Symposium MA04 : Advances in Additive Manufacturing—Materials, Processes and Devices

A great host of nanoscale amounts of matter (nanoparticles) with thermal, optical, mechanical and electrical properties and functionalities drastically different than those of their counterpart bulk materials are often available in colloidal solution from. Such nanomaterials are of critical importance to the development of technologies in many fields, ranging from energy and transportation to biology, electronics and photonics. There is a pressing need for novel, facile, maskless, high yield methodologies for their assembly, handling, characterization and device integration. Here, a remarkably simple process for the maskless direct printing of nanoparticles of all kinds through electrohydrodynamic NanoDrip printing will be presented and the related physics and thermofluidic transport phenomena leading to the tunable formation of in- and out-of-plane functional nanostructures as single entities or large arrays will be explained. Subsequently, a host of demonstrated applications enabled by NanoDrip printing will be discussed ranging from plasmonics, to light design through the controlled printing of quantum dots, to the printing of transparent conductive grids for solar cells and touch screen displays and to nanoscale force sensing devices for cells with unprecedented resolution. Through this, the great potential of EHD NanoDrip printing as an emerging advanced manufacturing methodology, alone or in an additive manner will be illustrated.

Two-photon lithography is a polymerization based direct laser writing technique that is capable of printing complex 3D parts with submicron features. During this process, submicron features are written within the interior of a photo-responsive resist via localized polymerization chemistries driven by nonlinear two-photon absorption. This capability has enabled researchers from diverse fields to fabricate functional micro/nano scale structures for applications such as photonic crystals, optical and mechanical metamaterials, microfluidics, miniaturized optics, and flexible electronics. The most widely implemented form of this process involves serially scanning a tightly focused laser spot in space to generate 3D parts. This serial scheme severely limits the scalability of the process. In addition, material scalability is limited by the optical constraints for transparency of the resist and refractive index matching with the focusing objective lens. These optical constraints limit printing of tall millimeter scale structures with submicron features to only a small set of resist materials. Herein, we have overcome rate and material based scalability limitations via (i) parallelization of the process that increases the rate by at least 50 times without adversely affecting the resolution or the depth resolvability observed in serial writing and (ii) modifications to the focusing optics that reduces the sensitivity of the process to index mismatch and resist opacity.

Although past attempts to parallelize two-photon lithography have been successful in increasing the rate of the process, those implementations have reduced the ability to fabricate complex 3D parts. Specifically, past demonstrations have either printed (i) the same feature into a periodic structure or (ii) an extrusion of an arbitrarily complex 2D plane (without depth resolvability). Here, we have overcome this scalability versus part complexity tradeoff by implementing a projection-based parallel writing scheme for printing of arbitrarily complex 3D parts. In this scheme, an image of an array of individually actuated micro-mirrors is projected onto a plane interior to the resist. We have achieved depth resolvability by ensuring that the femtosecond pulsed laser beam is both spatially and temporally focused. In addition, we have modified a commercial objective lens in such a way that the light propagation path length through the resist is only a fraction of the objective’s working distance. This has enabled us to print tall millimeter scale structures with index mismatched resists (such as acrylate monomers with fumed silica particles) via the dip-in printing mode. In combination, our work (i) increases the rate of two-photon lithography by a factor of at least 50 without adversely affecting feature resolution and (ii) broadens the applicability of dip-in printing mode to non-index matched resist materials.

Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-740672.

We report a high-resolution printing technique of Ag-nanowire (AgNW) -based electrodes on 1-µm-thick polymer films, which enabled the fabrication of transparent flexible organic thin-film transistors (OTFTs). All electrodes in the OTFTs were fabricated with this technique, where alcohol-based AgNW dispersion was applied on hydrophilic/hydrophobic-patterned surface. The processing temperature below 120°C allows the device integration with ultrathin (~1 µm) and transparent organic materials. Thus, the OTFTs exhibited mechanical stability under a bending radii of ~1 mm (~0.6% strain) and a visible transmittance around 80%.
Flexible transparent electronics will pave a way for novel optoelectronic applications such as paper-like displays, flexible touch panels and smart contact lens [1]. In this trend, AgNW-based transparent electrodes are expected as an alternative material to indium-tin-oxide (ITO) [1] owing to their mechanical flexibility, high conductivity and optical transparency. In addition, they can be fabricated with printing techniques such as ink jet, gravure, and screen printing, whose low-temperature processabilities can facilitate device integration with flexible polymer materials. However, printing AgNW-based electrodes with widths/spacings of less than 100 µm remains challenging.
The present work demonstrates the feasibility of printing AgNW-based electrodes with widths/spacings down to 20 µm in high accuracy of less than 3 µm, contributing to flexible transparent electronics. The AgNW-based electrodes exhibited bending stability to 1% strain. Capitalizing on such electrodes, we developed the transparent flexible OTFTs. First, AgNW-based electrodes with 50 µm width were patterned as the source/drain electrodes with channel lengths down to 25 µm on a 1-µm-thick film of poly(p-xylylene) (parylene). To form the active layer, 30-nm-thick layers of a benzothienobenzothiophene derivative (C8-BTBT) [2] was deposited on the source/drain electrodes. After that, a parylene layer with ~400 nm thickness was deposited as the gate dielectric. The AgNW-based gate electrodes were patterned on top of the gate dielectric. Finally, a 1-µm-thick encapsulation layer of parylene was deposited. The completed OTFTs exhibited an on/off ratio of ~10⁶ and a hole mobility of 0.4 cm²V^-1s^-1. Furthermore, they had a high visible transmittance around 80% and withstood a bending radii of ~1 mm. Due to these versatile performances, it is implied that the OTFTs based on printed AgNW-based electrodes can serve as an alternative device to recent p-channel transparent TFTs with oxide semiconductors [3] for large-area and wearable optoelectronic applications.
References
[1] T. Sannicolo et al., Small 12, 6052 (2016).
[2] Y. Yuan et al., Nat. Commun. 5, 3005 (2014).
[3] Z. Wang et al., Adv. Mater. 28, 3831 (2016).

3D printing of microstructures with controllable surface wettability and optical properties is valuable to both fundamental research and practical applications. Such 3D structures are envisioned for microfluidic devices, micro electro-mechanical devices, optical components, biomedical and tissue engineering applications. However, poor optical transparency and need of additional surface modification step are obstacle in printing multi-functional 3D structures. Here, we propose an in-situ photoinitiated copolymerization approach to print multifunctional high resolution 3D microstructures using micro stereolithography. The use of rationally designed copolymerization approach enables us to tailor the wetting behavior (hydrophilic/phobic), while the appropriate choice of photoinitaioter and cross-linker aids in tuning the optical transparency of the 3D printed structures. The versatility of this approach is the use of monomers with hydrophilic and oleophilic moieties to print the structures ranging from very high to very low surface energy, which avoids the interface mismatch after surface modification for optical microscopy imaging. This optimized approach will help us in prototyping submicro-or micro-models with complex geometries and well defined wettability.

Microfabrication is typically limited to 2D, planar designs. Such limitations can be overcome by microscale additive manufacturing (AM) techniques which have been developed in the last decade. Amongst them are several methods for the mask-less 3D deposition of metals [1]. Many of those techniques have proven their value for e.g. the deposition of microscale electronic conductors, optical metamaterials or mechanical components. Yet, there are considerable differences between the individual techniques – minimal feature size, speed or material quality vary notably.
On the one hand, electrochemical microscale AM methods enable the fabrication of materials with dense, nanocrystalline microstructures and excellent material properties. Unfortunately, they share a common disadvantage: average deposition speeds are two orders of magnitude lower than for many other techniques. On the other hand, ink-based techniques are much faster, but often struggle with synthesizing dense metals. Additionally, they often require post-deposition heat treatments that can cause warping and shrinkage of the printed geometries.
Here, we combine the best features of these techniques: the high quality as-deposited material of electrochemical techniques with the high speed of ink-based methods. We present a new approach that combines electrohydrodynamic (EHD) printing [2,3] with electrochemistry. The EHD ejection of solvent provides increased mass-transport, which is shown to result in deposition speeds an order of magnitude higher than previous electrochemical microscale AM methods. The electrochemical reduction provides metallic, conductive, and mechanically stable deposits without any additional processing.
We demonstrate that electrochemical EHD-printing can be used to deposit gold, copper and silver structures of various geometry (lines, pillars, and overhangs) with minimum feature sizes < 500 nm. The obtainable microstructure is polycrystalline with a density varying from ~50% to > 90%, depending on the ejection voltage.
The presented technique is purely electrochemical and requires no inks. Key benefits are the simple working principle of the technique and its potential applicability to a large range of metallic materials. Furthermore, we envision facile multimetal and alloy printing. This could provide valuable tools for engineering the local composition and microstructure and thus properties of additively manufactured parts at the microscale.

[1] A. Reiser, L. Hirt, R. Spolenak, T. Zambelli, Adv. Mater. 2017, 201604211, 1604211.
[2] P. Galliker, J. Schneider, H. Eghlidi, S. Kress et al., Nat. Commun. 2012, 3, 890.
[3] J.-U. Park, M. Hardy, S. J. Kang, K. Barton, K. Adair, et al., Nat. Mater. 2007, 6, 782.

Focused Electron Beam Induced Processing (FEBIP) enables direct-write nanoscale fabrication with a variety of materials. Compared to a similar technique – focused ion beam deposition – FEBIP can achieve much higher resolution, inflicts less surface damage, and involves more accessible tools. However, FEBIP is not widely used due to its limited deposition rate, low material purity, and limitation on a type of precursor materials, which can be delivered to a high vacuum environment of the FEBIP chamber. We are developing a family of multi-mode energized micro/nano-jet techniques as a method of local precursor delivery, which aiming to resolve both issues and to expand the range of useful precursors for applications in FEBIP. In this presentation, we will discuss the fundamentals of several new methods, including electrospray nano-jets and supersonic gas micro-jets, we have developed for delivery of energized precursor molecules into a vacuum environment of the FEBIP chamber. The results of experimental characterization of the jet behavior upon impact on the substrate will be presented with implications to FEBIP deposit growth rate, topology and purity.

Energized micro/nano-jets provide unique capabilities for localized delivery of precursor molecules to the substrate, thus establishing locally controlled deposition/etching/doping site for focused electron-beam induced processing (FEBIP). Not only this enables FEBIP from precursor materials of different kinds, but also affords tuning of sticking coefficients and adsorption/desorption activation energies of participating molecules. The latter is especially critical for substrates, which are sensitive to doping such as graphene, whose electronic properties change by adsorption of different molecules. This avenue for future FEBIP development is most promising from the application prospective, as an emerging multi-functional (electron/photon/molecule beam) FEBID/FEBIE operation establishes an intimately integrated multi-functional processing environment that enables one to define shapes (patterning), form structures (deposition/etching), and modify (cleaning/doping/annealing) properties with locally-resolved control on nanoscale within the same tool without ever changing the processing environment. This, in turn, should allow for (1) increasing the process throughput by minimization of a number of intermediate “handling steps” (which is a deficiency for all beam based techniques as compared to batch fabrication), (2) the possibility to create almost an arbitrarily diverse portfolio of different device structures/functionalities on the same substrate due to “direct-write” nature of the approach, and (3) the capability for local property control/modification while minimizing the parasitic substrate contamination in the course of processing, which is especially critical for graphene-like electronic materials, whose properties are highly sensitive to intended or unintended dopants.

As piezoelectric polymers, Poly (vinylidene fluoride) (PVDF) and its copolymers are attractive in energy conversion applications because of their highly efficient electromechanical interaction as well as their flexibility and biocompatibility. Such unique properties have been widely utilized in various applications such as sensors, actuators and energy harvesters. However, in order to achieve high piezoelectric phase in PVDF, traditional manufacturing methods (i.e. mechanical drawing and electrical poling) restricted the form of PVDF based devices within two-dimensional films. Additive manufacturing techniques of PVDF have been investigated in the past years to achieve three dimensional structured devices, yet challenges still exist in obtaining piezoelectric active phase in 3D printed PVDF and applying electrical poling on complicated structures. Here, we introduce a novel additive manufacturing method to fabricate high piezoelectric 3D structures of PVDF. The developed method utilized a setup combining electrospinning and 3D printing, where PVDF are first electrospun into nanofibers and then accurately deposited on a grounded substrate using an automated stage. During the electrospinning process, mechanical stretching and electrical poling take place simultaneously on the spun PVDF nanofibers through the electric field applied between the printing head and substrate. This allows the printed PVDF forms in high quality β-phase which is most preferred in piezoelectric application because of their plannar chain conformation, meanwhile aligns the dipoles in one direction. In this wat, sufficiently poled PVDF-based piezoelectric devices can be fabricated in only one step. This novel electrospun 3D printing method is expected to break the restrictions in PVDF fabrication and extract more potentials of PVDF as soft piezoelectric materials in advanced applications such as smart skins and artificial muscles.

