Symposium CP01—Advances in In Situ Experimentation Techniques Enabling Novel and Extreme Materials/Nanocomposite Design

We review the state-of-the-art in the molecular design and processing of nanomaterials at the extreme limits of molecular-scale confinement. A particular focus is provided on unique mechanical and electrical behavior that can be achieved in the limit of such intimate molecular mixing. We show that molecular hybrids can have marked asymmetric elastic and thermal expansion properties that are inherently related to terminal chemical groups in confinement. We describe a new nanoscale design principle using hyperconnected molecular architectures to achieve remarkable mechanical properties controlled by designing connectivity into the intrinsic molecular structure in innovative ways. We probe the mechanical and fracture properties of hybrids in the extreme limits of molecular confinement, where a stiff inorganic matrix phase confines polymer chains to dimensions far smaller than their bulk radius of gyration. Finally, we demonstrate the synthesis of 1D core-shell nanowires that have electrically conducting copper cores and insulating shells. Nanowires are synthesized via a facile solution process leveraging the strong chemical interaction between organothiol molecular precursors and copper. We use molecular dynamic and density functional theory computations together with atomic resolution characterization to reveal the molecular structure, and electrochemical impedance spectroscopy to characterize the electrical properties.

Nanoscale metal-graphene nanolayered composite is known to have ultra high strength due to its ability to effectively block dislocations from penetrating through the metal/graphene interface. The same graphene interface can simultaneously serve as an barrier interface for deflecting the fatigue cracks that are generated under cyclic bendings. Cu-graphene composite with repeat layer spacing of 100 nm was tested for bending fatigue at 1.6% and 3.1% strain up to 1,000,000 cycles that indicated ~5 times enhancement in robustness against fatigue induced damage in comparison to the conventional Cu only thin film. Fatigue induced cracks that are generated within the Cu layer were stopped by the graphene interface, which was confirmed using transmission electron microscopy images acquired ex-situ as well as during in-situ tensile strain testing. Molecular dynamics simulations for uniaxial tension of Cu/Gr showed limited accumulation of dislocations at the film/substrate interface, which makes the fatigue induced crack formation and propagation through thickness of the film difficult in this materials system.

Recent advances in nano-manipulation, environmental TEM and MEMS allow us to investigate coupled mechanical and electrochemical phenomena with unprecedented spatial and temporal resolutions. For example, we can now quantitatively characterize liquid-solid and gas-solid interfaces at nm-scale. These experiments greatly complement our modeling efforts, and together they help provide insights into how materials are transformed in synthesis and how they behave in service due to combined electrochemical-mechanical forces. Applying theory, modeling and lab-on-a-chip microscopy, together with cost modeling, can judiciously guide the scalable production of high-performance energy materials.

Flexible and stretchable devices are of significant interest due to their potential expansion of the application of smart devices into various fields, such as energy harvesting, biological applications and consumer electronics. Due to the mechanically dynamic operations of such electronics, their mechanical reliability must be thoroughly investigated to understand their failure mechanisms and lifetimes. Reliability issue caused by bending fatigue, one of the typical operational limitations of flexible electronics, has been studied using various test methodologies; however, electromechanical evaluations which are essential to assess the reliability of electronic devices for flexible applications had not been investigated because the testing method was not established. By employing the in situ fatigue test, we have studied the failure mechanism for various conditions and parameters, such as bending strain, fatigue area, film thickness, and lateral dimensions. Moreover, various methods for improving the bending reliability have been developed based on the failure mechanism. Nanostructures such as holes, pores, wires and composites of nanoparticles and nanotubes have been suggested for better reliability. Flexible devices were also investigated to find the potential failures initiated by complex structures under bending fatigue strain. The recent advances in test methodology, mechanism studies, and practical applications are introduced. Additionally, perspectives including the future advance to stretchable electronics are discussed based on the current achievements in research.

In this talk, we report a novel actuating material – nickel hydroxide-oxyhydroxide – that exhibits enormous actuation due to a volume change stimulated either electrochemically, or by illumination of visible light of low intensities. For electrochemical actuation, Ni(OH)₂/NiOOH is capable of undergoing fast, reversible, and large actuation in alkaline electrolytes under potentials of less than 1 V, due to a redox reaction involving volume changes. On the other hand, the light actuation of Ni(OH)₂/NiOOH is due to its turbostratic crystal structure which is capable of intercalating water molecules. It is shown that the intercalated water can be rapidly and reversibly desorbed into the environment under visible light of intensities lower than 1 sun, resulting in fast actuation driven wirelessly by light.

By electroplating the actuating material on passive substrates, we have fabricated film-actuators capable of undergoing reversible bending and curling with an intrinsic actuating stress of tens of megapascals at response rates in the order of tens to hundreds of degrees per second, which are comparable to mammalian skeletal muscles. Also, by intentionally electroplating the nickel hydroxide-oxyhydroxide on selected areas of the substrate, we have also fabricated actuation devices of varies shapes and functions, e.g. a hinged actuator that can lift objects ~100 times of the weight of the actuating material is achieved, and other examples showing the potential use in robotic devices. The light-induced actuation mechanism reported here has the potential for realizing wirelessly powered micro-robotic devices.

Dielectric thin films are crucial for countless computing, wireless, and sensor systems. As their dimensions continue to diminish, and engineered microstructure becomes increasingly refined, it is crucial to be able to investigate charging, discharging, and breakdown at the nanoscale. Based on Atomic Force Microscopy several new approaches have been developed for such investigations, reported here for BaTiO₃ as well as BaTiO₃ and SrTiO₃ superlattices. In this manner, the influence on charging and discharging can be mapped for grain boundaries as well as heterostructure interfaces. Furthermore, by applying voltages up to 150V, in a controlled environment, across films shallow-angle-polished to create thicknesses gradients from 200 down to 2 nm, scaling laws for breakdown become uniquely accessible on a single specimen. This is advantageous as the composition is reasonably consistent, as compared to investigations with multiple films. Such methods are equally applicable for investigations of breakdown mechanisms with inorganic as well as polymeric dielectrics.

Understanding plastic deformation in silicate glasses is important in the development of glasses with improved resistance to mechanical failure. Plastic deformation under a point contact determines the stress state, which determines whether a surface flaw forms, which in turn determines the strength of the glass. Unfortunately, relatively little is known about the atomic scale mechanisms that control plasticity in silicate glasses. To date, most effort has focused on the continuum concepts of shear and densification. Raman spectroscopy has been used to study structure of glass glasses deformed by indentation ex-situ. To investigate the structure of plastically deformed glass under load, we have developed an instrument in which the deformed material under the indenter can be investigated in-situ, while the load is being applied. Measured changes in spectra can be correlated to changes in ring size, bond angles, and other structural motifs with load. Transient structural changes which disappear after load is removed were identified.

Resistive random access memory (RRAM) has attracted considerable interest for next-generation non-volatile memory due to its fast switching speed, high storage density and excellent scalability. To effectively control the switching characteristic with excellent memory performance, a complete study of the switching behavior is essential. In this work, we utilize SrCoO_x (SCO) which grows on Nb-doped SrTiO₃ (Nb-STO) substrate as our dielectric layer since it exhibits large different resistance between SrCoO_2.5 and SrCoO_3-��, where the value of SrCoO_2.5 is higher than that of SrCoO_3-��. Based on this characteristic, we fabricated a Au/SCO/Nb-STO device, and measured the SET (~1.6 V) and RESET (~-5.3 V) process with more than 700 cycles. Meanwhile, the conducting filaments were observed via in-situ transmission electron microscope (TEM). From the HRTEM results and corresponding FFT-DP (Fast-Fourier-Transform Diffraction pattern), it is clearly demonstrated the formation of conducting filaments is from SrCoO_2.5 to SrCoO_3-��. Accordingly, the formation/rupture of conducting path is due to the movement of oxygen vacancy, and hence its resistance is able to switch reversibly between low resistance state and high resistance state. This study not only revealed the switching mechanism of SrCoO_x but also proved it to be the promising candidate for RRAM application.

When investigating films thinner than one hundred nanometers, X-ray diffraction at synchrotron sources has been the method of choice in order to achieve enough signal. When films, however, exhibit a nanocrystalline microstructure and consist of light elements also diffraction reaches its limits. Here we provide case studies on reflection anisotropy spectroscopy that is sensitive to elastic distortions of the lattice or the symmetry breaking of a crack pattern. Thin films only start reflecting less than their bulk counterparts, when their thickness reaches several tens of nanometers and thus experiments can be extended to the single digit nanometer thickness regime. The case studies focus on plastic deformation and fracture on the phase change media as well as colored and noble metal thin films for applications in wearable electronics.

Liquid electron microscopy is a new TEM-based characterization technique that allows us to apply the powerful capabilities of an electron microscope to probe/ image and understand real-time nanoscale morphological and structural evolution of nano-systems under certain electrochemical conditions (operando).[1] For in situ electrochemical experiments, the importance of using the liquid cell is related to the fact that experiments in liquid environments enable direct imaging of the key phenomena taking place during the battery operation, stage crucial for establishing a good correlation between the microstructure of nanocomposites and the performances of the aimed devices. The main challenge of our work is the assessment of the batteries performances when nanostructured electrode materials are considered for both the anode and the cathode. Nanostructured materials show great promise; they can open new possibilities for enhancing the energy density of Li-ion batteries.
The present study is focused on the realization of the in – situ liquid TEM observation using a closed electrochemical cell [2] of the electrochemical technique that activate the nanoparticle nucleation/growth mechanism of under certain electrochemical conditions (operando ) as the ionic solution is introduced into the cell(unit) [3, 4].
From the experimental point of view, two electrodeposition approaches have been used: the cycling voltammetry and pulsed chronoamperometry modes both under constant flow. The interpretation and the analyses of the recorded current versus time curves allowed us observing that the nucleation mode is strongly related to the deposition conditions. Thus we noticed when the pulsed chronoamperometry mode is used we have a homogeneous NP deposition on the working electrode. The NPs size is depending on the cycle number and the liquid flow rate within the cell. In contrast, under cycling voltammetry mode, we observe an inhomogeneous deposition of the NPs generated by the oxidation of different organic species that can be formed after the first cycle.

[1] Gohier A., Laïk B, Kim K., Maurice J.-L., Pereira-Ramos, J-P., Cojocaru C. S., Advanced Materials 84 (2012) 2592-2597
[2] Unocic R. R., Sacci R. L., Brown G. M., Veith G. M., Dudney N. J., More K. L., Walden II F. S., Gardiner D. S., and Nackashi D. P. Microscopy and Microanalysis( 2014), 20, 452-461.
[3] Zaluzec N. J., Burke M. G., Haigh S. J., Kulzick M. A. Microscopy and Microanalysis( 2014), 20, 323-329
[4] http://www.protochips.com/products/poseidon/

Carbon Nanotubes (CNTs) and Metal Organic Frameworks (MOFs) represent two of the most promising classes of emerging materials in engineering today. Together, they exhibit many useful properties such as high conductivity [1], catalytic reactivity [3], porosity [4], and strength [2]. Methods in integrating these materials towards electromagnetic and chemical applications is of strategic importance for defense. Innovative materials have often been designed by combining low and high dimension constituents, through compositing, as a means to improving the functionality. In this work, we were able to combine these nanomaterials by synthesizing novel MOF-CNT nanostructured composites, using zirconium containing MOF (MOF-808).
MOF-808 is composed of zirconium (IV) nodules, connected by organic carboxyl linkers, to create a nanoporous octahedral 3D crystal structure. Carboxylated functional groups are believed to be an important surface modification, due to its ability to bridge the zirconium nodules. Multiwalled CNTs, surface functionalized with carboxyl end groups, were used as the reinforcing agent with MOF-808 to create a MOF-CNT composite. The functionalities were chosen to create a MOF-CNT composite, linked by primary bonds.
Scanning transmission electron microscopy (STEM) and nanoarea electron diffraction are powerful techniques in identifying structural changes at atomic scale resolution. Using a NION UltraSTEM-200X and a JEOL 2200FS at the U.S Naval Research Laboratory, we are able to identify MOF-808 adhesion to the surfaces of functionalized CNTs. Analysis shows the existence of lattice defects at the interface of MOF-808, as synthesized with carboxylated CNTs. The disruption is believed to be caused by the carboxylated CNTs acting as heterogeneous nucleation sites, forming the MOFs around the nanotube center. In this work, STEM and electron diffraction are used as effective techniques to study the interface of surface modified nanostructure composites.

