear strength (ILSS) enhancement characterization. Taken together, we present sodium as a facile and new low temperature versatile CNT growth catalyst with the potential to expand the range of substrates and applications that may benefit from direct CNT synthesis.

Strain sensors are widely used as stretchable electronic, human motion detector, and health monitor due to their ability to convert large-scale mechanical deformation to the resistance change [1-3]. Forming the porous foam structure is a more promising method to build large-scale deformation strain sensor than forming the solid structure [4]. To quantitatively control the relationship between deformation and resistance change, carbon nanotubes (CNTs), with numerous surface area, are used as a component due to its good electrical conductivity [5]. Compared to traditional strain sensor manufacturing methods, material extrusion-based 3D printing (ME3DP) is a more suitable method to form viscous slurry into complex stacked models.
In this work, we fabricate a superelastic strain sensor based on CNTs/PDMS foam nanocomposites with controllable electrical properties by ME3DP. Low viscous PDMS Sylgard 184 and high viscous SE 1700 were mixed in the ratio of 3:2 for good moldability and printability of slurry. Sodium chloride (NaCl powder, 10-20 μm) was added into PDMS precursor as a sacrificial material to generate foam microstructures. Multi-walled CNTs (1 and 2 wt. % of PDMS) were added for adjustable electrical conductivity. ME3DP with a mechanically driven syringe extruder was used subsequently for the formation of 3D models. Both solid and hollow scaffolds were printed to investigate the influence of macro-voids on mechanical and electrical properties. NaCl microparticles were then removed by DI water bath for 24 hours to create porous open-celled microstructures. Based on our hypothesis, the interconnection rate of CNTs in micro-voids will affect the electrical conductivity when the structure is stretched or compressed, resulting in the different performance of the strain sensor. Further study will be focused on controlling density and size of micro-voids, the gap of macro-void scaffold structures, infill ratio of CNTs, and how these parameters affect stretchability, compressibility, and electrical conductivity.

Reference:
[1] Yamada, Takeo, et al. Nature Nanotechnology 6.5 (2011): 296.
[2] Liu, Hu, et al. Journal of Materials Chemistry C 4.1 (2016): 157-166.
[3] Ryu, Seongwoo, et al. ACS Nano 9.6 (2015): 5929-5936.
[4] Chen, Qiyi, et al. Advanced Functional Materials 28.21 (2018): 1800631.
[5] Zhang, Runzhi, et al. ACS Applied Energy Materials 1.5 (2018): 2048-2055.

Graphene has shown the potential to significantly impact a number of different technologies. Properties such as high surface areas and electrical conductivity make it a promising material for hydrogen storage, battery, and ultra capacitor applications. One route to realizing the full potential of graphene in these energy storage applications is the assembly of three-dimensional macroscopic graphene networks that retain the properties of individual graphene sheets. However, given the low density and nanoporous structure of these graphene aerogels, they suffer from mass transport and mechanical fragility, both of which are detrimental. Here, we use additive manufacturing (AM) to 3D print graphene aerogels with improved mass transport and mechanical robustness. Using techniques such as direct ink write and projection microstereolithography are shown as promising routes to improve the properties of graphene aerogels. The details of the synthesis and characterization of these 3D printed aerogels will be presented.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DEAC52-07NA27344.

Understanding the role of preparation protocol in morphogenesis of polymer nano-composites and developing theories quantitatively relating their nano-scale structural features and dynamics to the macro scale properties is vitally important. Here, we report on the effects of kinetic variables on the spatial organization of nanoparticles with strong and weak interfacial attraction to the polymer matrix. While individually dispersed NPs provide the largest elastic moduli and strength, chain bridged NP clusters endow the largest ductility. Hierarchical systems combining both chain bridged NP clusters and individually dispersed NPs exhibit simultaneous enhancement of stiffness, strength and ductility with its extent tunable by the cluster content. Our results provide an experimental insight into the role of processing and kinetic traps on the structure development in real polymer nano-composites which can enable their broader exploitation as building blocks of lightweight dynamic engineering structures.

In the strive to produce nature-inspired hierarchical materials with an enhanced combination of mechanical properties, supercrystalline ceramic-organic nanocomposites have been produced in bulk form and characterized from a variety of perspectives. Through an interdisciplinary collaboration at the crossroad between materials science, chemistry and mechanical engineering, a bottom-up approach has been designed. It consists of a sequence of self-assembly, pressing and heat treatment, and it leads to macroscopic poly-supercrystalline materials with exceptional mechanical properties and behavior. The crosslinking of the organic phase induced by the heat treatment does not only increase the materials’ stiffness, hardness and strength (elastic modulus up to 70 GPa, hardness up to 5 GPa and bending strength up to 630 MPa), but alters also their constitutive response. Fracture toughness values higher than theoretical predictions have emerged, implying the presence of extrinsic toughening mechanisms, such as the crack-path deviation observed at indents’ corners. Ex-situ nanoindentation and in-situ SAXS/microcompression studies also suggest the possibility for supercrystalline materials to accommodate compressibility and plastic-like deformation. Defects analogous to the ones typically observed in crystalline lattices, such as stacking faults, dislocations and slip bands, are detected at the superlattice scale (even if one order of magnitude larger than the atomic one, and with interactions among the nano-building blocks controlled by the organic phase). Correlations between defects, processing and mechanical properties have been drawn by adapting the classic theories of mechanical behavior of materials. These same materials are additionally being used as bricks for the development of novel hierarchical composites, via additive manufacturing or fluidized bed techniques.

Water-based anti-corrosion coatings have great potential as environmentally-friendly alternatives to solvent-based coatings due to lower volatile organic compound (VOC) emission during drying. However, state-of-the-art water-based coatings exhibit inferior corrosion resistance in marine and oceanside environments. Careful in-situ observations of acrylic-TiO₂ composite water-based coatings on carbon steel substrates aged by the ASTM B117 salt spray test, simulating a marine environment, suggest that these coatings fail by loss of substrate adhesion, crack propagation and corrosive chemical species (e.g. H₂O, O₂, NaCl) penetration. Boron nitride nanotubes (BNNTs) hold great promise as corrosion resistance-enhancing additives since they potentially bridge surfaces of crack tips and raise crack propagation energy, and potentially form an inter-coating scaffold offering torturous diffusion paths for corrosive species. Furthermore, the electrically insulating nature of BNNTs prevents the formation of galvanic couples with the steel substrate (as observed with conductive nanomaterials such as graphene), thereby avoiding accelerated anodic dissolution of the substrate. Water-based acrylic-TiO₂ coatings infused with 0.1 wt.% BNNTs will be fabricated by ball mill mixing of aqueous BNNT dispersion with water-based paint, followed by drawdown bar application of BNNT-infused paint onto carbon steel substrates. In-situ observations of unmodified and BNNT-infused coatings on carbon steel during salt spray aging will be compared to confirm superior corrosion resistance of BNNT-infused coatings. Enhanced crack propagation resistance of BNNT-infused coatings will be determined by in-situ scanning electron microscope (SEM) observation of a crack tip while the free-standing coating is subject to a tensile stress test. Solubility and diffusivity of H₂O, O₂ and aqueous Na⁺ and Cl^- ions in unmodified and BNNT-infused coatings will be measured using various permeation tests, and differences in these values will be correlated to high-resolution observations of unmodified and BNNT-infused coating structure. To understand the kinetic processes leading to the dried coating morphologies, the 3D structural evolution of unmodified and BNNT-infused coatings during the coating drying process will be tracked in-situ with nanoscale computed tomography (nano-CT) at the National Synchrotron Light Source-II (NSLS-II) at the Brookhaven National Laboratory (BNL).

Assemblies of canopy-entangled, polymer-grafted nanoparticles (PGNs) provide a method to avoid nanoparticle aggregation common in polymer-nanoparticle blends, and have enabled mechanically robust designs for a variety of structural, electronic, and optical applications. Recent studies have elucidated the relationship between PGN design (graft density (Σ), graft length (N), and nanoparticle size (r₀)) and the plasticity of the assembly below T_g, and identified the key design parameters for PGN assemblies to retain polymer-like plasticity (i.e. crazing). Herein, we discuss the extension of PGN mechanical robustness to ultra-high strain rates (>10⁶ s^-1) examined with Laser Induced Projectile Impact Testing (LIPIT). Assemblies of polystyrene-grafted silica and Fe₃O₄ nanoparticles exhibit unexpected deformation mechanisms at these extreme deformation rates, resulting in unprecedented energy absorption, and kinetic energy dissipation of the micro-projectile. Mechanistically, instantaneous plastic deformation of the nanocomposite glass adiabatically heats the impact site, triggering cumulative energy dissipation processes, such as melt-draw. Linear polystyrene exhibits similar behavior, and successful PGN design should fulfill the requirements for both instantaneous and cumulative mechanical dissipation processes, i.e. retention of polymeric-like plasticity, such as crazing, and optimization of elongational viscosity to maximize energy dissipation while avoiding premature melt-rupture. PGNs with high-molecular-weight, entangled canopies (N/N_e > 5, 2< r₀Σ^0.5<10) exhibit specific penetration energy (E_p*) >30% more than prior record-setting reports of comparable polystyrene thin films. Understanding the impact of PGN design on melt viscosity and elongational flow provides a framework to maximize the high-rate impact performance of PGNs through cumulative dissipation processes, and to guide gram-scale synthesis of next-generation PGNs via a novel mini-monomer encapsulated ARGET ATRP emulsion polymerization process.

Characteristics of nanocrystalline materials are known substantially dependent on the microstructure such as grain size, crystal orientation, and grain boundary. Thus it is desired to have systematic characterization methods on the various nanomaterials with complex geometries, especially in low dimensional nature. One of the interested nanomaterials would be a pure two-dimensional material, graphene, with superior mechanical, thermal, and electrical properties. In this study, mechanical properties of “polycrystalline” graphene were numerically investigated by molecular dynamics simulations. Subdomains with various sizes would be generated in the polycrystalline graphene during the fabrication such as chemical vapor deposition process. The atomic models of polycrystalline graphene were generated using Voronoi tessellation method. Stress strain curves for tensile deformation were obtained for various grain sizes (5~40 nm) and their mechanical properties were determined. It was found that, as the grain size increases, Young’s modulus increases showing the reverse Hall-Petch effect. However, the fracture strain decreases in the same region, while the ultimate tensile strength (UTS) rather shows slight increasing behavior. We found that the polycrystalline graphene shows the reverse Hall-Petch effect over the simulated domain of grain size (< 40 nm).

Reduced graphene oxide grafted with multi-walled carbon nanotubes (MWCNT-RGO) was prepared to reinforce mechanical properties of the nylon 6,6 (PA66). Aminopyrene (AP) and 1-pyrenebutyric chloride (PBC) were attached on the MWCNT (MWCNT-PBC) and RGO (RGO-AP) by physisorption, respectively, followed by reacting acyl chloride groups in the MWCNT-PBC with amine groups in the RGO-AP to graft MWCNT on the RGO. The resulting MWCNT-RGO was melt mixed with PA66 with an expectation of reaction between acyl chloride groups remained on the MWCNT-RGO and PA66. As a result, PA66 grafted MWCNT-RGO (PA66-g-MWCNT-RGO) was fabricated. Formation and properties of MWCNT-RGO were confirmed by several analyses including FE-SEM, XPS, TGA and its interfacial adhesion behavior with PA66. The interfacial adhesion energy of the composites was characterized by using drop-on-fiber method, and PA66 composite containing MWCNT-RGO exhibited enhanced interfacial adhesion energy compared with PA66 composite containing pristine MWCNT and RGO. The PA66/MWCNT-RGO composite showed better filler dispersion than the PA66/MWCNT/RGO composite owing to improved interfacial affinity. For fixed filler content in the composite, the tensile strength and fatigue life of the PA66/MWCNT-RGO composite were the highest among the composites examined.

The surface functions of living organisms have drawn attention because of their unique properties caused by their intrinsic topological structures as well as chemistry for various applications such as self-cleaning surfaces, water harvesting, and nano-micro robotics. Among the plants or animals having functional surfaces or structures, Mimosa pudica is well known as a sensitive plant that can react in response to environmental changes like mechanical touches of even small pressure by raindrops. Furthermore, for maintaining its pressure sensitivity, the leaf surface shows the extreme wettability of a strong superhydrophobicity as well as robust self-cleaning against water drops, which is attributed by the well-distributed microscale clusters made of nanoscale wax flake.
In this work, we present a hierarchical hybrid nanocomposite of TiO₂ nanoparticle encapsulated Aluminum (Al) flake clusters by mimicking the hierarchically grown flake clusters on the leaf of Mimosa pudica. These flake clusters formed by the aluminum hydrolysis process with TiO₂ nanofluid could be fabricated not only on a flat substrate but also on the curved structures of a 3D printed leaf surface. When Al was immersed in heated TiO₂ nanofluid with the 3D substrate, the Al source reacts with hydroxyl ion decomposed water in nanofluid to form flake-like AlOOH nanostructure during which TiO₂ nanoparticles functions as the nucleus of a single AlOOH flake structure and the hierarchical flake cluster structures. The TiO₂ -Al based flake has long-term stability (more than two month) in superhydrophilicity due to dual scale roughness by AlOOH flake clusters as well as TiO₂ effect while the Al base substrate showed mild hydrophilic but not lasted for a longer duration. Hierarchical structures with long-lasting superwetting properties were applied for the liquid collecting system by the 3D printed structures with fluid transport path, which were also mimicked from the leaf vein structure of Mimosa pudica.

Polymer conformal coating over the whisker-prone tin (Sn) surface significantly reduces the risk of electrical short circuits caused by tin whiskers. The choice of coating material and possible degradation with time/environmental exposure can impact the effectiveness of the coating. The conformal coating should be strong enough to buckle a metal whisker without being punctured through the coating but also sufficiently tough to avoid the cracking ahead of the tin whisker growth. In addition, the adhesion of the conformal coating should be high to avoid itself from being peeled from the tin surface during the vertical growth of the whiskers and nodules. In this study, polyurethane-based conformal coatings, which consist of the hard and soft segments, were prepared at various curing conditions to control the aggregation of the hard segments that result in the brittleness of the coating. In an effort to restore the toughness, a toughening agent was also added to the polymer to form the phase-separated domains. Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to examine the degree of curing and its impact on the microstructure developments of the coating. Mechanical deformation of the microstructure-engineered coatings was evaluated by universal testing machine and indentation techniques that mimic the growing whisker. In particular, the contact mechanics between the growing tin whiskers and the protective polymer coating surface will be highlighted. Further, the effect of long-term high temperature and high humidity exposure on the adhesion of the coating and tin whisker mitigation behavior will be presented. This study provides a potential source of developing the effective copolymer microstructure and optimal mechanical properties that can well entrap the growth of tin whiskers and nodules beneath its coating.

Thanks to its superior specific strength to weight ratio, carbon fibers have been extensively used to reinforce polymers in composites for aerospace, automotive and construction uses since the 1970s. However, the atomic cross-linking that allows for the incredible strength of PAN-based carbon fibers comes at the price of increasing phonon scattering points, thus sacrificing thermal conductivity. Here, we report a new polymer composite reinforced with both PAN-based carbon fibers and highly thermal conductive (up to 1290 Wm^-1K^-1) graphene fibers. Heavily oxygenated graphene oxide is chemically grafted to the fibers to improve interfacial properties by increasing fiber-matrix contact area and introducing large amounts of hydrogen bonds between the carboxyl functional groups on the heavily oxygenated graphene oxide and the epoxide groups within the epoxy-based polymer matrix. Such a configuration provides ample mechanical strength and thermal conductivity for a comprehensive range of applications in the aerospace industry.

Aircraft grade tetra-functional epoxy, which is inherently strong but highly brittle, was reinforced with polystyrene-graphene oxide (PS-GO) core-shell nanoparticles to improve its mechanical properties with a focus on fracture toughness. PS-GO core-shell particles were synthesized by in situ emulsion polymerization of styrene in the presence of an aqueous dispersion of GO. The size of the core-shell particles was in the range of 300-600 nm. Scanning electron microscopy (SEM) images of the core-shell particles showed wrinkled spheres which indicated that the PS spheres have been well coated by the GO sheets. Epoxy composites were synthesized by adding 0.1, 0.5 and 1.0 wt.% of the core-shell nanoparticles to neat epoxy resin. The cured composites were tested for fracture toughness and compressive strength and maximum improvements of 25% and 21% respectively for the 0.5 wt.% and 0.1 wt.% PS-GO epoxy composites respectively were observed. At 1.0 wt.% loading, the mechanical properties were on the decline due to the plausible agglomeration of the nano-filler in the matrix. SEM images of the fractured surface showed that the modified epoxy samples had a rough surface while virgin epoxy had a smooth surface. Toughening occurred due to filler matrix debonding causing crack arrest. The thermal behavior of the composites was tested using dynamic mechanical and thermal analysis (DMTA). It was observed that there was a ≤ 2% change in the storage and tan modulus of the modified epoxy as compared to the unmodified epoxy. Hence, the desired improvement in fracture toughness and compressive strength of tetra-functional epoxy was achieved by reinforcement with PS-GO nanoparticles without compromising the thermal properties.

Co superalloy Tribaloy T400 (CoCrMoSi: 8.5% Cr, 28.5% Mo, 2.6% Si, and Co as balance weight) is considered as a potential candidate in impeding the wear-assisted surface material loss from engineering modules such as combustion chamber gas engines, bearings, valves, etc. operating at a temperature as high as 800 °C. A hybrid suspension-powder plasma spray technique has been employed to deposit T400 (average powder size = ~10-45 µm) reinforced with Cr₃C₂ (d₅₀= 3.8 µm) of high hardness (600-1200 HV) and adequate tribo-corrosion protection for such application. A uniform and adherent T400-Cr3C2 coating of thickness ~100 micrometer with 34.4% reduced 2D theoretical inter-lamellar porosity (ImageJ Pro) with respect to T400 revealed in microstructural analysis infer a favorable higher densification characteristic to suspension plasma spray. Dendritic laves’ CoMoSi/Co₃Mo₂Si and hard intermetallic Co₇Mo₆/Co₂Mo₇ phases of T400 along with the corresponding Cr₃C₂ phases are confirmed in T400-Cr₃C₂ phase analysis. Synergistic reinforcement of Cr₃C₂ has further elicited an enhancement in elastic modulus (of ~188.5 GPa) by ~39.4%, and Vickers hardness (of ~10.6 GPa) by ~68.2% in comparison to that of T400 (~135.2 GPa, and ~6.3 GPa respectively); which unambiguously elucidates the consequent increase in plasticity index by ~15.7%, and a drop in maximum displacement amplitude (h_max) by ~21.2% in T400-Cr₃C₂. Correspondingly, an enhancement in COF from 0.37 to 0.43 at 40,000 fretting wear cycles with a 65% reduced experimental specific wear rate in fretting estimated from Hertzian contact theory is obtained for T400-Cr₃C₂. In conclusion, Cr₃C₂ reinforcement in T400 can cater heavy-duty load bearing application by providing substantially improved micro-hardness via retaining the smaller grain size, and a subsequent fretting wear augmentation through carbide phase strengthening.

2D nanomaterials have shown many unique and attractive properties of which mechanical properties play very significant roles in manufacturing, performance, coating, and integration in the development of their potential applications. In this work we study the mechanical properties of combinations of multi-layered heterostructures of 2D nanomaterials, including graphene and hexagonal boron nitride (hBN) by performing nano-indentation through atomistic molecular dynamic simulations. Different stacking order of the free-standing 2D films are chosen and simulated indentation analogous to indentation via atomic force microscopy (AFM) are performed. Elastic properties and intrinsic breaking strengths including Young's modulus, bending modulus, ultimate tensile strength, and fracture strain are measured through nanoindentation simulation and compared to monolayer and bilayer 2D nanostructures. Our results suggest the heterostructures are comparatively more robust than their mono- and bi-layer counterparts. However, the indented area of hBN is much smoother than graphene which had comparatively rough fractured area. The magnitude of strength and fracture strain of monolayer and bilayer graphene is marginally greater than their respective hBN layers whereas that of three layered heterostructures are substantially stronger than their counterpart homostructures. The computed results will be compared with the experimental results during the presentation.

Transparent materials are used extensively in vehicles, cameras, sensors, and displays due to their ability to transmit light and provide physical protection from external chemical and mechanical interactions. Currently, glass is the most commonly used transparent material, but its protection capability is inferior to metals or alloys due to its low toughness. Recently, transparent nanocrystalline ceramic, MgAl₂O₄, has been developed through environmentally controlled pressure-assisted sintering of ceramic nanopowders with grain sizes ranging from 3.7 nm to 80 nm, the smallest grain sizes currently reported. This nanocrystalline ceramic is regarded as an excellent candidate for transparent armor in military applications because mechanical properties of ceramics are usually better than those of glasses. Due to the small size of sintered ceramics, however, the determination of its mechanical properties has not been thoroughly investigated. The nanocrystalline structure contains a high density of grain boundaries, so it is critical to understand how grain boundaries influence plasticity and fracture behavior.
In this study, therefore, we performed nanoindentation and in-situ micromechanical tests on nanocrystalline MgAl₂O₄ to elucidate the role of grain boundaries in plasticity and fracture. For nanoindentation, the Hall-Petch (H-P) relation was observed up to 10.5 nm grain size with a peak hardness of 25.7 GPa, after which an inverse H-P relation was observed with decreasing hardness. These results imply that under the confined deformation mode of nanoindentation, the plasticity mechanism resembles that of nanocrystalline metals, where the grain boundary sliding occurs below the Hall-Petch limit. However, micropillar compression showed entirely different results. In-situ uni-axial micropillar compression showed brittle fracture without any noticeable plasticity. The fracture strength increases monotonically as the grain size decreases. Thus, the inverse H-P relation does not appear in fracture strength. The larger and sharper grain boundaries of 80 nm grain size samples could act as stronger stress concentrators which result in a lower yield strength based on the Griffith criterion. Thus, the grain boundary plays completely different roles; as fracture initiators in uniaxial compressions and plasticity barrier (H-P) or plasticity carrier (inverse H-P) in nanoindentation. High-resolution transmission electron microscopy analysis will also be presented to describe the unique amorphous-crystalline hybrid structure along grain boundaries, which could also be the critical factor controlling both plasticity and fracture processes. These results help provide a better understanding of the mechanical behavior of nanocrystalline MgAl₂O₄ and eventually lead to an improved design of transparent armor with the superior protection capability.

Magnetically susceptible nanoparticles have shown promise in diverse application areas such as shape memory polymers, membrane technology, and drug delivery. In the current work, the impact of surface coating of iron oxide nanoparticles on the bulk magnetization properties of poly(ethylene oxide), PEO, nanocomposites as well as the structure of the nanocomposites were explored. PEO was chosen as the matrix polymer due to its wide use in the healthcare industry. Two coatings were investigated in addition to bare nanoparticles: poly(ethylene glycol), PEG, coated and amine coated 10–nm–diameter iron oxide nanoparticles. Nanoparticles were dispersed in concentrations varying from 0.010–0.750% by weight in PEO. A significant increase in temperature was observed in all samples when PEO/Fe₃O₄ nanocomposites were exposed to an alternating magnetic field. Analysis of magnetization curves revealed an unusual result. The uncoated nanoparticles showed a stronger magnetization than the PEG coated nanoparticles even though the PEG coated nanoparticle containing PEO/Fe₃O₄ nanocomposites showed a much more significant and rapid magnetic heating response. Shape retention properties of samples were also investigated as a function of alternating magnetic field process parameters and iron oxide surface chemistry.