We present an additive manufacturing technique for deposition of nanotwinned (nt) metallic nanostructures. Nt metals offer remarkable mechanical and electrical properties compared to their nanocrystalline (nv) counterparts. Three dimensional (3D) freestanding nt-Cu structures with various geometries were fabricated using localized pulsed electrodeposition (L-PED) at the tip of an electrolyte containing glass capillary as nozzle. FIB and TEM analysis confirmed the presence of coherent twin boundaries (TBs) within the 3D printed Cu. The material and mechanical properties of the printed structures were investigated, and compared to nc-Cu structure printed using the same technique under direct current (DC). The results revealed that the printed copper was high quality and mostly free of impurities and defects. The mechanical properties of the 3D printed nt-Cu were characterized using in-situ SEM compression tests of the wires with sub-micron diameter. The results showed a flow stress of over 960 MPa, which is notably high for an additively manufactured Cu structure. This capability will be advantageous in different nanotechnology applications, in particular for 3D nanoscale electronic devices and sensors.

There is a consensus, both from commercial suppliers and academia, on the need for precise calibration of interparticle forces to design ceramic inks for additive manufacturing. These forces determine the “printability” of the suspensions by exhibiting a direct effect on the rheological response of the system (e.g, shear thinning, fluid-to-gel transition). The current calibration of these forces relies heavily on electrostatic repulsion and achieves the desired levels by changing the pH of the medium, adding salt, and utilizing polyelectrolyte species that are oppositely charged. Other coagulants, binders, defoamers, and organic solvents might also be present in the formulations, most of the time, in considerable amounts. However, i) organic solvents and other chemicals prevent the use of these inks in public spaces, ii) these solvents cannot provide the scalability and cost-effectiveness of water, iii) concurrent optimization of minimum 2 additives complicates the formulation of the ink, iv) having a high volume of additives raises questions on the precise dimensional control of the final object/feature and usually requires binder removal steps after printing, and v) current lack of systematic study for the formulation of the inks limits the type and nature of the nanoparticles that are to be used in these inks.
To design a single additive that can offer stability and viscosity-control, we utilized a grafted random copolymer by harnessing both electrostatic repulsion and steric hindrance We systematically changed the charge, ionization capacity, and structure of poly(ethylene glycol) (PEG) grafted random copolymer of acrylic acid (AA) and 2-acrylamido-2-methylpropane sulfonic acid (AMPS), to reach almost theoretical particle loadings as calculated by Krieger-Dougherty equation. This particle-specific design of an additive enabled the realization of alumina inks with more than 80 wt. % particle loading with less than 1,5 wt.% use of a single additive. The optimization route that we report here has the potential to provide insights for the design of other ‘single additives’ and massively expand the limited portfolio and performance of ceramic inks for 3D printing.

Metal additive manufacturing (AM) has been gradually accepted by the industry for manufacturing complex functional end-use components. While metal AM processes are increasingly being adopted, the most critical issue the industry encounters now is build failures resulting from residual stress and distortion induced by the laser process. To address these issues, a novel methodology is proposed to optimize the design of support structure, in order to reduce residual stress and ensure manufacturability. First, a modified inherent strain method is proposed and employed for fast prediction of stress and deformation. It is based on thermomechanical modeling at mesoscale and implemented as a one-time static mechanical analysis. In this manner, process simulation can be significantly accelerated and compatible with structural design. Second, lattice structure constrained stress topology optimization, coupled with fast process simulation, is performed to design the support structure. This not only can prevent build failure by limiting the residual stress to below the yield strength, but also reduce material consumption to build the support structure. Further, the self-support nature of lattice structure makes it ideal for support structure design. Once the density profile of support structure is obtained, a reconstruction scheme is employed to realize the variable density lattice structure for practical application. Several examples are used to demonstrate the effectiveness of the proposed algorithm. Both numerical simulation and experiments show that the proposed method can significantly reduce residual stress and ensure successful printing.

Mechanical metamaterials composed of periodic space-filling plate assemblies are designed such that they exhibit an isotropic elastic response at the macro-scale. Moreover, specific configurations are considered with elastic moduli close to the theoretical Hashin-Shtrikman upper bound for porous solids. The fabrication of such structures with standard additive techniques is particularly challenging due to the closed-cell nature of the meso-structures. In this work, a strategy is developed to make closed cell meso-structures through 2-photon polymerization in a direct laser writing system. Prototype metamaterials with relative densities ranging from 10 to 40% are produced with minimal feature sizes of a few microns. Cubic metamaterial specimens are subjected to uniaxial compression loading in a custom-made displacement-controlled in-situ testing device. Aside from confirming the numerical estimates of the elastic moduli, the large strain response of plate-based metamaterials is determined and the density scaling of their specific energy absorption under uniaxial compression is discussed.

Fundamental understanding of the effects of processing parameters on various as-printed material characteristics, such as component dimensions, grain size/morphology/orientation, and defect density/distribution, is essential for critical advances in the development of structural or functional 3-D printed items. A systematic study on powder bed fusion (PBF), selective laser melting (SLM) 3-D printing process has been conducted to investigate the role of volumetric energy density (VED) in as-printed material characteristics. However, the consideration of only the VED (a thermodynamic parameter) will not accurately depict the complex heat and mass transport phenomena, their influence on the melt pool geometry, and, in turn, the development of microstructure and defects. Therefore, a processing-parameter matrix was created covering a wide range of VEDs with a series of iso-VED cases with varying combinations of power and speed. For example, under a same VED condition, comparisons between high power/high speed cases and low power/low speed cases were made for multiple iso-VED cases. In this study, the laser-beam power was varied from 140 to 380 W, while laser scan speed was also varied from about 75 to 1200 mm/s at each respective laser-beam power using 316L stainless steel as a model system. Using synchrotron x-ray microtomography, quantitative analysis of defects (pores, microcracks, and lack-of-fusion defects) was performed and correlated to the microstructure development as a function of the laser power, scan speed, and the VED. The correlations among 3-D printing parameters, their influence on solidification process, microstructure development, and defect characteristics will be discussed. More specifically, in addition to the general understanding of the role of VED inputs during an SLM process, the effect of laser power and scan speed within each subset of iso-VED cases will be discussed in terms of the physics of melt pool behavior and its correlations to the development of grain structure (i.e., size, shape, and orientation) and pore/crack characteristics (i.e., count, size, aspect ratio, and volume fraction). [A portion of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. The abstract is released under LLNL-ABS-740854.]

The removal of support material is a laborious necessity for the post-processing of powder bed fusion printed (PBF) parts, typically removed by machining techniques. These sacrificial supports are included to relieve thermal stresses and support overhanging parts often resulting in the inclusion of supports in regions not easily accessed by machinists. Recent advances have introduced dissolvable metal supports to PBF printed parts through an electrochemical etching process. Dissolvable support is appealing since it reduces the costs and time associated with traditional support removal. However, the speed and effectiveness of this approach is inhibited by numerous factors such as support geometry and metal powder entrapment within supports. To fully realize this technology, it is necessary to model and understand the optimization of support structure design within the context of dissolvable supports.
This presentation will detail an investigation into the impact of various support parameters on trapped powder and time required for support removal. By designing supports to decrease powder entrapment, the time needed to electrochemically etch the supports will be decreased. The specific impacts of hatch spacing, perforation height, and fragmentation separation in Materialize Magics block supports on this process will be discussed. Overall, the findings of this work will provide an effective means of implementing a model into support generation software that enables the generation of block support material that is optimized for minimized necessary etching duration.

Hybrid Aluminum Matrix Composites (HAMCs) have been fabricated using powder metallurgy methods. The reinforcements used were alumina (Al₂O₃) particles and chopped carbon (c)-fibers. The reinforcements were mixed and homogenized in a rotary ball mill. Then the powder mixtures were pressed into a solid mass using uniaxial press and then sintered in a controlled environment. The microstructures were analyzed using scanning electron microscope (SEM) and the different phases formed were detected using X-ray diffraction technology. The mechanical properties such as hardness, tensile stress, and ductility were determined. The initial study found that the aluminum carbide formed in the matrix-fiber interface. This resulted a lower tensile strength of HAMCs. To prevent the carbide formation in the matrix-fiber interface, the fiber reinforcements were coated with metal nano particles.

Architected cellular materials (i.e., single or multi-phase periodic cellular materials with unit cell topology optimized for one or more functionalities) have been extensively investigated over the past two decades, for their potential to achieve combinations of properties unavailable in any existing monolithic material. Historically, the difficulty in manufacturing such materials (except for the simplest, large-scale topologies) have limited investigation and application of architected materials with complex microstructures. More recently, extraordinary advances in additive manufacturing technologies have enabled fabrication of macro-scale architected materials of unprecedented topological complexity and structural hierarchy, allowing a whole new level of mechanical and multifunctional performance, and opening the field of mechanical metamaterials. These manufacturing advances have catalyzed enormous research interest in two areas: (i) the hierarchical design and fabrication of macro-scale architected material with microscale – or even nanoscale – features, which enables translation of beneficial size effects on mechanical properties that only exist at small scale (e.g., strengthening of metals and toughening of ceramics) to a macroscopic material; (ii) the development of novel topology optimization tools that enable architected materials design that take full advantage of the new additive manufacturing capabilities, resulting in multi-phase designs, highly hierarchical designs, designs that circumvent additive manufacturing limitations (e.g., the need for external supports), and designs with non-linear objectives and constraints.
In this talk, I will review recent progress in both areas, with specific focus on the fabrication and optimization of micro/nano-lattices with exceptional specific strength and topologically complex architectures that enable non-linear effective mechanical response from linear elastic constituents.

Additive manufacturing typically necessitates the inclusion of support structures to provide a thermal pathway for heat dissipation and provide a base for overhanging surfaces. Directed energy deposition (DED) is a common method to 3D print metals, chosen since multiple materials can be printed serially. Dissolvable supports are a common method for removing these sacrificial structures in the printing of plastics due to the low effort and time associated with the process. The presented work adapts this process for DED printed structures and models the entire part as a functionally gradient material (FGM). FGM materials are chosen to provide varying properties across the gradient. By printing a gradient transitioning from an austenitic stainless steel build substrate to low chromium steel to stainless steel we successfully lower the corrosion resistance across the gradient.
This presentation will detail an investigation into the resultant properties across the gradient such as microstructure, chromium concentration, and etch rate. Additionally, we will show that incomplete mixing is prevalent within tracks, necessitating the consideration of properties across the gradient, as appose to just the outer layer. These variations across both layers, tracks, and within a single track are important to consider when developing a process for DED printed dissolvable supports.

Selective Laser Melting (SLM) is a powder bed fusion process in Additive Manufacturing (AM). Its freedom of design has given it an edge in the aerospace, automotive and medical manufacturing industry. With its wide range of applications, there has been an increasing demand for new materials to be incorporated in the SLM process. The optimized parameters for each material, such as laser power, laser scanning speed, hatch spacing and powder layer thickness, are commonly derived from experimental trial and error. With many parameters combinations, it can be expensive and time consuming to conduct experiments to find the optimum parameters which will give the best properties for the intended usage of the part. Since SLM requires a good interlayer bonding, the melt pool depth can be a good indicator of how well the current layer fuse with its previous layers. Modelling of the SLM process can be used to aid in this issue. Since SLM is a multi-physics process, there are many considerations when performing the simulations. In this paper, study of the melting of stainless steel 316L using a Computational Fluid Dynamics to observe the melt pool characteristics. As there are many experimental results on SS316L, they are used to validate the accuracy of the model. The simulation model allows the observation of the molten pool flow during the SLM process due to Marangoni’s effect and recoil pressure. Furthermore, different parameters are tested to show their effects on the melt pool and track formation. Different laser beam diameters, laser powers as well as scanning speeds were used to study the effects they have on the melt pool characteristics. The results were used to determine the relationships between these factors and the melt pool characteristics.

We recently demonstrated a simple two-step approach for fabricating net-shaped metallic interpenetrating phase composites (IPCs) with tailorable thermal and mechanical properties. In the first step of this approach, selective laser melting is used to create a lattice preform. Next, this preform is liquid metal infiltrated with a second material that has a melting point lower than that of the lattice. Here we report on the processing-structure-thermal conductivity relations for these additively manufactured composites. Using experiments, periodic homogenization theory and finite element simulations, we show how the mesostructure, porosity, and properties of the constituents influence the effective thermal conductivity. The spatial distribution of porosity is found to strongly influence heat transport, suggesting unit cell geometries and processing strategies that will optimize the thermal conductivity.

Accurate prediction of the material response associated with any laser materials processing technology begins with precise knowledge of the energy coupling mechanisms active during the laser-matter interaction. In laser powder bed fusion additive manufacturing of metal parts, complex hydrodynamics driven by vapor recoil and Marangoni convection lead to liquid metal interfaces that are steeply curved thereby affecting Fresnel absorptivity (near-keyhole mode absorption). Changes in absorptivity due to melt pool and powder motion can lead to fluctuations in energy coupling which drive excursions in melt pool depth, microstructure and local residual stress. Under certain circumstances, vapor recoil can lead to laser keyhole formation during laser powder bed fusion processing which in turn can lead to part pore defects which adversely affect mechanical properties. Furthermore, ejection of material from the melt pool and entrainment of powder from melt vapor flux can generate spatter particles that become incorporated into subsequent powder layers and can lead to lack-of-fusion defects. To clarify the complex physics involved, a combined experimental and simulation effort is required with sufficient energy, spatial and temporal resolution. In the present work, a laser calorimetric test bed is developed equipped with high speed optical and thermal imaging and used to study changes in energy coupling as a function of laser power above the melting point for bulk metal plates and metal powder layers of several commercially-relevant metal powders. Hydrodynamic finite element modeling of the powder bed is used to simulate the melt pool morphology and dynamics, providing insight to energy coupling, keyholing and spatter generation mechanisms. The measurements and simulations taken together offer powerful new insights into the laser powder bed fusion process which might be exploited to improve efficiency and overall process robustness. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This work was funded by the Laboratory Directed Research and Development Program at LLNL under project tracking code 18-SI-003.