[1] V. Trovato et al., "Carbon nanotubes textile coating for the development of wearable sensors," 2018 7th International Conference on Modern Circuits and Systems Technologies (MOCAST), Thessaloniki, 2018, pp. 1-4.
doi: 10.1109/MOCAST.2018.8376600
[2] Ho, Po-Hsun, et al. "Intrinsically ultrastrong light-matter interactions in crystalline films of carbon nanotubes." arXiv preprint arXiv:1803.01087 (2018).
[3] Brønsted Basicity in Metal–Organic Framework-808 and Its Application in Base-Free Catalysis Xiaodan Yan, Kai Wang, Xiaocheng Xu, Shuhua Wang, Qian Ning, Weiming Xiao, Ning Zhang, Zijuan Chen, and Chao Chen Inorganic Chemistry 2018 57 (14), 8033-8036 DOI: 10.1021/acs.inorgchem.8b01044
[4] Farha OK, Özgür Yazaydõn A, Eryazici I, Malliakas CD, Hauser BG, Kanatzidis MG, Nguyen ST, Snurr RQ, Hupp JT: De novo synthesis of a metal–organic framework material featuring ultrahigh surface area and gas storage capacities. Nat. Chem. 2010, 2:944–948.

An exploration of poly(lactic acid) (PLA), high density polyethyle (HDPE), and low density polyethylene (LDPE) composites containing liquid crystalline graphene oxide (LCGO), edge functionalised graphene (EFG), and laponite in a range of concentrations in both single and dual filler combinations was undertaken. These were extruded as composite filament and tested as feed material for fused deposition modelling (FDM).
Printable polymer nanocomposites have the potential to widen the scope for products made by FDM by improving physical properties. Graphene has been shown to provide improvements in PLA modulus, thermal stability, and tensile modulus [1,2]. LCGO has been developed in-house, and has shown great potential to improve the mechanical, thermal, and electrical properties of polymers [3]. In this study nanocomposites ranging from 0.1wt%-10wt% filler were extruded using a customisable counter-rotating mini twin screw extruder. As graphene has a tendancy to either form bonds with itself that cannot be broken during extrusion and may also restack to form graphite [1], separation of LCGO and EFG from itself prior to and during the filler addition is key.
Powdered polymer (HDPE/LDPE/PLA) was added to the first open port of the extruder. Once the polymer was molten, a dispersion of fillers (LCGO/EFG/laponite) in an aqueous solution was syringe pumped directly into the melted polymer in a second port along the extrusion screw. As each drop of solution was added the melt, the water evaporated and the filler was taken in to the polymer. This method prevents a lot of the filler-filler contact that occurs in typical powder coating methods, which in turn keeps agglomerate size low. Solution concentrations were up to 10mg/mL, added at at 6mL/min.
LCGO was made on site at UOW [4], to approximately 9mg/ml (aq. dispersion). EFG was made in house at UOW. Laponite was sourced from BYK Additives and Instruments (Germany). PLA was sourced from Filabot (USA), HDPE from Sigma-Aldrich (USA), LDPE from Visy (Australia). Each additive was diluted to 0.1, 0.5, 1, 2, 5 and 10mg/mL in an aqueous solution for both single and dual filler composites.
Direct solution additon mid extrusion has shown to generate average agglomerate size range from <500nm to 50μm. It is expected that the upper limit will lower to 25μm with a slower addition of filler.
Mechanical strength was seen to improve by 20% in composites of PLA/EFG at 8wt%, and by 10% in dual filler composites PLA/EFG/LCGO at 2wt%.
Dripping water on to the polymer melt did not lower the degradation temperature of any of the polymer, indicating it evaporates off immediately as intended.
Acknowledgements
The authors thank the Australian National Fabrication Facility - Materials Node for their provision of research facilities, acknowledge the use of facilities within UOW Electron Microscopy Centre and funding provided by the ARC Centre of Excellence for Electromaterials Science.

References:
[1]Norazlina. H, Kamal, Y. Graphene modifications in polylactic acid nanocomposites: a review. Polymer Degradation and Stability, 2010,95,pp. 889-900; [2] Gaska, K.; Xu, X.; Gubanski, S. & Kádár, R. Electrical, mechanical, and thermal properties of LDPE graphene nanoplatelets composites produced by means of melt extrusion process. Polymers, 2017, 9, 11; [3] Seyedin, M. Z., Razal, J. M., Innis, P. C., Jalili, R. & Wallace, G. G. Achieving outstanding mechanical performance in reinforced elastomeric composite fibers using large sheets of graphene oxide. Advanced Functional Materials, 2015, 25, pp. 94-104; [4] Naficy, S., Jalili, R., Aboutalebi, S. H., Gorkin, R. A., Konstantinov, K., Innis, P. C., Spinks, G. M., Poulin, P. & Wallace, G. G. Graphene oxide dispersions: tuning rheology to enable fabrication. Materials Horizons, 2014, 1, pp. 326-331

Defect engineering is capable of significantly enhancing catalytic performance of target materials, but very few high efficiency of defect-engineered catalysts were reported involving a facile and targeted-production synthetic technique. Here, a unique localized micro-deflagration strategy combined with postprocessing of heat treatment was reported to synthesis efficient oxygen reduction reaction (ORR) nanocarbon shells electrocatalyst. This micro-deflagration endows ultrafast and large scale production of highly porous nanocarbon equipped with rich nitrogen (11.86 at%) and parts of N-atoms can be further removed via post heat treatment for constituting targeted active sites of defects for ORR. Loaded with large specific surface area (540 m²g^-1) and 6.05 at% N-doping, the defective nanocarbon shells allows excellent electrocatalytic activity (E_onset of 0.89 V vs.RHE, E_1/2 of 0.79 V vs.RHE and J_k of 6.17 mA cm^-2), long-term stability, and tolerance to crossover effect, comparable to commercial Pt/C in alkaline electrolyte. Most importantly, this work has achieved the creative synthesis of metal-free defect-rich carbon materials and significantly enriched the research of defect engineering chemical electrocatalyst.

Defective oxygen vacancy aiming at surface self-modification for target materials is proved to be highly efficient for catalysis. Here, we reported in situ implantation of Ti³⁺ ions in TiO₂/graphene quantum dot nanosheets (Ti³⁺-TiO₂/GQDs NSs) as an efficient visible-light photocatalyst, using a facile sythesis of two-step calcination. By virtue of heat treatment for TiO₂ nanosheets and carbon source of citric acid, significant oxygen vacancy/Ti³⁺ sites can be introduced in TiO₂/GQDs NSs via carbothermal reduction reaction derived by GQDs, which contributed to provide critical active sites for photocatalysis. Such nanohybrids exhibited extented optical absorption in visible-light region and low recombination of photo-induced carriers. The photodegradation of methylene blue (MB) and rhodamine B (RhB) result both demonstrated higher visible-light photocatalysis performance than that of original TiO₂. Based on the analysis of experimental results, oxygen vacancy/Ti³⁺ sites and GQDs induced in TiO₂ nanosheets play a synergistic effect for the improvement in photocatalysis properties_. Finally, the formation mechanism and deep understanding for the carbothermal reduction induced Ti³⁺ sites is provided in DFT analysis.

Monolayer Graphene with extraordinary mechanical properties and super large specific surface area arouses extensive scientific interests as the potential reinforcement for fabrication of light-weight, high strength and stiffness metal-matrix composites. Previous experimental studies have demonstrated this reinforcement effect of graphene in a variety of metal matrix composites mainly in the shape of thin film or thick bulk, however, few talked about its reinforcement efficiency in the three-dimensional (3D) structure. Herein, to reveal the reinforcement effect of graphene in the 3D structure, a graphene-nickel composite foam has been fabricated through applying electrochemical co-deposition process. Specifically, a Polyurethane (PU) foam with the stochastic lattice structure is selected as the template, on which the graphene nano-flakes and nickel matrix were co-deposited through adjusting the electrochemical deposition conditions. Afterwards, the PU template was removed by a thermal decomposition process and the light-weight nickel foam in consist of hierarchy open cells and hollow triangle ligaments was obtained. Computational simulations and theoretical modeling were utilized to understand the underling mechanisms and provide a comparison to the experimental results. Our study on the 3D graphene-metal composite foam sheds light on fabricating novel high strength mechanical metamaterial.

Recently, the trend of miniaturization of electronic components is accelerated due to the spread of mobile devices. In particular, Multi-Layer Ceramic Capacitor (MLCC), made by stacking Ni paste layers and ceramic slurry layers alternately followed by a high-temperature sintering process, has a layer thickness of less than 1 μm. One of the biggest issue in MLCC is difference in sintering behavior between ceramic and metal layer and subsequent twisting or fracture due to that. In order to identify the origin of such problem and reduce it, a quantitative tool for analyzing the sintering behavior of the thin film form is required. However, at present, the analysis of sintering behavior of μm scale thin films is limited to indirect methods such as ex-situ microstructure change analysis. In this study, the sintering behaviors in thin film of Ni nanoparticle were successfully analyzed by measuring the stress variation of thin films using laser based optical analysis. First, a Ni paste of 80nm particles was applied evenly on a ceramic substrate by spin coating and it was heated up to 580°C in a chamber filled with N₂ gas mixed with H₂ gas for preventing oxidation. In this process, the stress curve versus temperature with 4 inflection points was obtained. Among them, the points related to the evaporation and decomposition of the solvents in the paste were excluded by comparison with the TGA data, and only one inflection point near 430°C remained. Then the microstructural analysis was carried out for the samples quenched at the temperatures below and above the point. Finally, it was found that the inflection point has a significant relation with the neck formation in the sintering process. The same results were obtained for Ni-alloy materials. This study suggests a method of in-situ analysis of the sintering behavior of metal nanoparticles through stress analysis, and it is expected to act as useful guide line for the study of materials requiring high-temperature sintering process.

The fundamental relationship between electronic transport properties and applied strain in piezoresistive materials permits their usage in a broad range of applications related to electromechanical systems. Furthermore, strain engineering has been demonstrated as a useful technique to improve the performance of electronic devices, as it allows for enhanced carrier mobility and direct control of the band gap of semiconductors. Quantitative characterization of the relationship between electrical properties and strain is essential to the development of piezoresistive-based devices and to the application of strain-engineered devices. To quantify fundamental piezoresistive properties and reach high strain levels, testing at the nanoscale is critical, as the size scales can fall below the threshold for external or bulk defects. However, proper characterization of the piezoresistivity is a challenge at the nanoscale due to the miniscule specimen size. In this work, in situ tensile tests inside a scanning electron microscope (SEM) are conducted to study the mechanical and electromechanical properties of tellurium (Te) and germanium (Ge) nanowires. The in situ tensile testing technique allows real-time observations of elastic/plastic behaviors and gives the resistance changes under different strains for the determination of the piezoresistivity. The reversibility and the repeatability of the observed piezoresistive properties can be confirmed from loading-unloading tests and measurements of multiple nanowires with different diameters. These tests offer accurate experimental data that yields insights into the fundamental mechanisms behind piezoresistivity, such as strain-induced changes in band structure.

