Search

Electrophoretic deposition (EPD) of platinum nanoparticles (PtNPs) on platinum-iridium (Pt-Ir) neural electrode surfaces is a promising strategy to tune the impedance of electrodes implanted for deep brain stimulation in various neurological disorders such as advanced Parkinson's disease and dystonia. However, previous results are contradicting as impedance reduction was observed on flat samples while in three-dimensional (3D) structures, an increase in impedance was observed. Hence, defined correlations between coating properties and impedance are to date not fully understood. In this work, the influence of direct current (DC) and pulsed-DC electric fields on NP deposition is systematically compared and clear correlations between surface coating homogeneity and in vitro impedance are established. The ligand-free NPs were synthesized via pulsed laser processing in liquid, yielding monomodal particle size distributions, verified by analytical disk centrifugation (ADC). Deposits formed were quantified by UV-vis supernatant analysis and further characterized by scanning electron microscopy (SEM) with semiautomated interparticle distance analyses. Our findings reveal that pulsed-DC electric fields yield more ordered surface coatings with a lower abundance of particle assemblates, while DC fields produce coatings with more pronounced aggregation. Impedance measurements further highlight that impedance of the corresponding electrodes is significantly reduced in the case of more ordered coatings realized by pulsed-DC depositions. We attribute this phenomenon to the higher active surface area of the adsorbed NPs in homogeneous coatings and the reduced particle-electrode electrical contact in NP assemblates. These results provide insight for the efficient EPD of bare metal NPs on micron-sized surfaces for biomedical applications in neuroscience and correlate coating homogeneity with in vitro functionality.

Carbon aerogels (CAs) combine unique properties including ultra-high surface area, high electrical conductivity, corrosion resistance, and robust mechanical properties making them ideal materials for electrochemical applications. Traditional CA synthesis results in isotropic, random nanoporous networks that work well for applications relying on diffusional mass transport. However, many applications would benefit from integration of engineered macroporous network structures that enable directed pressure-gradient-driven mass transport. Here, we report on using 3D-printed sacrificial polymeric templates to generate templated CAs (t-CAs) with integrated engineered nonrandom macroporous network structures. Specifically, we used projection micro-stereo-lithography (PμSL) and two-photon polymerization direct laser writing (2PP-DLW) to fabricate millimeter-to-centimeter-sized 3D sacrificial polymeric templates with features ranging from tens of microns (PμSL) to 100s of nanometers (2PP-DLW). T-CAs were fabricated by infiltrating the templates with resorcinol-formaldehyde (RF) precursor solution, followed by carbonization at 1050 °C to simultaneously convert the RF gel to a CA and decompose the 3D-printed template, leaving an embedded templated macroporous network structure behind. X-ray computer tomography confirms integration of the macroporous architecture defined by the template. The templated macroporous architecture improves mass transport in t-CAs compared to traditional bulk CA as demonstrated by more uniform activation and their response in electrochemical cyclic voltammetry and galvanostatic charge-discharge tests.

Nanofabrication techniques that can generate large and complex 3D structures with nanoscale features are becoming increasingly important in the fields of biomedicine, micro-optics, and microfluidics. Direct laser writing via two-photon polymerization (DLW-TPP) is one such technique that relies on nonlinear absorption of light to form nanoscale 3D features. Although DLW-TPP provides the required nanoscale resolution, its built height is often limited to less than a millimetre. This height limitation is driven by the need to tightly focus the laser beam at arbitrary depths within the photopolymer. This requirement necessitates matching the photopolymer's refractive index to specific values but the required techniques have not been disseminated widely in the open scientific literature. To address this knowledge gap, we test two universal, different approaches to generate refractive index-matched polymeric and preceramic resins and demonstrate their performance by printing of fine submicron features in 3D structures as tall as 2.5 mm. Specifically, we achieve index-matching by mixing commercially-available resins or covalent modification of functional monomers. This work investigates the relationship of voxel shape to RI mismatch, and presents tuning of RI through mixing and covalent modification to a nonconventional material system of preceramic resin which has never been demonstrated before. We demonstrate the material flexibility by generating 3D silicon oxycarbide structures from preceramic resists while simultaneously eliminating the part-height limitation of conventional DLW-TPP.

Electrochemical reactors utilizing flow-through electrodes (FTEs) provide an attractive path toward the efficient utilization of electrical energy, but their commercial viability and ultimate adoption hinge on attaining high currents to drive productivity and cost competitiveness. Conventional FTEs composed of random, porous media provide limited opportunity for architectural control and engineering of microscale transport. Alternatively, the design freedom engendered by additively manufacturing FTEs yields additional opportunities to further drive performance via flow engineering. Through experiment and validated continuum computation we analyze the mass transfer in three-dimensional (3D)-printed porous FTEs with periodic lattice structures and show that, in contrast to conventional electrodes, the mesoscopic length scales in 3D-printed electrodes lead to an increase in the mass correlation exponent as inertial flow effects dominate. The inertially enhanced mass transport yields mass transfer coefficients that exceed previously reported 3D-printed FTEs by 10 to 100 times, bringing 3D-printed FTE performance on par with conventional materials.