Additive manufacturing of Ti-6Al-4V ELI is becoming a leading method of producing orthopaedic devices with complex geometries and for patient–specific implants. Selective laser melting (SLM) is one of the leading choices for powder based additive manufacturing. The fatigue properties of SLM Ti-6Al-4V have been studied for a variety of processing conditions and geometries. However, these tests had varying heat treatments, geometries, R-values, and frequencies. The purpose of this work is to systematically determine the effect of surface treatments and varying structure on the fatigue behavior of SLM Ti-6Al-4V ELI. The monotonic tensile and fatigue behaviors were examined to determine the relationships between topography, porosity, and mechanical performance.
Dogbone test specimens were manufactured by selective laser melting of Ti-6Al-4V ELI (3D Systems DMP320). Three specimen geometries were built: solid, solid with an additional 0.5mm porous layer on all sides, and a 65% porous gage section. The porous structure was based upon a diamond lattice. All samples underwent a hot isostatic press. Then, samples underwent one post-processing treatment: as-built (no treatment), rotopolishing, or SILC cleaning. All samples were subsequently anodized according to SAE AMS 2488D Type 2. Samples were tensile tested to failure at a displacement rate of 1 mm/min using a MTS Satec 20 kip servo-controlled, hydraulically-actuated test frame. Fatigue tests were run at increasingly lower stress levels below the yield strength of the samples to generate fatigue curves and to determine the endurance limit. Fatigue tests were run on the same MTS Satec frame in axial stress control at a frequency of 5 Hz with R=0.1. Tests were run until failure or runout, which was defined as greater than 2,000,000 cycles. Roughness was measured with a laser confocal microscope.
Rotopolishing and SILC cleaning improved the failure strain (15-21%) of solid and surface porous samples due to the decreased roughness, 0.38 μm and 1.27 μm, respectively. As expected, porous samples had lower properties compared to solid and surface porous samples, regardless of post-processing treatment. Fatigue strength was dependent upon surface roughness, where the smoothest surface (0.38 μm) had the highest fatigue strength (450 MPa). As surface roughness increased (0.38 to 6 μm), the fatigue strength decreased from 450 MPa to 200 MPa due to increased number of sites for crack initiation. Once the porous structure is the dominant structural feature, the impact of post-processing treatment on fatigue is minimal. For SLM devices under high loading conditions, surface porosity can be added without a large decrease in monotonic properties, but fatigue strength will decrease due to the porosity even with a post-processing treatment. These results will be discussed in the context of an additively manufactured, FDA 510(k) cleared, orthopaedic device for foot and ankle applications.

Metal additive manufacturing processes using powder bed fusion have made incredible advances over the past decade. Many of these advances have been motivated by the needs of the aerospace community. While powder bed approaches are perfect for small or medium size parts, the extremely high cost of metal powders is a significant concern when large scale aerospace components are needed. Consequently, there has been a surge in metal AM research involving processes that use less expensive wire feedstock materials. The magnetohydrodynamic (MHD) liquid metal jetting process developed by Vader Systems is one such process. In MHD droplet jetting, an electromagnetic coil surrounds a reservoir where commodity metal wire is fed in and melted. Pulsed electromagnetic fields from the coil induce Lorentz forces in the electrically conductive molten metal that result in jetting of discrete droplets through a nozzle. Jetting frequencies of 500-1000 Hz with droplet diameters of 500 microns are typical. Process parameters such as reservoir temperature, substrate temperature, droplet firing frequency, and droplet overlap distance significantly affect how droplets spread and solidify upon impact with previously deposited material. This paper will describe results of process models that simulate the impact, spreading, and solidification of 4043 aluminum droplets during MHD deposition under different process conditions. Simulated material behavior will be compared with experimental results captured using high speed video, and results will be used to make recommendations on suitable process parameter windows that lead to preferred microstructures subject to the requirement of high density deposits.

In the field of metal additive manufacturing (AM, also known as 3D printing), the community, from the view of material science, are mainly facing two problems: one is how to eliminate defects (e.g. porosity) and residual stress, and the other is how to control the microstructures (e.g. grain and phase). Previously, we have demonstrated the applicability of high-speed synchrotron x-ray imaging and diffraction techniques at the Advanced Photon Source in probing the microstructural evolution during the metal AM process in real time. The temporal resolution can reach 100 ps and the spatial resolution can reach 1 μm/pixel. In this presentation, we will show the power of these hard x-ray techniques and map the road to rapid and precise controlling of the metal AM process. We believe our study will facilitate the understanding of laser-metal interaction, and pave the way to a more extensive application of metal additive manufacturing.

Additively manufactured (AM) objects are becoming increasingly prevalent in many commercial applications. This shift could lead to an influx of materials and devices of unknown origin flooding the globe, potentially increasing the prevalence of counterfeit goods. A thorough understanding of the chemical and structural composition of these types of objects as well as the raw materials from which they are produced yields useful information of provenance. Measurement techniques such as DSC, pyrolysis-GCMS, X-ray diffraction, and micro computed tomography can be used to determine the detailed chemical composition and structure of AM objects. This information (e.g., organic content of the device and structural features such as porosity or grain size) is being explored for the identification of the source of the raw materials, to elucidate the additive types and content, and the method of manufacturing. Furthermore, to reduce the likelihood of a counterfeit part, functional materials can be incorporated into the raw materials before manufacture to produce signatures that may be readout and maintained on a blockchain platform. As a proof-of-concept, fluorescent nanomaterials were incorporated into an AM object, which can be analyzed through non-destructive techniques to verify the authenticity of the part.

In the last decade, research has focused on 3D printing for not only creating conceptual models but functional end-use products as well. As patents for 3D printing expire, new low cost desktop systems are being adopted more widely. This trend is leading to products being fabricated locally and improving supply chain logistics. However, currently low cost 3D printing is limited in the number of materials used simultaneously in fabrication and consequently is confined to fabricating enclosures and conceptual models. For additively manufactured end-use products to be economically meaningful, additional functionalities will need to be incorporated in terms of electronic, electromechanical, electromagnetic, thermodynamic, and optical content. Research has recently focused on embedding electronic components and electrical interconnect into 3D printed structures either by interrupting the process or by inserting the additional content after the structure has been built. However, only until recently and with an investment from the national initiative on Additive Manufacturing – America Makes – has there been a concentrated research focus on developing technology that provides multi-functionality. This presentation will review work in multi-process 3D printing for creating structures with electromechanical actuation and electro-propulsion.

Sacrificial supports are an inconvenient necessity in traditional 3D printing processes. It is both time consuming and expensive to remove these supports, included to alleviate thermal strains and allow for the fabrication of overhanging features. Recently, dissolvable supports have been introduced for powder-bed fusion (PBF) printed stainless steel components. This processes was easily introduced during a post-processing heat treatment, in which a sensitizing agent introduced chromium carbides into the initial ~100 µm of material. The sensitized material was electrochemically dissolved resulting in the supports being detached from the component. Inconel 718 (IN718) is a commonly used superalloy that includes elements such as niobium and titanium to protect the alloy from sensitization through the formation of primary carbides. This work will present the adaptation of dissolvable metal supports to IN718 and report on the microstructure following sensitization and during dissolution. Specifically, the effect of microstructure on dissolution along with processing parameters will be reported.

Support structures are included in most 3D printing techniques, such as directed energy deposition (DED) and powder-bed fusion (PBF), in order to alleviate thermal stresses and allow for the printing of overhanging parts. Recently, dissolvable metal supports have been introduced as a solution to reduce the costs and time associated with production. Following PBF fabrication, parts were surrounded in a carburizing paste that introduced chromium carbides over the initial ~100 µm of surface, reducing the free chromium concentration. This sensitization makes the carburized regions very susceptible to corrosion in a solution of nitric acid and potassium chloride. Since sacrificial supports are typically <200 µm in width, they are easily dissolved during the process. This work will study the effect of processing parameters and microstructure evolution throughout the dissolution process. Specifically, discussing how the microstructure affects dissolution and the resultant microstructure after support removal is complete. Processing parameters such as potential will be varied to target different phases during dissolution.

Nanotwinned (nt) metals have a unique microstructure with grains that contain a high density of layered nanoscale twins divided by coherent twin boundaries (TBs). These metals exhibit superior mechanical and electrical properties compared to their coarse-grained and nanocrystalline counterparts. Since TBs can effectively block dislocation motion, the nanotwinned metals often show higher strength and ductility compared to their nanocrystalline counterpart. In addition, these metals show more resistance to electromigration which is a common problem for metals at the nano/microscale. Several processes including pulsed electrodeposition (PED), plastic deformation, recrystallization, phase transformation, and sputter deposition have been used so far to make nt-metals in film and bulk forms. However no additive manufacturing technique has been introduced so far to fabricate nanotwinned metals in microscales. Here we report on a new process for 3D printing of nt-metals, termed localized pulsed electrodeposition (L-PED) to perform microscale 3D printing of nt-Cu with high density of coherent twin boundaries (TBs). We also show that the twin density and grain size of the material can be controlled in this process. This cost-effective process, which is performed at ambient environment can be used for direct 3D printing of layer-by-layer and complex 3D micro-scale nt-cu structures with various applications including electronics, micro/nanoelectromechanical systems (MEMS, and NEMS), metamaterials, plasmonic, and sensors. We also show that L-PED process can be performed on non-conductive substrates to make interconnections between two conductive pads.
This process will enable incorporation of metals with high strength and ductility and low electromigration susceptibility into various applications. The characterization tests performed on the 3D printed samples show that the structure is fully dense, with low to none impurities, and low microstructural defects, and without obvious interface between printed layers, which overall result in good mechanical and electrical properties so there is no need to perform any post-processing steps. We show that the L-PED process can be performed with in situ control over twin lamella thickness (λ) and twin density during printing through control of the pulsed ED parameters. Such spatially varying microstructure control may enable spatial tuning of mechanical and electrical properties of the printed metal.

Ultrasonic additive manufacturing (UAM) is a solid-state manufacturing technology for producing near-net shape metallic parts combining additive ultrasonic metal welding and CNC subtractive machining. Even though UAM has been demonstrated to produce robust metal structures in Al-Al, Al-Ti, Al-steel, Cu-Cu, Al-Cu, and Al-NiTi material systems, UAM welding of high strength steels presents challenges. In this study, current progress on steel to steel welding via UAM is discussed. The stainless steel 410 is selected for this study due to its wide range of applications and good heat treatment properties. The effect of pre-heat temperature as a process parameter along with the influence of hot isostatic pressing (HIP) as a post process treatment on the UAM steel samples are investigated. A custom shear test was designed to characterize the mechanical strength of unique laminated UAM steel samples. The results show that increasing the preheat temperature from 100°F to 400°F improves interfacial strength and structural homogeneity of the UAM steel samples, while the HIP process significantly improves the shear strength of UAM samples by over twice that of as-welded samples. Additionally, optical images, scanning electron microscope (SEM) analysis and electron backscatter diffraction (EBSD) measurements are presented. Specifically, the EBSD results indicate that plastic deformation and grain refinement happen at the welding interface during the UAM process compared with the as-received material and the HIP process mitigate the interfacial defects compared with the as-welded samples, which leads to the increased interfacial shear strength.

Additive Manufacturing (AM) of metals is a growing technology base that has the potential to significantly impact product realization in aerospace, automotive, medical industries, and many others. Many challenges exist in powder bed fusion processes for metal AM (Selective Laser Sintering, SLS, Selective laser Melting, SLM, and Electron Beam Melting, EBM), due to thermal stresses within the printed parts, operator burden in completing the build process, and overall cost of the equipment. Binder jet 3D printing for metal AM production utilizes an inkjet technology to deposit a polymer binder into a powder bed of metal. The layers of the metal part are glued together, one layer at a time in this approach. The part is removed from the printer, cured in a low temperature oven. Then, the green part is sintered and infiltrated in a high temperature furnace. This project aims to improve the strength of binder jetted green parts since the structural weakness of the currently produced green parts is a bottleneck of this technology and limits its application. This study introduces a new system utilizing a difunctional monomer binder for printing stainless steel particles and others. Using difunctional monomer binder, more complex and stronger green parts were created. The monomer was jetted to form the shape and cured to induce in situ polymerization. This approach created green parts stronger than those made with a commercial binder, and allowed for the creation of more complex green parts. Many parameters including viscosity, surface tension, solution composition, drop size, curing temperature and time were tailored within the printing process.