*This material is based upon work supported by the National Science Foundation under Grant No. CMMI-1825254.

Although cement as a durable and affordable building material that has been widely used in construction environments, it still remains unclear on how their microstructures and chemical compositions at the molecular level affect the mechanical properties and time-dependent behaviors. The multiscale heterogeneities in the cementitious materials hinder the scientist from comprehensively understanding the mechanisms of the macroscopic phenomenon. With various hydration extent, irregular porous void and impurities like ettringite, portlandite and sulfate hydrates in the OPC (Ordinary Portland Cement), the study of the cement paste structure with accurate morphology from the molecular level to macroscale becomes challenging. In the light of recently proposed reactive modeling techniques of Calcium-Silicate-Hydrate (C-S-H), the elastic modulus of cement paste at the atomic scale as well as the effect of random nanopores on strength and toughness performance has been explored. However, fast and massive production of the full atomistic model is constrained by speed and size in that the reactive model using ReaxFF is computational intensive. In this study, we focus on straightforward modeling approach based on NMR experiments to study how defects at the nanoscale, for example, the intrinsic defective attribute of calcium silicate chains, including the mean chain length and the calcium-to-silicon ratio (Ca/Si), could strongly affect the mechanical properties of cement paste at macroscale. The combinations of calcium silicate hydrates with various Ca/Si ratios ranging from 1.2 to 2.1 are established and analyzed. From the bottom-up perspective, molecular dynamics simulations are carried out to simulate nanoindentation. By analyzing atomic structures and response under different loading, we reveal the relationships between the mechanical properties and the atomistic structure of C-S-H. This method helps us to bridge the atomistic structures with the material properties of C-S-H at larger scale level.

Aluminium (Al) based marine equipment go through a natural degradation when exposed to corrosive and erosive conditions. The reliability and maintenance of these marine based objects is found to be a major problem due to cavitation erosion. In addition, marine biofouling which is caused due to the presence of hydrogen sulphide in the microbes leads to Microbially Induced Corrosion (MIC) in objects in the marine environment. Together, the synergistic effect of erosion, corrosion and fouling leads to reduced lifespan of the structural and operational components in the marine renewable industry. Given the sheer scale of the marine renewable industry which is estimated to reach around €9 billion by 2030, the effects of cavitation erosion, corrosion and biofouling can cause large losses to the industry which will further increase the significant costs in the operation of such offshore technology. Numerous approaches have been developed to withstand the corrosion of the Al metals. Due to environmental concerns with regard to hexavalent chromium, chromate based coatings are being phased out and the present focus is on the advancement of chromate free coating systems. Conventionally, biocides were used in the antifouling coatings to stop the accumulation of fouling organisms like bacteria, fungi, barnacles etc. Nevertheless, these biocides were found to be toxic to the environment. This is the main driver behind developing eco-friendly multi-functional sol-gel coatings for marine renewable applications. The sol-gel process has the capability to develop coatings with low temperatures and provides uniform coatings with consistence thicknesses and also suitable in developing low fouling surfaces.
The main aim of this work is to develop hybrid sol-gel coatings which act as protective layers for the Al metal. Baseline coatings were synthesized using organically modified silicon precursor 3-methacryloxypropyltrimethoxysilane (MAPTMS) mixed with zirconium (IV) propoxide. The newly synthesized hybrid coating formulations will be compared with the developed baseline coating in terms of their corrosion and cavitation erosion resistance properties. The hybrid coatings were synthesized with a zirconium/alkoxide precursor which were then deposited by dip coating on Al panels. Characterization of the developed coatings included evaluation of properties such as surface finish, chemical composition, morphology, wettability and anti-corrosion properties. The structural and functional, thermal and electrochemical properties of the coatings were evaluated using ATR-FTIR, Differential Scanning Calorimetry (DSC), Open Circuit Potential (OCP) and Potentiodynamic polarization techniques. Furthermore, other properties such as cross-cut adhesion, hardness and Water contact angle (WCA) were determined. The baseline coating were shown to have hydrophobicity with the water contact angle (WCA) of 82°. Thermal analysis revealed that the baseline coatings were stable up to 220°C. Open circuit potential and potentiodynamic polarization measurements indicated a considerable improvement in the corrosion resistance of the coated substrates. Cavitation erosion and abrasion tests were completed on all coatings and rankings of these were produced. Mass loss measurements and surface roughness were used to compare and evaluate the relative degradation of the coatings. The tested coatings will be developed further to improve their cavitation erosion, abrasion and anti-contamination properties.

Two types (hard and soft) of the molds have been used in nanoimprint lithography (NIL) for a high throughput in a large area, and high-resolution parallel patterning. Although hard molds have proven excellent resolutions and high temperature strength, cracks of mold often occur, and high pressure is needed. On the other hand, although soft molds can operate at lower pressure without cracks, it has poor pattern resolution. Here, we introduced a novel hybrid mold of anodized aluminum oxide (AAO) template chemically connected with polydimethylsiloxane (PDMS) layer. Due to the flexible nature of PDMS, we could obtain various nanostructured polymers on not only flat substrate but also curved substrate under relatively lower pressure. Furthermore, the hybrid mold is successfully used for roll-to-roll imprinting for the fabrication of high density array of various nanostructured polymers in a large area.

Thermoplastic resin-based laminated composites with remolding/reshaping ability, recyclability, and multifunctionality, if combined with reinforcement agents, have drawn attention in the composite industry. However, incorporation problems of reinforcement agents such as carbon nanotubes (CNTs) into highly viscous thermoplastic matrices have been challenging due to the difficulties in processability at elevated temperatures and poor dispersion which leads to weak interfacial bonding with the polymer matrix. Utilizing the grown CNTs on nanofibers having high surface area provides an ability to tailor mechanical, electrical and thermal performance in various applications such as lithium-sulfur batteries, laminated composites, and supercapacitors. Hence, to effectively produce CNT-reinforced interleaves with a high CNT quality, it is required to propose an alternative method to overcome these implementation problems.
In this study, growth of radially aligned CNTs on curved polymeric nanofibrous substrates has been conducted for the first time to address one step CNTs synthesis on polybenzimidazole (PBI) nanofibers at high temperatures, which benefits from the synergetic effect of the nanofiber network and stiff and conductive CNTs. Additionally, direct synthesis of CNTs on polymeric nanofibers has the potential of being used as interleaves without tarnishing the quality of the CNTs, as delamination, collection, dispersion, and distribution steps would have been avoided, achieving a single-step approach. The growth of radially oriented CNTs on PBI nanofibrous substrates has successfully been achieved by chemical vapor deposition method (CVD) and these interlayers are used for toughening in CF/PEEK laminated composites.
PBI nanofibers were produced by firstly dissolving PBI solution (PBI Products, 26.2 wt.%) in N-dimethylacetamide (DMAc) solvent to prepare a 20 wt.% solution, followed by an electrospinning process (Argeteknolab). 10 mM catalyst solution (Fe(NO₃)₃.9H₂O (404.0 g/molar, Emir Kimya) and iso-proponal (Sigma Aldrich)) were prepared to deposit Fe³⁺ ions onto nanofibers which acted as a catalyst for CNT growth. Then, nanofibers were kept at 50 °C for 5 hours in an oven. The growth of CNTs onto PBI nanofiber was achieved at 600 °C with 15 min. nucleation and 10 min. growth times. The morphology of the produced CNT/PBI nanocarpets was investigated by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM).
After the production of CNT/PBI nanocarpets, compression molding was utilized to lay up CNT/PBI nanocarpets and Polyethersulphone (PES) films (GoodFellow, thickness 0.5 mm) with different sequences where CF/PEEK prepregs (Mir AR-GE) were used as top and bottom layers. The main reason for the integration of PES films into composites was to provide compatibility between the CF/PEEK prepregs and the CNT/PBI nanocarpets. Following the compression molding process, the samples were hot-pressed for 30 minutes at 320 °C under a pressure of 10 bars. The thickness of the final composite structures was measured to be 1.5±0.03 mm.
The mechanical performance of PES/PBI/PES and PES/CNT-PBI/PES integrated CF/PEEK composite samples under flexural loadings was investigated by a universal test machine (UTM, Shimadzu AG-X Plus) according to ASTM D790-18 standard. The initial results showed that the flexural strength of the neat PBI nanofiber integrated CF/PEEK laminated composite was 103.5±8 GPa, while CNT/PBI nanocarpet integrated laminated composite was around 144.3±10 GPa. In the final paper, to further investigate the effect of the CNT integration on the overall composite structure, a dynamic mechanical analyzer (DMA) (TA Instrument, Q850) will be used to determine storage and loss moduli and to evaluate their variation with frequency and temperature using a three-point bending fixture.

In this era of advanced manufacturing, a plethora of materials and methods have been developed to produce lightweight materials with unique mechanical properties. Record high strength/modulus to density ratios, flaw tolerance, flexibility of ceramic materials, and combination of seemingly conflicting properties (e.g. stiffness and damping) show the promise of nanoscale material architectures. Vertically-aligned single-walled carbon nanotube (SWCNT) forests is one of the best platforms to build these materials from, by the virtue of aligned, a-few-nanometer diameter, and long length (> 1 mm) struts with exceptional mechanical properties arising from strong C-C sp² hybridized bonds. However, mass production of SWCNT forests needed for their wider adoption is hampered by a poor understanding of how to scale CNT growth recipes from small benchtop reactors to larger scale systems.
We sought to address the SWCNT forest growth scalability by gaining a deeper understanding of the CNT growth process. We first identify the limiting step in the SWCNT growth rate and determine the dependence of the reaction kinetics on chamber pressure, gas composition and flowrates. We then employ this knowledge in translating the CNT forest growth to successively larger areas, ranging from 1 × 1 cm² pieces to 6-in. wafers. We demonstrate synthesis of high-density (> 10¹² CNTs/cm²), small-diameter (~ 2 nm) SWCNT forests with structural characteristics and growth kinetics that are uniform over large areas, up to 4-in. diameter.¹ Achieved carbon conversion efficiency far exceeds typical benchtop reactor processes and is on par with the best values reported in the literature. We also demonstrate that the carbon conversion efficiency increases with substrate area and decreases with total flow rate. This trend can be rationalized by accounting for the reactant gas flows in the showerhead CVD reactor, as shown by computational fluid dynamics simulations. We finally demonstrate that, by varying the gas mixture, flowrates, and chamber pressure, we can tune CNT growth kinetics as desired without affecting the forests’ structural properties. These results and insights gained from them will prove to be a valuable guideline for future scale up efforts of SWCNT forests for advanced mechanical/structural nanocomposites.

¹E. R. Meshot, S. J. Park, S. F. Buchsbaum, M. L. Jue, T. R. Kuykendall, E. Schaible, L. B. Bayu Aji, S. Kucheyev, K. J. Wu, F. Fornasiero, submitted (2019).

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-XXX

Cutting-edge technology such as flexible electronics and displays, biocompatible health monitoring systems, novel photovoltaic devices, and functional coatings rely on novel behavior of thin-film materials such as nanocrystal layers. While the optoelectronic properties of semiconductor nanocrystals are well-studied, deploying these materials in flexible/stretchable devices requires an understanding of the mechanical behavior of the nanocrystal layers, which can be challenging to investigate in-situ. Nanoindentation experiments and other probe-based techniques can be used to evaluate single-nanocrystal elastic/plastic deformation – but are more difficult to employ as in-situ methods for layers of nanocrystals on elastomeric substrates. Recently, our group created a novel in-situ framework for evaluating the mechanical properties of thin layers of luminescent silicon nanocrystals (SiNCs) by measuring the onset of instabilities in the SiNC layers on polydimethylsiloxane (PDMS) under finite bending deformation. Our work revealed a neo-Hookean coefficient (µ, analogous to shear modulus at low stress/strain) of a 4.5 µm-thick SiNC film to be 345 ± 23 kPa, which is several orders of magnitude smaller than the shear modulus of a single SiNC. While we hypothesize that the reason for this property difference is due to the porosity of the SiNC layer, there are many other parameters that could influence the mechanical behavior of the SiNC layers including layer thickness, SiNC size and surface functionality, and mechanical properties of the PDMS substrate.
Here we investigate these important parameters regarding their influence over the mechanical properties of SiNC layers on PDMS. We prepare the PDMS in-house using a Sylgard 184 elastomer kit. Next, we synthesize SiNCs using a low-pressure nonthermal plasma and then inertially impact them into thin layers onto the PDMS through a slit-shaped orifice. We control the SiNC size, surface functionality, and layer porosity using the gas flowrate, pressure, gas composition, and deposition stage parameters. The PDMS properties depend on the prepolymer/catalyst ratio and curing conditions during PDMS preparation. Our results indicate that the layer and PDMS parameters exert competing influences on the mechanical properties of the SiNC layers. For example, we found that reducing the stiffness (reducing µ) for PDMS leads to a decrease in µ for the SiNC layer. This change occurs simultaneously with a microstructural densification of the SiNC layer, although the relationship is not monotonic. Unraveling the interconnectedness of these parameters on the physical, mechanical, and optoelectronic properties of the SiNCs will allow for predictive engineering of nanocrystal films for flexible devices and thin-film technology going forwards. While we use SiNCs on PDMS as our testing system, our discoveries are applicable across material types and realize a non-destructive, in-situ approach for evaluating the mechanical properties of other multilayered architectures of elastomers and nanocrystals.

Zirconia is a representative example of ceramic materials that demonstrate outstanding strength and relatively high fracture toughness and is utilized engineering application. In addition to fields requiring physical and chemical stability, it has been considered a candidate fuel matrix and cladding material in the nuclear power sector because of its radiation tolerance when its crystal grain is reduced to the nano-scale level. Moreover, tetragonal stabilized zirconia has a crack-resistant mechanism so-called transformation toughening. This reversible phase transition is involved volume expansions (4-5%) of tetragonal ZrO₂ to monoclinic ZrO₂ phases, and has been widely studied to apply to composite materials. However, despite its advantages, zirconia has a brittle nature that can easily be broken by external impacts. Ceramics perform high strength and stiffness compared to other metals and polymers. On the other hand, the lack of plasticity and sensitivity to flaws are actually weak in terms of structural stability and thus limited in its use. In brittle ceramic materials, it is generally accepted that pores are considered as a detrimental flaw because of serving an occurring point that higher stress is locally concentrated. On the other hand, an insight for suppressing brittle fracture has been proposed recently by applying uniform porous and hollow structures on the nano-scale level. In order to improve the mechanical properties of the ceramic material, not only the morphological aspects, a thoughtful microstructure design is also required in terms of constituent atomic arrangements.
In this study, through the anodization technique, the zirconia layer with nanoporous structure was fabricated. It is aimed to examine the mechanical responses of nanoporous ceramics when manipulated in atomic arrangements. The annealing treatment and proton beam process were utilized as an effective tool inducing the crystallization to the amorphous ceramic oxide layer. To observe the mechanical properties of the zirconia ceramic with nanoporous structures, the micro-pillars were prepared and uniaxially compressed. The deformed morphology of the compressed pillars was also characterized to analyze the mechanical behaviors of each condition.

The zirconium oxide layer with a honeycomb-like structure was fabricated by optimized anodization technique. The microstructure of zirconium oxide layer can be manipulated by utilizing the annealing and ion beam treatment. It was observed that as-fabricated nanoporous oxide layer with amorphous structure was experienced to be fully or locally crystallized. Through micro-compression tests, the plasticity without generating cracks was exhibited in both as-anodized and irradiated pillars with amorphous structure. This fracture tolerant response with large plasticity could be caused by that the compressed pillar has an amorphous structure. The mechanical properties of conventional ceramics manifest in the case of annealed pillar, implying that crystalline structure brings out the brittle feature. In proton irradiated condition, the increased free volume induced by ion bombardments made the ceramic layer more plastically responded. From the results of the mechanical behaviors, the occurrence of plasticity without brittle failure of the ceramic material implies the great potentials that the nanoporous oxide layer can be utilized for various engineering applications by securing the material stability. Especially in ion beam process, it is expected that the ideal characteristics for high strength with remarkable plasticity can be designed by controlling the degree of crystallization with varying the dose of the beam irradiation.

After publication of first 2D material, Graphene, numerous journals treating with 2D nanomaterials were published. Boron nitride (BN) is a synthetic material made from boric acid or boron trioxide and consists of the same number of boron (B) and nitrogen (N) atoms. BN has an isoelectronic structure similar to that of a carbon lattice and shares the same number of electrons between adjacent atoms. The mechanical properties of BNNPs are comparable to those of graphene. The elastic modulus of BNNPs ranges from ~800–850 GPa, depending on the degree of chirality. Like graphene, the mechanical properties and thermal conductivity of BN nanostructures make them attractive as nanofillers in composite materials.
The major applications of BNNP is the polymer nanocomposite. Unlike other polymer composites, polymer nanocomposites show superb property despite of low volume percent of reinforcement material. In addition, polymer nanocomposite is easier to fabricate than conventional consolidation process. The major fabrication factor of polymer nanocomposite is dispersibility of reinforcement material in polymer matrix. However, BNNP agglomerates among themselves due to van der Waals force, so this problem needs to be solved.
To solve agglomeration and dispersion problem, we adopted functionalization process to give BNNP proper dispersibility in solvent. BNNP was covalently functionalized using hydroxide-assisted ball-milling processes, which both induce chemical exfoliation and apply strong mechanical shear forces. This covalent functionalization resulted in more stable dispersion than was attainable with non-covalent functionalization.
In this research we functionalized BNNP to prevent agglomeration and increase own properties by using hydroxide functional group. This functionalization process exfoliates BNNPs via the synergetic effect of chemical peeling and mechanical shear forces to overcome the limitations of the previous approaches. After functionalization of BNNP, functionalized BNNP/Epoxy (BNNP/Epoxy) composite were fabricated by solvent mixing. Mechanical properties of BNNP/Epoxy composites were characterized by using Microforce testing machine.
Mechanical properties of BNNP/Epoxy composites showed 1.8 times enhanced elastic modulus and 2.7 times enhanced fracture toughness, compared to pure epoxy. These results could be explained by hydroxides stacked on surface of BNNP flakes, which hindered agglomeration among BNNP flakes. Furthermore, hydroxides on BNNP improved interfacial bonding with epoxy matrices. This result leads to homogeneous dispersion of BNNP in Epoxy and successfully optimized mechanical properties of nanocomposites. Our team wish that these results could provide useful property criteria of Boron nitride Nanoplatelet for structural materials as industrial applications.

Ultralight materials are expected to be applied to structural materials, heat insulating materials, and filters, etc. Various composite materials such as carbon nanotubes, graphene, and polymers have been proposed for the improvement of mechanical properties for ultralight materials. Recently, ultralight materials have been proposed which have high recovery rates for compression. Since carboxymethylcellulose (CMC) acts as a dispersant for carbon nanotubes, an ultralight material consisting of carbon nanotubes (CNT) and CMC has been reported, but a high recovery rate against compression at low density was not observed. In this study, we fabricated the ultralight materials with the apparent density of 1.25 mg/cm³ consisting of CNT and CMC,and showed that the material has a high compression recovery rate. In this case, the low molecular weight CMC is necessary for the ultralight material to show a high compression recovery rate.
In this study, single-walled CNT (SWCNT) with diameter of 2nm was used. Then, two kinds of CMC with different molecular weights were prepared. The viscosity of low molecular weight CMC (l-CMC) and high molecular weight CMC (h-CMC) solutions at 1 wt% was 50 mPa s and 1734 mPa s, resepctivily. SWCNT was added to an aqueous solution in which CMC was dissolved, and subjected to ultrasonic treatment to prepare a CNT dispersion. The CNT dispersion was frozen while controlling the temperature with a heat insulating material, and freeze-drying was performed to produce a CNT/CMC ultralight material. The upper surface is controlled to −80 ° C, and the bottom surface is thermally insulated by a heat insulating material, whereby the hexagonal ice crystal structure formed on the upper surface reaches the lower portion, and vertical gaps are arranged. Structural observation was performed by a scanning electron microscope, and mechanical properties were evaluated by a compression test.
Samples were made with an apparent density of 5 mg/cm³, 2.5 mg/cm³and 1.25 mg/cm³as the target. No recovery after compression was observed in the composite materials with an apparent density of 5 mg/cm³, 2.5 mg/cm³ regardless of the two kinds of CMC. However, in the density of 1.25 mg/cm³, the clear difference of mechanical properties was observed between h-CMC and l-CMC. In CNT/h-CMC , no recovery was observed. On the other hand, in the CNT/l-CMC ultralight mateiral, the drastical recovery was observed. The recovery rate was 60% against 80% compression. Then, it has low compressive stress even in high strain range. The compressive stress was 0.0014 MPa for 80% strain. Moreover, the compression recovery rate was kept when the compression test was repeated.
In high density of CNT/CMC materials over 2.5 mg/cm³, hydrogen bonds are formed between adjacent CMC. In this range, the recovery rate is lowered because the hydrogen bond force is greater than the resilience by CNT. The low density of CNT/l-CMC materials shows a higher recovery rate than the other samples because the resilience by CNT was greater than the attraction of hydrogen bonds between CMC molecules.

Effective use of heat by heat insulation attracts attention from the aspect of energy saving. Insulating materials are used widely in homes, automobiles, and space transport aircraft. In recent years, development of silica aerogel has been promoted as an ultimate heat insulating material. Silica aerogel is a porous silica body that has both a high porosity (90% or more) and a pore structure with an average diameter of 20 to 40 nm. Heat transfer is generated by three mechanisms of convective heat transfer of gas, heat transfer of solid, and radiative heat transfer. Silica aerogel with low density and fine void structure can greatly suppress convective heat transfer of gas and heat conduction of solid, and achieve extremely low thermal conductivity of 0.02 W / (m K) or less at room temperature . ¹⁾ However, the low mechanical properties due to the low density, and the need for high pressure processes such as supercritical drying are problems. In this study, we focused on porous silica. Porous silica is an aggregate of silica particles, and has fine voids of several nm to several tens of nm, which is a powder material. Porous silica has very high thermal insulation performance due to its bulky, fine void structure. Furthermore, since porous silica is a powder, if it can be aligned sparsely in the material as much as possible, it has lower density than silica aerogel and high heat insulation performance is expected. However, as in the case of the silica aerogel, the low mechanical properties, and the poor formability due to the powder are problems. Therefore, we attempted to improve mechanical properties and formability by combining carbon nanotube (CNT), for which high mechanical properties are reported, with porous silica. The high thermal conductivity of CNT may increase the thermal conductivity of the heat insulating material, but in this study, this was suppressed by the coating of CNT with a polymer. Furthermore, since CNT are nanomaterials having a diameter of several nm to several tens of nm, there are extremely many interfaces and interface contact points. As a result, the thermal conductivity of the bulked actual material becomes significantly lower than the theoretical value due to the thermal resistance of the interface. Porous silica has a high transmittance in the infrared region and radiation heat transfer is not suppressed. The carbon material has a high absorption rate of radiation, and the addition of CNT is also expected to shield the radiation. ²⁾
Using Carboxymethyl cellulose (CMC) as a dispersing agent, CNT was dispersed in water by ultrasonication. This liquid was mixed with porous silica and freeze-dried to obtain a CNT / CMC / SiO₂ composite material. The structure of the material was observed by an optical microscope and a scanning electron microscope (SEM). The mechanical properties were measured by uniaxial compression test and three-point bending test. Thermal conductivity was measured as a measurement of adiabatic performance.
In the case where no CNT was added, the material was largely shrunk by fleeze drying , and it was not possible to form a compact. However, the addition of CNT made it possible to produce a high porosity (90% or more) CNT / CMC / SiO₂ composite material with almost no shrinkage. This material is composed of voids of several tens of μm derived from ice crystals upon freeze drying, and these fine voids are expected to suppress the convection of gas. In addition, the mechanical property was improved by the increase of the added amount of CNT. By increasing the amount of CNT added from 0.5 mg / cm³ to 2 mg / cm³, the bending strength is from 0.01 MPa to 0.02 MPa, and the strain until the material breaks is increased 4.1% to 8.5%. The increase of CNTs confirmed the improvement of the flexibility of the material.