Accompanying with doping of Ca, Zr, etc, BaTiO₃-based ceramics exhibited excellent dielectric behaviors. Especially, Sn has been found to increase the permittivity by 3 times in comparison with single BaTiO₃ and to induce the relaxor-type behavior.[1,2] Recent transmission electron microscopy studies[3] have shown that polar nano region (PNRs) embedded in the matrix is responsible of relaxor behaviors. The lattice constant increases with the doping of Sn, and the PNR region expands due to the stress of the matrix which has a large effect on the ferroelectricity of materials. Through high-resolution scanning transmission electron microscopy and first order calculation[4], they confirm the unit cells of the PNRs expand by the stress, which indicates that there is more space between Ti atoms and the oxygen octahedra to allow larger off centre displacements. This result shows an extreme improvement in dielectric permittivity. In addition, experimental methods of applying the temperature and electric field to control the stress in the PNR region have been proposed, and simulation has confirmed that these elements increase the piezoelectricity of the relaxor ferroelectric materials.[5,6] However, the relationship between PNR and electric field of the Ba(Ti_0.85Sn_0.15)O₃ (BTS) film was not confirmed experimentally. Therefore, we use a combination of time-resolved micro-X-ray diffraction (TRμXD), which is used to apply biaxial stress to the thin film in real time. As a results, we confirm the relationship between PNR with piezoelectric response through calculation of the correlation length in Ba(Ti_0.85Sn_0.15)O₃ film.

The (111)-oriented Nb:SrTiO₃ single crystal substrates with a Nb concentration of 0.5% were used as substrates to grow the epitaxial Ba(Ti_0.85Sn_0.15)O₃ thin films on top. The BaTiO₃ film were deposited using pulsed laser deposition (PLD) in an oxygen atmosphere of 10 mTorr. After the deposition, the samples were cooled down in a high vacuum. The dielectric constant was 412, which is twice as high as that of the conventional BTO (about 200). In Pohang Light Source, an X-ray beam with energy of 8 keV was focused to a spot with a size of 11.5 μm onto a circular Pt top electrode with a diameter of 100 μm. With an application of electric field (364 MV/m), (111) reflections of film shifted to the lower q_z value from 2.696 to 2.691 Å, which indicates the piezoelectric expansion with piezoelectric coefficient of 6.7 pm/V. We deposited the BTS thin film through PLD for the first time, and experimentally observed the piezoelectric properties as an electric field was applied. The change in the PNR region was confirmed from the results.

[1] C. Lei et al., J. Appl. Phys. 101, 084105 (2007)
[2] Wei Xiaoyong et al., Appl. Phys. Lett., 83, 10 (2003)
[3] Tao Shi et al., Sci. Rep., 5, 8606 (2015)
[4] Wang, D. et al., Nat. Commu. 5, 5100 (2014).
[5] Manley M. E. et al., Sci. Adv., 2, 1501814 (2016).
[6] Fei Li et al., Nat Commun., 7, 13807 (2016)

In the recent years, replacing the current Zr-alloy-based cladding with silicon carbide composites has gained traction in the Department of Energy (DOE) as well as in the industry. The most recent generation of SiC/SiC composites (III), defined by near-stoichiometric chemical composition and crystallinity with reduced oxygen content, has been extensively tested for nuclear applications. They are comprised of a SiC matrix (m) reinforced with long SiC fibers (f) that are woven or braided onto a tubular cladding configuration. The properties of SiC/SiC composites are greatly influenced by the matrix processing methods, fiber architecture and orientation, and interface materials. Oxidation reaction kinetics under steam attack is of critical importance under accident conditions. When exposed to steam, SiC exothermically forms silica, hydrogen and carbon monoxide releasing an energy of 223 kJ/mol, approximately. In general, a competition between a parabolic oxidation process and a linear volatilization process results in paralinear kinetics.

In this work, we capture the oxidation process in a model composite material using a combination of x-ray tomography and mesoscale phase-field simulations. First, the microstructure of the composites is faithfully extracted by x-ray tomography and a full three-dimensional reconstruction is performed. This nondestructive technique allows us to extract all the pertinent microstructural details. Starting with the digitized microstructure, we then simulate the evolution of the pores/pathways accompanying the transport of steam through phase-field modeling. Non-equilibrium chemical thermodynamic models are used to make accurate predictions on the pertinent chemical reactions. The evolution equations (Allen–Cahn and Cahn-Hilliard), which are fully coupled to heat conduction and deformation models, allow us to determine the extent of oxidation processes in the composites under anticipated accident conditions.

Nucleation and growth of oxygen precipitates in Czochralski Si crystals with different boron dopings has been investigated in-situ up to 1000°C to monitor their structural evolution from the very early stages to a stable configuration.
Thickness-dependent Pendellösung oscillations as described by the dynamical theory of X-ray diffraction are extremely sensitive to strain fields from defects in a host crystal. Based on this, we initiated a novel approach to study the precipitation kinetics of oxygen in Si. A diffusion-limited model of growing spherical precipitates with two growth regimes interprets the data. An initial diffusion-driven progression is followed by a long time precipitation behavior, which is interpreted as Ostwald ripening.

Cu composite reinforced by three-dimensional (3D) graphene network was synthesized in a simple two-step process; (1) an axial compaction of Cu particles in a mold, and (2) CVD at an appropriate condition leading to a Cu composite reinforced with 3D graphene network. Cu composites with different contents of either graphite or graphene were obtained by controlling the processing parameters. 3D graphene network reinforced Cu composite is presented with a possible explanation for and characterization of the formation of graphene. Lastly, the thermal conductivity data of 3D graphene-Cu composite, which is currently about 450 W/mK in both perpendicular and parallel directions, is presented along with a possible mechanism to explain the high thermal conductivity.

3D micro-architected structural meta-materials are instrumental in light-weight, high-strength, and damage-tolerance engineering applications. The reliability of such materials is often dictated by the mechanical properties of their truss components such as nanowires and nanopipes and their defects, such as notches and voids and also structure-level defects that disrupt the connection of the lattices. An in-depth understanding on the effects of component-level factors such as grain boundaries, surface roughness, sizes, shapes and man-made notches and structure-level factors such as the sizes and shapes of the connection disruptions on the deformation mechanisms and failure patterns of 3D micro-architected meta-materials is of great importance to their fabrication and services with high reliability. In this talk, we will first report our recent progresses in the study of the effects of component-level factors such as grain boundaries, surface roughness, and the sizes, shapes and man-made notches on the plasticity and failure of the components using both computer simulations and mechanical testing. We will then report our recent work on the effect of temperature and component size on the deformation and failure of micro-architected meta-materials under compression and tension using both computer simulations and mechanical testing. Finally, we discuss the effect of structure-level defects (connection disruption) on the failure behavior of 3D micro-architected meta-materials under tension using both finite element modeling and mechanical testing. The present works explore the size effect, temperature and flaw sensitivity of 3D nanolattices and their components using both simulations and experiments, and demonstrates various interesting and unique features of the architected structural meta-materials.

In this study, we explore the effect of electron-beam irradiation on the mechanical behavior of amorphous silica nanostructures. It has been reported that amorphous silica exhibits ductile deformation behavior under the electron-beam irradiation during the in-situ compression testing. However, the dependency of electron-beam conditions such as acceleration voltages and beam currents has not been thoroughly investigated. Hence, we performed in-situ mechanical testing for amorphous silica nanostructures at various electron-beam conditions. By systematically changing acceleration voltages and electron-beam currents, we found the thresh-hold electron-beam condition to induce plastic deformation of amorphous silica. Furthermore, our simulation study suggests that the electron-beam effect has clear correlation with the interaction volume of the electron-beam with the sample.

3D printing has become a popular fabrication technique for rapid prototyping of complex structures in recent years due to its simplicity, relatively short production time and low material wastage. Despite these advantages, it remains difficult to fabricate structures out of composite materials, such as nanofiller-reinforced polymer resins, using 3D printing. Here, we investigate the factors influencing the stereolithographic synthesis of 3D structures, such as concentration of filler and post-fabrication baking temperature and duration, and demonstrated, for the first time, the successful fabrication of 3D graphene-polymer composite lattices. Quasistatic mechanical tests showed that the stiffness of a polymer, reinforced with as little as 0.02 wt. % graphene, can increase up to 70.2 % compared to a non-reinforced polymer. Furthermore, enhanced energy absorption was also observed for octet-truss lattices 3D printed from the polymer-graphene composite material. In situ microscopic deformation and failure of the 3D printed graphene-polymer composite during mechanical testing will be presented and discussed as well.

Strength, ductility and toughness are key attributes of precipitate strengthened alloys, but remain challenging to achieve in many metallic systems. A detailed understanding of dislocation dynamics at precipitates is necessary for controlling work hardening, localized plasticity, plastic reversibility (e.g. Bauschinger effect), fatigue and failure, as well as the development of high performance steels, lightweight alloys, and high-temperature superalloys. Progress in these areas has been hampered by the lack of direct experimental evidence of detailed deformation mechanisms at precipitates, which prevents the verification and calibration of physics-based predictive computational models. Here, we use bimetallic core-shell nanocubes as model systems in which single precipitates (core nanocrystal) are isolated, and colloidal chemistry is used to tailor precipitate size, composition and defect structure. Nanocrystals are compressed using in-situ scanning electron microscope to obtain stress-strain behavior at individual precipitates, and characterized before and after compression using transmission electron microscopy. Molecular dynamics and discrete dislocation dynamics simulations are compared to experiments in order to determine the dislocation mechanisms responsible for the mechanical response. Here, we compare the mechanical behavior at semi-coherent precipitates in Au@Cu core-shell nanocubes and coherent precipitates in Au@Ag nanocubes. Au@Cu nanocubes have misfit dislocations at the core-shell interface and threading dislocations within the Cu shell, while Au@Ag nanocubes are defect-free. Au@Cu core-shell nanocubes are found to have higher strength, and strain hardening that is absent in Ag and Au@Ag nanocubes. We determine that deformation in Au@Cu nanocubes is governed by the motion of threading dislocations that extend from the bimetallic interface to the nanocube surface, that operate as single-arm sources. Plastic deformation in Au@Ag nanocubes occurs through the nucleation of dislocations at free surfaces.

This presentation will review the recent progress in the understanding of plastic deformation and fracture in metallic materials that have hierarchical composite structures across nano-to-micro length scales. Examples will be presented from different model systems such as laser-processed Al-Al₂Cu-Si eutectics, and vapor deposited Cu-Mo thin films. The role of in situstraining experiments in TEM and SEM, and integration with atomistic modeling, dislocation theory and crystal plasticity in understanding mechanical behavior will be emphasized. The common aspects in interface-dominated mechanical behavior in hierarchical metallic composites such as unusually high flow strengths, high strain hardening rates, plastic co-deformability and suppression of shear bands will be highlighted.

We use solid metal dealloying to synthesize TaTi/ZrTi nanocomposite thin films with widely differing morphologies of the constituent phases, ranging from particulate to bincontinuously interpenetrating, but comparable characteristic dimensions of microstructure features. We then investigate the indentation and helium implantation response of these composites. Our work shows a systematic dependence of indentation pile-up height on microstructure morphology, which we attribute to morphology-induced differences in strain hardening rates. We also find a reduction in the size and distribution of helium precipitates relative to un-dealloyed materials, which we relate to the characteristic dimensions of microstructure features of the composites. We conclude with a discussion of future prospects for tailoring mechanical and radiation response by changing microstructure morphology in dealloyed composites.