Patient-specific phantoms have a wide range of biomedical applications including validation of computational models and imaging techniques, medical device testing, surgery planning, medical education, doctor-patient interaction, etc. Although additive manufacturing technologies have demonstrated great potential in fabricating patient-specific phantoms, current 3D printed phantoms are usually only geometrically accurate. Mechanical properties of soft tissues can merely be mimicked at small strain situations, such as ultrasonic induced vibration. Under large deformation, the soft tissues and the 3D printed phantoms behave differently. The essential barrier is the inherent difference in the stress-strain curves of soft tissues and 3D printable polymers. This study investigated the feasibility of mimicking the strain-stiffening behavior of soft tissues using dual-material 3D printed metamaterials with micro-structured reinforcement embedded in soft polymeric matrix. Three types of metamaterials were designed and tested: sinusoidal wave, double helix, and interlocking chains. Even though the two base materials were strain-softening polymers, both finite element analysis and uniaxial tension tests indicated that two of those dual-material designs were able to exhibit strain-stiffening effects as a metamaterial. The effects of the design parameters on the mechanical behavior of the metamaterials were also studied. The results suggested that metamaterial 3D printing technique can be used to create patient-specific phantoms that mimic the mechanical properties of biological tissues. As a proof-of-concept study, we used the 3D printed tissue-mimicking phantoms to quantitatively assess the post-transcatheter aortic valve replacement (TAVR) aortic root strain in vitro. A novel indicator of the post-TAVR annular strain unevenness, the annular bulge index, outperformed the other established variables and achieved a high level of accuracy in predicting post-TAVR paravalvular leak, in terms of its occurrence, severity, and location. This work has promising applications in procedural planning for cardiovascular interventions.
Refs:
[1] Zhen Qian, Changsheng Wu, et al. Rapid prototyping of the aortic root in severe aortic stenosis for pre-TAVR planning, Circulation (2014) 130:A20259
[2] Kan Wang, Changsheng Wu (co-first author), et al. Dual-material 3D printed metamaterials with tunable mechanical properties for patient-specific tissue-mimicking phantoms, Additive Manufacturing 12 (2016) 31-37
[3] Zhen Qian, Kan Wang, Changsheng Wu, et al. Quantitative prediction of paravalvular leak in transcatheter aortic valve replacement based on tissue-mimicking 3D printing, JACC: Cardiovascular Imaging 10 (2017) 719-731

Complex vascular architectures have been manufactured through removal of sacrificial templates embedded in polymer substrates using the Vaporization of Sacrificial Component (VaSC) technique^[1]. Prior work with VaSC required high temperatures (ca. 200 °C) and long exposure times under vacuum, limiting application of the process to polymer matrices that remain stable under these conditions. In this work, the feasibility of using cyclic-poly(phthalaldehyde) (cPPA) as a sacrificial polymer for rapid vascularization of polymers and polymer composites at lower temperatures is investigated. cPPA is a metastable polymer which depolymerizes completely into monomer units upon exposure to acidic or elevated temperature conditions^[2]. As a first demonstration, rectangular solvent cast cPPA films (15 mm x 5 mm x 300 μm) were embedded in a poly(dicyclopentadiene(DCPD)) matrix and depolymerized at temperatures ranging from 95 – 110 °C within 3-1 hours, respectively. Successful vascularization was confirmed by optical imaging and pumping fluid through the resulting channel. Next, we investigated the simultaneous degradation of cPPA templates during an exothermic curing reaction of the matrix. DCPD monomer was partially cured as a gel with embedded cPPA fibers. A rapid exothermic Frontal Ring Opening Metathesis Polymerization (FROMP) of the gel was initiated with a thermal stimulus^[3]. The temperature of the front (ca. 150 °C) spontaneously depolymerized the cPPA fibers to create microchannels in fully cured poly(DCPD) matrix in less than 3 minutes. Current research is focused on creating more robust melt-processed cPPA fibers and 3D printed scaffolds to expand the range of vascular architectures for adaptive and environmentally responsive structural materials with properties like self-healing, self-cooling and electromagnetic reconfigurability.


References
[1] R. C. R. Gergely, S. J. Pety, B. P. Krull, J. F. Patrick, T. Q. Doan, A. M. Coppola, P. R. Thakre, N. R. Sottos, J. S. Moore, and S. R. White, Adv. Func. Mater. 25, 1043 (2014).
[2] H. L. Hernandez, S.-K. Kang, O. P. Lee, S.-W. Hwang, J. A. Kaitz, B. Inci, C. W. Park, S. Chung, N. R. Sottos, J. S. Moore, J. A. Rogers, and S. R. White, Adv. Mater. 26, 7637 (2014).
[3] I. D. Robertson, L. M. Dean, G. E. Rudebusch, N. R. Sottos, S. R. White and J. S. Moore, ACS Macro Letters 6, 609 (2017)

3D bioprinting has been introduced as a viable additive manufacturing technique for tissue engineering applications because it allows for patient-specific reconstruction of damaged hard and soft tissue. It is a desirable option because it allows for the ability to optimize bioinks to target specific tissue types and the potential to encapsulate cells in the bioinks for full-scaffold cell integration. A current limitation to bioprinting, as with most 3D printing, is that the printed material has poor tensile characteristics. Electrospinning, an alternative deposition technique, has been show to produce high tensile scaffolds for ligament tissue engineering. These densely packed structures form a favorable microenvironment that directs cell growth, migration, and proliferation. We hypothesized that a custom built 3D bioprinter + electrospinner hybrid system would allow for targeted scaffolds of the bone-ligament interface such that the 3D bioprinter would allow, with custom bioinks and optimized architecture, for a functionally-graded transition from bone to ligament phases and the E-spun fibers would allow for high tensile loading needed for the ligament phase.

We introduce a custom hybrid 3D bioprinter + electrospinner built in our lab, known as the E-spin Printer, to facilitate layer-by-layer, alternating bioprinting and electrospinning of bioink and fibers, respectively. This biocomposite scaffold is fabricated such that the tensile stiffness and strength better approximates that of native bone and ligament tissue with a functionally-graded transition from bone to ligament phases. Our bone phase is made from Polyethylene (glycol) Diacrylate (PEGDA)-based bioink, composed of PEGDA solution incorporating decellularized bone tissue to enhance the rheological and mechanical properties. The ligament phase is made from Polycaprolactone (PCL) fibers which enhance the structural integrity of the scaffold. We report preliminary findings of scaffold mechanical testing and cell viability.

The design of tissue engineering materials for both orthopaedic and dental implants is a great challenge in terms of desirable mechanical properties, biocompatibility, and osseointegration. Titanium has been an effective implant material due to its excellent strength to weight ratio, corrosion resistance, toughness, and bio-inert oxide surface. Selective laser melting (SLM) is an additive manufacturing process that fabricates constructs based on CAD Files by scanning powdered materials using the thermal energy supplied by a focused and computer controlled laser beam,. SLM allows the generation of complex 3D parts by a layer-wise material addition technique that selectively melts successive layers of metal powder on top of each other. This work reports the relationship between the cellular attachment and bacterial growth, simulated body fluid (SBF) growth with different inclined SLM part. The SLM printed Ti6Al4V samples were characterized first by the electron microscopy, Profilometer, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy. Cell viability, cell attachment, proliferation of chinese hamster ovarian cells (CHO) on Ti6Al4V SLM plates were confirmed by MTT assay, and confocal imaging in vitro experiments. Bacterial adhesion on SLM parts were also observed by scanning electron microscope and confocal laser microscopy. SLM printed samples were then incubated into SBF solution for 28 days and SBF growth on SLM parts were confirmed by SEM and XPS analysis.

A novel method for 3D printing of continuous fiber reinforced ceramic matrix composite (CFCMC) structures was achieved by fused deposition modeling (FDM) and pyrolysis of homogenized thermoplastic ceramic precursor inks consisting of thermoplastic resin and ceramic precursor. The inks were found to exhibit excellent thermoplastic properties for 3D printing and pyrolysis into SiOC ceramics when temperatures exceed 850 ^oC under inert atmosphere. Ceramic density was found to increase with the increasing of the percentage of ceramic precursor content. The pyrolysis caused uniform shrinkage of the samples by 20 % and the shrinkage was independent of the ceramic precursor content. The ceramics were amorphous and compact within the temperature range of 850 ^oC to 1300 ^oC. They decomposed into amorphous SiO₂ and β-SiC when the temperature reached to 1400 ^oC, and generated a lot of crack in ceramics. The novel inks were printed into complex lightweight structures with embedded continuous fibers through the continuous fiber FDM printer, which unravels the potential of 3D printing of high-performance CFCMC.

The fabrication and assembly of solid oxide fuel cell (SOFC) components into an integrated structure, including both support and functional layers, remains one of the primary challenges preventing the widespread adoption of SOFCs as an energy conversion technology. We present an efficient and highly scalable multi-material process for fabricating SOFCs using a combination of 3D-Painting (a room-temperature, extrusion-based 3D-printing process) and dip-coating of particle-laden, liquid 3D-inks. 3D-Painting is used to sequentially and precisely deposit anode and cathode materials, allowing unprecedented control over gas channel geometries. Depositing layers thinner than 100 µm using extrusion-based 3D-printing is impractical, so these 3D-inks are repurposed for dip-coating of mechanically robust and controllably thick multi-material films for electrolyte and interconnect layers. The 3D-inks used for both 3D-printing and dip-coating are synthesized through simple, room-temperature mixing of a polymeric binder, organic solvent mixture, and SOFC material powders. The volume percentage of particles contained in the inks can be tailored between 60-90 vol% to control shrinkage and porosity during binder burn-out and sintering; optimization of the particle content to achieve uniform shrinkage between materials is critical to prevent warping, cracking, or delamination during cell co-firing and to ensure optimal performance of each component. In addition, the effects of 3D-printed object geometry, residual solvent content, and different polymeric binders on overall material shrinkage are analyzed. The microstructural and electrochemical characteristics of the fired cells are analyzed and compared with cells produced entirely using traditional tape-casting techniques. Finally, we show how these processes can be extended beyond traditional, planar SOFC architectures to geometries designed to optimize electrochemical function and device performance.

Alkali alumino-silicate glasses are important technological materials due to their high heat resistance, thermal stability and dielectric properties. Further, porous alkali alumino-silicate structures have been used as thermal insulation barriers as well in catalysis. In this context, capabilities to print porous alkali alumino-silicate structures using a specialized 3-D paste-printing technique have been developed. Specifically, using silica and alumina powders in conjunction with NaOH, as well as appropriate surfactants and blowing agents, the ability to print sodium aluminum silicate (NAS) foams of controllable macro- and meso-porosity is demonstrated. In addition, we also incorporate certain fillers to produce composite foams with improved mechanical strength. The effects of key variables such as feed ink formulations and their rheology, print rates as well as cure temperatures on final microstructure, density and pore sizes are reported. SEM, FTIR and NMR studies were also carried out to examine the structural and bonding characteristics of the 3-D printed foams. Comparisons with analogous structures obtained by other conventional techniques such as direct curing and sol-gel process provide a basis to assess the relative advantages of using 3-D paste printing techniques for fabricating mechanically robust porous NAS foams.

Traditional thermoelectric device manufacturing uses bulk material processing with machining, assembly, and integration steps which lead to material waste and performance limitations. The traditional approach offers virtually no flexibility in designing the geometry of thermoelectric modules. Additive manufacturing can overcome these challenges. Although printing techniques, including 3D printing, have been explored for thermoelectric devices, these techniques have been limited to organic or organic-inorganic composite materials. Additive manufacturing solutions for inorganic thermoelectric materials, particularly those geared toward mid-/high-temperature applications, are scarce. The work presented here discusses selective laser melting (also known as laser powder bed fusion) of thermoelectric materials. Selective laser melting is an additive manufacturing process which locally melts successive layers of material powder to construct three-dimensional objects. When applied to thermoelectric materials, selective laser melting could enable new geometries and architectures, material-to-device integration, and large-area processing.
Selective laser melting was conducted on well-known thermoelectric materials such as bismuth telluride. The processing was conducted with commercial and custom-built systems, and the custom-built systems included both pulsed and continuous wave lasers. The powder preparation and laser processing parameters were explored to construct bulk, three-dimensional parts. Chemical and physical properties were characterized. X-ray diffraction results for pre- and post-processed material demonstrate the phase changes (or lack thereof) for different material types, providing insight into which materials would need post-processing to regain the favorable phases. Microscopy results demonstrate the extent of melting between layers as well as the variations in microstructure as a function of processing conditions. Thermoelectric property characterization was conducted. While Seebeck coefficient and thermal conductivity values are similar to traditionally-manufactured parts, electrical conductivity seems impacted by the unique microstructure developed from laser processing. The results demonstrate the feasibility of selective laser melting for inorganic thermoelectric materials.

Direct ink writing is a very promising additive manufacturing method that combines the efficient use of materials with bottom-up synthesis. This group of techniques relays strongly on the understanding of the relationships between inks’ processing and resulting materials’ properties.
We use this synergistic approach to explore the potential of additive manufacturing and push the boundaries in metal-oxides’ microstructure engineering. ZnO and TiO₂ inks have been investigated using solgel and hybrid (solgel/particle) based inks, respectively. From these, materials’ fundamentals coupled with printing and post-processing conditions are utilized to tune hierarchical microstructure features such as crystalline orientation, surface wrinkling, crystallization, cellular configuration and surface area properties.
These levels of control are of paramount importance for different applications including energy-related as sensors active layers, waste management/water purification systems as photocatalytic reactors, as well as in biomedicine as tunable bio-compatible scaffolds. Additional advantages of our approach include the use of environment-friendly precursors bringing a sustainability focus, imperative towards bridging the gap between lab developments and industry practice. This perspective also enables low-cost fabrication and highly safe materials.
We believe our efforts to constitute a pivotal foundation for shifting the focus of additive manufacturing towards changes in paradigms that enable novel materials with unprecedented characteristics.