1) A. S. Dorcheh and M.H. Abbasi, J. Mater Process Technol, 199, 10-26 (2008)
2) X. Tao, L. He and Z. J. Hu, Int. J. Heat Mass Transfer, 58, 551 (2013)

Self-healing materials offer a promising mechanism to create wearable electronics and soft robotics with high stretchability and durability, due to their intrinsic ability to recover after damage. Embedding different self-healing materials with distinct nanomaterials can impart different functionalities (e.g., conductors, semiconductors, insulators) to a self-healing polymer. Recently, researchers have designed composite systems in which healing restores not only the mechanical integrity of the device but also its electrical functionality. However, the mechanism for the self-healing of nanocomposites is not well understood, and a variety of factors including size, shape, and chemical interactions of the embedded particles can influence the rate of healing in a composite self-heaving device. In this poster, we present different examples of self-healing composite systems and discuss how the self-healing properties of the composite change based both on the polymer matrix and the embedded nanomaterials. This work highlights potential insights into the design of self-healing composites with improved properties and their potential to be integrated into robust wearable electronics.

In this work, the effect of additions of silver or titanium nanoparticles on the microstructure and mechanical properties of Al₂O₃-based composites was studied. The processing method for the manufacturing of alumina-based composites was a combination of RBAO and SPS processes. After milling stage, carried out in a high energy mill, we have that approximately 90% of the powders have a particle size less than 0.5 μm. After SPS process well densified bodies with almost full density were obtained. The microstructure observed by OM, shows that the reinforcement metal (silver or titanium) occupies intergranular positions and the grain size is very fine and homogenous. On the other hand, with respect to mechanical properties, addition of metals on alumina increases significantly its fracture toughness, being more significant the effect of silver, because K_IC rises from 3.2 MPam^0.5 for monolithic Al₂O₃ to 6.9 MPam^0.5 for Al₂O₃/1wt%Ag composite, whereas, for the Al₂O₃/1wt%Ti composite, the value of K_IC was of 5.7 MPam^0.5.

A carbon nanotube (CNT) yarn is attracting as strong and lightweight yarn because of impressive mechanical properties. Especially, among their yarns, tough performance, which is the ability to absorb mechanical energy before fracture, is considered an important mechanical property for the protection of damage to external force. We described high tough carbon nanotube yarn by inspired a muscle structure of nature, which called myofibrils. The muscle inspired CNT yarn is dramatically improved both tensile strength and strain, and yarn provides a high-energy absorption, which is higher than spider dragline silk (165 J/g) and Kevlar (78 J/g). Additionally, not only demonstrates high tough performance in the wetting and high temperature conditions but also shows ease in shape change by using water. This sewable, wearable, and shape controllable tough yarn has a potential for various applications such as bulletproof material, wearable device, stretchable electrode, aerospace industry, and artificial muscle.

In this study, we propose a unique actuator based on a polymer type actuator and carbon-nanotube (CNT)-composite papers.
The polymer actuators have different features from existing actuators. These are low voltage operation, light weight, flexible and molecular level movement. For the above reasons, energy saving is possible and more complex operation can be realized. In addition, other actuators using bucky gel that consists of CNTs and ionic liquid have been also developed.
CNTs are substance consisting of carbon and have many excellent properties such as high electrical conductivity, light weight, high strength and flexibility. Since CNTs have these properties, CNTs are expected to be used for various things. In contrast, CNTs are nanoscale powdery substance. Therefore, we may contrive to use it or develop the way for applications such as combining with other materials. Thus, we focus on “papers” as the material which is able to change shape easily and is a familiar material of our daily life. We made a paper mixed with CNTs. This material had many excellent properties based on CNTs. We have called this material “CNT-composite papers”.
Recently, development of a lightweight and flexible polymer actuator has been required for medical use, interior design, and so on. Here, the paper can be cut and pasted. So, if the polymer actuator made of CNT-composite papers can be developed, it can be used in various situations. And more complex movements can be performed than existing mechanical actuators. For the above reasons, we aim to develop the “paper actuator” using CNT-composite papers.
One of the polymer actuators contains ions and harnesses the force generated by the movement of ions for operation. We here focused on this type of the actuator in this time. This actuator composes of two types of layers, i.e., the electrode layer and the electrolyte layer. The electrolyte layer is sandwiched between the electrode layers and generally contains ionic liquids. Both the electrode layers and electrolyte layer contain the electrolyte. And the electrode layer contains CNTs. When a voltage is applied to the actuator, cations are attracted to the cathode and anions are attracted to the anode. In ionic liquids, there is a difference in the size of the cations and anions. If cations are bigger than anions, the cathode will be extended and the anode will be shrunk. From this process this device operates as an actuator.
Here our device has a three-layer structure, we designed. The outer layers consist of the CNT-composite papers as the electrode. And the inner layer bases on an ordinary paper containing no CNTs as the electrolyte layer. The ionic liquid was included in this device as the electrolyte.
The production method of the CNT-composite papers is based on Japanese washi papermaking method. First, we prepare the pulp dispersion by stirring pulp fibers in water and prepare the CNT dispersion by mixing CNTs and dispersants with ultrasonication in water. Next, we mix the pulp dispersion and the CNT dispersion. And then, we pour the mixture dispersion in the mold on the net, and scoop up the mixture by the net and dry it by heat pressing. Then, we can fabricate CNT-composite papers by this process. Then an ordinary paper containing no CNTs is sandwiched between CNT-composite papers. We layer the papers by heat pressing in this state. After that, we drip the ionic liquid. When voltage was applied this sample, we confirmed that our sample showed the operation as the actuator, as a result.
We believe our CNT-composite paper will be used as actuators in near future.

Slippery liquid-infused porous surface (SLIPS) technology provides unique capabilities that are unmatched by any other surface technologies. SLPIS comprises a smooth and slippery lubricating surface, where lubricant is trapped within the pores of a solid material to repel various substances, such as water and ice. Thanks to their ultralow ice adhesion strength, SLIPS materials have actively investigated in the field of icephobic coatings where the ice accreted on the coating is passively detached by ice’s own weight. The SLIPS coatings however are still far from being employed in the real environment because of limited durability of the slippery liquid that can be removed together with the detached ice.
Here, we investigate icephobicity of a SLIPS coating based on oil-infused polydimethylsiloxane (PDMS). In order to improve the durability, porous silica aerogel (specific surface area of 600 ~ 800 m/g², porosity above 90%) is used as the oil container. The different viscous oils (viscosity of 6, 100, and 350cst)-infused aerogel and PDMS are mixed and bar-coated, and the hardened films are followed by additional swelling of the oils.
We setup the reliable measuring instrument for icephobicity and characterize oil content, surface topography, ice adhesion strength during repeated icing/deicing cycles of the SLIPS films with different aerogel content. Finally, the inclusion of aerogel container is confirmed to increase the total oil content of the film, which results in more durable icephobicity.

Titanium Carbide (Ti₃C₂ - MXene) with surface termination (Ti₃C₂T_x; T_x: -F, -OH, -O) have created a lot of interest for applications in electro-mechanical engineering domain apart from material science investigations. This interest is primarily due to unique conductivity and hydrophilic behavior exhibited by MXenes which are useful for fabricating nanocomposites for multifunctional sensing applications. The investigation of mechanical properties of a single layer (flake) or few-layer pure (free-standing) Ti₃C₂T_x and Ti₃C₂T_x nanocomposites reported in the literature are limited to the estimation of elastic properties like Young’s modulus and tensile strength. Experimental identification of structural parameters of pure MXenes and MXene/Polymer nanocomposites from the point of view of mechanics is necessary for the design and development of smart structures for Structural Health Monitoring (SHM).
The focus of this paper is on the modeling of pure MXenes (Ti₃C₂T_x) and MXene nanocomposites. Characterization of Ti₃C₂T_x and Ti₃C₂T_x nanocomposites along with experimental validation of the models are performed. The characterization tests discussed in this paper are conducted with Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) and X-ray Diffraction (XRD). MXene flakes are modeled as thin plates in microscale while the macroscale samples subjected to testing are modeled as one-dimensional and two-dimensional structures. These models help in studying the mechanics of the samples and determining the mechanical tests to be performed. Nanocomposites with MXenes as fillers in polymer matrices like epoxy, polydimethylsiloxane (PDMS) and polyvinyl alcohol (PVA) are fabricated and subjected to mechanical loads. The flexible films formed by PDMS and PVA and rigid films formed by epoxy are subjected to tensile and bending tests. Results of these tests are compiled and discussed. The paper concludes with the discussion on the possibility of using MXenes for smart structure and smart coating for SHM.

The magnetic behaviour of NiCo coatings along with high hardness, corrosion resistance and excellent wear resistance makes them a suitable candidate for applications in sensors, actuators and protective coatings. Addition of foreign particles into NiCo matrix is known to promote grain refinement and randomization leading to enhanced corrosion resistance behaviour. In the present work, varying amount of multi-walled carbon nanotubes (MWCNTs) and graphene oxide (GO) was incorporated separately into the NiCo coating matrix which was electrodeposited over mild steel substrate. The formed composite coatings were characterized using scanning electron microscopy (SEM), x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS) techniques for morphology, phase identification and composition of the coatings respectively. Electrochemical impedance spectroscopy and potentiodynamic polarization studies in 3.5 wt.% NaCl solution revealed that the corrosion behaviour of the coatings was sensitive to the amount of graphene oxide and CNTs present in the coatings. With the continued addition of CNTs and GO the corrosion rate of the composite coatings first decreased to the lowest corrosion rate value and then increased to values higher than that of pristine NiCo coatings. This indicated towards the existence of optimum concentration of CNT or GO for achieving highest corrosion resistance performance. Addition of optimum CNT concentration provided hydrophobicity to the coatings and promoted growth along low energy (111) direction. Electron backscatter diffraction (EBSD) analysis of the coating cross-section prepared using SEM-FIB revealed that samples exhibiting maximum corrosion resistance (for optimum CNT/ GO concentrations) also had the highest fraction of low angle grain boundaries (LAGBs) with no additional strain in the matrix.

In this work we use molecular dynamics simulations to investigate mechanical behavior of silicon nitride nanoporous membranes upon tensile loading. We show that pore arrangement pattern, pore size, as well as pore separation distance play an important role in mechanical behavior of the nanostructure with hexagonal pore shape. A brittle to ductile fracture mechanism criterion is successfully developed for hexagonal pore pattern via introducing a new ultrastructure based parameter. For ductile fracture, it was found that von-Mises shear strain is localized into a network of narrow bands within the ligaments that connect the pores. In consequence, a compressive stress is produced within the nodes that connect ligaments and cause suppression of crack propagation and enhances the ductility. In contrast, the accumulation of tensile stress in nodes leads to brittle fracture.

In the recent years, 3D printing has been employed as an effective tool to study hierarchical structures that are difficult or elusive to be synthesized on atomic scale, such as the Schwarzites [1]. Schwarzites [2,3] are 3D carbon nanostructures with their shapes resembling triply periodic minimal surfaces (TPMS), which are porous structures possessing negative Gaussian curvature. In this work we investigated the mechanical properties of diamond Schwarzites through fully atomistic molecular dynamics (MD) and 3D printing techniques. The geometrically optimized atomic models were used to create macroscale models that were then 3D printed (cm size) in home-made device using PLA (polylactic acid) and ABS (acrylonitrile butadiene styrene) polymers. Interestingly, as it was also observed for other Schwarzite families (gyroid and primitive) [1], some qualitative trends of the mechanical behavior (in particular with relation to compressive/tensile deformations and energy absorption/dissipation mechanisms) are present at nano and macroscale. This is especially evident when the ratio between the number of octagons to hexagons (‘flatness’) increases. Another interesting feature is that they exhibit negative (auxetic) [4] and zero Poisson’s ratio regimes. Considering that our multiscale approach is completely general (to use atomic models to create macro models that can be 3D printed), it can be very useful to create new or to improve Schwarzite-based engineering functional materials.

[1] S. M. Sajadi, P. S. Owuor, S. Schara, C. F. Woellner, V. Rodrigues, R. Vajtai, J. Lou, D. S. Galvao, C. S. Tiwary, and P. M. Ajayan, Adv. Mater. 2017, 1704820.
[2] A. L. Mackay, H. Terrones, Nature 1991, 352, 762.
[3] R. H. Terrones and M. Terrones. N. J. Phys. 2009, 5, 126.
[4] R. H. Baughman and D. S. Galvao, Nature 1993, 365, 735

Carbon fibre reinforced polymer (CFRP) composites have been widely used owing to their superior specific strength and modulus. Meanwhile, graphene-based composites have drawn much attention since it’s discovered with exceptional mechanical, electrical and thermal properties. More recently, interest in hybrid composites combining continuous fibres with nanomaterials has grown for achieving ideal properties. For these hybrid composites, the nanomaterial dispersion as well as the interfacial bonding raise the challenges.

In this work, graphene/carbon fibre/epoxy hybrid composites were investigated. The graphene was achieved through in lab electrochemical exfoliation, followed by filtration, washing and drying. The obtained graphene has adequate in-situ oxygen groups, i.e. hydroxyl and carboxyl groups, which makes the dispersion easier as it could stay stable in either water or ethanol. On the other hand, in order to improve the interfacial bonding, silanization was introduced with the 3-Aminopropyl)triethoxysilane (APTES) as the coupling agent. The APTES could react with both oxygen groups of the graphene and amine groups of the hardener, thus build the bridge between them. After the surface modification, the graphene was dispersed in ethanol then sprayed on to carbon fibres, followed with vacuum assisted resin infusion to manufacture the composites. During the mechanical testing, compared with the sample without functionalization, the delamination was largely removed after the silanization. As a result, the mechanical properties, especially for the tensile and flexural strength, improved by more than 60%.

Additive manufacturing of continuous carbon fiber composites using thermosetting polymers is a challenge and so far very few work has been reported. Recently, we developed a novel 3D printing system allowing one-step manufacturing of continuous carbon fiber/thermosetting composites in a reapid and energy-efficient way. Our 3D printing system has good compatibility with commercial carbon tow and bundles, and most of epoxy resins. Our 3D printed composites exhibit high fiber volume fraction (>60%) and high tensile strength (>800 MPa). In this oral presentation, I will talk about our 3D printing technology, including the fundamentals and applications. In addition to carbon fiber, our 3D printing technology can be also extended to other high performance fiber composite manufacturing, including kevlar/thermosetting, and glass fiber/thermosetting.

Load bearing capability and lifetime of the epoxy composites can be enhanced by improving interfacial adhesion fiber and epoxy. In this paper, we aim to elucidate the mechanisms that create the interface of cellulose nanocrystals (CNCs)-bonded carbon nanotubes (CNTs)- carbon fiber reinforced polymer (CFRP) composites. Particularly, we will articulate the CNTs/CNCs’ synergistic effect on the interfacial strength of carbon fiber epoxy composite. To date, several approaches have been proposed to improve interfacial bonding of epoxy composites including sizing of reinforcing fiber [1], nanofiller coating [2, 3] and nanoparticle growth on the surface of fibers [4]. Among these, coating of fibers as a thin layer of polymeric components or nanoparticles is a widely adopted strategy for interfacial enhancement. However, the main challenge is still to develop scalable techniques to incorporate well-dispersed nanoparticle in polymer matrix composites to create a strong bonding between the matrix and reinforcing fiber to enhance interfacial properties. We have introduced a novel processing technique in which CNCs are exploited to control the dispersion and deposition of pristine CNTs onto carbon fibers. CNCs are spindle-shaped nanocrystals having 3-5 nm width and 5-500 nm length [2] with high mechanical properties (130 GPa modulus and 7 GPa strength). Our results show that the integrating 0.2 wt % CNC and 0.2 wt % synergistically increase the flexural and interlaminar properties of CFRP composites by 44%. The CNC-CNT suspension was prepared by sonication in deionized water and the suspension was used to coat the carbon fabrics prior to infusion of resin in a vacuum assisted resin transfer molding (VaRTM) process. To investigate the interfacial properties of CNC-CNT coated carbon fiber/epoxy, in-situ single fiber fragmentation test under an optical microscope is performed. Furthermore, AFM-nano-IR characterization will be used to correlate the chemical composition (with 200 nm resolution) and surface topology of the interphase to its mechanical properties such as strength and modulus.
References
1. Dai, Z.S., et al., Effect of sizing on carbon fiber surface properties and fibers/epoxy interfacial adhesion (vol 257, pg 6980, 2011). Applied Surface Science, 2011. 258(5): p. 1894-1894.
2. Asadi, A., et al., Improving the interfacial and mechanical properties of short glass fiber/epoxy composites by coating the glass fibers with cellulose nanocrystals. Express Polymer Letters, 2016. 10(7): p. 587-597.
3. Tamrakar, S., et al., Tailoring Interfacial Properties by Controlling Carbon Nanotube Coating Thickness on Glass Fibers Using Electrophoretic Deposition. Acs Applied Materials & Interfaces, 2016. 8(2): p. 1501-1510.
4. Liu, Y., et al., Comparison of different surface treatments of carbon fibers used as reinforcements in epoxy composites: Interfacial strength measurements by in-situ scanning electron microscope tensile tests. Composites Science and Technology, 2018. 167: p. 331-338.

Graphene is publicized as the game changing material of this millennium. As research continues to expand our knowledge of this 2D semimetal, interfacial properties are increasingly becoming an important characteristic for an idealized graphene integrated composited system. Our research suggest translating graphene’s properties at the nanoscale to the macroscale is best achieved by forming primary or secondary chemical bonds from the matrix to a graphene flake. In our previous work, we have invented a method of creating a graphene reinforced polymer matrix composite (G-PMC) from flake mineral graphite and PA66 in-situ, using high shear elongational flow. Due to our process, we were able to identify chemical bonding at graphene’s surface and edge by XPS and Raman Spectroscopy. By electron microscopy, our results shed light on the mechanism to the formation of graphene functionalities and the creation of unique modified nanostructured morphologies. This work highlights a method to green chemical routes for manufacturing scalable graphene composites.

Graphene oxide (GO) and reduced graphene oxide (rGO) aerogels have been studied intensively over the past decade for their immense potential speculated by the highly desirable mechanical, electrical, and physiochemical properties. Herein, we report the fabrication of an ethylenediamine (EDA)-reduced GO aerogel via lyophilization freeze-drying method from prepared rGO/EDA hydrogels. EDA is added to the GO solution to act as a partial reducing agent, in addition to serving as a crosslinker during the formation of the hydrogel. The resulting structures exhibited extremely high compressibility and structural stability, while additionally exhibiting significant compressive elasticity. Aerogel samples were reported to hold up to 3000x the sample mass, while simultaneously compressing less than 40% of the total volume. In addition, fabricated aerogel samples were measured to exhibit extremely little, even negligible fatigue over several testing cycles, which further describes the extent of the aerogel’s structural integrity. This gives rise to the potential of EDA/rGO aerogels towards mechanical applications such as shock absorbance or impact resistance.

Carbon fiber-reinforced polymer (CFRP) are lightweight composites and have massive potential towards aerospace and automobile component application. However, the delamination cracks of CFRP are still a challenging topic for the researcher. Numerous works have been done by adding nano-fillers to modify the polymer and improve the mechanical, electrical, and thermal properties. Understanding and minimizing the delamination cracks in CFRP are extremely valuable to reduce the failure in aerospace and automobile parts.
Also, CFRP has very low through-thickness conductivity in comparison to conductivity in the fiber direction due to limited the fiber contact surface. The study of through-thickness conductivity is essential because delamination activity in CFRP can be monitored with a change in electrical resistance in through-thickness direction. Investigation and improvement in through-thickness conductivity of CFRP will lead to understanding and reducing the delamination of CFRP during the catastrophic event like lightning strike.
In this present investigation, Polyaniline (PANI) doped graphene nanosheets (GNS) are used as nano-filler, and its effect on fracture toughness and through-thickness conductivity has been studied. The impact of variation in GNS (0, 0.2, 0.5, and one wt %) with respect to PANI is investigated. A double cantilever beam (ASTM D5528) experiment is carried out to characterize the interlaminar fracture toughness. Through-thickness conductivity is measured by impedance spectroscopy. Scanning Electron Microscopy (SEM) is used to study the cross-section of the laminate. Also, viscoelastic properties have been studied in DMA.

Thermally conductive but electrically insulating materials become more and more important for the thermal management. However, the traditional high filler loading to build a thermally conductive network usually leads to significant increase in the viscosity of the precursor mixtures and mechanical deterioration of the polymer composites. Therefore, it is necessary to increase the thermal conductivity of the composites with as little filler as possible. In this work, the surface of BN was firstly coated by SiO₂ via hydrolysis and condensation reactions with tetraethylorthosilicate (TEOS). Then, atomic layer deposition (ALD) technique was used to coat alumina (Al₂O₃), which would play a catalytic role in the subsequent reaction, on the surface of BN coated by SiO₂ (BN@SiO₂). After that, the BN@SiO₂ treated by ALD (BN@SiO₂-Al₂O₃) was mixed with a certain proportion of super P. Si₃N₄ nanofibers grown in situ on BN can be obtained through reduction and nitridation heat treatment in a nitrogen (N₂) atmosphere. In this structure, which Si₃N₄ nanofibers grown on the surface of BN (BN/Si₃N₄), BN can form the main thermally conductive paths and the Si₃N₄ nanofibers can connect the neighboring BN like “bridges” to construct phonon transmission pathways. This special structure can be used to enhance the thermal conductivity of the polymer composites.