Annealing twins can have a strong impact on the mechanical performance of Ni-base superalloys used in turbine disks. While they are recognized for helping to increase yield strength, annealing twins are also known to localize strain and initiate microcracks in high-cycle fatigue. Using a combination of in-situ deformation DIC and 3D crystal plasticity based computational modeling, we investigate the microstructural factors leading to strain localization and subsequent slip band initiation. The analysis focuses on the coupled role of elastic anisotropy, grain neighborhoods, and grain shape and size in determining the location of the exceptionally preferred points of high elastic strain concentration and localized slip, when the applied strain is under but near the macroscopic elastic-plastic transition. We find that the very few localized slip bands are correlated to the development of only the highest elastic strain concentrations. The latter develops in grains that have an outstandingly compliant orientation relative to all its neighbors. We discuss some important microstructural features responsible for microcrack initiation as revealed by the model calculations.

Nanoscale metallic multilayers formed by alternating layers of two dissimilar metals with layer thicknesses in the nanoscale range have very attractive properties including high hardness, strength and thermal stability. Nevertheless, most of the mechanical characterization of metallic nanolaminates has been carried out by means of nanoindentation or micropillar compression and very little information is available on the tensile behaviour of free-standing nanolaminate films. Thus, there is a lack of information on the deformation and fracture mechanisms of these systems in tension.
In this investigation, the mechanical behaviour of Cu/Nb nanolaminates manufactured by accumulated roll bonding were studied by means of tensile tests carried out within a transmission electron microscope (TEM). Tensile specimens with the loading axis parallel and perpendicular to the laminate were milled using a focus ion beam gun and attached to a push-pull loading device which was introduced in the TEM. Load and deformation were recorded during the in situ tensile deformation, together with the mechanisms of deformation, crack propagation and crack arrest at the Cu/Nb interfaces. Evidence of plastic deformation by confined layer slip was found in both orientations for Cu and Nb, but important differences were found in the fracture mechanisms between both orientations. Crack nucleation and propagation was localized in the thickest Cu layer when the nanolaminate was perpendicular to the loading axis. In the parallel orientation, crack was nucleated in a Cu layer but was stopped at the Cu/Nb interface, leading to a rough crack path. The influence of the orientation on the deformation and fracture mechanisms was discussed to the light of the observations provided by the in situ mechanical tests.

Twinning is an essential carrier of plastic deformation and critically influences the mechanical behavior of materials. Due to its atomic-scale complexity, the atomic mechanism of twinning nucleation and growth in hexagonal close-packed (HCP) crystals remains theoretical contention to date, which largely impedes the design and processing of high strength alloys. Here, by using in situ transmission electron microscopy, twinning mechanism was directly captured in HCP crystals. In stark contrast to the classical twinning dislocation mechanism, it was found that {1 0 -1 2} twinning nucleation could be dominated by atomic shuffles which were manifested by the direct transformations between prismatic and basal planes; this process established the lattice correspondence of the {1 0 -1 2} twin. During the twin growth, the incoherent B│P-type twin boundaries transformed to coherent twin boundaries which propagated by the movement of classical twinning disconnections. The findings resolve long-standing controversies in twinning of HCP crystals, paved the way for twinning-based design and processing of high strength HCP alloys, and hold broad implication to the deformation in complex crystal structures.

Radiation induced damage in the core structural materials that are made of crystalline materials are microstructural/crystal defects. Conceptually, these defects can be visualized as regions where there is either a deficiency of lattice atoms (voids, vacancies, edge dislocations, vacancy type dislocation loops) or an excess of lattice atoms (self interstitial atoms, interstitial type dislocation loops). All these defects lead to changes in the mechanical properties of the material. For example, radiation induced swelling (by vacancies and voids) causes dimensional distortion and embrittlement, and is a life-limiting materials issue for structural materials in nuclear power reactors. Austenitic steels were used in fast reactors but could not reliably serve beyond ~150 dpa. Ferritic and ferritic-martensitic (FM) steels have been found to swell much less than austentitic steels. Nano-structuring of both austenitic and FM steels appears to be a promising avenue for further improvement of swelling resistance, providing such structures are stable under ion irradiation. Advanced oxide dispersion strengthened alloys were found to be a promising core structural material, but amorphization and dissolution of oxide particles under high dpa irradiation challenges potential applications. Interfaces (interphase boundaries and grain boundaries) between the metal matrix and nanoscale oxides in oxide dispersion strengthened (ODS) steels systems prove to benefit swelling resistance and creep resistance. Nanoscale metallic interfaces have shown strong defect sink strength and the ability to suppress He bubble formation. However, all of these do not change the intrinsic issue — radiation induced damage in crystalline materials. Compared to crystalline solids, amorphous materials possess no translational symmetry, and amorphous materials do not contain conventional crystal defects such as vacancies, interstitials, or dislocations.
There are two unique radiation effects which must be considered for the irradiation response of amorphous materials: thermal spike formation associated with damage cascades. Another effect is from creation of excessive free volume along an ion track, particularly in the damage cascade core which is rich in open volume temporally due to ballistic collisions. The excessive free volume can thermally enhance atomic mobility and initialize correlated movements needed for crystal nucleation. To prevent crystallization of amorphous associated with the two unique radiation effects, ceramics with high crystallization temperatures is a promising candidate. However, such kinds of amorphous ceramics often exhibit high strength without measurable plasticity. Based on our recent discovery – extreme irradiation-tolerant amorphous Fe-SiOC ceramics that achieve a superior strength-plasticity combination. In this talk, we report our recent discoveries and understanding regarding strength, plasticity, and irradiation properties of amorphous ceramics containing nano-sized metal additions.

Two-dimensional (2D) sharp interfaces with distinct boundaries demarcating an abrupt discontinuity in material properties in nanolayered composites have been studied for almost twenty years and are responsible for enhanced behaviors such as strength, radiation damage tolerance, and deformability. However, 2-D interfaces have their limitations with respect to deformability and toughness. 3D interfaces are defined as heterophase interfaces that extend out of plane into the two crystals on either side and are chemically, crystallographically, and/or topologically divergent, in three dimensions, from both crystals they join. Here, we present the mechanical behavior of two different classes of nanocomposites: 1.) nanolayered Cu/Nb containing interfaces with 3D character and 2.) Tungsten-based 3D ordered nano/mesoporous composites consisting of a porous W scaffolding with silicon carbide or silicon nitride infill. Micropillar compression results show that the strength of Cu/Nb nanocomposites containing 3D interfaces is significantly greater than those containing 2D interfaces. Shear banding in 3D Cu/Nb is observed during pillar compression with retention of continuous layers across the shear band. We will present our recent results on deformation of such 3-D interfaces and structures, and describe this evolution mechanistically through the use of atomistic simulations.

Fatigue of metals at the nanoscales has drawn extensive attention due to not only the long-term service reliability of nanoscale metallic components in micro/nano-devices, but also the fundamental fatigue mechanisms of the materials at the nanoscales. It has been demonstrated that the decrease in the length scale from micron scales to submicron scales strongly constrains the formation of typical dislocation structures (dislocation walls and cells), and thus suppresses cyclic strain localization, leading to the enhancement of fatigue strength. In this talk, we will present ex-situ TEM investigations on cyclic plasticity and damage behavior of metals at the nanoscales, such as nanocrystalline metal films and metallic multilayers. For nanocryatalline metal films with a thickness ranging from 20 to 930 nm, we found an evident transition of cyclic plasticity from full dislocation-dominated to partial dislocation-controlled behavior, meanwhile grain boundaries exhibit the unstable behavior related to the film thickness. Furthermore, the fatigue damage behavior was characterized, and slip irreversibility was estimated theoretically based on the observations of dislocation activities. For the nanoscale metallic multilayers, we found that the heterogeneous layer interface in the Cu/Ta multilayers with different layer thicknesses exhibits different stability under cyclic loading. The fatigue damage mechanism and the scaling effect on the slip irreversibility, grain boundary as well as heterogeneous layer interface instability of the metal films and multilayers at the nanoscales will be discussed.

Plasticity and fracture of materials at the nanoscales can deviate significantly from the same phenomena in bulk properties, which may have important implications if the materials are to be used in real-world engineering systems. Nanoscale metal-metal multilayered materials, where interfaces can be created and modulated in a highly controlled manner and with relative ease, provide a model material system platform to understand plasticity and fracture based on their interactions with other microstructural features at the nanoscales. Recently, there is a growing trend to understand fracture of multilayered materials to see the interactions between the crack and multilayered interface through novel experimentation techniques. In this review, we will introduce the rationale behind this trend and the current microfracture testing methods to test and analyze multilayer fracture behavior. Four examples of in situ fracture techniques are highlighted in this work – the in situ observation upon tensile testing of film on a substrate, the in situ microfracture clamped beam bending technique across the multilayers for accumulative roll bonded as well as physical vapor deposited multilayers and the in situ delamination of metal-metal multilayers along the multilayered interface.

In present synchrotron radiation facilities, the trend towards fast on-the-fly experiments [1,2], made possible in particular thanks to faster low noise hybrid pixel area detectors, opens many avenues for in situ evaluation of structure, strain and defects in various nano-materials. Pole figure measurements can now be performed fast enough to follow intriguing texture changes during phase transformations [3]. Combination of curvature measurements with x-ray diffraction allows for a detailed understanding of strain evolution during crystallization of phase change materials [4]. Thanks to focused beams strain mapping in devices can be performed in situ as a function of temperature [5]. I will also show how small x-ray beams may be used for investigating the mechanical behavior of nano-objects in situ during mechanical testing [6,7]. All these recent evolutions will benefit a lot from the development of new and even more brilliant sources which will appear in the coming years.

[1] G. Chahine, M.I. Richard, R. Homs et al., J. Appl. Cryst. 47, 762 (2014).

[2] C. Mocuta, MI. Richard, J. Fouet, S. Stanescu, A. Barbier, C. Guichet, O. Thomas, S. Hustache, A. Zozulya, D. Thiaudiere, J. Appl. Cryst. 46, 1842 (2013).

[3] MI. Richard, J. Fouet, M. Texier, C. Mocuta, C. Guichet, O. Thomas, Phys. Rev. Lett. 115, 266101 (2015).

[4] T. Ouled Khachroum, MI. Richard, P. Noe, C.Guichet, C. Mocuta, C. Sabbione, F. Hippert, O. Thomas, Thin Solid Films 617, 44 (2016).

[5] B. Vianne, MI. Richard, S. Escoubas, S. Labat, T. Schülli, G. Chahine, V. Fiori, O. Thomas, Appl. Phys. Lett. 106, 141905 (2015).

[6] C. Leclere, T. W. Cornelius, Z. Ren, A. Davydok, J.-S. Micha, O. Robach, G. Richter, L. Belliard and O. Thomas, In situ bending of a Au nanowire monitored by micro Laue diffraction, J. Appl. Cryst. 48, 291 (2015).

[7] T. Cornelius and O. Thomas, Progress in Materials Science 94, 384-434 (2018).

The Smart Cut^TM technology, which is based on the implantation of light ions in a crystalline matrix, is well-suited for the production of Silicon-On-Insulator (SOI) substrates used in electronics. Under annealing the gas implanted in the crystal forms defects that evolve into micro-cracks, which eventually leads to a subsurface fracture and the delamination of a thin crystalline layer. Bonding the implanted crystal to an oxidized substrate prior to the splitting, allows one to obtain a SOI substrate with a nanometer-thick top silicon layer.

However, the physics leading to delamination is still not fully understood and it is difficult to selectively measure the brittle, implanted layer when a wafer is bonded on top. The aim of this study was to find a way to measure in situ the effect of these buried micro-cracks in terms of lattice strain and tilt on the crystalline matrix under annealing.

A new method called Full Field Diffraction X-ray Microscopy (FFDXM) has been used to observe the deformation induced by the gas-filled micro-cavities in the Smart Cut^TM technology. This method has been developed at the European Synchrotron Radiation Facility (ESRF) at the ID01 beamline. It consists in using a well-collimated X-ray beam to diffract on the sample and post-sample lenses to obtain a real space image on a two-dimensional detector of the Bragg reflection, quite similar to dark-field imaging in electron microscopy. It allows direct imaging of crystalline domains with a space resolution of some hundreds of nanometers over hundreds of microns, with possible orientation and/or strain contrast. Using hard X-rays (10 to 20 keV), it is possible to image buried features, e.g. at the interface of two bonded wafers.