Due to their excellent mechanical, electrical, and thermal properties, carbon fibers (CFs) have been used in nearly all-engineering fields, promoting vast research to improve their mechanical properties. However, the improvements of the mechanical properties of CFs are now saturated; thereby researchers pursue a new direction for improving the mechanical properties of the CF reinforced composites. On the other hand, carbon nanotubes (CNTs) have been regarded as a new generation reinforcement material, stimulating a considerable amount of research. However, the application of CNTs to polymer composites has brought many problems related with aggregation of CNTs that allow their low volume fraction in the composites. Thus, the hybridization of CNTs and CFs has been considered to be a versatile method to develop new advanced materials by hierarchically combining their excellent thermal, electrical, and mechanical properties at nano and micro scales . Direct growth of CNTs on CF surface is regarded as an effective hybridization method that can improve the reinforcing effect of CFs in composites and solve the dispersion problems of CNTs, becasue radially grown CNTs on CFs can improve the radial stiffness and axial tensile strength of CF-reinforced composites. In addition, the interfacial shear strength (IFSS) of polymer composites and the improved electrochemical performance as CF electrodes were also reported. In this presentation, we will report on a fabrication method of CNT-grafted CF without its degradation using bi-metallic catalysts and low-temperature grafting process . CF reinforced composites and carbon/carbon composites using CNT-grafted CF were manufactured and their mechanical properties such as modulus, tensile strength and flexural strength and electrical/thermal conductivity were characterized, demonstrating significant improvement. Finally, the mechanism behind such improvement will be presented in detail at the Conference.

Photosensors responsive to the short wavelength infrared (SWIR) spectra are used in a variety of applications including environmental monitoring and medical diagnosis. However, conventional SWIR sensors are limited by complex die transfer and bonding processing. Here we are advancing SWIR photodiodes by using a new generation of narrow bandgap conjugated polymers that are processed by solution processing techniques and allow simple direct deposition. The polymers are processed into bulk heterojunction photodiodes with photoresponse up to wavelength of 1.8 micron. The performances of devices with different polymer structures were compared through metrics including detectivity, quantum efficiency, response time and rectification ratio. Example applications including blood pulse measurements and spectroscopic identification will be demonstrated.

In addition to optoelectronics, I will also show an example of an instrumented glove for augmenting movement disorder assessments. The system is based on capacitive pressure sensing, and the validation allows an objective, repeatable metric that improve resolution over the current best practices. The glove measures the power required to move a patient’s arm and shows reduced inter- and intra-rater variability. Our approach using wearable sensors offers an objective route for the characterization of movement patterns, which would permit the effective evaluation of intervention outcomes, as well as provide a platform for novel motor interventions in the future.

3D-printing of composites offers advantages over single component systems for developing and producing radio frequency (RF) devices by synergistically combining the properties of the matrix and filler components. To date, there has been limited success in printing composite structures with suitable electromagnetic characteristics for operation in the millimeter-wave range (>30 GHz). Here, we describe a generalized block copolymer based strategy to incorporate ceramic and conductive materials into 3D-printable inks. The behavior of these inks can be tuned by altering the filler particle to polymer ratio in a plug-and-play fashion and can be deposited with both positional and compositional control using a custom active mixing printing nozzle. The performance of RF devices for operation in the Ka band (26.5 - 40 GHz) printed using this technology will be discussed.

Advances in additive manufacture (AM) processes such as fused deposition modelling (FDM) and direct ink writing (DIW) may offer a flexible, cost-effective approach to address conventional manufacturing limitations, such as time-consuming, high work-in-progress, multi-step assembly. In principle, AM can also allow more novel geometric or even bespoke designs of structural and functional products. However, in terms of energy storage devices, such as batteries and supercapacitors, the benefits of AM have yet to be exploited fully and current manufacturing approaches remain concerned only with planar large area electrode assembly, through many stages, into simple rolled or planar cell configurations.

In this paper, we first assume increased design flexibility from additive manufacture for all key sub-components or materials of an electrochemical double-layer capacitor (EDLC, supercapacitor) in order to propose a range of novel supercapacitor materials arrangements and overall device geometries (form-factor). We then use mean-field local-density approximations based on dilute-solution theory to model the non-linear induced charge electro-kinetics and the overall supercapacitor energy storage behaviour. We predict performance in various novel arrangements of the constituent materials and their spatial arrangement at the electrode length-scale, for various unusual geometric form-factors at the packaged device scale.

To explore the model predictions we have designed, built and commissioned a novel, modular AM system, extending typical AM functionality by combining FDM and DIW techniques simultaneously, together with various in-situ monitoring tools. The system automatically deposits supercapacitor elements including an encapsulating housing (PLA/ABS/PMMA), binder-less electrodes (activated carbon, surfactant), aqueous electrolyte (dilute sodium sulfate), and current collectors (silver based) in a single non-stop operation, allowing for finished cell manufacturing times of less than 20 minutes. The manufactured EDLCs demonstrate high aerial capacitance of 1240 mFcm^-2 at 50mVs^-1 and good cycling stability of 94% over 1000 cycles. The design factors affecting gravimetric performance were also revealed.

We show that single-step manufacture of encapsulated supercapacitors removes the need for the traditional discrete and time wasting steps of cell assembly, suggesting supercapacitors and other energy storage devices can be directly integrated into 3D-printed systems and components. At a more fundamental level, the flexibility AM of supercapacitor sub-elements provided an opportunity, not available from conventional manufacture, to validate designs for optimised ion-diffusion pathways based on simulation. AM also showed how changes in the overall cell geometry can be used to generate unusual combinations of energy and power in terms of areal, gravimetric and volumetric performance.

Significant advancements in microfluidics have enabled miniaturization of microfluidic and nanofluidic biosensors for applications in DNA analysis, genomics study, and more. This project seeks to reduce fabrication complexity of microfluidic devices by combining reactive ink chemistries with drop-on-demand printing. In this work, we demonstrate Drop on Demand (DOD) fabrication of microfluidic mixer devices with integrated sensors using Polydimethyl Siloxane (PDMS) reactive inks, silver reactive inks, and fugitive inks. Fugitive phase change inks were used to define the microfluidic channels. Ag reactive inks were used to define sensing electrodes at the inlet and outlets of the device to quantify the degree of mixing of two electrolytic solutions. PDMS reactive inks were used to both confine the fugitive inks and print caps between layers for multi-layer devices.
A fully functional microfluidic mixer device was fabricated using DOD technique, calibrated and tested using NaCl solutions with concentrations ranging from 0.01 M to 1.0 M to show that electrolyte concentration and mixing completeness can be accurately measured. Overall, this work demonstrates a simple and inexpensive process to fabricate a passive microfluidic mixer device with integrated electronics using affordable phase changing materials and reactive inks that can be mixed in the lab. Unlike paper-based microfluidic devices often fabricated using printers, our process demonstrates that flushable, “hard” microfluidic devices can be fabricated using drop-on-demand printing. This process should facilitate low-cost microfluidic prototyping and fabrication.

Reactive ink chemistries offer unique solutions to numerous issues in printed electronics and have the potential to address other areas in additive manufacturing as well where dense, pure films are required. While the initial work on reactive silver ink showed high conductivities, it was limited to relatively low viscosities that were particularly well-suited for inkjet or aerosol printing. By tuning these complexes one can increase the viscosity by several orders of magnitude without the use of polymers. This allows the user to see the same conductivities and film performance as the originally proposed chemistry but gives access to a wider breadth of deposition techniques. Furthermore, a wider material palette can be accessed by tuning the complexes for various metals resulting in a robust formulation strategy for numerous applications.

In 2014, TIME Magazine heralded Additive Manufacturing / 3D printing as one of the 25 best inventions of all-time signifying that it can lead to a decentralized and highly customizable manufacturing technique in the future. Polymers are the most common materials to be 3D printed today due to the simplicity of extruding them in molten form that quickly cools and hence solidifies. However, there is a great demand for developing methods to easily print metals. Currently available commercial methods for additive manufacturing of metals tend to be prohibitively expensive requiring upwards of $500,000 in capital investments, and use energy-intensive lasers with techniques that need high sintering temperatures in excess of 800°C. In addition, they need special environments including vacuum-like low pressures to avoid oxidation while handling metal nanoparticles, making it a very messy process leading to porosity in finished parts, low resolution and poor electrical conductivity due to presence of some non-sintered powder particles with organic viscous binders, apart from having slow printing speeds as compared to conventional subtractive manufacturing methodologies. Finally, the operating and processing procedures are almost impossible to be integrated with co-printing of various polymeric, organic, soft and biological materials on the same equipment. Here, we present an alternate but simple approach that utilizes low melting point gallium-based alloys as complements to the existing materials for 3D printing metals and co-printing them with polymers at room temperature. Gallium-based liquid metal alloys offer the electrical and thermal benefits of various metals like gallium and indium, combined with the ease of printing due to its low viscosity (~2x water). Despite having high surface tension (~10x water), these metals build mechanically stable structures due to the formation of a thin (~3 nm thick) surface oxide. The oxide skin is passivating, forms spontaneously in presence of air or dissolved oxygen on the surface of the metal and allows us to direct-write planar as well as free-standing, out-of-plane conductive microstructures down to a resolution of ~10 μm, on-demand, using a 4-axis pneumatic dispensing robot customized from a desktop CNC machine at relatively low pressures (~10s of kPa). We have demonstrated rapid prototyping of functional electronics such as flexible and stretchable antennas for radio-frequency defense communications, as well as consumer-based electronic devices like laser pointers and inductive power coils for wireless charging of smartphones, and wearable thermoelectric generators for energy-harvesting applications. We have also exhibited the patterning of 3D multilayered microchannels with vasculature using these printed liquid metals as a sacrificial template at room-temperature that can be employed in numerous lab-on-a-chip devices to enable inexpensive fabrication of personalized healthcare sensors.

Ferrite-based magnetic material has wide applications due to excellent chemical stability, good mechanical hardness, and remarkable magnetic properties. However, it is limited to be processed into thick films or simple shapes by the exisiting approaches. To obtain complex geometries for the extensive applications, extrusion-based three-dimensional (3D) printing is attractive. Herein, a soft magnetic ferrite ((Ni,Zn)Fe₂O₄) with the desired mesh architecture is obtianed and applied as a magentic filter for the treatment of heavy-metal-contaminated water due to its serious toxicity. This filter can effectively capture the quasi-superparamagnetic Fe₃O₄ nanoparticles after sufficient adsorption of heavy metal ions under an external magnetic field as low as 0.07 T instead of the traditional strong field up to 2 T. Interestingly, after one single filtration process, the heavy metal concentration could be significantly decreased from 1 mg L^-1 to satisfy the drinking water standard recommended by the World Health Organization (e.g., <0.01 mg L^-1 for Pb(II)). In the end, the fabricated filter is convenient to be recovered by simple ultrasonic washing and nanoparticles can be well-regenerated by the adjustment of pH, which can avoid byproducts and secondary pollution. Overall, a proof-of-concept magnetic device for effective heavy metal removal is demostrated by a combination of adsorption and subsequent low-field separation using the 3D-printed magnetic filter. It will also have a great impact on other environmental and biomedical issues, for example, antibiotics contamination.

Terahertz technology has now received more and more attentions due to its wide applications in various scientific and technical fields. Because of their capability to prohibit the propagation of terahertz irradiation for all directions in their band gaps, three dimensional terahertz photonic crystals (3D-TPCs) with a periodic diamond structure could create specific functionalities. Various techniques had been developed to assembly these complex structures difficult to fabricate by conventional processes, including the particle manipulation assembly and lithographic techniques. However, the particle manipulation assembly is inherently complex and time-consuming, while few materials are suitable for lithographic techniques. Novel approaches should be developed for the fabrication of 3D-TPCs with the easy operation, low cost, and the flexibility on the structure/material design.

The direct-writing technology could create complex 3D structures via a layer-by-layer building process with a broad range of materials at the microscale level, which had demonstrated great potentials for emerging applications. Thus, it could provide a powerful alternative for producing complex 3D-TPCs with various material systems. A recent progress of the direct-writing technology (biomimetic 4D printing) demonstrated that the shape of complex 3D structures could be modulated via a careful design of the ink system, which opened new avenues for the creation of “smart” 3D structures responsive to external motivations. Thus, if 3D-TPCs could be created with tunable terahertz properties under proper external stimulations, novel functions could be integrated to these 3D-TPCs for cutting-edge terahertz technology.

Here, we developed a composite ink system composed of polydimethylsiloxane (PDMS) and barium titanate (BaTiO₃) nanoparticles for the creation of mechanically flexible 3D-TPCs with tunable terahertz properties under external force field by the direct-writing technology. By the solid state shear milling process, BaTiO₃ nanoparticles of high refractive indices (RI) were well dispersed into PDMS polymer matrix to create these composite inks. In this composite ink system, PDMS serves as the flexible matrix component to provide the reversible mechanical deformation capability for these 3D-TPCs, while BaTiO₃ nanoparticles of high refractive indices provide the terahertz response with proper 3D structure design. By varying the content of BaTiO₃ nanoparticles to modulate the RI of this composite ink or changing the geometry of these 3D-TPCs, different terahertz properties could be obtained. More interestingly, these 3D-TPCs demonstrate a unique tunable terahertz property under external force field due to their mechanical flexibility from the PDMS matrix of the composite ink. Thus, their terahertz property is responsive to external force fields reversibly, which can find novel applications in terahertz technology and other relative technological applications.