Photoactive deformable polymers have drawn a significant amount of attention due to their distinct advantages such as fast responsiveness, local and wireless control, and environmentally-friendly actuation. Photo-responsive polymers (PRPs) can convert the light energy into the mechanical work. Accordingly, these functional soft materials have been utilized as the bio-mimetic devices and light-responsive soft robots. Among them, the liquid crystalline polymer (LCP) doped with the azobenzene molecules exhibits large and reversible mechanical deformation in response to the UV/visible light irradiation. When the 365 nm-light ray is illuminated, the rod-like trans-moieties are excited and then, isomerized into bent cis-molecules. The photo-chemical reaction induces the collapse of the initial symmetry of the LCP network, which results in the macroscopic deformation. In order to analyze and design the deformation of the PRPs, the multiscale simulation framework, which integrates the mesoscopic light-triggered response and macroscopic mechanical behavior, is systematically developed. Especially, we consider the effects of the diverse design parameters such as the initial LC phase, morphology of the polymer network, and geometry of the specimen to carry out multiscale simulation-based design of the photo-mechanical deformations. First, the coarse-grained molecular dynamics (CG MD) simulation is performed to investigate the changes in properties of the macromolecular network in response to the photo-isomerization reaction. The mesoscale photo-switching potential is firstly developed by using the iterative Boltzmann inversion (IBI) technique to reflect the light-induced molecular shape change and LC phase transition. As a result, we successfully reproduced the light-activated transition between 3 phases (Smectic A (Sm A) – Nematic (N) – Isotropic (I)) and corresponding mesoscale deformations. The light-induced polymeric shape change and softening effect on the elastic properties are parameterized by the photo-isomerization ratio, which represents the extent of the photo-chemical reactions. Then, the mesoscale parameters are upscaled to the continuum scale stress-strain relationship, which is derived from the neo-classical elastic free energy of the LCPs. In order to efficiently reflect the light-induced rotation of the LC mesogens and geometric nonlinearity, a co-rotational formulation is implemented to the finite element (FE) shell model. The presented multiscale analysis efficiently realizes the exotic 3D deformations as well as the simple bending behavior. In addition to these deformation prediction capability, the instability of the snap-through deformations are systematically investigated in terms of the material parameters. We expect that the present scale-bridging computational study can help to practically design the deformed topographies of the photo-responsive mechanical actuators and light-stimuli soft robot components.

Acknowledgements

This work was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Korea government (MSIP) (Grant No. 2012R1A3A2048841).

Understanding and predicting the thermomechanical behaviors of polymer nanocomposites are challenging as they are greatly influenced by many factors, such as interfacial energy and filler volume fraction, giving rise to the presence of nanoscale interfaces. To better design of polymer nanocomposites, we have recently developed a predictive multiscale modeling approach, namely the energy-renormalization method, to examine how the nanoscale interfaces and molecular characteristics influence the mechanical and glass transition properties of polymer nanocomposites. Taking nanofiller reinforced glassy polymer as a model system, I will present a multiscale modeling framework based on coarse-grained modeling, in conjunction with machine learning, to achieve improved and tunable performance of nanocomposites.

Biological and bio-inspired materials are an example of composite materials with superior mechanical properties. Nature can provide inspiration for mechanical optimization of artificial matrix-reinforcement systems, often involving high tunability and synergetic properties. One of the emerging topics of interest in this respect is the study of the mechanical behaviour of composite adhesive structures. This involves the full characterization of the interaction between complex surfaces, a problem that remains to be fully understood in three-dimensional cases, with adhesive and frictional properties emerging from effects from the nano- to the macro-scale. Indeed, unless specific symmetries or simplifying conditions are considered, it is almost impossible to exactly predict the interaction between generic composite surfaces, due to the large number of parameters involved, including geometrical features, mechanical properties of the materials, substrate properties and the adhesive potentials determining the contact properties. If composite materials are considered, the complexity of the mechanics involved is further increased. To tackle this problem, we have developed a theoretical-numerical approach to simulate the behaviour of a heterogeneous composite adhesive membrane-like structure interacting with a rigid substrate. The model is based the framework method to discretise continuous bodies using structured spring lattices. An in-house developed C++ code has been developed, which allows the model to have a great portability and versatility. As an example, the model is applied to simulate the peeling behaviour of a composite spider disc attachment, composed by a stiff tread fused with a softer silk plaque. The adhesive interface is described using a 3D cohesive law, validating the solutions with analytically predictions for symmetrical problems. Results show how the geometrical properties of the membrane determine the maximal pull-off force and extensibility of the system, and how tearing phenomena and the heterogeneity of the structure change the overall behaviour of the spider disc attachment. Numerical predictions are compared with experimental results from collaborating groups.

The quest for alternative biodegradable materials such as cellulose to replace plastics is attracting tremendous attention among the materials science community in addition to being directly aligned with the public interest. Most cellulose nanomaterials comprise of a combination of small (~nm) but strong nanofibers and long (~ μm) but weak microfibers. We investigate the role of interactions of such fibers, spanning across different length-scales, in improving the mechanical properties of the nanocomposite. To qualitatively understand the interface mechanics in the composite material, we devise a three-tier multi-scale coarse-grained (CG) modeling scheme. The levels of the CG scheme are as described: (a) Cellulose molecular chain level (~ Å); (b) Elementary fibril level (~ nm); and (c) fiber level (μm). We characterize the length and diameter of the fibers using Atomic Force Microscopy and thereby choose the 2^nd and the 3^rd level of the CG scheme to model the nano and the microfibers respectively to reproduce analogous experimental conditions. We construct hybrid models by mixing the nano and microfiber models. Changing the mass % of the separate nanofibers help explore the fundamental reason behind why the hybrid show greater strength and toughness than the material separately fabricated using only nano or microfibers. From the quantitative aspect, we obtain that the fracture resistance enhances efficiently by around 57 times when the nanofiber content is about 85.7%. Independent molecular simulations of inter-fiber sliding in the nano-, micro- and hybrid fibers elucidate the formation and breaking of hydrogen bonds and clearly explain the role of hydrogen bonding across their interfaces which directly imparts superior mechanistic properties in the hybrids. Implementing the above CG scheme, we also study the interaction between two neighboring fibers (nano & micro) shedding fundamental insight on the mechanical response of cellulose nanomaterials under various representative loads. Such multi-scale modeling investigations, in addition to being very timely, can also be extended to other fundamental building blocks and directly influence the material design of novel hybrid materials having tunable interfacial properties.

Recent technology of developing membrane-type electronic devices which can deform without losing electrical performance enables stick-and-play system. Because thinner structure is more bendable, a number of researchers tend to make tremendous efforts to develop printable electric devices thinnest possible and transfer printing process. Most efforts work well by thin structure directly when the target applications require mechanically high flexibility but confront some problems of the lack of rigidity to realize three dimensional (3D) electronics devices such as omnidirectional sensory or display system. Here, we report the reliable strategy of handling membrane-type electronic devices, which can deform easily but cannot maintain the shapes by themselves. The method uses linear thermal plastic frameworks that can mechanically support the thin device and also undergo transition from glassy state to rubbery state, in which their Young’s moduli generally decrease from several GPa to several MPa and migration of polymer chains are also possible. We carried out solvent- or heat-assisted plasticization to allow traveling into the rubbery state temporarily for controlled transformation. The strong advantage of this method is the possibility for positioning membrane-type electronics not only neutral mechanical plane but the top or bottom of the plastic frameworks, which is not available without plasticization. Using this method, we successfully developed bezel-less tetrahedral image sensors or curvilinear arrays of IGZO transistors with nonzero Gaussian curvature from planar forms.

What if we could design materials that integrate powerful concepts of living organisms – self-organization, the ability to self-heal, and an amazing flexibility to create astounding material properties from abundant and inexpensive raw materials? This talk will present a review of bottom-up analysis and design of materials for various purposes – as structural materials such as bone in our body or for lightweight, strong and resilient composites, for applications as coatings, and as multifunctional sensors to measure small changes in humidity, temperature or stress. These new materials are designed from the bottom up and through a close coupling of experiment and powerful computation as we assemble structures, atom by atom. Materiomics investigates the material properties of natural and synthetic materials by examining fundamental links between processes, structures and properties at multiple scales, from nano to macro, by using systematic experimental, theoretical and computational methods. We review case studies of joint experimental-computational work of biomimetic materials design, manufacturing and testing for the development of strong, tough and mutable materials for applications as protective coatings, cables and structural materials. We outline challenges and opportunities for technological innovation for materials and beyond, exploiting the use of artificial intelligence as a way to complement conventional physics-based modeling and simulation methods. Altogether, the use of a new paradigm to design materials from the bottom up plays a critical role in advanced manufacturing, providing flexibility, tailorability and efficiency.

Composites with superior mechanical properties are widely used in advanced engineering applications. In the hopes of designing high-performance composites, various analytical, numerical, and experimental methods are used to predict the mechanical properties of composites in terms of reinforcement arrangement and external shape. However, the vast design space of composites, limited accuracy of analytical and numerical approaches in fracture, and time-consuming nature of experimental methods make the rational optimization of composite properties almost infeasible, especially when considering the entire failure mechanism. In this presentation, we introduce two representative works on composite design problems where we utilize machine learning techniques to predict material properties and use the trained models for optimization. First, we use convolutional neural network (CNN) to predict and optimize the stiffness, strength, and toughness of a model composite system consisting of two different linear elastic-perfectly brittle materials, with a reasonable amount of training data (full stress-strain curves of 100,000 random configurations obtained by crack phase field simulation). Despite the astronomically larger combinatorial design space, CNN’s fast inference speed enables the prediction and the optimization of composite design with acceptable accuracy. Second, we leverage Gaussian process regression (GPR) for the design of bio-inspired composites with relatively periodic and regular arrangements with a small training set (full stress-strain curves of 3D-printed samples with 50 different configurations obtained by experiments). The GPR-based design strategy provides a route for strength and toughness optimization when analytical or numerical approach is limited due to the nonlinearity of constituent materials and the uncertainty in manufacturing processes. Our studies demonstrate the potential for machine learning techniques to accelerate the composite design optimization process considering inelastic properties.

The complexity of nanocomposites poses tremendous challenges for the quantitative characterization of the properties of the individual components within the composite. The need for multifaceted characterization of the nanocomposite (mechanically, chemically, electrically, magnetically, etc) requires a multitude of nanoscale characterization methods to be applied to a single sample. While a myriad of excellent techniques exist that can provide information about specific aspects of the material, combining these individual techniques sequentially is particularly difficult for nanoscale composites for reasons of colocalization, contamination, and the hierarchical structures that can span several length scales (from nm to mm).

Combining multiple measurement techniques into one in-situ nanocharacterization instrument can solve these issues to enable multiparametric analysis of nanocomposites. For this purpose, we have developed a combined AFM/SEM/FIB instrument that combines the broad range of physical nanocharacterization methods of atomic force microscopy (mechanical, electrical, magnetic) with the chemical characterization capability of scanning electron microscopy and the micromachining capabilities of focused ion beam milling. Of particular interest for nanocomposites is the correlation of chemical composition, with structure and mechanical properties mapping. The vast differences in Young’s moduli between the individual components in many nanocomposites makes nanomechanical characterization particularly challenging. We therefor use advanced AFM based mechanical property measurement techniques (off resonance tapping in combination with contact resonance) to span three orders of magnitude in Young’s moduli.

In this presentation we will discuss examples of this in-situ nanocharacterization for 2D and 3D study of polymer blends, biological nanocomposites and high-performance self-healing synthetic materials.

For heterogeneous polymer samples like polymer composites and thin films, it is often of interest to understand the mechanical properties of microscopic domains within the material and near the boundaries between one component and another. Atomic Force Microscopy (AFM) has the nanometer level resolution and sensitivity needed to investigate these domains, but established AFM measurement modes do not yield results that allow direct comparison to established rheological techniques like Dynamic Mechanical Analysis (DMA). Contact resonance [1] provides mechanical property maps at well-defined frequencies, but cantilever resonances are many orders of magnitude higher than DMA, making comparisons indirect at best. Intermittent contact methods like TappingMode[2], force volume, and PeakForce Tapping [3, 4] face challenges in calculating intrinsic mechanical properties like storage and loss modulus (or tan delta) due to the non-linear process of making and breaking contact [5].

AFM based nano-DMA (AFM-nDMA) provides viscoelastic results that can be directly compared with bulk DMA. Like bulk DMA, it provides spectra of storage and loss modulus across frequency and temperature allowing construction of master curves through Time Temperature Superposition (TTS) [6]. In addition, it allows high resolution measurements of the microstructure of heterogeneous samples to allow better understanding of their properties and behavior. This presentation will examine the capabilities of this new mode with examples in a wide range of polymers and composites.

[1] U. Rabe, S. Amelio, E. Kester, V. Scherer, S. Hirsekorn, and W. Arnold, Ultrasonics, 2000, 38, 430.
[2] O. Sahin, C. Quate, O. Solgaard, and A. Atalar, Phys. Rev. B, 2004, 69, 1.
[3] T. J. Young, M. A. Monclus, T. L. Burnett, W. R. Broughton, S. L. Ogin, and P. A. Smith, Meas. Sci. Technol., 2011, 22, 125703.
[4] B. Pittenger and D. G. Yablon, Bruker Application Note, 2017, AN149, 1. doi: 10.13140/RG.2.2.15272.67844
[5] M. Chyasnavichyus, S. L. Young, and V. V Tsukruk, Langmuir, 2014, 30, 10566.
[6] M. L. Williams, R. F. Landel, and J. D. Ferry, J. Am. Chem. Soc.,1955, 77, 3701.

Rooted from its heterogeneous microstructure, composite materials possess high strength to weight ratio and, therefore, they are applied in a wide range of industry. However, their complicated microstructure makes it difficult to predict the failure mechanism and residual material life. The in-situ health monitoring system has received much attention in recent years as one of the promising solutions for the aforementioned limitations of composite material. In this research, electrical resistance measurement and IR thermography is utilized simultaneously as a unified non-destructive testing system to monitor the initiation and evolution of damage inside the composite materials. Through the experiment where we combined our coupled NDT system and a uniaxial tensile test of GFRP, the deformation and failure timeline of GFRP under tensile test could be subdivided into three different levels and finally enabled us to identify a characteristic stress value called ‘Damage stress(σ_D)’. In this research, we suggest that the Damage stress value, characterized by our coupled NDT system, can be a reasonable estimation of the fatigue strength of the studied material. In contrast with the conventional fatigue tests that requires tens of thousands of loading cycles, application of our coupled NDT system can effectively characterize the fatigue strength of composite materials in a single uniaxial tensile test. Series of analysis on the experiment result were carried out to understand how the damage stress value can be equivalent to the actual fatigue strength of the studied material. Furthermore, the correlation between the thermal, electrical and mechanical behavior of composite materials was thoroughly understood through multiphysics modeling study.

The introduction of vasculature to polymers and composites enables adaptive and environmentally responsive materials with properties like self-healing, self-cooling, and electromagnetic reconfigurability. Previously, we demonstrated thermal degradation of sacrificial poly (lactic acid) (PLA) templates embedded in thermoset matrices to create multifunctional composites with interconnected vascular networks [1]. However, this vascularization method is a two-step process that requires significant energy input and time to first polymerize the thermoset matrix and then to depolymerize the PLA into volatile products at elevated temperatures (ca. 200 °C for 12 hours). Here, we introduce a biologically inspired, simultaneous depolymerization of sacrificial templates during frontal polymerization of the surrounding matrix [2]. As the reaction front propagates to polymerize the matrix, the released heat is sufficient to concurrently degrade and volatilize the sacrificial template. We explore sacrificial materials such as cyclic poly(phthalaldehyde) (cPPA) and poly(propylene carbonate) (PPC) , which depolymerize at relatively low temperatures (~100°C). This new manufacturing process enables freeform fabrication of complex, porous vascular networks with potential for highly functionalized interfaces.

1. Esser-Kahn AP, Thakre PR, Dong H, Patrick JF, Vlasko-Vlasov VK, Sottos NR, Moore JS, White SR. Three-dimensional microvascular fiber-reinforced composites. Advanced Functional Materials, 2011; 23:3654-3658.

2. Robertson ID, Yourdkhani M., Centellas PJ, Aw JE, Ivanoff DG, Goli E, Lloyd EM, Dean LM, Sottos NR, Geubelle PH, Moore JS, White SR. Rapid energy-efficient manufacturing of polymers and composites via frontal polymerization. Nature, 2018; 557:223-227.

Recently, we developed a method for quantifying the complete uniaxial stress-strain relationship for ultra-thin polymer films. The key to this method is the use of liquids to help support ultra-thin, often fragile, films. Although this method has provided new insights into mechanical properties and deformation mechanisms for dimensionally-confined polymer systems, for some polymers the presence of a liquid may influence the ultra-thin properties. Here, we introduce a new method, freestanding tensile tester to directly measure the uniaxial stress-strain response of freestanding polymer thin films. Using polystyrene thin films, with thickness 15 nm-100 nm, we observe and quantify large strain deformation mechanisms, as well as yield stress and elastic moduli, and compare these results to the liquid supported measurements. We find that the liquid acts as a craze stabilizer for polystyrene thin films which leads to lower strains in the freestanding method compared to the liquid supported method. These results provide new fundamental insights into how the surface interactions can alter polymer behavior in thin confined polymer films.

Current manufacturing of carbon fiber reinforced polymers (CFRPs) consumes a significant amount of energy due to high temperature and pressure cure cycle requirements. Frontal ring opening metathesis polymerization (FROMP) reduces the total energy cost of manufacturing CFRPs by orders of magnitude while reducing the fabrication time. Using a resin system comprising of a ring-strained dicyclopentadiene (DCPD) monomer and a latent second-generation Grubbs catalyst, site-heating generates a self-propagating polymerization front that consumes the monomer and creates rigid, tough thermosets and composites with greater than 50% fiber volume fraction. pDCPD composites currently display suboptimal mechanical properties attributed to weak adhesion of the hydrocarbon matrix to the carbon fibers with commercially available surface chemistries. In this work, several olefinic sizing agents are synthesized and applied to carbon fibers to covalently bond the matrix and reinforcement for increased interfacial shear strength (IFSS). The IFSS, debond length, and fiber slippage at the interface are all characterized in single fiber fragmentation tests under cross-polarized light. Sizing agents applied to the carbon fiber increase the amount of norbornene moieties on the surface, resulting in a reduced critical fiber length. This improved surface functionality increases the IFSS by over 50% from the epoxy-sized fibers. Due to the high fracture toughness of the pDCPD matrix, fiber debonding along the weak interface occurs preferentially to matrix cracking. In addition, we investigate the impact of the unique thermal processing history during FROMP on interfacial properties and compare to conventionally cured samples.

Adaptive, responsive and self-regulated 2D and 3D materials hold promise for a variety of applications – from energy-efficient transducers and energy harvesting to autonomously moving devices and homeostatic systems. From a wide variety of designs that have been utilized to engineer such materials, a hybrid bioinspired system comprised of high-aspect-ratio skeletal elements embedded in a responsive gel “muscle” has proven very versatile. One limitation of this system is its reliance on the responsiveness of the gel, in order to achieve the macroscopic defornations provided by the movement of the skeletal elements. We will present several active temperature- and light-responsive systems where microstructures made of liquid crystal elastomers (LCEs) are themselves the responsive elements. By varying the molecular structure of the LCE precursors and cross-linkers and by predetermining nematic ordering of the resulting elastomers through choosing the orientation of the prepolymer samples in the external director – magnetic field – we have realized a wide range of systems capable of temperature- and light-induced deformations, many of which are unattainable in traditional gel-skeletal element responsive designs or in well-known cellular structures. We will demonstrate predetermined and self-regulated deformations in these LCEs, as well as designs capable of chirality generation and switch, and unprecedented nonmonotonic responses to monotonic stimuli. We foresee that this platform can be widely applied in switchable adhesion, information encryption, autonomous antennae, energy harvesting, soft robotics, and smart buildings.

The mechanical properties of thin polymer films are of scientific and technological interest to diverse communities of researchers. This interest is mainly driven by the development of flexible and stretchable electronics with a wide range of applications in flexible, wearable, and implantable devices. While the moduli of thin polymer films are known to deviate dramatically from their bulk values, the nature of such size effect still remains debatable. Specially, indentation technique gives rise to contradicting results from both buckling experiments and molecular dynamics calculations, which is claimed to result from the substrate effect. Herein, freestanding ultrathin polymethyl methacrylate (PMMA) films with thickness ranging from 6 nm to 129 nm were measured by atomic force microscope (AFM) based nanoindentation. Obvious plate-to-membrane transition is observed in the deformation characteristics with decreasing thickness. Combined with the classical plate theory and contact mechanics model, we demonstrate the great enhancement in both Young's moduli and yield strength even in the absence of substrate, which is further evidenced and interpreted by molecular dynamics (MD) simulations. Additionally, the broadening of the elastic strain range allows higher stretchability, which is anticipated to further advance the development of flexible and stretchable electronics.

In the use of advanced manufacturing, targeting certain mechanical performances by tailoring inherent material properties is a success factor, and material diversities are emphasized as the application area expands to non-metallic materials and mixed composites. Multi-phase polymers or phase-separated block copolymers (BCPs) consist of two or more mechanically distinctive nanoscale phases. The diverse nanostructures of BCPs, which can be tailored through thermal annealing process, cause obvious changes in mechanical properties. Moreover, the high-strain-rate deformation characteristics are generated and controlled by impact tests with different collision conditions. Polymers are emerging materials in additive manufacturing such as the cold spray technique, however, mechanical behaviors including extreme plastic deformations resulting from collisions have not been demonstrated thoroughly.
In order to obtain insight into the novel phenomena of BCP’s mechanical behaviors with anisotropic nanostructures, single-particle impact experiments of polystyrene-block-polydimethylsiloxane (PS-b-PDMS) block copolymer (BCP) microparticles with different nanostructures were performed to demonstrate the effects of nanostructures of BCP microparticles before and after impact against rigid substrates. BCP particles with different volume fractions of PS and PDMS were annealed at diverse conditions to obtain different degrees of ordered nanostructures in two phases, such as cylinders and lamellae. Both annealed and non-annealed BCP particles were tested by using the laser-induced projectile impact test (LIPIT) method. A single BCP micro-particle was accelerated to a high velocity (70 – 600 m/s) and impacted onto a rigid substrate, and the collision was monitored with an ultrafast imaging system using femtosecond illumination pulses.
Coefficients of restitution, critical velocity, acceleration-force-induced inelastic deformation and collision-induced extreme plastic deformation features of the BCP particles were investigated for different nanostructures and impact conditions. Electron microscopy and focused ion beam milling were used to demonstrate ordered nanostructures and impact-induced morphological changes of the BCPs before and after the collisions. Acceleration-force-induced inelastic deformation was investigated by analyzing dimension changes during acceleration and until the moment of impact. As critical velocities and coefficients of restitution of BCPs were changed depending on the status of the nanostructures, the morphologies of nanostructures were important factors in deciding the mechanical characteristics of BCP before and after impact. One of the most critical findings of this study was that nanoscale structural changes of BCP microparticles cause microscale changes in mechanical behaviors, and it furthers research into how BCPs can be tailored to satisfy target performances in additive manufacturing.

* This material is based upon work supported by the National Science Foundation under Grant No. CMMI-1760924.