The acquisition time of FFDXM (on the order of the minute) is compatible with the annealing times in the Smart Cut^TM technology (on the order of the hour). Thus it has been used in situ to evidence and follow the micro cavities coalescence mechanism and the spatial extent of their tilt/strain fields during annealing, a key point to improve understanding of silicon fracture mechanism after light ion implantation.

The imaging of crack propagation in mechanically stressed bulk samples and thin film stacks with high spatial resolution is essential for the validation of models based on materials physics and chemistry as well as on fracture mechanics, and eventually for the design of advanced materials with tailored mechanical properties. Information about the kinetics of crack evolution is needed for ensuring the in-service mechanical performance and reliability of advanced products. Laboratory X-ray microscopy and nano X-ray computed tomography (nano XCT) offer intrinsic advantages for 3D in-situ imaging of cracks in multi-component materials and structures, since they are nondestructive, i.e. the material itself and the local strain state are not modified by sample preparation. We will present experiments that combine nano XCT and mechanical loading for visualization of crack evolution in composites and in 3D patterned microchip structures.

The combination of nano XCT with micro-mechanical experiments merges the advantages of high-resolution 3D imaging of the microstructure of materials with the observation of phenomena like crack propagation during mechanical loading. The application of in-situ mechanical loading of multi-component materials is demonstrated. A customized micro-mechanical test setup is designed for in-situ micro-indentation experiments in such a way that it fits into the limited space of the nano XCT tool and that the indenter tip and the sample fit into the field of view of the X-ray microscope. Cracks in carbon fibre reinforced aluminum matrix composites along interfaces are imaged using XCT, to evaluate the fracture behavior of MMCs.

A miniaturized double cantilever beam test in an X-ray microscope is used for in-situ imaging of crack evolution in microchips. High-resolution 3D image sequences based on nano XCT are acquired during mechanical loading to visualize crack opening and propagation in on-chip interconnect structures of integrated circuits with about 100 nm resolution [2]. As a result, the weakest layers and interfaces are identified and the robustness of the Cu/low-k interconnect stack against chip-package interaction, a serious reliability concern for microelectronic products, is evaluated. This setup can be used to study the dynamic response of samples during low-cycle fatigue too.

Perspectives to overcome three major limitations of state-of-the-art nano XCT tools, i.e. the need of sample preparation (typically less than 50 mm thickness, depending on the material composition, if 8 keV photons are used), limited space for in-situ experiments and low imaging throughput, are discussed. A novel tool concept for X-ray microscopy at high photon energies, using advanced X-ray sources with high flux and the option of multi-energy photons, i.e. sources with microstructured metal/diamond anodes [3], and of advanced X-ray optics with high efficiency at photon energies > 10 keV, i.e. multilayer Laue lenses [4], will be presented.

[1] E. Zschech et al., "Laboratory Computed X-ray Tomography – A Nondestructive Technique for 3D Microstructure Analysis of Materials“, Pract. Metallogr. 55, 539–555 (2018)
[2] K. Kutukova et al., "A Laboratory X-ray Microscopy Study of Cracks in on-chip Interconnect Stacks of Integrated Circuits“, Appl. Phys. Lett. 113, 091901 (2018)
[3] W. Yun et al., “Impact of new high flux laboratory systems”, Synchrotron Radiation Instrumentation Conf. SRI, Brookhaven (2015)
[4] S. Niese et al., “Full-field X-ray Microscopy with Crossed Partial Multilayer Laue Lenses”, Optics Express 22, 20008-20013 (2014)

This talk will highlight recent advances with in situ Transmission Electron Microscopy (TEM) nanomechanical testing and imaging techniques that provide insight into small-scale plasticity and the evolution of defect structures in materials. With the advent of fast direct electron detectors, it is possible to perform strain mapping during in-situ deformation in a TEM with the precision of a few nanometers using the 4DSTEM technique. This talk will describe our recent results from in situ TEM nanomechanical testing that provide insight into multiscale metallurgical phenomena using these techniques, such as the role of short range ordering in Ti alloys and deformation phenomena in metallic glasses.

Advances in materials design often begins with a hypothesis or simulation, and ends with a structure/process/property correlation. In between is synthesis and characterization, often with a very limited sample size necessitated by either the production of a large numbers of samples(e.g. combinatorial studies) or expense (cost of base materials plus synthesis). It is important to get as many structural parameters with that small volume of material as possible, therefore correlative and in situ studies are of primary interest. Here the authors present a nanomechanical testing technique which is capable of very large throughput, capable of collecting more than one million mapped data points on a damascene steel sample in the cm2 dimensions. The mechanical property map is correlated to both chemical and structural mapping that can be used to generate a ful structure/property/processunderstanding. This indentation mapping technique can also be compared with in situ microbeam bending tests, for example the transition in a BMG to Ti dissimilar weld from ductile Ti rich, to a ductile BMG with Ti lamella, to fully brittle BMG.

Grain growth in metallic thin films is commonly achieved with high temperature (greater than 30% of the homologous temperature) annealing. In this study, we explore an alternate route that is non-thermal in nature. Specifically, we study the synergy of electrical current and tensile strain instead of temperature. Our approach involves passing electrical current in the specimens at much lower current density than the electro-migration damage limit. This directly causes Joule heating, which is eliminated by both passive and active heat removal techniques to keep the specimen temperature at or near ambient conditions. We then study the role of mechanical strain on the grain growth at room temperature. In-situ Transmission electron microscopy (TEM) shows more than >10 times grain growth under the application of elastic strain (about 0.1%) dramatically increased the grain size by more than 10 times in a few seconds at the room temperature regions. This suggests that the synergy of elastic strain energy and electrical current density may achieve grain growth in metallic materials even at room temperature. We investigated this phenomenon using molecular dynamics simulation, which reveals that externally applied strain is localized at the grain boundaries in nanocrystalline metals, which promote the effects of electron wind force on the grain boundary atoms. We conclude that synergy of two or more stimuli can achieve grain growth at low or even room temperature.

Our experimental study begins with fabrication of a micro electro-mechanical system (MEMS) device with 100 nm thick Palladium thin film. Palladium melts at 1555 ^oC and is used in catalysis and electronics applications. It is therefore an attractive specimen for room temperature annealing. To decouple the contributions from the electrical current, temperature and stress, we developed a unique experimental setup, where the same specimen could be divided into two regions. The center region experiences high temperature and the applied current density. The two edges of the freestanding thin film specimens remain at the same current density, but at around room temperature. The negligible grain growth at the edges suggests the dominant role of temperature on grain boundary mobility. The initially nanocrystalline (5 nm grain size) specimens were exposed to various levels of current density with and without tensile strain inside a TEM. Transmission electron microscope (TEM) images and diffraction patterns show that even elastic strain energy can not only strongly influence the grain growth, but also lower the required electrical current density. The resulting microstructure showed very high density of nano-twins compared to the purely thermally annealed ones.

In addition to the experimental study, we employed classical molecular dynamics (MD) simulation to investigate the underlying mechanism on grain growth using embedded atomic model (EAM) potential. We model the electron flow by imposing an additional equivalent wind force on each atom at room temperature. This equivalent electrical wind force enhances the atomic motion and diffusion at defective regions (grain boundaries), which in turns facilitates the grain growth. The role of mechanical stress is to promote this mechanism through localization at the grain boundaries. Displacement vector analysis of individual grain from molecular dynamics simulation reveals that grain grows due to the grain boundary sliding and rotation during tensile straining accompanied by AC current.

We have used in situ TEM to investigate deformation and failure of 3C-SiC as synthesized and after irradiation with carbon ions. Radiation-induced embrittlement is a significant problem in all kinds of nuclear materials (e.g. [1]). 3C-SiC has shown severe embrittlement and loss of strength under high fluence at relatively low temperature [2]. Our in situ nanoindentation tests on as-synthesized 3C-SiC show that this brittle ceramic can show some plasticity at room temperature. The continuous load-displacement curves confirmed that no fracture happened during indentation, which is also reflected in the TEM videos. Bend contours moved away from the surface during loading, representing the stress field inside the sample under applied force. SEM observation post indentation shows metal-like extrusion around indents, which demonstrates that samples experienced some plastic deformation. In dark-field TEM images after indentation, there are bend contours left in the sample, which is further evidence for residual strain, and thus plastic deformation of the samples. In addition, during unloading, after the force dropped to zero, the contours were still moving backward, which is an indicator of plastic deformation recovery. Very small sample size can decrease the brittle-to-ductile transition temperature to room temperature for some brittle materials [3][4], and room-temperature plasticity has been observed previously in different polytypes of SiC with different geometries due to phase transition [5], gliding of existing dislocations [6], amorphization [7], and other mechanisms.

In situ TEM nanoindentation tests of 3C-SiC irradiated at 600°C with 3.15 MeV carbon ions up to up to 5 displacements per atom (dpa) in the first one micron show different fracture behavior. In situ indentation along the irradiation profile probes different levels of radiation damage. Indenting at deeper irradiation depth with more irradiation damage (high dpa and larger diameter defects) causes quick and sharp crack and a small residual indent, while indenting at non-irradiation effected area causes a larger residual indent and more debris. These preliminary results suggest that irradiation induced defects make the sample more brittle and reduce its fracture toughness.

Reference:
[1] B. Z. Margolin et al., “Radiation embrittlement of support structure materials for WWER RPVs,” J. Nucl. Mater., vol. 508, pp. 123–138, Sep. 2018.
[2] Y. Katoh et al., “High-dose neutron irradiation of Hi-Nicalon Type S silicon carbide composites. Part 2: Mechanical and physical properties,” J. Nucl. Mater., vol. 462, pp. 450–457, Jul. 2015.
[3] P. Gumbsch, S. Taeri-Baghbadrani, D. Brunner, W. Sigle, and M. Rühle, “Plasticity and an Inverse Brittle-to-Ductile Transition in Strontium Titanate,” Phys. Rev. Lett., vol. 87, no. 8, p. 085505, Aug. 2001.
[4] W. Kang and M. T. A. Saif, “In Situ Study of Size and Temperature Dependent Brittle-to-Ductile Transition in Single Crystal Silicon,” Adv. Funct. Mater., vol. 23, no. 6, pp. 713–719, Feb. 2013.
[5] B. Chen et al., “Deformation-induced phase transformation in 4H–SiC nanopillars,” Acta Mater., vol. 80, pp. 392–399, Nov. 2014.
[6] S. Kiani et al., “Dislocation glide-controlled room-temperature plasticity in 6H-SiC single crystals,” Acta Mater., vol. 80, pp. 400–406, 2014.
[7] I. Szlufarska, R. K. Kalia, A. Nakano, and P. Vashishta, “Nanoindentation-induced amorphization in silicon carbide,” Appl. Phys. Lett., vol. 85, no. 3, pp. 378–380, Jul. 2004.

Strain engineering of the electrical and optical properties of semiconductor nanomaterials is an effective way to tailor the performance of nanowire-based devices such as high-speed transistors and sensor. However, the direct characterization of the mechanical and electrical properties of individual nanomaterials, in particular, the correlation between them is still a challenge. In this work, we directly investigate the intrinsic mechanical and electromechanical properties of individual InAs nanowires under uniaxial tensile stress using in situ transmission electron microscopy.