The prevalence of devices used for the Internet of Things (IoT) relies heavily on the development and incorporation of integrated power sources and energy storage. Printed batteries are an emerging solution for on-device power requirements using low cost, high accuracy fabrication techniques. While several printed batteries have been previously shown, few have designed a battery that could be incorporated into an integrated device. Specifically, a printed battery with a small active electrode area (< 1 cm²) demonstrating high areal capacities (> 10 mAh cm^-2) at high current densities (1-10 mA cm^-2) has not been developed. This work addresses these challenges by investigating the scaling limits of a printed Zn-Ag₂O battery and determining the materials and processing limitations for developing a mm²-scale battery.
Zn-Ag₂O is the chosen battery chemistry given its inherent air stability, high energy density (130 Wh kg^-1), and high discharge rate capability. Unlike Zn-MnO₂ or Li-ion, Zn-Ag₂O batteries also maintain a steady discharge voltage over a wide range of discharge rates, which is desirable for IoT applications to minimize power electronics requirements and lower overall power consumption. To assemble the battery, stencil printing was chosen based on its low cost and compatibility with various substrates including flexible plastics. Stencil printing is also better suited than other printing methods such as inkjet or gravure in order to print the thick active layers (10s-100s µm) necessary to achieve high areal capacities. Processing temperatures of each battery component were below 150°C to remain compatible with low cost, flexible substrates. Mass loading of the anode and cathode inks was optimized to maximize cell capacity while maintaining ink viscosities compatible with stencil printing. Batteries were printed with active areas between 0.01 and 0.25 cm² and discharged at current densities between 1-20 mA cm^-2. Electronic conductivity of the electrodes and ionic conductivity of the electrolyte were measured as a function of active area. In addition, electrochemical testing including impedance spectroscopy and cyclic voltammetry was performed to determine the impact of scaling on cell degradation mechanisms.
The fully printed Zn-Ag₂O batteries demonstrated a steady discharge voltage of 1.45 to 1.5 V and high areal capacities of 8-12 mAh cm^-2 at current densities between 1-10 mA cm^-2. Self-discharge lifetimes of one month were obtained with the use of a PDMS encapsulation layer and internal resistances were typically less than 30 Ω. This work represents the first demonstration of a small, packaged, fully printed Zn-Ag₂O battery with high areal capacities at high current densities, a crucial step towards realizing integrated energy storage for printed electronics systems.

Single-walled carbon nanotubes (CNTs) are one of the most versatile electronic materials ever discovered. Electronically, they can be semiconducting or metallic; mechanically, they are flexible yet have a tensile strength greater than steel; and physically they can be centimeters long to just a few nanometers. For nearly two decades, these diverse possibilities have excited and motivated researchers pursuing CNTs for electronic applications. However, thus far the versatility of CNTs has also been their greatest obstacle in terms of purification, precise positioning, and so forth. In this talk, I will discuss how the inherent versatility of CNTs can be appropriately harnessed for enabling certain applications. The tremendous progress in solution-phase processing of nanotubes has opened a path for their most suitable, near-term use as printed thin films. Three recent advances will be presented, including: 1) print-in-place additive electronics; 2) printed sensors for harsh environments; and 3) point-of-care printed biosensors operating in whole blood. Each of these is made possible by drawing from distinct properties of thin-film CNTs; properties unavailable from any other printable material. The fact is, CNTs offer sufficiently unique and reproducible behavior when printed into thin films that they should be given much greater consideration from the printed electronics community. In the company of organic semiconducting inks, and even that of non-printed metal-oxide semiconductors, printed films of CNTs are a standout with significant advantages.

The main trend in modern medicine, made possible by the recent advance in medical technology, is minimally invasive surgery. Traditional surgery typically requires incision of living tissues, which causes extensive local damage that requires time to heal. On the contrary, by using techniques that strongly limit tissue incision, complications can be considerably reduced. Many minimally invasive techniques are already in use [1], while more innovative ones are currently under development.

One of the most attractive approaches currently under investigation consists in the use of remotely controlled microrobots. These devices are able to perform a variety of tasks in vivo, including drug and cell delivery, microsurgery or diagnosis. By using microrobots, the operation is performed exclusively in the target zone, with minimal damage for the surroundings. To operate inside human body, microdevices must be moved wirelessly in the less invasive way possible but with high control over position and speed. Magnetic actuation has been proposed as optimal to achieve a controlled motion for microrobots [2].

An intriguing application for these magnetically controlled microrobots is cell delivery: cells are loaded on a microdevice, which brings them in the place inside human where they must exert their therapeutic action. Examples of such devices are available in literature [3] and present a classical scaffold structure, able to accommodate cells.

The aim of this work is the manufacturing of a magnetically controllable cylindrical scaffold potentially usable for cell delivery. Stereolithography is used to model microsized scaffolds, which are subsequently covered with functional layers. Electroless metallization, a costless and scalable approach, is used to deposit such layers. In particular, a semi-hard magnetic alloy is applied to allow magnetic actuation. A gold coating constitutes the final layer to impart biocompatibility to the surface of the microrobots. Magnetic control and human cell biocompatibility of obtained devices is investigated, as well as their microstructure.

The work presented was carried out in the framework of the interdepartmental laboratory MEMS&3D of Politecnico di Milano, Italy.

[1] C.T. Frantzides et al., Atlas of Minimally Invasive Surgery, Saunders (2008)
[2] F. Qiu et al., Engineering 1(1): 21-26 (2015)
[3] S. Kim et al., Adv. Mater. 25: 5863–5868 (2013)

Electrochromic materials and films can be used as a non-emissive technique for display applications. Such technique has already been used in cars’ antiglare rearview mirrors, smart windows intended for energy savings for buildings. However, most of the electrochromic materials are deposited on rigid substrates, which will prevent it from being used in flexible and stretchable electronics applications, where low temperature deposition technique is desired. In this work, we explore the electrochromic materials and their inkjet-printing process onto flexible and stretchable substrates. A set of four inks based on the combination of synthesized WO3 nanoparticles, com- mercial WO3 nanoparticles, commercially obtained W-TiO2 and TiO2 nanoparticles, PTA, and oxylic acid dihydride (OAD) were mixed in isopropanol. The relative weights of each mixture were based on a previously reported D-optimal ink formulation. The ink was diluted 1/10 after initial mixture, and patterns were printed 33 times using a Microfab Jetlab II printer. An Autolab Potentiostat/Galvanostat was used to perform Cyclic voltammetry mea- surements at the specified voltages. A four-channel Arbin system was used to perform Galvanostatic chargedischarge measurements. The micrstructure of the nanoparticles used in this study were examined under scanning electron microscopy for examining nanoparticle morphology , x-ray diffraction for chemical and structural characterization, and dynamic light scattering for particle size determination. Electrochromic layers are then ink-jet printed on flexible and stretchable PDMS substrates, using synthesized Ag nanowires as conductive, yet highly transparent electrodes. The stretchable printed electrochromic device under various stress conditions are studied and electrochromic performances are evaluated that demonstrates clear switching behavior under external bias voltage. Detailed performance will be discussed.

Printed electronics is a disruptive manufacturing technique that combines functional materials and printing to make electronic devices in new form factors and enables innovative products. Printed electronics will yield breakthrough technologies in sensing, displays and wireless communication. In this regard, self-reducible metal-organic decomposition (MOD) inks have been synthesized for the formation of metal conducting layers. The main advantage of MOD inks over conventional flake/nanoparticle inks is that MOD compound allows smooth films at low temperature sintering with high conductivity. The molecular silver inks have ease of sintering and excellent electrical properties; however, the high cost of silver is becoming an issue for printed applications where the main driver for printing is cost. Thus, there is need to develop a low cost ink as an alternative.
In this work, we would like to present results on molecular screen printable copper ink which produces highly conductive, robust to oxidation and solderable printed traces compatible with sintering on PET and Kapton substrate. The sheet resistivity of ~8 to 15 mΩ /sqare/mil can be obtained for 4-20 mil screen printed lines.

Metal oxide (MO) semiconductors have attracted extensive attention for printed thin film transistors (TFTs) due to their higher performance than competing organic semiconductors. To achieve MO-based high performance circuits, forming unipolar circuits with n-type TFTs having different threshold voltages (V_t) is required owing to the lack of feasible p-type MO semiconductors. Several approaches have been reported to control the V_t of MO TFTs such as varying the active layer thickness and depositing metals on active layers. However, they have the disadvantages of degrading performance of TFTs in thicker film and being unsuitable for printing processes, respectively. This work addresses these challenges by selectively printing polymers on the back-channel of printed MO TFTs; by using printed polymers as back-channel control layers, TFT V_t can be controllably and selectively adjusted, thus allowing for the realization of printed NMOS logic circuits.

Polymers such as polyethyleneimine (PEI) and epoxy-based resists SU-8 were inkjet printed on the back-channel of the TFTs having printed indium oxide semiconductors and ITO source/drain electrodes on SiO₂/Si wafers. These layers have been found to passivate the back-channel of the oxide semiconductor, thus controllably altering V_t. The average V_t of MO TFTs was negatively shifted from 3 V in bare TFTs to -15 V or -37 V in the case of printing of SU-8 single or SU-8/PEI double layer, respectively. The V_t could also be shifted back over a wide range using additional post-annealing below 150°C. Based on the results above, NMOS inverters with depletion-mode TFT loads and enhancement-mode TFT drivers were fabricated on glass substrates with solution-processed aluminum oxide dielectrics and ITO gate electrodes. The resulting inverters showed excellent performance, delivering a gain of 23 V/V and a propagation delay of 0.5 ms at a 10 V supply, this performance was achieved despite the use of relatively long printed channels of 220 μm channel length.

This work thus represents an effective methodology to adjust the V_t of MO TFTs by selective polymer printing, thereby presenting an efficient strategy for realizing printed oxide logic circuits.

The presence of high conductivity ohmic contacts is of paramount importance to the performance of optoelectronic devices such as solar cells. The formation of these highly conductive contacts often requires high temperature treatments for extended periods of time to sinter the conductive particles and to evaporate the organic residues from conductive pastes which inhibit conductivity. This makes this process tedious and time-consuming. Many emerging optoelectronic technologies require low temperatures to accommodate use of thermally sensitive substrates such as flexible and lightweight printed electronics on polymer, cloth or paper in order to potentially overcome the challenges posed by high temperature techniques. Reactive inks enable printing of highly conductive features at low temperatures through Drop-on-demand printing without the need for a post-annealing step.
In this work, the authors evaluate the adhesion performance of these reactive metal inks on Indium Tin Oxide (representative of the top layer of Silicon-Hetero-Junction solar cells). A 180 °, ASTM standard peel testing method was used to evaluate the adhesion performance of these metal reactive ink contacts on ITO. The silver metal contacts were printed using two different reactive ink dilutions: 1:1 and 1:10 by volume with ethanol, and two different droplet sizes (35 µm and 28 µm) were used to further control the amount of silver present in each droplet. The amount of silver within a contact line was varied by printing multiple number of layers. Failure analysis was done on the metal fingers for varying amount of metal in each contact line to study and quantify the different adhesion failure modes of these metal reactive inks on ITO. We learned that the metal contacts printed with higher concentration inks showed lower adhesion to ITO and diluting the ink 1:10 by volume with ethanol showed significant improvements in adhesion strength with less than 5 % of the film failing adhesively during peel tests. This approach introduces new techniques to deposit front contacts on solar cells and understand their adhesion properties.

This presentation will discuss customized manufacturing of Printed Circuit Board (PCB) using 3D printing technology (3D-PCB) for the module system of portable sensing applications. 3D printing technologies with various conductive materials will be presented and summarized along with development of printed sensors and their system applications. An optimized Directly Ink Writing technique is adapted in a novel 3D-PCB fabrication platform using silver nanoparticle based ink. An electro-chemical circuit called potentiostat, as an example, is designed and optimized from an open source circuit. Using the same 3D printing platform, a reliable lactate sensor with 3-electrodes is printed on the flexible substrate. By combination of 3D printed sensor and 3D-PCB, the 3D printed sensor system demonstrates the electrochemical test including cyclic voltammetry (CV) and amperometry. It is demonstrated that 3D-PCB technology can significantly accelerate the large-area fabrication process of conventional electronics, and merge its capability into electro-chemical applications.