The quality of metal objects fabricated via laser powder bed fusion are highly affected by process parameters, and their influence on final products is yet to be fully explored. In this work, pyrometry signals of the melt pool were collected from a set of stainless-steel samples during manufacturing and the effect of laser power on porosity and roughness of final printed parts was analyzed. Results show that the melt pool pyrometry signal of contours increases with higher laser power, whereas it is lower and decreases for the infilled part. Post-built X-ray computed tomography imaging reveals that porosity decreases while sample roughness increases upon increasing laser power. The decrease in porosity with increasing laser power is attributed to the larger size of the contour welds that were printed first, leading to an increase in dimension of the final products.

High-precision Hugoniot measurements of heterogeneous materials are a key tool in creating accurate equations of state, but such measurements are prone to larger uncertainties than their homogeneous counterparts. To efficiently reduce these uncertainties one must estimate the relative contributions of different error sources. Sample cracking is one likely source. Here we estimate its contribution through modeling, using computed tomography scans of powdered single crystal quartz samples to estimate typical packing variation within and between samples. Samples were prepared using the same materials and methods. Cracks were prominent in the samples. We describe a simple model to estimate shock transit time variation for a single shot due to cracking, and estimate that up to 3% variation can be attributed to these structures. This suggests that decreasing variation requires addressing such packing heterogeneities during target fabrication rather than experimental diagnostics.

Recent advances in additive manufacturing, specifically direct ink writing (DIW) and ink-jetting, have enabled the production of elastomeric silicone parts with deterministic control over the structure, shape, and mechanical properties. These new technologies offer rapid prototyping advantages and find applications in various fields, including biomedical devices, prosthetics, metamaterials, and soft robotics. Stereolithography (SLA) is a complementary approach with the ability to print with finer features and potentially higher throughput. However, all high-performance silicone elastomers are composites of polysiloxane networks reinforced with particulate filler, and consequently, silicone resins tend to have high viscosities (gel- or paste-like), which complicates or completely inhibits the layer-by-layer recoating process central to most SLA technologies. Herein, the design and build of a digital light projection SLA printer suitable for handling high-viscosity resins is demonstrated. Further, a series of UV-curable silicone resins with thiol-ene crosslinking and reinforced by a combination of fumed silica and MQ resins are also described. The resulting silicone elastomers are shown to have tunable mechanical properties, with 100–350% elongation and ultimate tensile strength from 1 to 2.5 MPa. Three-dimensional printed features of 0.4 mm were achieved, and complexity is demonstrated by octet-truss lattices that display negative stiffness.

PyPhase is a free and open-source Python package for propagation-based near-field phase reconstructions, which implements some of the most popular phase-retrieval algorithms in a highly modular framework supporting the deployment on large-scale computing facilities. This makes integration, development of new phase-retrieval algorithms, and the deployment on different computing infrastructures straightforward., X-ray propagation-based imaging techniques are well established at synchrotron radiation and laboratory sources. However, most reconstruction algorithms for such image modalities, also known as phase-retrieval algorithms, have been developed specifically for one instrument by and for experts, making the development and diffusion of such techniques difficult. Here, PyPhase, a free and open-source package for propagation-based near-field phase reconstructions, which is distributed under the CeCILL license, is presented. PyPhase implements some of the most popular phase-retrieval algorithms in a highly modular framework supporting its deployment on large-scale computing facilities. This makes the integration, the development of new phase-retrieval algorithms, and the deployment on different computing infrastructures straightforward. Its capabilities and simplicity are presented by application to data acquired at the synchrotron source MAX IV (Lund, Sweden).

Laser powder bed fusion (LPBF) is a highly dynamic multi-physics process used for the additive manufacturing (AM) of metal components. Improving process understanding and validating predictive computational models require high-fidelity diagnostics capable of capturing data in challenging environments. Synchrotron x-ray techniques play a vital role in the validation process as they are the only in situ diagnostic capable of imaging sub-surface melt pool dynamics and microstructure evolution during LPBF-AM. In this article, a laboratory scale system designed to mimic LPBF process conditions while operating at a synchrotron facility is described. The system is implemented with process accurate atmospheric conditions, including an air knife for active vapor plume removal. Significantly, the chamber also incorporates a diagnostic sensor suite that monitors emitted optical, acoustic, and electronic signals during laser processing with coincident x-ray imaging. The addition of the sensor suite enables validation of these industrially compatible single point sensors by detecting pore formation and spatter events and directly correlating the events with changes in the detected signal. Experiments in the Ti–6Al–4V alloy performed at the Stanford Synchrotron Radiation Lightsource using the system are detailed with sufficient sampling rates to probe melt pool dynamics. X-ray imaging captures melt pool dynamics at frame rates of 20 kHz with a 2 µm pixel resolution, and the coincident diagnostic sensor data are recorded at 470 kHz. This work shows that the current system enables the in situ detection of defects during the LPBF process and permits direct correlation of diagnostic signatures at the exact time of defect formation.