InAs nanowires, both bare InAs nanowires and InAs/Ga_0.4In_0.6As core-shell nanowires, were used in the investigation. Simultaneous stress and current-voltage (I-V) measurements were enabled by an electrical push-to-pull (EPTP) device in a Hysitron PI95 nanoindentation TEM holder. In situ nanoscale lattice strain mapping within the nanowire was performed using scanning transmission electron microscopy (STEM) combined with nanobeam electron diffraction (NBED). Direct and quantitative stress, strain and electrical transport measurements were carried out on individual nanowires. The results showed that the resistivity of the InAs nanowires decreased continuously with uniaxial tensile stress. Large piezoresistance effect was found in InAs nanowires. However, bare InAs nanowires and InAs/Ga_0.4In_0.6As core-shell nanowires show different electromechanical behavior. The large piezoresistance effect is partly due to the small Young’s modulus of the nanowires (~11 GPa), but band gap narrowing of InAs is considered to be the main reason for the resistivity change. Moreover, despite apparent elastic deformation of the nanowires, significant inhomogeneous strain distribution within the InAs/Ga_0.4In_0.6As core-shell nanowires was unveiled by STEM-NBED strain mapping at different tensile stress levels. The inhomogeneous strain distribution at nanometer scale can increase the resistivity of the nanowire by enhancing electron scattering. The findings demonstrate unique mechanical and electromechanical properties of the nanoscale InAs wires and provide new insights of the correlation between mechanical strain and electrical transport properties in free-standing nanostructures [1].

Financial support from Swedish Research Council and Nano Area of Advance and Energy Area of Advance at Chalmers University of Technology are acknowledged.

[1] L.J. Zeng et al., Nano Lett., 2018, 18 (8), pp 4949–4956.

A MEMS based technique for in-situ TEM nanomechanical tensile testing of ultrathin films has been developed which allows for high magnification TEM observations of the deformation mechanisms in the thin film at the nanoscale while simultaneously measuring the macroscale behavior of the thin film quantitatively. Consecutive stress relaxation transients were performed in-situ in the TEM on ultrafine grained 100-nm-thick gold and 200-nm-thick aluminium films using this MEMS device. The plastic deformation in the gold films was observed to be mediated via intragranular dislocations originating from grain boundaries and was very localised. Local grain growth was also observed in the aluminium films in regions of severe plastic deformation along with dislocation pile-ups in some of the grains. The physical activation volume of the gold films was obtained around 3-15b³ while activation volumes between 5-40b³ were measured for aluminum films. The observed plastic deformation mechanisms are consistent with the low activation volumes calculated for these ultrafine grained thin films.

Pre-stressing aluminum (Al) particles is linked to increased reactivity by altering the physical and mechanical properties at the core-shell interface of the Al-Al₂O₃ particle. Changes in quenching rates during pre-stressing have been shown to change the reaction mechanism of micron Al particles. Faster quenching rates (i.e., 900 K/min) lead to shell delamination and a reduced oxidation barrier. Slower quenching rates (i.e., 200 K/min) result in increased hoop stress in the shell. To examine the effects of pre-stressing under high velocity (850-1200 m/s) impact conditions, 3-4.5 micron diameter Al particles were annealed to 573 K, then quenched at different rates (200 and 900 K/min), mixed with 1 wt. % polyvinylidene fluoride (PVDF), and formed into pellets. The projectiles were fired from a High-velocity Impact-ignition Testing System (HITS) into a steel anvil. Data for flame spreading and pressure in the impact chamber were collected for two stages of ignition and reaction corresponding with impact and rarefaction due to after blast. Results show that shell delamination in the faster quenched Al samples contributes to faster reaction rates during impact as well as during rarefaction in the after blast for both impact velocities. In contrast, increased hoop stress in the shell from slower quenching significantly delays energy release at the lower impact velocity by delaying shell failure. At the higher impact velocity, the increased stress in the slower quenched particles contributes to more rapid energy release than untreated particles during impact and after blast, but the alumina diffusion barrier is not removed as effectively from the particles compared to the delamination present in the faster quenched particles. These results establish a link between altering mechanical properties of fuel particles towards influencing fuel particle reactivity under the extreme loads of high velocity impact ignition and reaction.

Quantitative nanoscale force measurement using atomic force microscopy (AFM) has been well established [1]. However, these measurements are limited to cantilever-based AFM which suffers from a snap-to-contact when tip-sample separations are on the order of 10 nm. In order to probe forces when the tip-sample separation is in the sub-10 nm range, a force sensor which has vertical rigidity orders of magnitude larger than conventional AFM cantilevers is required. Quartz tuning forks (QTFs) satisfy this condition while also achieving higher sensitivity due to the coupled oscillation of their tines which produces a resonant Q-factor that is typically an order of magnitude larger than conventional AFM cantilevers. However, the coupled oscillation complicates the mechanical modeling and characterization. QTFs exhibit multimodal resonances: although previous models have accurately described the fundamental resonance [2, 3, 4], no model has been able to explain other oberserved resonant modes. The present study aims to create a lumped model capable of explaining all of the mechanical resonances of a QTF and extracting the relevant parameters. The QTF spectrum is measured electrically (via piezoelectric effect) and simultaneously measured optically (via Fabry-Perot interferometry). Electrical measurements are performed with two different lead configurations to separately extract the in-phase and anti-phase oscillations of the tines. Optical measurements are performed in two orthogonal directions to extract the true amplitude of the in-plane and out-of-plane oscillations. By combining these measurements with an in-plane model and an out-of-plane model, the lumped parameters can be extracted. Preliminary measurements indicate that the coupling is an order of magnitude larger than previously reported [3]. Complete characterization of the QTF will enable predictions of the behavior of the QTFs when a tip-sample interaction is present. This will allow quantitative extraction of the interaction force at sub-10 nm gap separations.

References:
[1] Sader, John E., et al. "Quantitative force measurements using frequency modulation atomic force microscopy—theoretical foundations." Nanotechnology 16.3 (2005): S94.
[2] Babic, Bakir, et al. "Mechanical and electrical characterization of quartz tuning fork force sensors." Sensors and Actuators A: Physical 223 (2015): 167-173.
[3] Castellanos-Gomez, Andres, Nicolas Agraït, and Gabino Rubio-Bollinger. "Dynamics of quartz tuning fork force sensors used in scanning probe microscopy." Nanotechnology 20.21 (2009): 215502.
[4] Ng, B. P., et al. "An improved dynamic model of tuning fork probe for scanning probe microscopy." Journal of microscopy234.2 (2009): 191-195.

Characterising the structural properties (strain gradients, chemical composition, crystal orientation and defects) inside nanostructures is a grand challenge in materials science. Bragg coherent diffraction imaging (Bragg CDI) can be utilized to address this challenge for crystalline nanostructures. A resolution of the structural properties of less than 10 nm is achieved up-to-date [1]. The capabilities of the Bragg CDI technique will be demonstrated on single nanoparticles for enhanced catalysis.
As an example, the Bragg CDI technique allows understanding the interplay between shape, size, strain, faceting [2], composition and defects at the nanoscale. We will demonstrate that Bragg CDI on a single particle model catalyst makes it possible to map its local strain/defect field and directly images strain build-up close to the facets. The localized strain modifies sorption energies of the reactants. In situ [3] and operando Bragg CDI was also performed: it was possible to track a single particle in liquid and gas phase environments, to monitor its facet changes and to measure its strain response to gas or electrochemical reaction.
This technique opens pathways to determine and control the internal structure of nanoparticles to tune and optimise them during catalytic and other chemical reactions.
[1] S. Labat, M.-I. Richard, M. Dupraz, M. Gailhanou, G. Beutier, M. Verdier, F. Mastropietro, T. W. Cornelius, T. U. Schülli, J. Eymery, and O. Thomas, ACS Nano 9, 9210 (2015).
[2] M.-I. Richard, S. Fernandez, J. Eymery, J.-P. Hofmann, L. Gao, J. Carnis, S. Labat, V. Favre-Nicolin, E. J. M. Hensen, O. Thomas, T. Schülli, and S. J. Leake, Nanoscale 10, 4833 (2017).
[3] M.-I. Richard, S. Fernández, J. P. Hofmann, L. Gao, G. A. Chahine, S. J. Leake, H. Djazouli, Y. De Bortoli, L. Petit, P. Boesecke, S. Labat, E. J. M. Hensen, O. Thomas, and T. Schülli, Rev. Sci. Instrum. 88, 093902 (2017).

Financial support to this work by ANR Charline (ANR-16-CE07-0028-01) is gratefully acknowledged.

The mechanical strengthening of metals has been a long-standing challenge and topic of materials science in academia and industry. Previous studies show that size (grain size or twin thickness) softening could occur at the sub-10 nm scale1-5, i.e. below a critical size, smaller grains or thinner twins lead to the softening of metals, which has been referred to as the inverse Hall-Petch effect. Here we report that size strengthening of nickel continues all the way down to 3 nm. Radial diamond-anvil-cell x-ray diffraction experiments show that ultrafine nickel has larger yield strength than coarser counterparts, and that the size strengthening becomes even stronger in the sub-10 nm size range. Computational simulations reveal that partial dislocation-mediated strengthening takes over the full dislocation-associated Hall-Petch mechanism below a critical grain size, resulting in further strengthening at the sub-10 nm scale. Partial-dislocation-associated stacking faults and nano twins are seen in the transmission electron microscopy examinations of the deformed sub-10 nm nickel samples, providing compelling evidence for the strengthening mechanism shift. Ultrahigh strength of materials can be potentially achieved through size strengthening, a significant advance for industrial applications of nanomaterials.

Acknowledgement: The authors acknowledge support of National Natural Science Foundation of China (Grant Nos. 51527801 and U1530402) and that of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
REFERENCES:
1. Schiøtz, J., Tolla, F. D. D. & Jacobsen, K. W. Softening of nanocrystalline metals at very small grain sizes. Nature 391, 561 (1998).
2. Schiøtz, J. & Jacobsen, K. W. A maximum in the strength of nanocrystalline copper. Science 301, 1357-1359 (2003).
3. Lu, L., Chen, X., Huang, X. & Lu, K. Revealing the maximum strength in nanotwinned copper. Science 323, 607-610 (2009).
4. Chokshi, A., Rosen, A., Karch, J. & Gleiter, H. On the validity of the Hall-Petch relationship in nanocrystalline materials. Scripta Metallurgica 23, 1679-1683 (1989).
5. Conrad, H. & Narayan, J. Mechanism for grain size softening in nanocrystalline Zn. Applied physics letters 81, 2241-2243 (2002).

Because of unique sensitivity to lattice, Bragg coherent x-ray diffraction imaging (BCDI) has been employed on various nanoscaled materials such as metal, metal oxide, and mineral, to image three dimensional map of strain distribution and lattice distortion with non-destructive measurements. In-situ and operando imaging became a major driver for BCDI to address scientific questions on physics, chemistry, and materials science in recent years.
In this talk, I will introduce current state-of-art of BCDI and recent experimental results on in-situ and operando BCDI. High temperature BCDI reveals annealing effect on gold grain on thin film and relaxation of strain inside quantum materials such as nanodiamond and silicon carbide. Strain and dislocation development has been observed on palladium during phase transition under hydrogen flow and on gold-silver alloy for dealloying process. In addition, some estimates of BCDI in the future will be discussed.