Hybrid organic-inorganic perovskites (HOIPs) have gained a great deal of notoriety for unique opto-electronic properties in addition to comparatively low production costs. Continued improvement in power conversion efficiencies of HOIP-based solar cells has also enabled tangential research into other optical devices; such as light-emitting diodes and solid-state radiation detection. A fundamental issue with HOIP based optical devices is in the inherent instability of the HOIP material itself. Environmental factors such as humidity, heat and ultra-violet light initiate degradation in HOIP material and limits lifetime and deployment of HOIP technology.
Previous work on improving stability has shown that compounding HOIP materials with polystyrene (PS) can drastically increase resilience of HOIPs in high humidity environments (RH~98%, 20 °C). Subsequently, the percolation threshold for conduction in these composite systems was determined (~ 70 wt% loading). The focus of this work is on fabrication of functional fiber mats of HOIP/PS composite media through melt electrospinning. Melt electrospinning is a solvent-less fabrication technique which utilizes an electrostatic field to draw-down a polymer melt into microfibers. The polymer itself may also act as a vehicle to carry out reactions and carry dopants through the draw-down process. Temperatures necessary to create a polymer melt from PS are in excess of the temperature at which thermal degradation occurs in the lead iodide HOIP species and its precursors. However, previous work has shown successful synthesis of HOIP microcrystals embedded in PS fibermats produced from melt electrospinning. At operating temperatures in the sealed melt chamber of the electrospinner methyl ammonium iodide degrades into methyl amine gas in addition to other organohalodic species. Methyl amine gas has been shown to act as a flux, converting HOIPs into a liquid state which then revert to a solid upon removal of the methyl amine gas. If correct this would be the first instance of the utilization of HOIPs in the liquid state during fabrication, as opposed during a post-processing step. The environment in the melt electrospinner will be investigated to determine the presence of liquid HOIP in situ and its effect on fiber morphology and functionality. Additionally, the use of secondary dopants, such as graphene, in the polymer melt will be investigated to incorporate a greater degree of functionally to melt electrospun HOIP/PS fibermats for use in opto-electronic devices.

Advancements in 3D printing technology have the potential to transform the manufacture of customized optical elements, which today relies heavily on time-consuming and costly polishing and grinding processes. However, the inherent speed-accuracy trade-off seriously constraints the practical applications of 3D printing technology in optical realm. In addressing this issue, here, we report a new method featuring a significantly faster fabrication speed, at 24.54 mm³/h, without compromising the fabrication accuracy required to 3D-print customized optical components. We demonstrated a high-speed 3D printing process with sub-voxel-scale precision (sub 5 mm) and deep subwavelength (sub-7 nm) surface roughness by employing the PµSL process and the synergistic effects from the grayscale photopolymerization and the meniscus equilibrium post-curing methods. Fabricating a customized aspheric lens 5 mm in height and 3 mm in diameter was accomplished in four hours. The 3D-printed singlet aspheric lens demonstrated a maximal imaging resolution of 373.2 lp/mm with low field distortion less than 0.13% across a 2-mm field of view. We attached this lens onto a cell phone camera and captured the colorful fine details of a sunset moth’s wing and the spot on a weevil’s elytra. This work demonstrates the potential of our method to rapidly prototype optical components or systems based on 3D printing.

We have developed silica inks that may be three-dimensionally (3D) printed and thermally processed to produce optically transparent glass structures with sub-millimeter features in forms ranging from scaffolds to monoliths. The inks are composed of silica nanoparticles suspended in a liquid and are assembled using direct ink writing (DIW). The assembled structures are then dried and sintered at temperatures well below the silica melting point to form amorphous, solid, transparent glass structures. This technique enables the mold-free formation of transparent glass structures previously inaccessible using conventional glass fabrication processes.

This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 within the LDRD program. LLNL-ABS-733253.

This talk will discuss methods to directly print liquid metal alloys into 3D structures at room temperature and embed it in functional polymers to create conductors that are soft, self-healing, and ultra-stretchable. The metal is a gallium-based metal alloy that is a low-viscosity liquid at room temperature with low toxicity and negligible vapor pressure. Despite the large surface tension of the metal, it can be printed into non-spherical shapes due to the presence of an ultra-thin surface oxide skin. We have harnessed these properties to form a number of electronic devices encased in polymer matrices. For example, the metal can be printed to create stretchable interconnects between rigid thermoelectric ‘legs’ encased in elastomer; the resulting device is flexible and can be used in wearable devices to convert body heat into electricity. We have also utilized the ability to withdraw the metal from 3D printed structures as a sacrificial, fugitive ink to create microvasculature in polymer monoliths. The talk will discuss these examples as well as the key parameters that make printing possible. We found that the gap between the nozzle and substrate is important since printing is shear-driven. The ability to print metals at room temperatures is promising because it can enable co-printing of metal with temperature sensitive materials such as polymers, elastomers, and biological materials.

The Alliance for the Development of Additive Processing Technologies (ADAPT) was started in 2016 with a grant from Colorado’s Advanced Industries Economic Development Program, together with administrative support from Manufacturer's Edge, Colorado's Manufacturing Extension Partnership; cost share from founding members Colorado School of Mines, Lockheed Martin, Ball Aerospace, and Faustson Tool; and a research partnership with Citrine Informatics. This presentation will review the founding of ADAPT, as well as the growth of the center and the return on investment for the state advanced manufacturing infrastructure in the first two years of operations.

Graphene, a two-dimensional (2D) nanomaterial consisting of a monolayer of sp² hybridized carbon atoms, has attracted recent interest due to its unique material properties, notably its high electrical and thermal conductivities and exceptional mechanical strength. In order to pursue high mass and volume demanding applications, it is necessary to integrate the properties of 2D graphene into macroscopic, three-dimensional (3D) structures. Several different methods have been developed to produce 3D graphene macrostructures, dubbed graphene foams (GF). The current fabrication process of graphene foam can be categorized in one of two categories: 1) growth of graphene in porous metal foam and 2) printing and reduction of graphene oxide (GO) dispersion. A direct approach without the need of metals or graphene oxide, or that is not made from graphite using wet chemistry, would be desired. Laser induced graphene (LIG), a graphene structure synthesized by a one-step process through laser induction of commercial polyimide (PI) film in ambient atmosphere, has been shown to be a versatile material in applications ranging from energy storage to water treatment. However, the major drawback of this material is its limitation in the 2D form on the PI substrate. In this study, we have developed a 3D printing process of LIG foams based on laminated object manufacturing, a widely-used additive manufacturing technique. We also developed and show here a subtractive laser milling process to add further refinements to the 3D structures. By combining both techniques, we are able to print various 3D graphene objects. The LIG foams show good electrical conductivity and mechanical strength, as well as viability in various applications of energy storage and flexible electronics applications. This includes a Li-ion capacitor where the LIG foam anode has a total gravimetric capacity of 345 mAh g^-1, which is 95% of graphite's theoretical capacity. It has a comparable full cell energy density of 64 Wh kg^-1. As a pressure sensor, an LIG composite device is able to record arterial pulses waveforms with a clear dicrotic notch.

The disturbance by Electromagnetic interference (EMI) may degrade the performance of the circuit or even stop it from functioning. EMI shielding is a way to reduce the effects of EMI on a desired space. In general, materials with high electrical conductivity are effective for EMI shielding. But for this reason, circuits or devices that use high-conductivity materials also serve to block electromagnetic signals unintendedly. For example, when a metal or conductive polymer electrode is coated on a commercial touchscreen, the electric field is disturbed and can not be recognized as a 'touch' when the finger approaches. PEDOT: PSS, one of the conductive polymer, also has EMI Shielding Efficiency (EMI SE) due to its high electrical conductivity. Absorption loss is negligible when PEDOT: PSS is thinly coated on a scale of several hundred nanometers, but thickness-independent reflection loss still interferes with the electromagnetic wave signal. We fabricated thin-film electrodes of tens of nanometers to make different electrical tendencies and structures than bulk by diluting in solution. At that thickness range, the conductive pathways are changed and thus the electrical conductivity can be controlled. Therefore, we modifyed the structure of PEDOT: PSS to fabricate electrodes with very low reflection loss. We also fabricated a capacitance pressure sensor using this optimized electrode and attached it to a touch screen to produce a transparent film capable of 3D force input.

Additive manufacturing (AM) is rapidly advancing due to the large demand for custom parts, quick turnaround times and/or components with complex geometries. In AM, a 3D model is sliced into a series of 2D cross sections. 3D Components are formed by fusing material in the shape of each cross section. This process repeats as each layer is stacked on-top of each other to form the 3D model. One dominant technique, laser sintering (LS) utilizes a laser to heat and fuse thermoplastic powder together. Laser scanning requires fast traverse speeds and high heating rates in order to economically bond the materials. However, the resulting temperature gradients and short processing time (<1 ms) limit the materials that can be sintered and the properties that are achieved. A new AM technology, Large Area Projection Sintering (LAPS), creates parts by fusing an entire layer of material simultaneously. By fusing an entire layer, longer exposure times can be employed without compromising overall build time. This allows for significantly lower peak temperatures (and thus less degradation) and allows more time for the material to densify. For example, Peak surface temperatures in LAPS are less than 20 ^○C above preheat temperature compared to over 100 ^○C reported for LS. This work reports on the sintering kinetics of the most common LS material: polyamide 12 (PA12). The results of processing at longer times (~1-10s) using the LAPS process are compared to similar materials processed with commercial sintering equipment. LAPS is shown to more fully melt the powder than LS.. It also maintains or improves on LS strength while significantly increasing ductility. Thissuggests that projection sintering could open the door to processing new materials with greater temperature sensitivity and increased opportunities for spatial control over sintered properties.

As increasing of Greenhouse gas (GHG) emission, global warming has been accelerated in the world. In order to reduce the emission of carbon dioxide (CO₂), many researchers make an effort on development of new technology. Our group has been interested in UV curable coating materials using biomass derived isosorbide and its derivative due to lower birefringence, highly UV resistance and transparency etc. In this research, we synthesized eco-friendly urethane acrylate (UA) using isosorbide and other acrylate monomers. Urethane acrylate were polymerized isosobide methacrylate and poly(tetramethylene ether) glycol (PTMG) using two kinds of isocyanate of 4,4′-Methylenebis(cyclohexyl isocyanate (H₁₂MDI) and Isophorone diisocyanate (IPDI) which are non-yellowing type chemicals. It were found that the molecular weight average (Mw) for PUA has a range of 2,000~6,000 g/mol and PDI of 2.1~2.2 via Gel permeation chromatography (GPC). We will investigate curing kinetics of isosobide methacrylate and their mixture and apply to 3 dimensional structure of combination rear lamp for automotive using DLP-SLA 3D printing technology.

Additive manufacturing (AM) is transforming the way we design and fabricate structures on many scales. At small length scales, additive micromanufacturing is expected to expand the capabilities of microfabrication significantly. This enabling character of microscale AM has been demonstrated by the rise and spread of microstereolithography (MSL) [1].
In the past decade, a multitude of additional microscale AM techniques have been developed. Many of them aim to expand the range of materials available for microscale AM from the organic photoresists of MSL to inorganic materials and metals [2].
Determining the nature and the quality of these synthesized materials is a key aspect for establishing AM in microfabrication, since long proven quality standards have to be met. Additionally, knowledge of a material's microstructure and its influence on the material’s properties is the first step towards engineering and optimizing the material’s performance.
In order to determine the relationships between microscale AM methods and the deposited materials with a consistent methodology, we undertook the first, comprehensive comparison of the microstructure and the mechanical properties of metals fabricated with most of the presently suggested microscale metal AM technique. This work is a collaborative effort of multiple groups in the field of microscale metal AM: the range of techniques studied includes well established methods, e.g., focused electron beam induced deposition and direct ink writing, as well as more novel approaches, e.g., electrohydrodynamic printing and electrochemical deposition. The mechanical performance of the printed structures was evaluated using nanoindentation and microcompression, and the materials’ microstructure was analyzed using cross-sectional electron microscopy.
Both the elastic and plastic properties were found to vary by orders of magnitudes between the individual techniques. We show that these differences can be related to the large variations in microstructure of the deposited materials. These microstructures in turn are coupled with the various physico-chemical principles exploited by the different printing methods. Some microscale AM techniques are demonstrated to deliver materials with dense and crystalline microstructures with excellent mechanical properties, comparable to those of bulk nanocrystalline materials.
This study demonstrates that metallic materials with a wide range of microstructures and properties can be synthesized by contemporary microscale AM techniques. It is intended to provide practical guidelines for future users of these methods and help to establish AM techniques in microfabrication.

[1] J. K. Hohmann, M. Renner, E. H. Waller, G. von Freymann, Adv. Opt. Mater. 2015, 3, 1488.
[2] A. Reiser, L. Hirt, R. Spolenak, T. Zambelli, Adv. Mater. 2017, 201604211, 1604211.

Direct ink writing of metal-oxide hydrocolloid systems enables the manufacturing of biologically inspired, hierarchical architectures using environmentally friendly and biocompatible materials. Furthermore, direct ink writing allows for the previously unattainable incorporation of delicate materials, such as proteins and peptides. In this work, we report on the development of solution-based inks designed around food-grade hydrocolloids, such as Xanthan and Guar gum, compatible with direct ink writing. With this advance, the additive manufacturing of biomedical devices, including tissue scaffolds and stents with embedded drugs and proteins, is possible.
Within this system, the hydrocolloids supply a porous matrix and template suitable for the incorporation of organic molecules, while the metal-oxides provide the necessary mechanical properties. It is found that the porosity, pore-size distribution, and viscosity of the inks may be engineered through variation of the hydrocolloid weight percent, as well as through the refinement of the foaming process. Viscosity measurements, dynamic mechanical analysis, and compressive stress-strain analysis are used to characterize the materials’ rheological and mechanical properties. Moreover, close analyses of the printing parameters are used to create processing maps of the inks, identifying those parameters which result in spanning, free standing, and planar structures, all while maintaining the engineered porous microstructures. Ultimately, this work enables the direct ink writing of biologically inspired, hierarchically porous architectures with embedded organic materials for use in biomedical devices.