Cellulose is an attractive material resource for the fabrication of sustainable functional products, but its processing into structures with complex architecture and high cellulose content remains challenging. Such limitation has prevented cellulose-based synthetic materials from reaching the level of structural control and mechanical properties observed in their biological counterparts, such as wood and plant tissues. To address this issue, we report a simple approach to manufacture complex-shaped cellulose-based composites, in which the shaping capabilities of 3D printing technologies are combined with a wet densification process that increases the concentration of cellulose in the final printed material. Complex-shaped composites with cellulose concentration up to 27.35 vol % can be created through the wet densification of 3D printed scaffolds. The densification process involves the exchange of the aqueous phase of the printed wet scaffold by a liquid mixture that works as a poor solvent for the cellulose particles. Because of their high cellulose concentration, composites obtained via infiltration of wet densified scaffolds with an organic phase show significantly higher fracture strength and stiffness compared to state-of-the-art 3D printed cellulose-based materials. This strengthening effect arises from the very high concentration of cellulose achieved in the final composite and is also partly affected by the strong alignment of the cellulose particles along the extrusion direction during the printing process. The high level of structural complexity and control achieved with this combined process opens the way to the fabrication of cellulose-based materials that capture some of the design principles of biological structures like wood and morphing plant structures. Since mechanical stability and high cellulose content are key to achieve long-term durability and to fully benefit from the sustainable nature of this material resource, the proposed manufacturing workflow is also expected to have a major impact in future cellulose-based structural, biomedical and energy-related products.

Polymer/inorganic filler composites have attracted a great deal of attention as a light-weight structural material with excellent mechanical properties. So far, it has been widely accepted that the interface between a polymer and a filler is a key to control the material performance. However, it is still open how the interfacial layer composed of adsorbed chains is formed. In this study, the direct observation of the formation processes for the interfacial adsorbed layer was performed by atomic force microscopy (AFM). As a model polymer, deoxyribonucleic acid (DNA) was selected because of its relatively-large molecular size, leading to observation with relative ease. Lambda phage DNA with 48.5k base pairs, hereafter referred to as DNA, was used. Basically, DNA molecules used take a random coil conformation in a solution state. The radius of gyration (R_g) and the persistence length determined by small-angle X-ray scattering measurement were 284 nm and 29.3 nm, respectively. DNA solutions at various concentrations were prepared by diluting a stock solution of DNA with 40 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer solution containing 5 mM NiCl₂ (5Ni). Mica disks were incubated with the diluted DNA solutions for a given time and then washed with 40 mM HEPES containing 10 mM NiCl₂ (10Ni). To study the morphology of single chains, AFM observations for the sample incubated in 0.1 mg/mL DNA solution with 5 Ni for 2 min were carried out. A single chain of DNA adsorbed on a mica surface was clearly captured. It took a two-dimensional (2D) extended coil conformation. The radius of gyration estimated from the AFM image, defined as R_g,2D, was approximately 630 nm. This value was twice larger than the corresponding value in a solution state determined by SAXS. For statistical analyses, the wide AFM images, containing two or three DNA chains, were acquired. The R_g,_2D and the persistence length evaluated by fast Fourier transform (FFT) analysis were 796 nm and 362 nm, respectively, meaning that DNA chains were more extended onto the mica surface than in the solution. This is probably due to create greater contact points between DNA and mica surface. To discuss the adsorption processes of DNA chains, AFM observations for the samples incubated in a 0.1 mg/mL DNA solution with 5 Ni for 5 - 45 min were carried out. With increasing incubation time, the amount of adsorbed DNA chains increased. Interestingly, DNA chains were inhomogeneously adsorbed, although there were a lot of empty sites on the mica surface. To obtain a better understanding of this finding, we focused on the distance between adsorbed DNA chains. First, the AFM-FFT analysis was made for the sample incubated for 2 min. Since DNA chains were highly isolated for one another, the power spectrum as a function of spatial frequency (L⁻¹) so obtained corresponds to the form factor spectrum. Then, the structure factor spectra for the samples incubated for several hours were obtained from each FFT spectrum after dividing it by the form factor spectrum. In these spectra, a peak corresponding to the distance between adsorbed DNA chains was observed at L⁻¹ of approximately 6×10⁻⁴ nm⁻¹, namely 1.5 μm, which was slightly smaller than twice R_g,2D described above. This peak position was not dependent on the incubation time. This result means that DNA chains were inhomogeneously adsorbed since the earlier stage of adsorption process. Also, entanglements between two DNA chains were clearly observed. Taking into account that the concentration of the solution here used was much lower than the overlapping concentration of DNA, the results indicate that the entanglements between precedingly adsorbed chains and free chains occurred on the mica substrate, called “cooperative adsorption”. In our presentation, effects of molecular weight of DNA and ionic strength of DNA solutions on the adsorption process will be also discussed.

The viscoelastic behavior of polymer grafted nanocomposites (PGNs) with significantly different glass transition temperatures (T_gs) between the graft and matrix polymers is investigated with molecular dynamics simulations. These types of PGNs have been shown to have reversible and repeatable stiffening behavior upon heating (Senses, E.; Isherwood, A.; Akcora, P. ACS Appl. Mater. Interfaces 2015, 7, 14682). This unique thermal stiffening behavior was attributed to the dynamic coupling of the high-T_g adsorbed chains and low-T_g matrix chains. The PGN studied in the current work consists of a nanoparticle with grafted high-T_g polymer chains in a low T_g polymer matrix. The effect of the dynamic coupling of the grafted and matrix polymer chains is studied by molecular dynamics simulations. The influence of the graft density on viscoelastic properties is investigated to identify the mechanism of the observed stiffening in these types of PGNs.

Fragrance oils were encapsulated in a variety of porous particles by preparing emulsion of fragrance oil/hydrophilic polymer and by kneading the emulsion with porous particles. Depending on the hydrophilic polymers, the evaporation rate was slowed down and was controlled by the addition of water. Corn starch, polyvinylalcohol, chitosan, carboxymethyl cellulose were effective because the evaporation rate of fragrance oil was low (lower than 20 %) in air and was high (higher than 80%) in water when the particles were stored at room temperature for 2 hours. The evaporation rate was similar in air and in water in case of sodium alginate, xanthan gum and carbopols. Among the porous particles including activated carbon, powdered charcoal, zeolite, porous silica and magnesium aluminum metasilicate, powdered charcoal had the lowest evaporation rate of fragrance oils. However, the water-triggered evaporation was not observed since fragrance oil did not evaporate even in the presence of water.
Choice of fragrance oils was also important in controlling the evaporation rate. In air, more than 50 % of limonene, citronella and peppermint oils evaporated when the particles were stored at room temperature for 2 hours. Only less than 20 % of lavender, tea tree and ylang-ylang oils were evaporated under the same experimental condition. When water was added to the magnesium aluminum metasilicate particles containing corn starch, 80 % of the lavender oil evaporated in 2 hours while 50 % was evaporate in 24 hours in air.
The water absorbent particles retained absorbent power after the emulsion containing the fragrance oil and hydrophilic polymer. The prepared particles were mixed with superabsorbent polymers to be used in manufacturing the pet pads. The particles and the superabsorbent polymers had different densities and therefore had to be shaken from time to time to ensure the mixing.
Water-absorbent particles such as Syloids and Neusilins were effective in controlling the water-triggered evaporation of fragrance oils because the particles deprive water from the hydrophilic polymers and help form the outer protective polymer layer. When the particles were used with the superabsorbent polymers the urine odor was eliminated by absorbing the chemicals in the urine, by odor-masking of the evaporated fragrance oil, and by the antimicrobial activity of the fragrance oil.

Contemporary research based on electrospinning fabrication has continued to grow as this technique has proven to be an efficient and highly reproducible method for nanofiber production. Electrospinning is the process of forming nanofibers from viscous polymer solutions exposed to electrostatic forces that drive continuous jet formation as solvent is removed and the jet is stretched. The use of dual-electrospinning, involving a two-solution delivery system, in reducing fabrication time and fabricating composite fibrous mats has continued the advancement of electrospinning-based research. This study aims to utilize such advancements to demonstrate triple shape memory via two distinct stimuli that is possible in dual-electrospun poly(vinyl acetate) (PVAc) and poly(ε-caprolactone) (PCL) films.
The term shape memory (SM) refers to the ability of a material to form and recover from a secondary shape to its permanent shape through the application of an external stimuli. We have reported that isotropic PVAc:PCL blends prepared by electrospinning exhibit dual and triple SM, meaning that they can form two distinct temporary shapes. Hydration of the composite films was found to reduce the PVAc glass transition to a subambient level as water molecules disrupt intramolecular bonding and mobilize the network chains. This reduction produces a rubbery material that has a well-separated glass transition temperature (of PVAc) and melt temperature (of PCL), thus allowing this material to exhibit triple SM using two different external stimuli: heat and water.
We report on hybrid shape memory behavior involving both heat and mass transport for PVAc/PCL blends. Heating blends above the melting point of PCL, deforming the sample, and cooling will result in one temporary shape. Deforming the sample again in a water bath, followed by drying, forms a second temporary shape. Rehydration of the composite returns the sample to the first temporary shape and reheating the sample returns it to the original shape. Variables considered include the concentration of PCL, the overall sample geometry and thickness, and the extent of the shapes formed. This hybrid SM is quantified using a Dynamic Mechanical Analyzer (DMA) in conjunction with uniaxial tensile testing with a water immersion bath to separately quantify the SM response to heat and the response to water, respectively. Qualitative analysis is performed through image processing with visual confirmation through video documentation. This provides valuable information on a hybrid process of two well-defined and well understood phenomena, aiding in future research and product concepts that have the potential to revolutionize multiple different fields ranging from textiles to medicine.

A class of polymer-absorbed silica nanoparticle reinforced polymer nanocomposites were found to have a peculiar thermal stiffening response with increasing temperature that not only provides a new method to manipulate mechanical properties as a function of temperature but also offers the opportunity to develop new products based on thermal stiffening. The next step towards mass production requires testing these new class of materials’ processability via traditional polymer processing techniques such as extrusion. In the current work, a laboratory mixing extruder was used to evaluate the processability and the effect of processing conditions on thermal stiffening behavior. Polymers with different rigidities were absorbed on to silica nanoparticles before they were dispersed in a poly(ethylene oxide), PEO, matrix. Then these nanocomposites were melt extruded under various extrusion conditions. The resultant extrudates were characterized using Fourier-transform infrared spectroscopy (FTIR) and electron microscopy. Thermal stiffening behavior of the extrudates was characterized using a parallel plate rheometer. Systems with highly rigid adsorbed polymers, the storage modulus values dropped drastically after extrusion. Systems with less rigid adsorbed polymers recovered and even experienced a slight reinforcement in storage modulus after extrusion. The current work probes the processability of a novel nanocomposite system and also provides new insights towards its dynamics under complex deformation conditions.

Interfacial adhesion has been regarded as a key factor for mechanical peeling and transfer of ultra-thin films which are necessary to fabricate next-generation electronics. Ultra-thin films have strong adhesion because their extreme flexibility enables conformal contact to the topography of a substrate. Whereas, strong interfacial adhesion between a film and a substrate causes both structural damage to the film during delamination process and difficulty of interfacial crack initiation. To reduce adhesion energy and structural damage, solvent assisted transfer methods have been developed by using water and ethanol recently. However, quantitative control of adhesion and its application to damage-free, selective transfer have not been demonstrated yet.
In this work, we show damage-free and selective transfer of ultra-thin films by solvent-assisted interfacial adhesion control. Double cantilever beam fracture testing is performed in various solvent environments including water, ethanol, diiodomethane to measure quantitative adhesion energy of graphene-Cu and Au-Si interfaces. Moreover, two-component surface energy analysis and Owen-Wendt method are exploited to calculate work of adhesion, which is required energy to separate an interface and create new surfaces. Various solvents introduce dramatic change of the interfacial adhesion energy owing to the surface energy difference in the crack-tip environments, and the variation is corresponded to the calculated work of adhesion. Accordingly, the interfacial adhesion can be controlled by modulating surface energy of solvents, and it can be estimated by work of adhesion.
Solvent-assisted adhesion control enables structural damage-free and selective transfer of ultra-thin films. Degree of structural damage in transferred graphene and Au nanofilm is investigated by Raman area mapping and optical images. The solvent environments allow effective mitigation of structural damage in the transferred films by reducing interfacial adhesion energy, and the transferred graphene electrodes with less damage have outstanding mechanical reliability due to uniform stress distribution. Furthermore, novel method for selective transfer of patterned thin films is developed by modulating wettability of thin films and utilizing surface tension of a solvent. Diverse patterns of Au nanofilm such as serpentine electrodes, thin film spiral inductors, and characters are achieved, and the patterned thin films are transferred onto a flexible substrate. We expect that this study will provide an advanced transfer technology to integrate a patterned ultra-thin film without damage onto an arbitrary substrate.

Extreme environments comprised of elevated temperatures, harsh chemical species such as acids and bases, mechanical stresses, and ionizing radiation are of serious concern in every industry relying on materials retaining their inherent properties throughout their lifetime. Furthermore, some industries or uses require these materials to protect its workers and instrumentation from such aggressive conditions. In the current aerospace, defense, and nuclear industries, commercial products are readily used as protective barriers, but there are circumstances when these are less than ideal at providing optimal shielding against extreme conditions, especially neutrons and gamma rays^[1],[2]. In addition, these non-optimized materials may not possess desired properties such as ideal topology and geometry and thus mechanical behavior. As innovations to aerospace, defense, and nuclear technologies continue to progress, developing new engineered materials and composites for extreme environments grows in importance and need.

In the present work, we used an advanced manufacturing (AM) technique known as Fused Filament Fabrication (FFF) to create novel feedstock composite materials for 3D printing. FFF is a layered AM process whereby thermoplastic filaments are heated up to their melting point and extruded into cross-sections of the end product^[3],[4]. Because FFF has the capability to create prototypes and end-use parts with fine resolution details and excellent strength-to-weight ratios, the technology is ubiquitous across many enterprises requiring enhanced manufacturability and unique features.

Difficulties in creating composite filaments for FFF arise from fabricating a homogenous wire that has uniform thickness and a smooth surface. If a filament does not have these initial properties, then either the FFF process will not work or the end product will not be as desired. Creating a homogenous wire proves more difficult when different base and filler materials are used in the fabrication process, however, this can be solved if the organic materials are solvated in a liquid solution and then mixed with the inorganic phase. Creating a wire of uniform thickness relies heavily on the extrusion process, whereby the temperature and extrusion speed are controlled.

In this study, we have prepared homogenous composite filaments for AM from a variety of polymer bases and incorporated a high weight percent loading of metal fillers. Thermal, mechanical, chemical, and radiative properties were evaluated to determine the efficacy of this process of creating AM feedstock material for FFF technologies.

References

1. McAlister, D.R., Gamma Ray Attenuation Properties of Common Shielding Materials. PG Research Foundation, Inc., 2018. Revision 6.1.
2. Shin, J.W., et al, Thermochimica Acta, 2014. 585: p. 5-9.
3. Guo, N. and M.C. Leu, Frontiers of Mechanical Engineering, 2013. 8(3): p. 215-243.
4. Srivatsan, T.S. et al., Additive Manufacturing: Innovations, Advances, and Applications. CRC Press. 2016. p. 1-48.

Due to the advantages of electrospinning process including facility, versatility, cost-efficiency, it has been widely used to fabricate a polymeric nanofiber mat in the engineering field. Especially, a wide range of material selectivity, high surface area to volume ratio and porosity of electrospun nanofibers have contributed to remarkable developments in tissue engineering by recapitulating the hierarchical architecture of the extracellular matrix (ECM).

Significant development of electrospun nanofibrous scaffold in tissue engineering has been associated with the alignment of electrospun nanofiber with respect to topographical effects on cellular activity of cell proliferation, migration, and differentiation. More specifically, the aligned nanofiber mat has been demonstrated to promote the regeneration or repair of various in vivo tissues (e.g. skin, cardiac muscle, and peripheral nerve) and to be used for medical applications.

To produce aligned nanofibers, a number of approaches including electric field-controlled technique (e.g. parallel ground electrodes) and mechanical technique (e.g. high speed rotation of mandrel) have been proposed previously. Among them, the electric field-controlled method can easily produce an aligned nanofiber mat without an additional motion control system of the mechanical method. Moreover, diverse configuration of the alignment, like uniaxial and radial alignments, could be achieved by arranging the ground electrodes. However, the aligned nanofiber mat fabricated by the electric field-controlled method is too mechanically weak to handle. In addition, the charge retention of as-spun nanofibers, which cause unstable electric field and repulsive force near the aligned nanofiber mat, impedes the fabrication of thick aligned nanofiber mat in the process. In order to resolve the limitations of mechanical weaknesses, electrospun nanofibrous bilayer which is composed of the aligned nanofiber layer (functional layer) and the random nanofiber layer (supporting layer) was introduced in previous studies. Though several method such as transfer method and charge retention-based method have been suggested to generate the electrospun nanofibrous bilayer, the disadvantages (e.g. additional transferring process, misalignment, time-consuming, non-uniformity etc) of each method have not been improved.

In this study, overcoming the aforementioned disadvantages, we have developed a novel electrospinning process for the fabrication of electrospun nanofibrous bilayer based on electrolyte-assisted electrical discharge. Interestingly, the newly suggested electrospinning process enabled in situ fabrication of aligned and random nanofiber multilayer (e.g. triple, quadruple layer) by sequentially changing ground electrode from arranged metal electrodes to electrolyte with a simple liquid filling system and without additional transferring process to induce misalignment of the nanofiber. The adoption of the electrolyte was found to provide a stable electric field by an electrical discharge from the deposited nanofibers and allowed the focused electric field on upper surface area of electrolyte. Consequently, the fabrication time was reduced by 2.5 times compared to conventional method. In addition, more homogeneous morphology of the deposited nanofibers was shown. The improved elastic modulus of 4.5 MPa due to the compactly deposited random nanofiber layer demonstrated the sufficient mechanical properties for handling compared with the the elastic modulus of 2 MPa of conventional method. To examine its possible utilization in the field of regenerative medicine, biological in vitro wound healing assay was proceeded with NIH3T3 fibroblasts. The accelerated fibroblasts on the aligned nanofibers suggest the potential of the electrospun nanofibrous bilayer as a wound healing patch and indicated the wound closed faster than the control.

Effective properties of a porous material comprising solid and pore phases can be varied between upper and lower bounds at a given porosity by varying its pore structure. However, achieving low thermal conductivity and high Young’s modulus at the same time is difficult because there is a trade-off. Nevertheless, there must be an optimal pore structure. In this work, we try to find the optimal structure for a porous metal using finite element simulation. Relations between structural parameters and properties are characterized for several pore structures with porosity of 50%. The structural parameters are local curvatures of interfaces (morphology) and Euler characteristics (topology). Then, some rules will be extracted and applied to design an optimal structure.

Nanoimprint lithography (NIL) is a low-cost, high-throughput method used to manufacture products at the nanoscale. The benefits of nanoscale research are the improved physical characteristics of materials at smaller scales. Two of the most prominent methods of NIL are thermal (T-NIL) and UV. In general, T-NIL processes utilize a pre-fabricated rigid mold to imprint a pattern onto a substrate coated with a thermoplastic resist. This research focuses on the molecular dynamics (MD) simulations of T-NIL for four candidate polymers as resist materials: poly methyl methacrylate (PMMA), polyacrylic acid (PAA), polyethylene terephthalate (PET), and polyacrylonitrile (PAN). This is a novel method that considers more than two polymers as possible resists. In order to make inferences about the effect of temperature, force, and pressure on the quality of T-NIL results, the polymers are operated upon using MD. The specific force field applied to this system is a modified version of the consistent valence force field (cvff). It is shown that PMMA yields the most consistent imprints given a certain set of simulation parameters. The simulation parameters were modified to produce changes in: penetration depth, imprint depth, and recovery behavior. The ability to manipulate multiple polymer materials for T-NIL processes is demonstrated by this research.

Photo-actuation is of great interest because the light stimuli are capable of transferring remote controlling signals and high energy density power supply spontaneously with great spatial selectivity. Azobenzene, as one of the molecular scale photo-strain generator, offers direct light-mechanical responses with high energy conversion efficiency and fast responding kinetics. The direct utilization of this molecular-level photo-strain phenomenon is challenging. Conventionally, azobenzene monomers are homogeneously copolymerized with other host materials (such as liquid crystalline polymers), therefore by disturbing the local polymer network structure, the molecular-level photo-generated strain could be translated to mechanically applicable macro-scale actuation. While this strategy offers great optical efficiency, material selection feasibility and programmability, the translation of the molecular-level strain to mechanically applicable macro-scale is inefficient.
In this study, we describe a versatile heterogeneous polymer fabrication strategy to achieve significant enhancement in the translation process, resulting in amplified the photo-actuation responses in azobenzene incorporated polymers. The strategy employs a facial two-step polymerization process based on the sequential orthogonal thiol-ene Michael addition (thiol-acryloyl) and free-radical (thiol-allyl) reactions. In the first step, a combination of di-acryloyl and AB-type allyl/acryloyl functionalized azobenzene moieties undergo thiol-acryloyl addition reaction in the presence of the catalyst. This step offers accurate size control of azobenzene interconnected oligomers with a wide molecular weight range (from 2,300 to 23,000 Da). In the second step, free radical polymerization was carried out for the embedment of the azobenzene oligomers with the host polymer matrix, which provides free-standing actuators with heterogeneous polymer networks. Because of the great chemoselectivity between thiol-acryloyl and thiol-ene reactions, one-pot synthesis scheme is implemented. To better understand the contribution of isomer interconnectivity for photo-actuation performances, free-standing polymer film with different azobenzene interconnectivity (2,300, 5,000, 7,000, 9,000 and 23,000 Da) are tested under various working conditions (irradiation parameters, external loadings, and strains). The power output limits are also measured and fairly compared with the homogenous cases. Results of our study show significant photo-actuation performance enhancement in the isomer-interconnected heterogeneous network comparing to the homogenous cases. In extreme cases (23,000 Da), the maximum power output performances enhancement is up to 500%. Studies within different azobenzene interconnectivity also suggest that the increase of the interconnectivity of isomer significantly benefit the photo-actuation performances. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2012R1A3A2048841).

In this study we investigate a toughening mechanism in epoxy filled with silica based on nanoparticle debonding and subsequent cavitation. The fractographic analysis suggests that such processes take place in the crack tip region due to the high localized hydrostatic stress, and support enhanced dissipation and enhanced toughness. We study the toughening mechanism using models of composites with stochastic microstructure, aiming to identify the distribution of nanofillers which produces the largest toughness enhancement. To this end, we consider composite microstructures with spatial fluctuations of porosity/nanofiller concentration and of elastic moduli, and explore the design space defined by the amplitude and spatial correlations of these stochastic parameters. Based on this combination of experimental and modeling results, we develop design rules for nanocomposites with non-uniform distributions of fillers.