Nanocomposites of light-weight metals and ceramic Nanocrystals (NCs) as reinforcements are an important class of structural materials. Compared to other ways of fabricating these materials, liquid metal route results in nanocomposites with superior mechanical properties, greater industrial compatibility and chemical versatility. From thermodynamic point of view, introducing ceramic NCs directly into liquid metal is challenging because surface tension of liquid metals greatly exceeds that of ordinary solvents. Hence, in order to satisfy the wetting condition, strong chemical bonding between NCs and liquid metal is required. At the same time, the interfacial reaction should not compromise thermodynamic stability of the NCs. Among existing ceramic NCs, the recently discovered 2D transition metal carbides (MXenes) offer a unique solution to fabricating next generation light-weight nanocomposites due to their high-strength (Young Modulus 0.33 TPa), metal-like conductivity, solution-processability and surface group tunability. Here we demonstrate that 2D sheets of titanium carbide (Ti₃C₂T_x, where T_x stands for surface groups such as =O, -OH and -F) can be successfully integrated into ultra-light-weight Mg-Li alloy (final density ρ ≈ 1.8 g/cm³) via the liquid metal route. 2D sheets are uniformly distributed throughout the Mg-Li alloy matrix as evidenced with FIB-SEM and TEM analysis. The recovered NCs retain 2D morphology and ultrathin thickness of the starting MXene sheets (≈1 nm) and can be preliminary indexed as a face-centered cubic Ti₃C₂ with the preferred (111) orientation. Interestingly X-ray diffraction analysis of the composite shows that Ti₃C₂ NCs undergo an in-situ phase transition to a body centered tetragonal phase probably due to the strain from the metal matrix. The composite shows enhanced specific yield strength and Vicker's hardness over the base metal without compromising base alloy's malleability. The same composite in its liquid state exhibits gel-like viscoelastic properties associated with networks of MXenes spanning the entire sample volume. Formation of particle networks is believed to be responsible for the lack of particle phase segregation during processing in liquid metal and melt solidification. MXene induced liquid metal gelation is proposed as a general route to fabricating uniform light-weight nanocomposites suitable for semi-solid metal processing.

PARC has recently invented a novel composite, Chemically Linked Graphene Networks, wherein graphene sheets are chemically bonded to each other by flexible polymer linkers. Inter-particle chemical bonding restricts their movement under stress. This feature cannot be achieved with conventional particle-filled polymer composites in which the dispersed particles can move easily within the polymer matrix on impact or load. Chemically linked graphene networks produced cured architected structures with remarkably high strength when compared with the base polymers or with conventional graphene composites. These structures are enabled by a new, low cost synthetic approach to highly dispersible, chemically reactive functionalized graphene particles. In addition to enhanced mechanical strength, we have found that this material possesses additional valuable properties including outstanding gas barrier performance and high thermal conductivity. The technology is targeted for direct 3D printing of multifunctional lightweight load-bearing structural parts that may be used as a direct carbon-fiber reinforced polymer replacement, or as a replacement for some metal parts in industries such as aerospace and automotive.

Observations of strained nanocrystalline materials have shown that discontinuous grain growth can occur at room temperature with as little as 2% strain [1]. Understanding such changes in nanocrystalline material properties at room temperature is vitally important and has, until now, required detailed TEM observations of the resulting microstructure and inferences regarding the processes involved.

Transmission Kikuchi diffraction (TKD) in the scanning electron microscope (SEM) has advantages in resolution over conventional electron backscatter diffraction that have resulted in a wide range of applications of TKD across the materials and Earth sciences [2]. However, to date TKD has not been applied to the study of dynamic processes, utilizing in-situ deformation. Here we present results from a prototype in-situ tensile deformation stage that has been custom modified for use with TKD, enabling the investigation of discontinuous processes on the nanoscale, such as grain boundary sliding, twinning and stress-assisted grain growth.

Nanocrystalline copper samples were deposited with a range of grain sizes and thicknesses, and these were then separated from their substrate and floated onto TEM-compatible push-to-pull (PTP) holders (Hysitron Inc., USA). A focused ion beam (FIB) was used to shape the film into a suitable dog-bone configuration, so as to enable analysis in transmission. Samples from a surface treated steel were also prepared via FIB lift-out and attached to a PtP device using platinum deposition. In both cases, the PTP holder was then mounted on a custom-modified Hysitron PI-85L in-situ nanoindentor, designed for tensile testing of thin film samples with simultaneous TKD mapping. The experiments were carried out using a Zeiss Ultra Plus field emission SEM equipped with an Oxford Instruments AZtec EBSD and TKD system. Tensile tests were carried out using standard TKD conditions (typically 30kV accelerating voltage, 5-10 nA beam current, step sizes ~ 5 nm and mapping speeds ~ 66 Hz), with loading up to ~ 500 uN. In all cases in-chamber plasma cleaning enabled multiple scans of the same area of the sample by minimizing contamination.

Multiple nanocrystalline Cu film with a bimodal grain size and a thickness of 55-75 nm was deformed up to failure (up to 450 uN depending on specimen size),and mapped 5-8 times using TKD at different strains. A surface treated steel film with a thickness of 100-200 nm was also deformed up to failure (at 120 uN) and mapped 8 times using TKD at different strains.

The results show a variety of different nanoscale processes, including grain growth, sub-grain formation and detwinning in the nanocrystalline copper and twinning and significant intragranular deformation in the surface treated steel. Although the preference for different nanoscale processes at different locations has not yet been linked to the microstructure in this case, this shows the potential of this approach characterisation of nanostructural changes throughout a deformation experiment using TKD. In this example we show how the TKD mapping was able to track the changes (or lack of changes in places) in the grain structure and the eventual failure. The system was sufficiently stable to enable accurate quantification of nanoscale changes.

We believe that this in-situ TKD technique can become an effective tool for understanding the complex nature of discontinuous stress-assisted grain growth, allowing us to identify the exact nature of the grains and boundaries that are most affected by this mechanism.

References:
[1] D.S. Gianola et al., Acta Materialia 54 (2006) p. 2253–2263.
[2] G. Sneddon et al., Materials Science and Engineering R: Reports, 110 (2016), p. 1-12.

Nanocrystalline (nc) metals with grain sizes smaller than about 100 nm exhibit outstanding mechanical strength and fatigue properties compared to their coarse grained (cg) counterparts [1], and are promising candidates for application as structural or functional materials. Thin nc metal films in particular are employed as hard coatings on bulk components, structural components in MEMS/NEMS and conductive layers in microelectronic devices [2]. These structural components and devices are often subjected to cyclic stresses or fatigue loading. Aiming at ensuring a high reliability and mechanical robustness for optimum performance of these components, there has been a strong motivation for understanding the mechanical properties and governing deformation mechanisms in nc metals.

Nc metals have been reported to exhibit an extended microplastic regime during loading unlike in case of the cg metals that show a macroscopic yielding already at 0.2% strain [3]. In addition, these materials have been shown to exhibit Bauschinger effect (BE) during unloading, where a pronounced reverse plastic deformation is observed [4]. So far, such an effect has been explained by stress gradients developing between differently sized grains or by back stresses that develop due to piling up of dislocations at grain boundaries that aids their reverse motion during relaxation [5,6]. However, in case of nc metals it is still a subject of open discussion whether the deformation processes are mainly dislocation or grain boundary mediated. Thus, in the context of cyclic loading-unloading, we intend to explore the deformation mechanisms that contribute to the BE in these class of materials.

We performed in situ cyclic loading-unloading experiments on 100 nm thick sputtered nc Pd thin films inside a TEM. The stress strain curves show a nonlinear unloading deviating from ideal elastic unloading behavior, which is a characteristic of the BE reported in case of thin metal films. The corresponding bright field TEM (BFTEM) series show local contrast changes in the grains, which are partly reversible during unloading. Aiming towards a quantitative understanding of these events, we used automated crystal orientation mapping in STEM mode (ACOM-STEM) in conjunction with quasi static loading-unloading experiments. Analyses of series of HAADF-STEM images that were acquired along with the ACOM series indicated reversible contrast changes in several grains indicating changes in crystal orientations. This was confirmed from the quantitative analyses of the ACOM series consisting of a large number of grains that indicated grain rotation. The average rotation was found to progressively increase during loading while a significant number of grains showed complete or partially reversible rotations on unloading. Local analyses of the STEM series also shows that the contrast changes traverse multiple grains in direct vicinity indicating that these rotations are likely to be operating in a co-operative manner. Relating these effects to the observed mechanical response of the sample, we conclude that grain rotation is a potential mechanism contributing to the BE in nc metals. With our in situ ACOM approach, we are developing a closer link of the competing deformation mechanisms and the microstructural heterogeneity in terms of grain size, crystallographic texture and grain boundary character to further elucidate on the observed mechanical response of nc metals.


[1] K.S. Kumar et al., Acta Materialia, 2003, 51, 5743.
[2] E. Hosseinian et al., Nanoscale, 2013, 5, 12532.
[3] S. Brandstetter et al., Advanced Materials, 2006, 18, 1545.
[4] J. Rajagopalan et al., Scripta Materialia, 2008, 59, 734.
[5] J. Rajagopalan et al., Acta Materialia, 2010, 58, 4772.
[6] F. Mompiou et al., Acta Materialia, 2012, 60, 3402.

The creation, clustering, and ordering of oxygen vacancies (VOs) in perovskite-structured oxides (ABO₃, B being a transition metal) give rise to a special class of materials, such as Brownmillerite (BM) structured SrCoO_2.5, SrFeO_2.5, and rhombohedral structured SrCrO_2.8. A topotactic phase transition (e.g., SrFeO_2.5 ↔ SrFeO₃) can occur as a result of oxygen content variation, which leads to dramatic changes in their functional properties, such as electronic conductivity, ionic conductivity, and electrocatalytic reactivity. Understanding the detailed structural and phase transition processes is critical for the rational design of functional energy materials that are used in batteries and solid oxide fuel cells. In this study, using in situ environmental transmission electron microscopy (ETEM), we directly imaged the oxygen diffusion and phase transition processes in epitaxial BM-SrFeO_2.5 thin films at elevated temperatures. The high spatial and temporal resolution of the TEM allowed us to correlate the onset of phase transition, reaction rate, and transition pathways with the annealing temperature. The phase transformation from BM-SrFeO_2.5 to perovskite SrFeO_3-d, triggered by the intercalation of O ions, exhibited a clear reaction front, indicating the oxidation of FeO₄ tetrahedra to FeO₆ octahedra occurs preferentially at the phase boundaries, which is supported by our density functional theory calculations.

Controlling the micro/nanostructure of thin films would enable us to explicitly tailor their mechanical behavior. Here, we describe a new process to synthesize thin films with precise microstructural control via systematic, in-situ seeding of nanocrystals, and subsequent crystallization of amorphous precursor films. Using this process, we synthesized austenitic NiTi films with submicron grain sizes. We then co-fabricated freestanding samples of the films with MEMS testing stages and performed cyclic tensile load-unload experiments. Samples seeded with Cr showed a high phase transformation stress (> 700 MPa) during the first cycle that increased even further during subsequent cycles. More interestingly, the film showed a continuous decrease in stress-strain slope during unloading rather than the pseudo-elastic behavior typically observed in microcrystalline NiTi. Preliminary in-situ TEM straining studies suggest that this unusual loading behavior is caused by a combination of reverse phase transformation and reverse plasticity.

The topic of soiling of photovoltaic module (PV) and concentrated solar power (CSP) collectors has recently gained increasing attention due to its impact on solar power production, especially in arid and semi-arid areas with high concentrations of airborne dust. Soiling describes the deposition of dust and other contaminants on surfaces, reducing solar irradiation by absorbing or reflecting the sunlight, causing energy yield losses which can exceed 1 % per day. The amount of soiling is influenced by complex interactions of many factors which can vary significantly from site to site. Dust accumulation and corresponding soiling rate is determined by the interaction of particle deposition, rebound and resuspension, which are strongly influenced by particle adhesion forces and removal forces.

A focus is made on soiling mechanisms that increase particle adhesion such as cementation, particle caking and capillary aging. A detailed analysis and model of dew-driven soiling mechanisms of cementation, particle caking, and capillary aging are given on the basis of microstructural material and dust characterization of outdoor exposed glass samples. To identify the underlying physical and chemical processes of cementation, glass samples were exposed to outdoor conditions in various locations (Qatar, Chile, Morocco) and investigated at microstructural level using electron microscopy (SEM and TEM) and elemental analysis with energy-dispersive X-ray spectroscopy (EDX). For samples from Qatar, the formation of nanoscopic needle-like structures was observed, which can be attributed to the clay minerals and somehow contradicts the common perception that cementation occurs via the precipitation of salt.
Overall the study aims to provide a useful, in-depth but also global overview and fundamental understanding of soiling processes on PV modules down to the microscale, which could be used to develop appropriate soiling mitigation strategies.