The application space for additive manufacturing (AM) has grown significantly through the use of high-performance composite materials. While the mechanical, thermal, optical, and electrical properties of AM polymer composites are being actively studied, the magnetic properties of AM parts have seen much less attention. Even so, AM magnetic composites have a chance to impact a variety of industries that make magnetic components, such as transformer cores, electric motors, and electromagnetic interference shielding. Recent work in magnetic polymer composite AM has shown that the structural print settings for a fused deposition modeling (FDM) process influence the magnetic properties of the printed part [Bollig17]. However, the structural hierarchy present in these AM composites complicates a simple analysis of how these differences arise. Further investigation of the structure-property relationships is required to understand exactly how and why these property changes arise. Here, a variety of samples were investigated to disentangle how the macroscopic sample shape and the mesoscopic filament infill orientation affects the magnetic properties. A magnetic filament consisting of polylactic acid (PLA) polymer and 40 wt.% iron was used to print the 3D magnetic samples via FDM. The array of samples systematically covered different aspect ratios (length, width), overall geometry (rectangular vs. ellipsoidal), and two filament print orientations (long axis alignment vs. short axis alignment). In-plane magnetic hysteresis loops were collected with a vibrating sample magnetometer (VSM) to determine what effects these macroscopic and mesoscopic structural factors had on the magnetic properties. Since previous research demonstrated that the 2D layering of FDM led to an out-of-plane magnetic hard axis, only the in-plane magnetic properties as a function of structure were probed in this work. The samples were measured with the field applied along the longitudinal and transverse directions, and the resulting hysteresis loops were used to compare the magnetization, magnetic moment, and susceptibility for the various structural factors. The results show that the macroscopic shape (aspect ratio and geometry) of the sample has a prominent effect on the magnetic properties. The longitudinal (long) axis of the sample yields a higher saturation magnetization (Ms) than the transverse (short) axis, and shapes with ellipsoidal edges yield a higher Ms than rectangular shapes. The mesoscopic orientation of the printed filament within the sample had a subtler effect; magnetic susceptibility increases in the direction that the filament was printed. Knowledge of how these structural factors within the FDM structural hierarchy affect the magnetic properties of printed parts is helpful for designing parts with optimized performance. Further preliminary studies into magnetic annealing before, during, and after part printing will also be discussed.

The properties of lateral Ni film etching by TFB etchant at room temperature were examined by using samples with patterned lamellae layers consist of Ni, Al₂O₃, and SiO₂ for potential application of fabricating hierarchical structures. Lateral etching length was increased with increasing etching time, despite of the difficult passage through nanoscale gap in lamellae layers. However, higher etching rate (2.1 nm/s) was observed in lower Ni film, which has contact with SiO₂ and Al₂O₃ layers, compared to that (1.6 nm/s) of upper Ni film, which has contact with only Al₂O₃, due to stronger wetting property of SiO₂, inducing easier penetration of etchant into nanoscale passage between surrounding layers. The influence of the type of the contact material on the lateral Ni film etching was confirmed by the insertion of ~20 nm Al₂O₃ at the interface between the bottom SiO₂ and lower Ni film, resulting in a similar lateral etching rate of both the upper and lower Ni films. In addition, the effect of Ni film thickness on the lateral etching characteristics was examined. Despite different contact materials on the upper and lower Ni films, almost no difference in the lateral etching lengths was founded in the sample with 150-nm-thick Ni films, which implies that the interface region affected by the contact materials is negligible.

ZnO nanowires materials are being developed commercially for photocatalytic applications due to their wide and direct band gap, large exciton energy, high electron mobility, and high thermal conductivity. Recent experiments growing ZnO through chemical vapour deposition observe the autocatalytic growth of nanoscale [0001] aligned ZnO wires from the apex points of much larger pyramidal shaped ZnO₂ islands. Motivated to understand the mechanisms behind this abrupt transition in growth morphology, we here report on density functional theory calculations to predict the structure and surface energy of these facets at different stages of growth.

Reducing the surface roughness of an additive manufacturing (AM) component is one of the most critical factors in determining the suitability of an AM component. The typical as produced surface roughness of an AM component range from 100-500 micron inch. However, for most of the engineering application surface roughness must come down below 10 µ inch. Reducing surface roughness is exponentially more challenging for the internal surfaces of a component. This paper reports our research in the area of postprocessing of the interior surfaces of the AM component. We have applied electropolishing and chemical polishing methods to reduce the surface roughness of the internal surface. We found that chemical polishing route was very effective in simultaneously reducing the internal and external surface roughness of the steel AM components from ~300 µ inch to ~20 µ inch range. Chemical polishing is found suitable for any complicated AM shape and geometry. Our electropolishing methodology was very effective in reducing the surface roughness of the internal or external surface individually but not simultaneously. However, electropolishing produced the relatively better performance on the outer surfaces with the optimized process parameters. The surface roughness of the 316 steel AM components reduced from 300 µinch.to 3 µinch. However, electropolishing of the internal surfaces of the cubical or spherical cavity was not uniform and was very much dependent on the counter electrode geometry, electrolyte flow rate, and bath temperature. In this paper, we will summarize our research efforts to tackle the critical issue of reducing the surface roughness of the complex AM components.

This study presents 3D printing and testing of pressure/strain/temperature sensor using poly(vinylidene) fluoride (PVDF), BaTiO₃ (BT), and multiwall carbon nanotubes (MWCNTs) through the fused deposition modeling (FDM) 3D printing technique. PVDF polymer and BT ceramics are piezo-, pyro- and di-electric materials extensively investigated for sensor and energy storage/harvesting applications due to their unique characteristic of dipole polarization. However, when combined they rarely provide good piezoelectric performance due to the low coupling coefficient between their interfaces. In this study, MWCNTs were utilized to resolve the low coupling coefficient issue by dispersing MWCNTs in the PVDF matrix to create stress reinforcing network, dispersant, and electron conducting functions for BT nanoparticles. Moreover, due to high thermal conductivity characteristics of MWCNTs it can provide high thermal sensibility when subject to temperature change by influencing capacitance on nanocomposites. In addition, since MWCNTs and BT provide very high mechanical strength and strain respectively in PVDF matrix high change of poison ratio when subject to tensile force can be utilized to sense a strain as impact of capacitance and resistance. Various BT and MWCNTs percentages of nanocomposite film were fabricated by the FDM 3D printing which can simplify the fabrication process while providing lower cost and design flexibility. The increasing MWCNTs and BT particles gradually increase the piezoelectric coefficient (d₃₁) by 129 pC/N with 0.4wt.%-MWCNTs/18wt.%-BT/PVDF. The measured d₃₁ of the printed nanocomposites is comparable with pure BT ceramics or composites (79 ~ 185 pC/N). It was observed that 1wt.%-MWCNTs/12wt.%-BT/PVDF and 1.7wt.%-MWCNTs/60wt.%-BT/PVDF nanocomposites show highest strain and temperature sensibility. These results provide not only a technique to 3D print piezoelectric nanocomposites but also unique combination of BT and PVDF with MWCNTs for applications in multiple sensing application.

The unique properties of aligned nanomaterials make them attractive for a number of applications such as in flexible electronics and sensing. When anisotropic nanofillers such as carbon nanotubes, graphene and cellulose are aligned in the final composite and when deposition of nanomaterials is coupled with subsequent processing (thermal, laser annealing) it gives rise to enhanced mechanical strength, thermal and electrical properties. As a first step towards achieving this goal, we report on three different projects which deal with developing anisotropic fillers – depositing carbon nanotubes (CNT) on flexible substrates for electronics, developing graphene composites for thermal management devices and combining cellulose nanocrystals with thermoplastic polyurethane (TPU) to form conducting composites for use as sensors.
Due to the 2-D geometry, sp2 bonding structure, and the large aspect ratio of graphene, formation of electrically percolated networks at low nano-filler loadings is possible. Graphite composed of a few layers of graphene can therefore serve as structural components in polymer composites with multiple functionality. Utilization of CNCs has also gained popularity due to their excellent mechanical properties, high aspect ratios, and reactive surfaces that permit conductive metal particle surface functionalization. We take advantage of these properties through additive manufacturing where alignment is a result of shear forces during printing.
A method was developed to deposit CNT’s on flexible polydimethoxysilane (PDMS) and TPU and the samples were characterized by microscopy and Raman Spectroscopy to identify the CNT’s. Alignment is induced in the nanotubes by printing a concentrated sample onto the flexible susbstrate. For the cellulose project, an ink was successfully developed using Dimethyl Sulfoxide (DMSO) as a solvent and the samples successfully 3D printed using a printer. An interplay of cellulose concentration, nozzle diameter and print speed was used to achieve optimal print resolutions. By maintaining the solids volume fraction at 20% by weight, we are able to extrude CNC inks of varying cellulose concentrations using the 100 micron nozzle for the best 3D printing resolutions. Mechanical testing of the printed structures was initiated to test the effect of alignment of cellulose crystals on mechanical properties. Graphite nanoplatelets and a two-part epoxy resin (EPON 862) were mixed in a planetary centrifugal mixer to form conductive ink dispersions. The rheology of the ink was tuned by varying concentration of graphite, filler materials (carbon fibers) and solvent (acetone) to achieve multi-layer printability and thermal conductivity. Once the samples were thermally cured, the electrical, thermal and mechanical properties were measured.

Multi-layered structures of AZ31/Ti have been successfully fabricated by single pass hot-rolling. The microstructure and mechanical behavior of the AZ31/Ti multilayers are studied by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), nanoindentation, tensile, and shear tests. The individual sheets were heated to 450°C for 10 min in an argon atmosphere furnace, and then hot-rolled with a single-pass at thickness reductions 25%, 38%, 50%, and 55%. Heat treatment was applied to the rolled samples at 400 °C for 6h, 12h and 24 h in an argon atmosphere. The results showed that the Ti/AZ31 was well bonded. Loading and unloading curves were obtained at AZ31/Ti interface by a nanoindentation test. X-ray diffraction patterns verified that there was no intermetallic compound formed at the interface of the as-rolled and heat-treated samples. EDS results indicated that a thick bonding layer was formed.The shear strength of the interface bonds increases with the heat-treatment.

The conventional techniques for 3D printing ceramic parts are the ceramic powder laser sintering and lithography process that are usually costly in equipment and materials. A low-cost technique for 3D inkjet printing ceramic has been developed via polysilanzane preceramic precursor incorporated with γ-alumina nano powders to form a gel-like preceramic precursor with the aim to be used as the raw material for 3D inkjet printing ceramic parts. The gel-like preceramic precursor is simply extruded for 3D shaping and then is pyrolyzed at 700, 850 and 950 °C, respectively to transform from a polymeric composite to silicon aluminum oxycarbonitride ceramic part that has been characterized by XRD representing an amorphous silicon oxycarbonitride structure coating on the surface of γ-alumina. The Vickers hardness of the silicon aluminum oxycarbonitride (SiAlCNO) ceramic pryrolyed at 850°C is about 225~275 twice higher than that of 6 and 7 series aluminum alloy. The organic liquidic polysilanzane preceramic precursor pyrolyzed at 600 °C to form an amorphous SiCNO ceramic coated on the surface of the anodizing aluminum alloy provides the high resistance to wearing. After the SiCNO coated-anodizing aluminum alloy is worn back and forth by the steel velvet head with the wearing distance, 1.44 km and 2.88 km, the weight loss with each wearing distance is 0 g and 1.8 mg respectively that this result indicates that the SiCNO surface coating on the aluminum alloy provides an extremely hard surface to enhance the anti-wearing capability.

The possibility of using fused deposition modeling (FDM) to produce electromagnetic interference (EMI) shields may reduce drawbacks of current EMI shields made from sheet metal. These drawbacks include cost, weight, and freedom of design. However, the performance of these FDM-printed composite materials for EMI applications must first be assessed. Here, we focus on electrically conductive polymer composite parts made via FDM. In this work, we directly compare compounded samples of the most common polymer EMI shielding composite formulations found in the literature, including graphite, stainless steel, Ni-coated carbon fibers, and Ni-coated carbon nanotube fillers. Additionally, in order to examine the influence of structural print parameters, studied samples consist of a variety of relative print orientations (45° and 90°) and thicknesses (2mm, 6mm, and 1cm). Because electrical conductivity is a main characteristic of successful EMI shields, we test the electrical properties of these various polymer-matrix composite materials via both DC and AC methods in order to assess their shielding effectiveness. First, we measure the individual FDM filament samples and printed parts via DC conductivity measurements. Conductive sample contacts are made by using sputtered metallic contacts and metallic paint, and the measured electrical resistance is converted to electrical resistivity. For AC measurements, we conduct testing of the electromagnetic interference shielding effectiveness as a function of frequency. AC fields are emitted and received through sample sheets using a spectrum analyzer and two loop antennas. Preliminary results show that the Ni-coated carbon nanotubes and the graphite samples show the most optimal characteristics for EMI shielding, while the stainless steel samples also show promising performance. Results also show that varying print orientation does have an effect on the performance of the printed parts.