With the advancement in the technology, Composite materials are picking up paramountcy in its expansive overall application from Automobile structural material to Aerospace application, from Household to Marine application and several others like biotechnology, and so forth. Everywhere composite materials are finding its place by superseding conventional materials like metal, ceramic, etc. Low weight and high specific mechanical property make a composite material attractive for developing high load bearing-bridge, bulletproof armor, and high energy efficient process, etc.
In the present work, Composites have been fabricated with the same matrix and reinforcement material (i.e single polymer composite) having different molecular mass and degree of crystallinity. Purpose of choosing the same reinforcement and matrix material is to enhance its interface strength by increasing fiber and matrix compatibility because the same material has high affinity to make strong bonding in comparison to a different material (that is an increase in adhesive/cohesive force). Here, Composite material is chosen which is completely recyclable which makes it environment-friendly.
In this work, the main objective is to optimize mechanical and impact property of different composite flat-panel prepared by using the same matrix and reinforced material. So, By changing the internal architecture of reinforced material through different geometrical weaving like uni-directional and bi-directional fabric reinforcement structure. For the comparison purpose, the composites are produced having the same fiber/volume fraction. Composite’s mechanical and impact behavior keeps on changing as the fiber alignment changes. Mechanical strength and modulus have been calculated through prescribed ASTM standard.
Due to the high viscosity of thermoplastic material, it is difficult to obtain a uniform flow of a matrix during production time throughout the composite which leads to low load transfer from a matrix to the reinforcement material. To sort out this problem, Composite preform is prepared in such a way that melt flown distance of matrix is reduced to very small(through Hybrid yarn technology) which lead to a uniform flow of the matrix material which in turn gave excellent quality (High interlaminar/intralaminar strength) of a composite.
Detailed study of the specimens has been done after mechanical and impact testing. Mechanical testing has been performed using DIC software (Digital image correlation) and mechanical strain gauge. CT (computed tomography) scan has been performed for a thorough analysis of damage to different composites occurred during testing. XRD (X-Ray Diffraction) analysis has been performed to study the crystal structure of fiber to check the temperature sensitiveness on a crystal structure. SEM (Scanning electron microscope) Images of the cross-section of different composites have been taken to view the interface between fiber and matrix and quality of the composites. Numerical Simulation has been performed through ANSYS software to validate the experimental results.

The increase in the consumption of plastic materials is currently one of the major environmental concerns worldwide. To minimize this environmental impact generated by the plastics has been developing polymers from plant sources. Among these biodegradable polymers, the development of polyurethane (PU) using renewable raw material is growing every year, since PU is one of the most versatile polymers in the world today [1]. Despite the advantages that biodegradable plastics can bring to the environment it is necessary that these plastics have adequate durability in their final application especially when exposed in outdoor environments. For this, it is necessary to use stabilizing additives of UV light [2]. In this context, the objective of this work was to synthesize and evaluate the structural, morphological and mechanical properties of a nanocomposite of plant origin PU reinforced with fiberglass (FG) and additivated with Titanium Dioxide (TiO₂) nanoparticles. For the nanocomposite formulations, the micronized rutile TiO₂ was used, whose particles are in the range of 10 to 25 nm, which ensures that these micropigments reflect the UV rays without reflecting any visible light. The rutile form is considered the crystalline form more photo-stable, besides having a refractive index greater than the anatase [2]. The synthesis of PU/FG/TiO₂ specimens was performed by adding 0, 5 and 10% of TiO₂ and 50% of FG to a certain amount of polyol and diisocyanate in the ratio 1:1 stirring until completely homogeneous. The mixture was then placed in molds which were placed for 48 hours in a pressure vessel. The characterization of the nanocomposite was done by Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM) and Tensile Tests, which were performed followed the ASTM D3039/3039M 14 standard [3]. DSC analyses showed that, regardless of the amount of nanocarga used, there was an increase in the melting temperatures of the TiO₂ materials compared to PU containing only FG, indicating that the doped materials appeared to have a higher thermal resistance. SEM images showed that the PU/FG samples containing TiO₂ presented a surface with small agglomerates of small spherical particles which is indicative of the presence of TiO₂. According to tensile tests, the TiO₂ decreases the mechanical properties, like tensile strength and ultimate strain, of the composite.


[1] da Silva, E.H.P. et al. Manufacture and Mechanical Behavior of Green Polymeric Composite Reinforced with Hydrated Cotton Fiber JETI, 2, 26-34 (2019)
[2] Longstreth, J. et al. Health risks. J. Photochem. Photobiol, 46, 20-39, (1998).
[3] ASTM, D3039/D3039M - Standard Test Method for Tensile Properties of
Polymer Matrix Composite Materials System. West Conshohocken: ASTM -
American Standard Test Method, (2014)

Radiation poisoning causes serious problems such as sterility, leukemia and death. Recently, radiation shield research has been in great need to prevent the danger of radiation exposure danger in various fields such space engineering, nuclear generation, and environmental engineering.Radiation shield is typically composed of metals, such as lead, tungsten, and tin which are high atomic numbers and densities to block the transmission rate of radiation. However, such heavy metals are too heavy, rigid and bulky to be used commercially.
In this study, we fabriated a highly stretchable hydrogel shield which was fabricated from acrylamide polymer with PbO₂ metal particles and crosslinked by clay. By utilizing metal nanoparticles and incorporating them into hydrogels, the hydrogel nanocomposite exhibit attenuation coefficient (0.1343 cm^-1) and was stretched to 1400%. Furthermore, because hydrogel composite is physically crosslinked by clay, it has a self-healing property with a maximum self-healing efficiency value of 96.55% at 55°C for 2 hours. However, self-healing efficiency tended to decrease at high radiation dose. Accordingly, XRD, rheometer, GPC were used to investicate these tendency.

Inspired by a variety of camouflage mechanisms in nature, much effort has been made to develop rapid and reversible color changing systems for smart windows, displays, sensors and biomimetic devices. However, current technologies largely depend on modulating the periodicity of photonic structures, thereby the degree of deformation, the magnitude of the spectrum shift, and the color-tuning range are innately restricted. Here we demonstrate a novel type of main-chain chiral liquid crystalline elastomers (MCLCEs) that exhibit outstanding color changing capabilities under mechanical strain. When subjected to a tensile strain perpendicular to the helical axis, MCLCEs contract along the helical axis with a Poisson's ratio greater than 1, causing the Bragg wavelength significantly moves across infrared, visible, and UV spectra. A camouflage system is assembled using an array of patterned MCLCE films that can be actuated by pneumatic inflation. This device applies a bi-axial transverse strain to lead a highly efficient change of structural colors in accordance with the background pattern. Our research will provide new insights for designing smart windows and camouflage systems based on CLCEs.

Over the last decade, a significant attention has been paid to design the adhesive fibril with excellent adhesive properties, by searching for the geometry which give rise to flattened stress distribution because the detachment initiates at a location with high stress concentration. However, the previously optimized fibril geometries were obtained from a limited design space, and the stress distributions were significantly dissimilar with the optimal flat stress distribution. In this study, we introduce the ultimate adhesive fibril shape covering extensive design space by using machine learning technique, kernel support vector machine (SVM) and random forest repressor (RFR). To explore the vast design space, first, we conducted numerical simulation to obtain the stress distribution with 200,000 different fibril shapes defined on the grid. Using the numerical results as a training set, we trained kernel SVM to classify and exclude the geometry with stress singularity at the edge of adhered interface. We then define a score function that represents the similarity with the optimal flat stress distribution, and adapted RFR to predict the score of a given geometry. After establishing the machine learning model, we combine the genetic algorithm and the greedy algorithm to search for the optimized adhesive fibril shape with vast design space. The optimization is repeated until it converges, and the optimized results are compared with the optimized adhesive-fibril in previous studies. Our study demonstrates that the stress distribution at the adhered interface is highly sensitive to the subtle change of the shape, and provides a new strategy of optimizing the adhesive fibril geometry.

Hierarchical polymer composite materials are used in numerous applications that require high stiffness and high specific strength. Poly(urethane urea) polymers are high performance segmented polyurethanes and polyureas elastomers, particularly attractive as matrix materials because of their versatile properties and high tailorability achieved by tuning their microstructure. To further enhance the properties of PUUs, nano-scale fillers such as carbon nanotubes (CNT) are incorporated into PUUs, which form polymer nanocomposites (PNCs). The mechanical properties of these materials are controlled by their complex morphology caused by association of the hard segments (urea segments), which serve as physical cross-link sites due to the inter-chain joining that reinforce the soft matrix (urethane segments). Morphology of PNCs also depends on the nanofiller loading and orientation. Supplementary to experimental efforts, modeling approach provides a powerful means to investigate a broad range of parameters, which is challenging to explore experimentally. We employed dissipative particle dynamics simulations to systematically study impact of chemical and compositional variables on structural and mechanical properties of PUU-based composites. The effect of soft to hard blocks ratio, chemistry and molecular weight as well as CNT concentration and orientation were studied. Our results demonstrated that inclusion of nanofillers is able to alter morphology of PNCs from mixed-phase to interface-mediated microphase separation states, which alter the mechanical response of these materials. Our simulation results are in agreement with recent experimental data.

Capacitors are two terminal devices for energy storage. Polymers like polyethylene (PE), biaxially oriented polypropylene (BOPP), polystyrene (PS), polyethylene terephthalate (PET) are widely used in capacitor applications because of their good film forming properties such as low cost, high dielectric strength and low loss. State of art of polymeric dielectric films are biaxially oriented polypropylene with breakdown strength of 600-700 V/µm (for 10 µm thickness) and with dielectric constant of ~2.2^1,2. Various strategies have been used to increase the dielectric constant and the dielectric strength of polymeric dielectrics. One of them is to orient crystalline domains of polymers to make a torturous path, by applying mechanical deformation in semi-molten state. However, it is challenging to understand the polymer responses under deformation. One way to characterize the microstructural morphology is to measure the birefringence during deformation. By also looking WAXS and SAXS patterns these findings can be supported.
In this research, two different blends are investigated; PBT (polyethylene terephthalate/PEI (polyether imide) and PET/PEI. PBT and PET which are semi crystalline thermoplastic polymers, blended with different ratios of PEI which is an amorphous thermoplastic polymer with Tg ~220⁰C for high temperature applications. By doing so, processability of PBT also increased. It is found that up to 30/70 (PBT/PEI) blend can still crystallize on the other hand for PET/PEI above 30 % PEI the blend disappears. In the absence of orientation, the increase of PEI fraction leads to decrease of crystallizability as expected from the dilution effect that spatially prevents the crystallizable polymer chains from coming together. Blends with high PBT & PET amount can be stretched more and during stretching in blends, amorphous orientation takes place rapidly, followed by crystallization. Stress optical behavior which exhibits a multi stage behavior that depends on process conditions is also investigated³. At low stretching temperatures and high rates the stress optical behavior was found to start with an initial glassy photoelastic behavior.

1. Barshaw, E. J. et al. High Energy Density ( HED ) Biaxially-Oriented Poly-Propylene (BOPP) Capacitors For Pulse Power Applications. 43, 223–225 (2007).
2. Offenbach, I., Gupta, S., Chung, T. C. M., Weiss, R. A. & Cakmak, M. Real-Time Infrared-Mechano-Optical Behavior and Structural Evolution of Polypropylene and Hydroxyl-Functionalized Polypropylene during Uniaxial Deformation. Macromolecules 48, 6294–6305 (2015).
3. Kanuga, K. & Cakmak, M. Nonlinear mechanooptical behavior of poly(ethylene naphthalate)/poly(ether imide) blends. Dynamic phase behavior. Macromolecules 38, 9698–9710 (2005).

Nanoscale thin films with improved toughness, stiffness, and fracture resistance are crucial to numerous technologies, ranging from devices for human-machine interfaces to soft robotics, medicine, energy storage, and smart separation membranes. However, techniques that rapidly quantify the plasticity and failure mechanisms of these thin films, especially in relevant environments, are limited; and thus, hinder development of structure-composition-processing-performance relationships. In this work, we will discuss a high throughput concept to measure the elastic moduli, plasticity and failure strain of thin polymer and nanocomposite films. Building from the Wrinkling-Crack method, we design and additively manufacture compliant lattices to replace the traditional elastomeric support. The geometry of the lattice transduces macroscopic, uniform, in-plane deformation into a wide range of local deformation fields at each lattice cell via differing Poisson’s ratios. By placing a thin film on top of the lattice structure, each cell acts as a unique deformation stage, allowing simultaneous mapping of the yield and fracture envelope. Combining this with optical techniques, automated image processing, and pixelated films with varying composition, thickness, or process history, enables statistically robust analysis of many different parameters in parallel. The use of polystyrene and associated nanocomposite thin films facilitate the identification of opportunities and challenges for this concept.

Poly-Ether-Ether Ketone (PEEK) is a high-performance thermoplastic with excellent chemical resistance and mechanical performance under high-temperature and high-pressure environments. Therefore, PEEK has been commonly used in severe conditions such as aerospace, automotive, oil and gas field. In addition, PEEK has good biocompatibility and is easy to sterilize, which makes it become an important group of biomaterials for medical applications and a strong candidate as an alternative to replace metal and ceramic implants [1]. In recent years, FDM (fused deposition modeling) 3D printing has offered a new manufacturing technology which is able to fabricate complex and customized PEEK products that are either impossible or too expensive with conventional methods. Despite the widespread use of FDM technology, very few studies have investigated the effect of process parameters on the interlayer bonding of PEEK printed objects [2], where the interlayer bonding is a critical factor in order to create strong functional parts.

To address this problem, we adopted a DOE (Design of Experiment) approach to investigate printing parameters on the interlayer bonding using a three-point bending test and the crystallinity in the polymers. In this research, the aim is to identify significant main effects among four independent factors, including nozzle temperature, layer height, print speed, and layer time for the 3D printed PEEK. A 2^4-1design (resolution IV) was chosen which include fewer runs than a full factorial design but is efficient enough for identifying key factors. The samples were printed in an upright position, and were loaded in the weakest direction (layers are perpendicular to the tensile stress in the convex side of the specimen) in the three-point bending fixture. Flexure strength, modulus, and strain at break were evaluated.

The results show that the nozzle temperature is the most important factor which affects the flexure strength, strain at break and crystallinity of the PEEK part greatly, but has very little effect on modulus. The flexure modulus mainly depends on layer height. In addition, wait time and layer height had a significant effect on the flexure strength and crystallinity respectively. The examination of fracture surface through scanning electronic microscopy (SEM) shows different fracture types for specimens printed under different conditions. Overall, the combination of DOE and three-point bending test offers a systematic and efficient paradigm to investigate the effect of printing parameters on the interlayer strength of 3D printed PEEK.

[1] Panayotov, Ivan Vladislavov, et al. "Polyetheretherketone (PEEK) for medical applications." Journal of Materials Science: Materials in Medicine 27.7 (2016): 118.
[2] Dizon, John Ryan C., et al. "Mechanical characterization of 3D-printed polymers." Additive Manufacturing 20 (2018): 44-67.

Polydopamine (PDA) is used as a surface modifier for a wide variety of applications. Many experimental and computational studies have investigated the use of PDA and its adhesive properties, and it is believed that hydrogen bonding and van der Waals forces contribute to the adhesion of polydopamine. The application of PDA between a substrate and a surface coating has dramatically increased the wear resistance of thin film coatings, specifically polytetrafluoroethylene (PTFE). PTFE has been studied as a low friction surface coating since its discovery; using an intermediate layer of PDA between stainless steel and PTFE produces an incredible improvement in wear rate of the PTFE film. While experiments have shown that PTFE and PDA adhere quite strongly, the adhesive mechanisms between the two polymers remains relatively unknown. In this study we investigate the adhesive properties of PDA and PTFE via density functional theory and molecular dynamics. Various PDA oligomers are used to investigate the adhesive forces between PDA and PDA, and PDA and PTFE. Indentation and scratch tests are then performed on PDA/PTFE models and the deformation and wear mechanisms are studied between the interfaces. This study will elucidate the cause of tenacious adhesion between PDA and PTFE, as well as observe the influence of PDA molecules on the friction and mechanical properties of PTFE thin films.

Polytetrafluoroethylene (PTFE) has a low coefficient of friction but also a high wear rate. Numerous experimental and computational studies have investigated wear and the deformation mechanisms of PTFE films, however, severe limitations exists which prevent the modeling of PTFE on length scales which experimentalists are able to investigate. In this work, a coarse-grained model is developed to enable the computational modeling of PTFE particles at the microscale. The coarse-grained potential parameters are derived from first principle based ReaxFF force field simulations, while the PTFE particles are modeled after experimental observations. Indentation and scratch tests are performed using the newly developed coarse-grained models; it was found that the indentation force is dependent on the density of PTFE, and a smooth surface topography decreased the coefficient of friction of the particle. The development and use of the coarse-grained model of PTFE will enable numerical modeling of PTFE on length scales similar to experimental PTFE thin films, thus allowing for a deeper understanding of the deformation, wear, and failure mechanisms of microscale PTFE particles and films.

Recently, advanced composite structures have been widely used in various engineering applications due to their light-weight and high-stiffness characteristics. With increasing utilization of laminated composite structures, there are many theories developed to accurately predict their elastic responses, such as the well-known conventional theories (FSDT; first-order shear deformation theory) and many other refined shear deformation theories(EHOPT; efficient higher order plate theory, EFSDT; enhanced first-order shear deformation theory).
Meanwhile, composite structures have viscoelastic characteristics such as creep strain, stress relaxation and time-dependent failure because composite material is composed of elastic fibers and viscoelastic matrix. Thus, viscoelastic effects of the laminated composite structures should be considered for the reliable analysis. This brings us to develop a new type of enhanced first-order shear deformation theory (EFSDT) by employing the concepts of the strain energy relationship and Laplace transformation. This will allow us to analyze accurately the viscoelastic behavior of laminated composite plates under mechanical loading and to investigate viscoelastic effects such as creep strain and stress relaxation.
In this paper, viscoelastic finite element implementation based on the EFSDT will be performed, and the results of the present theory will be compared to those reported in the open literature. The process of strain energy transformation in Laplace domain and finite element formulation will be described in detail.

Organic and inorganic nanoparticle reinforcements have garnered widespread attention for polymer nanocomposites to yield properties enhancement useful for wide ranging modern and future technologies including photovoltaics, catalysis, optics, and renewable energy. Recent experiments and computational simulations revealed the macroscopic properties are governed by mesoscale structure and interfacial layer dynamics due to the interactions between the polymer matrix (host) and nanoparticle reinforcements (guest). However, a clear fundamental understanding of the role of size, shape, loading (volume fraction) in controlling the structure and dynamics of polymer-nanoparticle interfacial layer is limited. Moreover, ‘forward’ engineered polymer-nanoparticle composites often require higher volumetric density and better dispersions remains a challenging task. We report on developing polymer nanocomposites engineered to minimize dielectric losses and investigating structure and dynamics of interfacial layer to predict macroscale properties. The nanocomposites consist of poly(2-vinylpyridine) (P2VP) polymer matrix with (1) spherical silsesquioxane molecule clusters (~2-5 nm diameter) and silica nanoparticles (~10-20 nm diameter) and (2) planar nitrogenated graphene nanoribbons (~20 nm wide), having dimensions comparable to polymer matrix characteristic length i.e. gyration radius (R_g ~5 nm). This approach will enable improved nanocomposites properties due to enhanced interfacial interactions and identify key molecular parameters governing non-linear dielectric loss mechanisms while studying structure and dynamics using broadband dielectric spectroscopy and small-angle X-ray scattering. The transmission electron microscopy will reveal microscopic structure and the lattice bonding, interfacial stress transfer and conjugation length will be determined from micro-Raman spectroscopy. The exact loading and glass transition temperature, T_g, will be obtained using thermogravimetric analysis and differential scanning calorimetry, respectively. We will gain fundamental insights into the interfacial layer and diffusion dynamics above and below T_g and establish quantitative microscopic structure-property correlations, while predicting macro-scale properties. KY NSF EPSCoR and KY NASA EPSCoR subaward Grants.

Polymer based nanocomposites with excellent mechanical property and high energy density are crucial enablers for numerous applications in modern electronic and electrical industry. The energy density of parallel plate capacitors is determined by breakdown strength and dielectric permittivity of the inner dielectrics. Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)), with the highest permittivity among all the dielectric polymers, is promising candidate for high energy density capacitors. However, its poor mechanical property and relatively low breakdown strength restrict the applications. In this work, we propose a novel method combining combinatorial-electrospinning and hot-pressing to fabricate P(VDF-TrFE-CFE) based all-organic dielectrics with ferroconcrete-like structure. In this structure, continuous fibers of polysulfone (PSF) with high Young's modulus act as tough scaffold to improve the mechanical properties of nanocomposites, and an over 750% enhancement of Young's modulus is obtained. The enhanced mechanical properties bring about significant improvement in Weibull breakdown strength to 485 MV/m, more than 50% higher than neat terpolymer. Besides, the suppressed leakage current and conduction loss thus improved discharge energy efficiency under moderate electric field are achieved due to the high insulation of PSF and interface regions restricting the mobility of space charges.

Structural ceramics provide exceptional mechanical properties at elevated temperatures. The inherent drawback of ceramics is its brittle nature, which paves way for catastrophic failure and is therefore, undesired for practical applications. However, at the nanoscale, materials properties have the capability to be enhanced through careful control of structure and morphology. This talk will precisely focus on nanocomposites based on polymer-derived ceramic reinforcement with tailored architectures for improved mechanical strength. Various polymer/ceramic and ceramic/ceramic systems will be considered and its corresponding structural and mechanical properties will be discussed. Ultimately, these structure/processing/property relationships will provide key insight in the design and development of structural, aerospace components that experience extreme conditions, such as in hypersonic flight.

There is great interest to develop materials with properties that resemble those of biological systems such as hierarchical organization, high stiffness, and the capacity to grow or self-heal. While supramolecular chemistry offers an exciting opportunity to grow materials with nanoscale precision, the ability to transform molecular design into functional devices with enhanced utility at the macroscale remains a challenge.

To tackle this problem, our group is developing supramolecular fabrication tools that combine self-assembly with engineering principles to enable the design of materials with innovative properties. These approaches take advantage of phenomena such as the interplay between protein order and disorder, compartmentalization, diffusion-reaction processes, and host-guest interactions to guide assembly of multiple types of building blocks hierarchically and with molecular and supramolecular control. Some of these building blocks include ions, peptides, proteins, and other components such as graphene oxide.

The resulting materials exhibit properties such as hierarchical organization^1-3, the capacity to grow^2,3, tuneable mechanical properties^2,4, and anisotropic bioactivity⁵ and are being used for the regeneration of tissues such as enamel, bone, and blood vessels as well as the fabrication of more biologically relevant in vitro models.

References
1. Hedegaard et al (2018). Advanced Functional Materials 10.1002/adfm.201703716.
2. Elsharkawy et al (2018). Nature Communications 10.1038/s41467-018-04319-0.
3. Inostroza-Brito et al (2015). Nature Chemistry 7(11), 897-904. 10.1038/nchem.2349.
4. Redondo et al (2019). Biomacromolecules. 10.1021/acs.biomac.9b00224.
5. Aguilar et al (2017). Advanced Functional Materials 10.1002/adfm.201703014.