Fracture of crystalline silicon solar cells in photovoltaic modules is a big concern to the photovoltaics (PV) industry. Cell cracks cause performance degradation and warranty issues to the manufacturers and hence, they are a limiting factor to the applicability of silicon PV as a main stream power source with high long-term reliability. In such a scenario, it is important to evaluate and quantify the fracture of the crystalline silicon cells to address it. The roots of the cell cracking process lie in the manufacturing and integration process of the cells and modules as they go through a series of elevated temperature and pressure processes, involving bonding of dissimilar materials, causing thermomechanical stresses, thus leading to cell fractures. Evaluation of the mechanics of these thermomechanical stresses and the role of different materials and processes in a sequential manner is highly essential to quantify these stresses and optimize the PV modules to address them and hence, the cell fractures. We present a novel synchrotron X-ray microdiffraction based techniques combined with finite element modelling to evaluate and characterize the stress and fracture in the crystalline silicon PV modules. Our results show that the thermomechanical stress builds up over the module integration processes such as soldering and encapsulation. We show the detailed stress evolution at each step of the lamination process, using the finite element simulations, validated by the synchrotron X-ray microdiffraction results. We further show the effects of the material and process parameters which affect the cell stress and thus, could be used to reduce the cell fractures and pave the way for the next generation crystalline silicon solar PV modules with enhanced fracture resistance.

The existing performance limitations of Li-ion batteries can be tracked to slow transport and irreversibilities in the changes undergone by the electrode materials, often coupled with mechanical events such as fracture. Tools that provide insight into the onset and propagation of these transitions are critical to identify the mechanisms of electrochemical function. This information must be generated at the level of single particles, where irreversibilities trigger degradation of the electrode architecture. Synchrotron-based X-ray microscopy combines morphological observations at high spatial resolution with a suite of chemical probes, such as diffraction and spectroscopy. Therefore, it is ideally suited to relate electrochemical observations with changes in nanoscale chemistry, linked to mechanical degradation. In this talk, we will justify the need for sophisticated connections between these phenomena, and we will proceed to discuss recent developments in X-ray microscopy aimed at visualizing them in single particles of battery electrodes. The talk will highlight the new fundamental insight generated by the tools, involving local dynamics of heterogeneity and, where relevant, their relationship with fracture events. While significantly valuable insight will be gleaned from samples harvested at relevant points of the electrochemical reaction, these measurements create uncertainty due to the possibility of relaxation of components from the kinetically controlled functional state to one that is more stable under open circuit conditions. Therefore, we will introduce our latest efforts to carry out time-resolved measurements simultaneous to the electrochemical reaction.

Nanostructured alloys are currently of much interest for their potential applications in extreme environments. Most research has focused on their high strength and ductility, however, for high temperature applications, their susceptibility to creep damage is also important, and much less work has been performed on this topic. Creep may be significant in these materials as diffusional creep scales inversely with grain size, e.g., Coble creep varies as 1/r ³ , and in situ creep measurement is one of the technique highly demanded these days.
In the current work we have examined grain growth and creep behavior in Al alloys- AlW, AlSc and AlSb- using our in- situ creep measurement apparatus. We prepared our alloys as thin films using magnetron sputtering. These films were stabilized at a grain size ~150 nm by first annealing them at 250°C. Creep measurements were performed using a bulge test (JJ Vlassak and WD Nix, J. Mats. Res. 7, 3242, (1992)); the grain size and precipitate structure were examined by transmission electron microscopy. The creep measurements revealed that the creep rates increased linearly with applied stress, while the temperature dependence showed approximately Arrhenius behavior with an activation enthalpy of 0.2 ~ 0.5 eV, which is somewhat lower than that of grain boundary diffusion in large grained Al.

Microstructure design at the nanoscale affords tailoring materials behavior by mixing interfacial contributions into the otherwise bulk-dominated behavior. The strengthening at small scale has been linked to interfacial effects, as have observations of stiffening or enhanced compliance in nanoscale objects. Yet, the underlying mechanisms remain under investigation. While numerical studies provide a wealth of suggestions, meaningful experimental verification is more elusive. This talk discusses experiments designed to verify the suggested contribution of capillary forces to the plastic and elastic response of small bodies. Dealloying-made nanoporous metals provide model systems in the form of homogeneous metal networks with struts or ligaments at the nanoscale. The mm-size nanoporous bodies are highly deformable in compression and can be tested with reliable macroscopic methods. In-situ tests of such samples in electrolyte and control of the electrode potential afford variations of the surface state during ongoing deformation processes. Typically, the variation of surface state tension and surface stress are known; the response of the mechanical behavior then affords robust conclusions on the role of capillary forces in small-scale mechanics, including their impact on strength, compression-tension asymmetry, and size-dependent stiffness. At the same time, electrochemical cycles can be tuned to provide a stringent control over the purity of the surface, which is rarely available in experiments in the field.

The intricate deformation and failure of seamless gradient nanoglasses are investigated with tensile loading large-scale molecular dynamics simulations. Cu₆₄Zr₃₆ metallic glasses are used as parent alloys for the construction of functionally graded nanoglasses with grain sizes from 3 to 15 nm. Tensile loading simulations are used to characterize the mechanical behavior of the heterogeneous metallic glass and shed light on the plastic deformation mechanisms. Results indicate a strong localization of the plastic deformation beyond the yield point for loading parallel to the gradient direction in soft regions with high concentration of reduced grain sizes. Loading perpendicular to the gradient direction indicates overall significant delocalization of plasticity throughout the sample. Results indicate a synergy of the gradient design and a superior combination of strength and ductility for gradient nanoglasses with a hard core (large grain size) compare with gradient nanoglasses with soft core (small grain sizes), in agreement with reports for gradient nanocrystalline systems.

In recent years, polymer matrix nanocomposites have gained an increased amount of interest due to the scalability and manufacturability provided using polymers and the highly desirable properties from their fillers. The incorporation of ceramic, metallic, and/or organic fillers have given way to a variety of applications by enabling the improvement of thermal, electrical, mechanical, and dielectric properties. Such applications include, but are not limited to, energy storage materials, piezoelectric pressure sensors, and self-healing materials. Due to their high electrical conductivity and mechanical properties, carbon nanotubes (CNT) have attracted a great amount of attention for use as nanocomposite reinforcement; however, the fabrication of composites of this nature through additive manufacturing is a concept that has not been explored thoroughly. CNT have the tendency to agglomerate and become points of failure for composites rather than the reinforcements. For this reason, additive manufacturing of CNT-polymer nanocomposites typically uses low percentages of CNT. In this study, the synthesis and fabrication of multi-walled carbon nanotubes (MWCNT) composites was realized through stereolithography additive manufacturing with in situ, electric field-assisted dispersion. With the introduction of MWCNTs into the polymer, a 77% increase in elastic modulus was observed. Furthermore, in situ dispersion through electric field improved the average elongation at break of the samples with MWCNTs by 32%. An increase in ultimate tensile strength of the MWCNT from in situ dispersed composites of 42%, more than 20% over the pure polymer material, was also observed when applying an electric field. Engineering of the magnitude and direction of electrical conductivity in MWCNTs was accomplished through the application of in situ electric fields during the 3D printing process. The electrical conductivity of the material saw an increase of 50% with the introduction of MWCNTs which improved further by 26% with electric field induced dispersion. The feasibility using in situ electrical field in 3D printing to tune electrical and mechanical properties of MWCNT reinforced, polymer nanocomposites was demonstrated.

Advances in material science and engineering has bolstered technological advancements of the twenty first century. Nature constantly provides solutions to overcome materials and structures related obstacles. Cuticle of Mantis shrimp dactyl club, scarabaei, lobster, and epicarp of diplazium crenatoserratum and pollia condensata fruit, have inspired scientists to mimic their microstructure for enhanced optical and mechanical properties. The structural arrangement of the fiber layers plays crucial role in enhancement of toughness of the composite. In past, attempts have been made to design helicoidal structures at macroscopic levels. We have successfully fabricated helicoidally oriented fibers in microns range (20-25 microns) using near field electrospinning. For our first study, melt near field stage-controlled electrospinning was used to assemble dense Polycaprolactone (PCL) fibers (150-200μm) stacked helicoidally at 15° angle offsets. The toughness of the surface-treated helicoidal PCL was found to be two times higher than the surface-treated unidirectional sample and five times higher than the helicoidal sample without surface treatment. Alternatively, in our second study, near field solution electrospinning was used to fabricate much smaller (av. ~22μm) Polyvinylidene fluoride (PVDF) fibers fused together and oriented at angle offsets of 15°. Numerous interface changes between different planes and directions, arrests crack propagation in helicoidal layers, allowing material to absorb more energy before fracture. These fibers show superior tensile characteristics, impact resistance and toughness as compared to its bulk counterparts. Near field electrospinning provides a promising future pathway to fashion helicoidal architectures in nano range increasing the toughness and impact resistance of the materials furthermore.

Polymer fibers usually possess high tensile strength, anti-corrosion, and heat resistance properties. Combining them with “nanofillers”, such as graphene, would further introduce and enhance properties including mechanical reinforcement, energy dampening, electrical conductivity and thermal dissipation. Most of the previous works in this area involved homogeneously mixing graphene or graphene oxide (GO) dispersions in polymers, following by chemical or thermal reduction. However, the finalized graphene nanoparticles are not, strictly speaking, single-layered due to their tendency to aggregate during the mixing process and forming defects and unoxidized functional groups during post reduction. Besides, graphene particles are not always continuously bridged and their orientations are not anisotropic, leading to the limited mechanical strength, thermal and electrical conductivity in composite fibers.
We are currently working on a cost-efficient method suitable for mass production of single-layered graphene-included polymer composite (GPC) fibers through an in-situ graphite exfoliation method. During the fabrication process, spinneret was designed and fabricated to include polymers and original graphite in different channels. The relative molecular movements on two sides of the graphene channel can initiate and promote graphene exfoliation and orientation in a higher degree than those composite fibers with only polymers appearing on one side of the graphite. Upon stretching the spun fibers, polymer channels separated by the graphene channel will generate shear stresses, the degree of which will depend on polymer-graphene interfacial bonding and relative displacements of molecular chains on both sides of graphene layers, along with the graphene channel direction. Mechanical tests showed an increase of both modulus and strength (>200%) as comparing multi-phase composite fibers with pure polymer fibers. Scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD), thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC) were used as main characterization methods. We believe these properties, along with mass scale production, of GPC have extensive applications in automobile, aerospace, sensors, energy devices, displays, thermal exchangers, filters, catalysts, just to name a few.

Conductive composites are being investigated for use in applications such as electromagnetic shielding. Typically nanoengineered composites can have varying electrical conductivities and this variation can be correlated to the microstructure of the corresponding nanocomposites. In this work, conductive composites consisting of polyurethane acrylic and nickel nanoparticles embedded in it were fabricated and tested for their electrical conductivity. In the production process, half of the suspensions that the conductive composites came from were sonicated and half of them were not sonicated. After the conductive composites were tested for electrical properties with a four-point probe, EDS images were acquired of the top and bottom surfaces of four conductive composite samples from each of the six batches of samples. There were three concentrations of nanoparticles that were sonicated and three more of the same concentrations that were non-sonicated. The EDS images of the same sample were matched to the conductivities that corresponded to that sample. As a result, each sample had a group of images and conductivities. Each group of images and conductivities was compared to the groups of images and conductivities of other samples to assess the effect of the production process on the microstructures of the conductive composites and their resulting conductivity properties.