The majority of natural tissues in human body are elegantly structured to bear a range of functional loads in diverse conditions. Cartilage, ligament, tendon, and muscle, to name a few, possess a set of combinational mechanical properties, including high strength (~1 MPa), superior compliance (~100 kPa) and high toughness (~1000 J/m²), which are unmatched by their engineering counterparts (i.e., synthetic hydrogels). These unprecedented mechanical properties, particularly fatigue resistance, are highly desirable for emerging applications of synthetic hydrogels as diverse as tissue adhesives, gastro-retentive devices, hydrogel electronics, hydrogel robots, and optical fibers. The study of mechanics and design of tissue-like hydrogels are both scientifically interesting and technologically important.
This talk particularly focuses on one extreme property, i.e., fatigue resistance, which is rarely achieved in synthetic hydrogels. The reported fatigue threshold (i.e., the minimal fracture energy that crack does not propagate) for synthetic hydrogels is on the order of 1 to 100 J/m². We propose a general principle that introduction of high-energy nanostructures in hydrogels can substantially enhance their anti-fatigue-fracture properties by fracturing beyond amorphous chains. We demonstrate that the controlled introduction of crystalline domains (1) and/or aligned nanofibrils (2) can substantially enhance hydrogels’ fatigue threshold as high as 1000 J/m². We further show that the principle can be readily employed to achieve fatigue-resistant adhesion by bonding ordered nanocrystalline domains of synthetic hydrogels on engineering materials, which gives a world record high interfacial fatigue threshold of 800 J/m². The principle for achieving anti-fatigue-fracture properties in synthetic hydrogels can evoke a number of applications including ingestible gastro-retentive hydrogel devices (3), fatigue-resistant hydrogel coatings, and robust hydrogel robots (4).
References
1. Lin S, et al. (2019) Anti-fatigue-fracture hydrogels. Science advances 5:eaau8528.
2. Lin S, Liu J, Liu X, & Zhao X (2019) Muscle-like fatigue-resistant hydrogels by mechanical training. Proceedings of the National Academy of Sciences 116:10244-10249.
3. Liu X, et al. (2019) Ingestible hydrogel device. Nature communications 10.
4. Yuk H, et al. (2017) Hydraulic hydrogel actuators and robots optically and sonically camouflaged in water. Nature communications 8:14230.

The enamel of grazing animals represents one of Nature’s most remarkable biological materials — a ceramic-like composite showing exceptional strength, toughness, wear-resistance and controlled-crack propagation. In this work, we study these multi-functional, damage-tolerant biomaterial composites that preserve and ensure life-long reliability for survival (feeding). The goal of our interdisciplinary research is to understand the biomechanical form, function and performance of aprismatic wavy enamel present in hadrosauroid dinosaurs. We test the hypothesis that these wavy enamel structures in hadrosauroid dinosaurs served the same function as those of current grazing mammals through comparative fracture experimentation. Preservation of fracture properties in fossil enamels, many of which are not found in living animals, allows exploration of past form and function and provides a novel source for biomimetically inspired next-generation ceramics.
Hadrosaurid (duck-billed) dinosaurs are the only the only animals known that independently evolved a grinding dentition in the absence of prismatic enamel. Unlike mammals (such as grazing horses and bovids (mammals from cattle family)) hadrosaurids with grinding dentitions utilized undulating wavy enamel (= folded layers of parallel hydroxyapatite crystallites separated by thin layers of loosely aggregated inter-layer matrix), the most complex enamel known in any reptilian taxon.
Wavy enamel has a highly complex hierarchical internal structure and is a particularly challenging material to characterize mechanically, especially at the micron length scales where the mineral and voids once occupied by collagen components are closely intertwined and the available testing methods are limited. We utilized a suite of small scale testing techniques for this work, which involve (a) high throughput nanoindentation testing, to more-specialized FIB-fabricated (b) micro-pillar compression, and (c) micro-tensile loading scenarios. The mechanical data obtained from these tests is correlated with the structure information at complementary length scales using techniques such as BSE-SEM and Raman Spectroscopy.
Our indentation results show that the trends of both the elastic (indentation modulus) and plastic (yield strength) properties are highly complementary to each other and closely reflect the 2D optical profilometry map of the harder and softer enamel tissue layers. The structure-property maps reflect the unique morphology of the wavy enamel layering, where the periodic variations in properties between the layers, combined with the enamel layer undulations, is postulated to promote the remarkable fracture resistance, localized damage and strategically controlled crack directionality of this structure. More-specialized micro-pillar compression testing of the wavy enamel tissue shows clear differences between the fracture behavior under compression of the harder (higher compressive yield and fracture strength) vs. softer tissue layers in hadrosaurid wavy enamel. Intriguingly, the harder tissue layers show initiation of multiple cracks before fracture, suggesting another level of hierarchy where this layer could be composed of weaker interfaces and subsequent sub-layers.
Such insights are crucial for effective approaches to bio-mimetic designs originating from the wavy enamel structure. We utilize the knowledge gained from this work to incorporate the undulating architecture of the hadrosaurid wavy enamel on to a Cu-TiN multilayered nanolaminate structure, synthesized using physical vapor deposition techniques. Our initial results show that, similar to the enamel layer undulations in hadrosaurids, the addition of the waviness to the layering of the metal-ceramic multilayer is instrumental in undulating the crack propagation, thus robbing the crack tip of the required strain energy and controlling its propagation through the laminate.

Articular cartilage plays an important role in synovial joint function, but this function is diminished when cartilage breaks down in osteoarthritis. Tissue engineering is a promising approach for replacing failed cartilage, whcih has no blood supply or intrinsic capacity to heal. Here, a biomimetic composite scaffold is developed, composed of a composite poly (vinyl alcohol) (PVA) and poly (acrylic acid) (PAA) hydrogel, reinforced with gelatin nanofibers. The PVA-PAA hydrogel is cross-linked by repeated freeze-thaw cycles and contains pH-activated charges, which increases the compressive stiffness of the hydrogel. The gelatin nanofibers provide reinforcement against tensile loads, mimicking the function of collagen fibrils in cartilage. Mechanical characterization of the novel composite includes indentation tests, tensile tests and trouser tear fracture tests to measure stiffness, time-dependent behavior, strength and toughness of the composite. Properties approach those of natural cartilage and exceed values obtained for single component hydrogels. By combining biomimicry and composite engineering practices, a better solution for osteoarthritis can be developed.

Self-healing is a characteristic property of many biological materials, leading to enhanced mechanical strength and toughness. Various efforts have been devoted to emulating this property in composite materials for applications in structural engineering. Modelling approaches are essential for this goal, helping to understand how failure processes and global mechanical properties can be modified by self-healing processes and their parameters. We present various approaches developed recently using lattice spring or random fuse models with the addition of self-healing characteristics. We highlight how these influence the scaling of mechanical properties of the corresponding composites, identifying some characteristic signatures from a statistical mechanics point of view, focusing on observables like avalanche distributions, maximum strength and crack roughness. We show how some of these observables can be exploited as indicators of imminent failure and discuss the influence of self-healing parameters on the time evolution of damage in composites, e.g. in determining a transition from a more ductile to a more brittle fracture.

The teeth of limpets are reported to be the strongest natural material, with tensile strength values ranging from 3 to 6.5 GPa. However, the origin of ultrahigh strength of limpet teeth is still unknown. Limpets use conveyor belt-like radula to scrape rocks and extract algae during feeding. These processes require extremely strong teeth. Limpet teeth show characteristic composite nanostructures consisting of high volume fraction of reinforcing goethite crystals and softer amorphous hydrated silica matrix. The volume fraction and morphology of goethite crystals are heterogeneous at different locations of the tooth, which leads to site-specifically heterogeneous mechanical properties. The present work reports on the relationship between microstructures and deformation mechanisms of limpet teeth, using transmission electron microscopy (TEM) and in-situ TEM deformation. At the leading part of a limpet tooth, goethite crystals are mainly aligned along the principal direction of a tooth. The goethite crystals are rod-shaped, with approximately 30 nm in diameter and 300 nm in length. The volume fraction of the goethite crystals is approximately 50 %. TEM characterization of a longitudinal section at the tip of a limpet tooth shows both normally and laterally aligned goethite crystals, with some clusters of the normally aligned crystals. In addition, transition areas at the interfaces between goethite crystals and amorphous matrix are frequently observed. Atomic scale scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) analyses show that the transition areas have different atomic structures and chemical composition from the original goethite crystals. The trailing part of a limpet tooth shows larger goethite crystals, with lower volume fraction than the leading part. To understand deformation behavior of limpet teeth, in-situ TEM deformation experiments were conducted using the samples taken from the tip of a limpet tooth. Upon tension, the sample shows both positive tensile and transverse strain, which indicates negative Poisson’s ratio. Through a digital image correlation (DIC) analysis, the rotation of laterally-aligned goethite crystals appears to result in the observed negative Poisson’s ratio. Inspired by the observation on its structure and deformation pattern, we constructed a conceptual finite element model to mimic its behavior, which can be extended into the framework of micropolar elasticity. When the sample fractures, a crack propagates very fast right after its initiation. The ultrahigh strength of limpet teeth is expected to delay initiation of cracks. The present work will discuss the relationship among microstructures, deformation/fracture behavior and mechanical properties of limpet teeth. This could provide an insight into design of bioinspired engineering composite materials with superior strength and toughness.

The Italian artist, inventor and scientist Leonardo da Vinci (1452–1519) can probably be considered the father of bio-inspired mechanical design, as illustrated by his artificial wings and flying machines, based on bird observation and dissection. Today, 500 years from his death, bioinspiration is attracting widespread attention worldwide, both in academia and industry. This lecture provides a short overview of my group's research activity at the University of Trento, Italy, in line with the Leonardo's legacy on bio-inspired nanomechanics, including nano (i) and bio-inspired materials (ii), as well as their natural evolution towards what we have defined as "bionicomposites" (iii).

Heart valve disease with major symptoms of stenosis and regurgitation is prevalent worldwide. Surgical replacement of diseased heart valves at the end-stages has been widely performed with mechanical valves (MVs) or bioprosthetic heart valves (BHVs). All these current devices have significant limitations with risks of further morbidity and mortality. For example, MVs may cause hemorrhage and thromboembolism, and require anticoagulation for the lifetime of the patients. BHVs show better hemodynamic behavior due to the composition and structural similarity to native heart valves when compared to MVs, however, they do show limited durability because of calcification and progressive degeneration [1]. Thus, polymeric heart valve (PHV) prostheses combining the advantages of MVs and BHVs with long-term durability and no necessity for permanent anticoagulation are of great interest and also show potential applications in advanced transcatheter devices.
In this study, two types of silk fibroin (SF) fiber membranes with anisotropic (ASF) and isotropic (ISF) properties were prepared by electrospinning methods, and were further combined with poly(ethylene glycol) diacrylate (PEGDA) hydrogels to serve as polymeric heart valve (PHV) substitutes (PEGDA-ASF and PEGDA-ISF). The uniaxial tensile tests showed obvious anisotropy of PEGDA-ASF composites with elastic moduli of 10.95 ± 1.09 MPa and 3.55 ± 0.32 MPa, respectively, along the direction parallel and perpendicular to the fiber alignment, close to those of native aortic valve leaflets, while PEGDA-ISF processed isotropic property with elastic moduli of 4.54 ± 0.43 MPa. These novel PHVs consisted of polymeric fibers to mimic the fibrous networks in the fibrosa and ventricularis layers for stress bearing, as well as PEGDA hydrogels to improve anti-fouling function [2,3]. Furthermore, the presence of PEGDA hydrogels in the composites improved the resistance to progressive calcification of the embedded fibers in vitro, likely due to prevention of large-size hydrated ions to pass through by the polymeric networks of the hydrogels [3]. The non-fouling PEGDA hydrogels encapsulated the surfaces of the composites and prevented contact between platelets and the underlying fibers [4].
Pulse duplicator tests presented good hydrodynamic characteristics of these PHVs from PEGDA-ASF and PEGDA-ISF composites according to the ISO 5840-3 standard. Finite element analysis (FEA) revealed the PEGD-ISF valve with anisotropic property showed a lower peak maximum principle stress value (2.20 MPa) in commissures during diastole compared to that from the isotropic PEGD-ISF valve (2.37 MPa). In systole, the bending area of the PEGDA-ISF valve was close to free edges, however, which appeared in the belly portion and near the attachment line for the PEGDA-ASF valve. Hence, our results revealed that anisotropic properties played important roles not only in mechanical properties, but also in hydrodynamic performance of these artificial PHVs. These novel PHVs with good biocompatibility and hemodynamic property can likely be used for heart valve replacement in future.
References
[1] Bertazzo S, Gentleman E, Cloyd KL, et al. Nano-analytical electron microscopy reveals fundamental insights into human cardiovascular tissue calcification. Nat Mater 2013;12(6):576-83.
[2] Zhang X, Xu B, Puperi DS, et al. Integrating Valve-Inspired Design Features Into Poly(ethylene glycol) Hydrogel Scaffolds for Heart Valve Tissue Engineering. Acta Biomater 2015;14:11-21.
[3] Li Q, Bai Y, Jin T, Wang S, et al. Bioinspired Engineering of Poly(ethylene glycol) Hydrogels and Natural Protein Fibers for Layered Heart Valve Constructs. ACS Appl Mater Interfaces. 2017;9(19):16524-16535.
[4] Guo F, Jiao K, Bai Y, et al. Novel transcatheter aortic heart valves exhibiting excellent hemodynamic performance and low-fouling property. J Mater Sci Technol. 2019;35:207-219.

Atomically thin hexagonal boron nitride (h-BN) nanosheet (BNNS), a structural analogue of graphene, has received great attention due to its remarkable properties, which includes which include superior mechanical, thermal conducting and electrical insulating properties, high surface area, excellent chemical stability, high oxidation resistance, and potential biocompatibility.
Regardless of the development of facile and scalable BNNS fabrication processes, the impressive properties of BNNSs have not been exploited at the industrial scale because of the challenges associated with assembling the 2D crystals into macroscopic materials that can leverage the excellent properties of the nanoscale building blocks. On other hand, in nature, nacre exhibits surprisingly high mechanical properties with elegant simplicity in structure consisting of a brick-and-mortar assembly of inorganic and organic constituents. Furthermore, recent developments in nanomaterials and bioinspired materials revealed that excellent mechanical and functional properties can be simultaneously achieved by assembling the nanomaterials into brick-and-mortar (B&M) structure from the nacres.
Herein, artificial nacre-like materials were fabricated using hyperbranched polyglycerol (HPG)-functionalized BNNS and gelatin through a vacuum filtration-induced self-assembly technique. The electrostatic interaction-induced self-assembly of BNNSs and gelatin resulted in the formation of a layered B&M nanocomposite structure resembling that of natural nacre. In contrast to previously reported traditional polymer nanocomposite materials, BNNSs constitute a major load bearing hard component in the nanocomposite material, similar to the brick phases in the nacre structure, while the soft gelatin phases bind the hard brick phases together. Alignment of the BNNSs into a B&M structure was achieved by linking interlayer bridges through electrostatic interactions between anionic hydroxyl functional groups on the BNNSs and cationic amine functional groups on gelatin. The resulting nacre-like nanocomposite exhibited a controllable nanoscale alignment and mechanical properties similar to those of human cortical bones; these properties could be tuned by changing the amount of BNNSs, and by using a functionalization process. Moreover, biological characterization of the nacre-like nanocomposite revealed excellent biocompatibility with signs of negligible cytotoxicity and cell differentiation induction. Furthermore, we believe that various drugs and bioactive molecules could adhere to the surface of BNNSs via a functionalization process to improve biological functionality. The excellent mechanical properties and high biocompatibility of the nacre-like BNNS/gelatin nanocomposite suggests biomedical applications, including in bone implants, bone fillers, and biocompatible biomaterial coatings.

Despite considerable recent interest in micro- and nanofibrillated cellulose as constituents of lightweight structures and scaffolds for applications that range from thermal insulation to filtration, few systematic studies have been reported to date on structure-property-processing correlations in freeze-cast nanocellulose-based composite scaffolds, in general, and their application in tissue regeneration, in particular. Reported in this study are the effects of the addition of plant-derived nanocellulose fibrils (CNF), crystals (CNC) or a blend of the two (CNB) to the biopolymer chitosan on structure and properties of the resulting composites. Nanocellulose-chitosan composite scaffolds were freeze cast at 10°C/min and 1°C/min, and their microstructures were quantified in both the dry and fully-hydrated states using scanning electron and confocal microscopy, respectively. The modulus, yield strength, toughness (work to 60% strain) were determined in compression parallel and the modulus also perpendicular to the freezing direction to quantify anisotropy. Observed were the preferential alignment of nanocellulose crystals and/or fibrils parallel to the freezing direction. Additionally, observed was the self-assembly of the nanocellulose into micro-struts and micro-bridges between adjacent cell walls (lamellae), features that affected the mechanical properties of the scaffolds. When freeze cast at 1°C/min, chitosan-nanocellulose fibril scaffolds had the highest modulus, yield strength, toughness, and smallest anisotropy ratio, followed by chitosan and the composites made with the nanocellulose blend, and that with crystalline cellulose. These results illustrate that the nanocellulose additions homogenize the mechanical properties of the scaffold through cell-wall material self-assembly, on the one hand, and add architectural features such as bridges and pillars, on the other. The latter transfer loads and enable the scaffolds to resist deformation also perpendicular to the freezing direction. The observed property profile and the materials’ proven biocompatibility highlight the promise of nanocellulose-based composites for a large range of applications, including those for biomedical implants and devices.

Few systematic structure-property-processing correlations for directionally freeze-cast biopolymer scaffolds are reported. Such correlations are critical to enable scaffold design with attractive structural and mechanical cues in vivo. This study focuses on freeze-cast collagen scaffolds with three different applied cooling rates (10, 1, and 0.1 °C/min) and two freezing directions (longitudinal and radial). A semi-automated approach for the structural characterization of fully hydrated scaffolds by confocal microscopy is developed to facilitate an objective quantification and comparison of structural features. Additionally, scanning electron microscopy and compression testing are performed longitudinally and transversely. Structural and mechanical properties are determined on dry and fully hydrated scaffolds. Longitudinally frozen scaffolds have aligned and regular pores while those in radially frozen ones exhibit greater variations in pore geometry and alignment. Lamellar spacing, pore area, and cell wall thickness increase with decreasing cooling rate. Both longitudinally and radially frozen scaffolds possess higher mechanical property values, when loaded parallel rather than perpendicular to the ice-crystal growth direction.
Systematic trends and correlations become useful, when they allow predictions of performance and thus, the custom-design of new materials and structures. Traditionally, in the freeze-casting community, structural features, such as the lamellar spacing, λ (which in our case is equivalent to the short pore axis), are correlated with the freezing front velocity, v, during processing. Since the accurate measurement of the freezing front velocity for a given mold design and freezing setup is an elaborate undertaking that either requires specialized molds or cryo-X-ray tomography, it is more practical and useful to obtain correlations between structural features and the applied cooling rate. This is what we propose and report, here, for the case of longitudinal solidification, which results in a particularly reproducible and regular pore architecture. Correlations between the short axis of the pore, the pore area, and the applied cooling rate, ċ, are presented as well as correlations between cell wall thickness, modulus and yield strength. Collated, these correlations enable the custom-design of freeze-cast collagen scaffolds, which are ideally suited for a large variety of tissue regeneration applications.

The fouling and drag force are major issues in fluid transport systems because they cause additional costs for cleaning and fuel consumption, respectively. However, the biggest problem is drag force and anti-fouling ability have conflicting relations so it is difficult to find the surface which satisfies both. For example, a superhydrophobic surface shows drag-reducing effect due to the formation of air layer underwater but this empty air layer could easily collapse by air diffusion and hydraulic pressure. On the other hand, a superhydrophilic surface shows outstanding anti-oil fouling ability due to its high affinity to water but this causes the drag-enhancing effect. To overcome this conflicting relation, we suggest a unique method which is employing electrochemical gas generation system to both bio-inspired wettability controlled surfaces. A superhydrophilic surface has a great advantage for electrochemical water splitting (EWS) due to its large surface area and superaerophobicity underwater. Because of superaerophobicity underwater, a superhydrophilic surface generates the small air bubbles (~ 30 μm sizes) on the surface which form an air layer. This air bubble layer lowers the near surface fluid density which induces an almost 26.5% drag reduction efficiency compared to the flat surface. This value is comparable to the superhydrophobic surface which is the well-known drag reduction surface for fluid transport. In the case of superhydrophobic surface, when the air layer is collapsed the nanostructure could contact with water which means EWS could occur on the surface. Thus, the air bubble is generated on the surface which induces regeneration of the air layer. This study presents an idea that combines the bio-inspired wettability surfaces with EWS bubble generation system for drag-reduced superhydrophilic surface and long-term stability superhydrophobic surface which is useful for an application on the fluid transport system.

Bone is a hierarchically structured composite tissue spanning several length scales. It consists of collagen molecules, calcium phosphate minerals and water. Mineralized collagen fibrils (MCF) are embedded into an extrafibrillar matrix and form lamellar bone. Lamellae are 3 to 7 µm thick layers of MCFs arrays and constitute the majority of cortical bone. To better understand the mechanical properties of bone, it is necessary to characterize the material over several length scales, as well as under conditions resembling a physiological environment [1]. It is important to note that not only are bones subjected to various strain rates on a daily basis, but especially so upon impact, e.g. falling [2]. Therefore, we investigated the strain rate dependency of bone at the length scale of a single lamella via micropillar compressions under hydrated conditions. The micropillars were produced in ovine bone using focused-ion beam (FIB) in axial and transverse orientations, using a well-established procedure [3]. The pillars had diameters of 5.493 +/-0.158 µm and heights of 10.557 +/- 0.744µm. Compression experiments were performed under ambient pressure with a relative humidity of over 90% using a customized nanoindenter setup. Compression tests were carried out under displacement control at different strain rates, varying from 10^-3 to 10^2 1/s, to cover the mechanical response of bone over 6 orders of magnitude in strain rate. Within this range, the change in yield stress, as well as post-yield characteristics were assessed. The anisotropy of the strain rate sensitivity was investigated and post-test HRSEM analysis was performed to image the deformation and failure mechanisms and correlate them to the measured mechanical behaviour.
These experiments highlight the importance of physiological condition testing and the influence of strain rate dependency on bone yield stress and ductility. The generated data may enhance existing micro-finite element models for the clinical evaluation of fracture risk in osteoporotic patients in the future.

[1] J. Schwiedrzik et al., Acta Biomaterialis, 2017
[2] D.R. Carter et al., Science, 1976
[3] J. Schwiedrzik et. al., Nature Materials, 2014

In the papermaking industry, a significant fraction of fibres that cannot be re-utilized is wasted, which raise economic and environmental concerns. On the other hand, the development of renewable polymeric materials became a priority for the sustainability of several industries. Bacterial nanocellulose (BNC), a biopolymer extruded by Gluconacetobacter xylinus as a 3D nanofibrillar network, provide interesting properties as high porosity, high water retention, biocompatibility, non-toxicity and biodegradability. These properties have sustained promising applications in the biomedical field, pulp & paper, composites and foods. However, large-scale BNC production remains a challenge, due to the low productivities, ineffective fermentation systems and high operating costs. Therefore, the production of BNC through lignocellulosic residues has been studied. Recycled paper sludge (RPS) composed of small fibres with 40% of carbohydrates were hydrolysed and used as a carbon source in culture media formulation. Then, a Response Surface Methodology (RSM) optimization with RPS was assessed in order to maximize BNC production, through static fermentation with K. hansenii ATCC 53582. Overall, the results suggest that RPS had the potential to be an alternative carbon source for BNC production with a maximum BNC yield of 5 g/L. BNC produced as described above was then used for the development of novel green thermoplastic nanocomposites, combined with starch. When mixed with water and glycerol (with heat and shear), starch undergoes spontaneous destructuring, forming thermoplastic starch (TPS). In particular to food packaging applications, BNC has remained unexploited in spite of being considered to have enormous potential. In this work, two approaches for composite production were assessed. Firstly, BNC 3D membrane was filled with biodegradable bio-based thermoplastic starch (TPS), where the production was achieved in a two-step process: impregnation of TPS in the BNC membrane, followed by drying. Different thicknesses of BNC membrane were studied (1-5 mm) as two impregnation time (24h;72h). The second approach consisted on the use of glycerol-TPS as a matrix, where different concentrations (0.05 -0.5% w/v) of cellulose (Plant (PC) and BNC) was added. TPS-BNC and TPS-PC films were prepared by solution casting method. All nanocomposites manufactured were then characterized in terms of mechanical properties, morphology and permeability to water vapour (WVT). Overall, enhanced mechanical and barrier properties were obtained with composites composed by BNC membrane filled with TPS. In comparison to TPS-BNC and TPS-PC films, higher young modulus and tensile strength were obtained with the BNC membrane filled with TPS. Being longer and thinner, the BNC fibres offer greater mechanical resistance than the ordinary TPS-cellulose composites. In addition, the elasticity remains similar to the TPS-cellulose composites, despite having a lower concentration of starch/glycerol. The authors gratefully thank funding through the project Multibiorefinery PAC (SAICTPAC/0040/2015) and SkinShip with reference PTDC/BBB-BIO/1889/2014 (financiado pelo Fundo Europeu de Desenvolvimento Regional (FEDER) através do Programa Operacional Competitividade e Internacionalização - COMPETE 2020, do Programa Operacional Regional de Lisboa e por Fundos Nacionais através da FCT - Fundação para a Ciência e a Tecnologia no âmbito do projeto POCI-01-0145-FEDER-016595).

Fabrication of environment-friendly polymers are in great demand as they are derived from naturally occurring molecules which degrade easily into the environment without causing any harm and can be useful for solving environmental problems. One such environmentally benign, high performance polymer poly(4HCA-co-DHCA) was synthesized using phenolic monomers (4-hydroxycinnamic acid (4HCA) and dihydroxycinnamic acid (DHCA)) has high thermal and mechanical strength than usual aliphatic polyesters such as poly(lactic acid) which makes the resulting polymer highly rigid in nature. Also, these polymers show liquid crystalline behavior which sometimes dramatically increases the mechanical strength. Because of the high rigidity, these polymers cannot be used in applications which require flexible polymers. Poly(4HCA-co-DHCA) is brownish in color and look alike wood but it is very rigid in nature. This polymer can be a good candidate to be explored further to obtain a flexible polymer. To increase the flexibility of poly(4HCA-co-DHCA), some aliphatic chain such as polybutylene succinate (PBS) can be introduced. PBS is also derived from biobased monomers and it is biodegradable as well. In this report, we will present the synthetic protocol for copolymerization of rigid poly(4HCA-co-DHCA) with PBS. Flexibility and toughness of composite can be controlled by changing the wt.% of the PBS chain. The results will be discussed using various characterization techniques using NMR, GPC, TGA and tensile measurements.

Introduction: Every year more than 2.2 million bone graft procedures are performed to treat the bone defects worldwide, which costs approximately $2.5 billion. Recent approaches for segmental bone defect reconstruction are limited to autografts, which are considered as gold standard due to their osteoconductivity, osteogenicity and osteoinductivity properties. However, there are some restrictions and concerns of this procedure results in donor site morbidity, secondary infection, restricted availability, and surgical limitations. In this context, synthetic scaffolds have proved to be a potential candidate for the bone grafts due to their unlimited supply. However, the major challenge to repair the segmental bone defect is the development of synthetic porous scaffold using biodegradable biomaterial for proper cell adhesion and proliferation in abundance in order to get required porosity and better mechanical properties. A variety of techniques and scaffolds are being made for this engineering. The choice of material, mechanical properties, and porosity of the scaffold can affect the cell progression after implantation [1].Therefore, we have developed a novel biodegradable biocomposite scaffold based on polycaprolactone-poly lactic-co-glycolic acid (PCL-PLGA)-Brushite by using compression molding methods. Its structural and biological properties have been assessed. Importantly, faster degradation of PLGA than PCL led to the creation of in situ pores in the matrix of PCL, which can help in the neovascularization.
Materials and Methods: The biocomposite scaffolds were prepared by heating the mixture of PCL, PLGA and brushite at 150 °C in PTFE mold, followed by compression. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were used for the phase analysis and surface characterization of the as-sintered samples. The energy dispersive spectroscopy (EDS) was used for the analysis of brushite distribution in the PCL-PLGA matrix. The percentage porosity and density were measured by using liquid displacement method. For the mechanical testing, cylindrical samples of 6 mm diameter and 12 mm height were tested at a speed of 1.3 mm/min using the universal testing machine. Furthermore, the samples were tested for its degradation behaviour in 1×PBS at 37 °C for 2, 4, 6, 8, 16, 24, and 48 weeks.
Results and Discussion:
As-sintered cylindrical-shaped scaffolds were characterized by a dense structure with density ~1.07 g/cm³, which is comparable to the cancellous bone (0.1–1.0 g/cm³). The SEM carried out in the secondary electron mode revealed a micro-rough surface. This rough surface is expected to play an important role in cell adhesion and proliferation. A comparison of XRD results confirmed the presence of the amorphous phase of PLGA and crystalline phases of PCL as well as brushite. The mechanical testing of scaffolds showed a significant effect of PCL and PLGA ratio on the yield strength and young’s modulus. Furthermore, the detailed SEM-EDS mapping confirmed the uniform distribution of brushite (Ca, P, and O). The mechanical testing of scaffolds showed a significant effect of PCL and PLGA ratio on the yield strength and young’s modulus.
Conclusions:
In this study, we have prepared a novel biodegradable scaffold composed of PCL, PLGA and brushite and investigated its structural and biological properties. Based on our findings, we believe that this biomimetic scaffolds are suitable for bone tissue engineering applications and could be used a promising material for future endeavors.
Acknowledgements: The work was partly supported by the National Institute of Biomedical Imaging and Bioengineering of the National Institutes of Health (award number R01EB020640), and the Assistant Secretary of Defense for Health Affairs, through the Peer Reviewed Medical Research Program under Award No. W81XWH-16-1-0132.
References: Schaefer D, Martin I, Jundt G, Seidel J, Heberer M, Grodzinsky A, et al. Arthritis Rheum 2002;46:2524–34.

The adhesion of soft connective tissues (tendons, ligaments, and cartilages) on bones in many animals can maintain high toughness (~ 800 J m-2) over millions of cycles of mechanical loads. Such fatigue-resistant adhesion has not been achieved between synthetic hydrogels and engineering materials, but is highly desirable for diverse applications such as artificial cartilages and tendons, robust antifouling coatings, and hydrogel robots. Inspired by the nanostructured interfaces between tendon/ligament/cartilage and bones, we report that bonding ordered nanocrystalline domains of synthetic hydrogels on engineering materials can give an extremely fatigue-resistant adhesion with a record-high interfacial fatigue threshold of 800 J m-2, since the fatigue-crack propagation at the interface requires a much higher energy to fracture the ordered nanostructures than amorphous polymer chains. Our method enables fatigue-resistant hydrogel coatings on diverse engineering materials with complex geometries. We further demonstrate that fatigue-resistant hydrogel coatings on metallic joint replacements can give lower friction and better wear-resistance against natural cartilages than the commonly used polyethylene coatings.

Diatoms constitute the largest and the ecologically most significant group of organisms on Earth. The unique silicified cell structure as well as intricate morphology of these microorganisms attract scientists attention for over 100 years. The details of their structure and the functionalities are still under debate and applications of diatom shells are in progress.
One of the widely spread diatom species is a freshwater diatom - Didymosphenia geminata, composed of siliceous cells and extracellular polymeric fibrous stalks. The present paper [1] concerns the possibilities of using frustules of this diatoms as a novel filler in epoxy composites. Preliminary results of the research aiming at achieving this goal have been presented in [1,2].
The current study, extends earlier investigations on the possibility of using the diatom frustules in innovative composite materials with metallic matrix. The results are presented of the exploratory research on the properties of the composite, which show a great potential for the development of bio-based metallic matrix composites.

[1] Zglobicka I., Jablonska J., Suchecki P., Mazurkiewicz-Pawlicka M., Jaroszewicz J., Jastrzebska A., Pakiela Z., Lewandowska M., Swieszkowski W., Witkowski A., Kurzydlowski K.J. (2018): Frustules of Didymosphenia geminata as a modifier of resins. Inzynieria Materialowa, 5(225): 172-177. DOI: 10.15199/28.2018.5.2
[2] Zglobicka I., Li Q., Gluch J., Plocinska M., Noga T., Dobosz R., Szoszkiewicz R., Witkowski A., Zschech E., Kurzydlowski K.J. (2017): Visualization of the internal structure of Didymosphenia geminata frustules using nano X-ray tomography. Scientific Reports 7, 9086. DOI: 10.1038/s41598-017-08960-5

Cellulose is the most abundant natural polymer on earth and exists numerously in wood, crop, and cotton. It shows potential to be a candidate for the sustainable material in next generation due to its bio-compatibility, bio-degradation, chemical stability, non-toxicity, and renewable ability. Cellulose nanofiber (CNF) extracted from plants has been reported that it exhibits the unique optical properties, good mechanical strength, and high aspect ratio. TEMPO-oxidation is an effective way to separate the CNF in cellulose microfibril bundles due to the formation of the regioselective repulsive force between each nanofibrils by modification of C6 carboxylate. For extending the application, combining the photocatalysts might provide multi-functional properties with solvent-resistance and degradation capability of pollution. One-dimensional TiO₂ nanomaterials can achieve the network structure easily that provides an efficient charge transport path to inhibit the recombination of electron-hole pairs. Moreover, nanofiber-shaped materials can be probably combined with CNF to form the dense, flexible, and freestanding composite film because of the accordance size between each other. In this study, we proposed a simple way to prepare the TiO₂ NFs/CNF nanocomposite film with flexibility, solvent resistance, and photocatalytic performance. The relationship between the chemical, physical properties of the TiO₂/CNF film and the morphological-matching were discussed. CNF is fabricated by TEMPO-oxidation method and post-reduction by NaBH₄. Various TiO₂ based materials with different dimension and morphologies, such as P25 nanoparticles, anatase TiO₂ nanowires, microparticles, and Ag:TiO₂ nanowires were incorporated with cellulose nanofibers. To obtain the uniform and freestanding TiO₂/CNF film, the well-dispersion TiO₂/CNF mixture solution was activated by UV-ozone treatment to improve the TiO₂ hydrophilicity. After the drop-casting and drying, various TiO₂/CNF films with 10 mm-thickness were obtained. With the various TiO₂, the surface on two sides of film was totally different, which is caused by the TiO₂ distribution and dimension in CNF. From the cross-sectional FESEM images and EDS mapping, 1-D TiO₂ were entangled with CNF near the edge of film, but P25-TiO₂ NPs and TiO₂ MPs were embedded and assembled in the CNF hierarchical structure. The distribution of TiO₂ in the CNF matrix directly rearranged the CNF ordering stack. We also investigated the crystal structure as well as grazing-incidence wide-angle X-ray scattering (GIWAXS) for the detailed preferred orientation of cellulose nanofiber film. In particular, the TiO₂ nanowires entangled with CNF improved the hierarchical stack due to the similar morphology and stacking orientation. Therefore, the mechanical properties including tensile strength and young’s modulus can be improved and higher than that of TiO₂ particles/CNF. In the testing of solvent resistance for 30 days, these various TiO₂/CNF film showed the outstanding resistance and stable in the acetone and dimethylformamide (DMF). For the better photocatalytic activity, Ag:TiO₂ nanowires were employed. In the photodegradation of 1.0 ppm methyl orange in ethanol, Ag:TiO₂ NWs/CNF film presented the high degradation efficiency of 87% and showed the excellent stability of morphology and activity after 3 cycles. In addition, Ag:TiO₂ NWs/CNF film presented remarkable degradation behavior and stability toward the VOCs including methanol, n-butanol, and DMF with 5,000 ppm.

We present a study on the structural organization and mechanical proprieties of clay colloidal particle films at the air-liquid interface. The film self-assembly and stability is monitored employing the Langmuir Trough (LT) method. Both pure clays and modified clays are studied in an LT using synchrotron X-ray Grazing Incidence techniques (GID and GIXOS). The Langmuir film thickness and in-plane organization are monitored in order to investigate mechanisms for control of adsorption on different interfaces. These results are important for particle coating studies and for the development of new routes for assembling clay colloidal particles on liquid surfaces/interfaces. Surface properties are particularly important in relation to Pickering emulsions where particle coatings on droplets effectively prevent droplet coalescence and produce mechanically very stable surfactant-free emulsions. The adsorption of colloidal particles at the surface of liquid droplets [1] has applications in several areas such as the food-, cosmetics-, pharmaceutics-, and the petroleum sector. [2]. Here we demonstrate how the ordering of clay nanoparticles on a confined two-dimensional aqueous surface can be investigated in detail. In particular, we recreate the process of Janus clay-platelets exfoliation and clay platelet re-assembly, using synthetic Fluorohectorite clays [3], in the interfacial confined environment provided by the LT.

[1] Dommersnes, Rozynek, Mikkelsen, Castberg, Kjerstad, Hersvik, Fossum. (2013) Nature Comm, 4, 2066
[2] Gholamipour-Shirazi, Carvalho, Huila, Araki, Dommersnes, Fossum (2016) Scientific Reports 6, 37239
[3] Stoeter, Goedrich, Feicht, Rosenfeldt, Thurn, Neubauer, Seuss, Lindner, Kalo, Moeller, Fery, Foerster, Papastavrou, Breu. (2016). Angew. Chem. Int. Ed., 55, 7398–7402.

In the recent years we have observed a fast increase in the demand for personal flexible and wearable devices, for sensing, health monitoring and leisure. With the raise of 2D materials, the pursuit of high-performance flexible devices seems even closer to realisation. 2D materials seem perfect candidates for flexible nanotechnology because of their unique physical and mechanical properties. One of the issues innately related to flexible electronics is the ability of the active components to handle reasonably high strain. Therefore over the years it has been crucial to exploit the properties of 2D materials for the proper function of the flexible devices. But, while single crystals of 2D materials possess outstanding elastic properties, point and line defects will play an important role in potential technological applications of large-area samples grown, for example, by chemical vapour deposition.

In this work we investigate the crack propagation on micro- and nanoscale in polycrystalline MoS₂ films in a very small grain limit. The study is performed on two types of MoS2 samples differing in the grain orientation with respect to the film plane, i.e., horizontal (in plane)-vertical (out of plane) and purely horizontal, with thickness of 17 nm and 3 nm, respectively. Using electron microscopy techniques we determine critical uniaxial strain as approximately 5%, independently of the sample morphology and thickness. We also study the nanoscale crack propagation under the transmission electron microscope and show that they propagate along the grain boundaries as well as through the grains preferentially along van der Waals bonding. Our results provide an insight on the fracture of polycrystalline 2D materials as well as new means for tailoring the critical strain and nanofabrication of ultra-thin MoS₂ devices, by using well-developed tools and of great interest of flexible electronics industry.

Firefighters continually endanger their lives in order to rescue others. This can leave them with severe burns; in 2017 alone, 2,835 U.S. firefighters suffered from burn-related injuries. In this research, a flame retardant hydrogel was synthesized using biodegradable, non-toxic materials: xanthan gum (XG) and resorcinol bis(diphenyl phosphate) (RDP) and starch. RDP was first coated onto the xanthan gum and starch surface. Fourier-transform Infrared Spectroscopy confirmed the hydrogen bonding between RDP and XG/starch. To evaluate the protection of FR hydrogel, sheepskin was embedded in aluminum pans, covered with hydrogel, and burned continuously for 150s. Temperature change of the sheepskin was recorded during burning test. Results showed the formation of a uniform char layer from the FR hydrogel which protect the underlying gel layer and skin. The FR hydrogel sample outperformed its pure XG gel control sample by 29% in terms of the final temperature. The FR hydrogel helped to remain the skin temperature below 45 °C for over 50 seconds and below 55 °C for 114 seconds upon direct burning with a propane torch. TGA tests showed that the FR hydrogel had more residue after test which is in agreement to the char formation during the burning test. Data from the viscosity tests revealed that all samples displayed shear-thinning behavior. Thermal protective performance (TPP) tests were also done to evaluate heat transmission through the FR hydro gel when exposed to a continuous heat source, and result compared to the Stoll Curve which represent the heat level for causing second-degree burn. The TPP test result showed that the FR hydrogel provided a prolonged protection time comparing to the control sample.

Nanocomposite fibers in the form of scaffolds are candidate materials for several applications such as tissue engineering, regenerative medicine, controlled drug release and multifunctional surfaces. The performance and service life of such scaffolds is significantly affected by the mechanical properties of the nanocomposite fibers and the adhesion to the surface of their substrate material. Thus the understanding of the mechanical behavior of the fibers and scaffolds is critical for potential applications.
In this work, we focus on the fabrication of nanocomposite polycaprolactone (PCL) fibers and scaffolds via electrospinning and on their structural and mechanical characterization using X-Rays Diffractions (XRD), Atomic Force Microscopy (AFM) and depth-sensing Nanoindentation (NI), respectively. The scope is to correlate the structure of the nanocomposite fibers and scaffolds with their mechanical behavior and their antibacterial properties in terms of cell / bacteria adhesion and colonization.
For the Electrospinning of the nanocomposite PCL fibers we used: a) ZnO nanoparticles (NPs) with 15 nm average diameter, developed by precipitation method using Zinc Acetate Dehydrate, Sodium Hydroxide and Polyvinylpyrrolidone (PVP) as a surfactant and b) drugs such as Curcumin and Vancomycin.
X-ray diffraction characterization showed the addition of 1% w/v ZnO NPs results to more crystalline PCL fibers. The NI testing, using a Berkovich type indenter and penetration depth up to 1/10 (~ 100 nm) of the fibers thickness ( > 1 μm), and analysis of the NI Load-Displacement curves showed that the Elastic Modulus values of the PCL:ZnO(NPs) nanocomposite fiber increases from 1.2 GPa (pristine PCL) to 1.7 GPa, whereas the Hardness values remains almost the same (0.7 GPa). In addition, the tip of the AFM cantilever was used to deform the nanocomposite fibers and to estimate the critical load (P_c) for the onset of the elastic/plastic deformation and the formation of a clear imprint on the PCL:ZnO(NPs) fibers surface. It was found that P_c decreases from 1.25 μΝ to 1.07 μΝ, after the addition of the ZnO NPs.
The adhesion of the nanocomposite PCL:ZnO(NPs) fibers to glass substrate was also tested by AFM and it was found that although the fibers are subjected to plastic deformation and cutting, they remained adhered to the glass substrate. In conclusion, the incorporation of ZnO NPs into PCL fibers was found to affect both the crystallinity and the mechanical behavior of the nanocomposite scaffolds and fibers.

Many biological composites (e.g., bone, tendons, seashells) possess superior mechanical properties (e.g., strength, toughness) mostly due to their hierarchical organization. These structures grow through unique self-assembly processes, via a 'bottom-up' approach. Such an approach may inspire the synthesis of novel man-made multiscale composites, with enhanced mechanical properties. A key material for 'bottom-up' construction of multiscale composites is carbon nanotubes (CNT). However, up until today, the methods for assembling CNTs into hierarchical structures have not led to sufficient improvement of the mechanical properties with respect to conventional composite materials. Here we introduce a method termed Evaporation Driven Self Assembly (EDSA) patterning, assisted by high surfactant concentration. This technique enables the deposition of thick, oriented CNT layers onto quartz fibers, which are then used as a reinforcing component in polymer-based composites. We describe a series of experiments that are intended to characterize the composites microstructure and assess the interfacial adhesion of the CNT-coated quartz fiber to the surrounding matrix. These experiments include visual analysis by light and electron microscopy, as well as μCT scanning, and fragmentation testing to evaluate the interfacial shear strength. The results reveal that, although the nanoscale deposition is random (i.e. single nanotubes do not show any preferred orientation), the composite microstructure exhibits a ribbon-like, thick coating (thicker by an order of magnitude compared to the quartz fiber) that is approximately oriented parallel to the fiber long axis. The mechanical tests reveal that when this layer is impregnated by epoxy, it adopts a 'fibrous' behavior, namely the layer undergoes sequential fragmentation much like classic single-filament composites. A modified Cottrell-Kelly-Tyson (CKT) model is used to assess the interfacial shear stress and the results exhibit a threefold increase in composite strength. This type of multiscale composites could further realize the high potential of CNT mechanical properties, mainly due to the EDSA deposition that solves, to some extent, fundamental issues such as CNTs dispersion and low volume fraction.

A complete understanding of materials at nanoscale facilitates the possibility of nanomanipulation to improve the materials macroscale properties. Although cement paste is widely used and studied, due to its complexity as a hierarchical random composite, there are still several unknows related to the effects of molecular features on its macroscopic behavior. Molecular dynamics modeling with a reactive force field was used to study one of these unknown features: the effect of cement paste phases interactions at molecular level on the response to loading conditions at macroscale. Tricalcium silicate (C₃S) and amorphous calcium silicate hydrate (CSH) are modeled as minerals nanolayers. Under compression, the calcium atoms in the C₃S interact with water molecules from the CSH at the interface. Also, the water in the CSH favors the displacement of silicate chains and the breaking of Ca-O bonds, therefore, the CSH phase is more compressible than the C₃S phase in the composite.

The goal of this study is to develop and characterize a polymer blend consisting of polymethyl methacrylate (PMMA) and polybutylene adipate terephthalate (PBAT) using graphene as a compatibilizer. A polymer blend of this nature has the potential to be utilized in many aspects of the medical field, including antimicrobial bone implants and provisional dental crowns, while offering the ease of customizable manufacturing through 3D printing. PMMA is a nonconductive material with low impact toughness and high modulus, while PBAT is biodegradable, ductile, and has antibacterial properties; so by blending the polymers we aim to form a high-modulus, ductile material that can easily decompose in the environment. It is, however, difficult to form such a blend due to PMMA and PBAT being immiscible with each other. To overcome this problem, graphene is added as a compatibilizer to decrease interfacial tension and increase polymer–polymer adhesion. Graphene is expected to work well as a compatibilizer due to it having a high aspect ratio, and its work function with PBAT and PMMA is higher than that of PMMA with PBAT, indicating that the graphene will, critically, align along the polymer interfaces. Furthermore, graphene has high electrical, tensile, and antibacterial properties, which are desirable traits for the bulk blend. Based on previous studies working with similar polymer blends¹, we will start with a 70/30 mixture of PBAT/PMMA that will be tested with varying concentrations of graphene. Thermal, mechanical, and electrical properties will be examined with an objective to find optimal polymer concentrations to maximize these traits. In addition, to determine enhanced properties of the composite for biological applications, we will be testing antibacterial efficacy using E. coli and S. aureus and biocompatibility by testing cell viability of dental pulp cells. This novel blend of common low cost polymer gives a unique edge in fulfilling applications held traditionally by PMMA while also being more environmentally conscientious.

[1] Guo Y, He S, Yang K, Xue Y, Zuo X, Yu Y, Liu Y, Chang CC, Rafailovich MH. 2016. Enhancing the mechanical properties of biodegradable polymer blends using tubular nanoparticle stitching of the interfaces. ACS Appl. Mater. Interfaces 8(27): 17565–17573.