As-processed metals and alloys by selective laser melting (SLM), or laser powder bed fusion (L-PBF), are often full of defects and flaws such as dislocations, twins, elemental segregations, impurities and porosities, which can positively or negatively impact mechanical properties. Here, we systematically characterize the tensile behavior of L-PBF Ti-6Al-4V at quasi-static strain rate and room temperature, including state-of-the-art in situ synchrotron X-ray diffraction (SXRD) and computed tomography (SXCT). These studies reveal that the tensile yield strength and uniform elongation are mainly dictated by the as-built microstructure, while the strain-to-failure is sensitive to the porosity, even in very high-density samples (>99.5%). in situ SXRD reveals that the micro-plasticity in as-built Ti64 initiates at a stress level well below its macroscopic yield strength, signified by the early lattice strain deviation behavior of {0002} and {112¯0} reflections. SXCT reveals pore growth mechanisms when the tensile axis is perpendicular to the build direction, whereas no such behavior is observed as the tensile axis is along the build direction. These anisotropic pore growth mechanisms result in vast differences in the strain-to-failure of L-PBF materials. Our melt-pool dynamics modeling with similar laser conditions to the experiments identifies a previously unknown pore source; i.e., edge-of-track pores. We present a normalized energy diagram to identify the optimized processing window for high quality samples. Keywords: Selective laser melting, Ti-6Al-4V, Synchrotron X-ray diffraction, X-ray computed tomography, Pores, Plasticity

Two-photon lithography (TPL) is a high-resolution additive manufacturing (AM) technique capable of producing arbitrarily complex three-dimensional (3D) microstructures with features 2-3 orders of magnitude finer than human hair. This process finds numerous applications as a direct route toward the fabrication of novel optical and mechanical metamaterials, miniaturized optics, microfluidics, biological scaffolds, and various other intricate 3D parts. As TPL matures, metrology and inspection become a crucial step in the manufacturing process to ensure that the geometric form of the end product meets design specifications. X-ray-based computed tomography (CT) is a nondestructive technique that can provide this inspection capability for the evaluation of complex internal 3D structure. However, polymeric photoresists commonly used for TPL, as well as other forms of stereolithography, poorly attenuate X-rays due to the low atomic number (Z) of their constituent elements and therefore appear relatively transparent during imaging. Here, we present the development of optically clear yet radiopaque photoresists for enhanced contrast under X-ray CT. We have synthesized iodinated acrylate monomers to formulate high-Z photoresist materials that are capable of forming 3D microstructures with sub-150 nm features. In addition, we have developed a formulation protocol to match the refractive index of the photoresists to the immersion medium of the objective lens so as to enable dip-in laser lithography, a direct laser writing technique for producing millimeter-tall structures. Our radiopaque photopolymer resists increase X-ray attenuation by a factor of more than 10 times without sacrificing the sub-150 nm feature resolution or the millimeter-scale part height. Thus, our resists can successfully replace existing photopolymers to generate AM parts that are suitable for inspection via X-ray CT. By providing the "feedstock" for radiopaque AM parts, our resist formulation is expected to play a critical role in enabling fabrication of functional polymer parts to tight design tolerances.

Rapid solidification cellular structures are known to play a crucial role in helping achieve high strength and high ductility in 316L austenitic stainless steels fabricated by laser powder-bed-fusion (L-PBF). Despite this, the understanding of their intrinsic characteristics (e.g., crystallographic orientations, dislocations, precipitates, elemental segregations) and the respective impacts on the material's strength and thermal stability remains nebulous. We conduct several dedicated transmission electron microscopy (TEM) studies to investigate these strengthening mechanisms and identified that cell walls follow specific crystallographic orientations. The high density of tangled dislocations inside cell walls are found to have a higher tendency to dissociate, forming wider stacking faults while oxide precipitates are confined inside cell walls. These features act as barriers to moving dislocations upon plastic deformation and contribute to the high strength. Our dislocation dynamic simulations indicate that segregated particles are effective in blocking dislocations locally, helping the formation of dislocation cells and participating to the material strengthening. To study the thermal stability of L-PBF 316L SS, we perform systematic post-processing heat treatments from 400-1200°C. Microstructure characterizations using electron backscatter diffraction, TEM, and synchrotron X-ray diffraction coupled with dislocation dynamics and CALPHAD simulations and tensile testing reveal three heat treatment zones where the structure-property relationship can be tuned. After annealing up to 600°C, the microstructure remains stable; but the work hardening behavior is altered with a material that retains high strength and high ductility. Annealing between 600-1000°C activates elemental diffusion and gradual disappearance of cell walls, leading to a sharp drop in yield strength and a tradeoff between strength and ductility. Low-angle grain boundaries remain stable up to 1000°C while the average grain size defined by high angle grain boundaries is near constant at annealing temperatures up to 800°C. Annealing above 1100°C removes all L-PBF microstructure footprints and renders a conventional-like microstructure. Compared to conventional materials, L-PBF 316LSS displays substantially higher thermal stability and superior performance at elevated temperatures.

Laser Powder Bed Fusion (LPBF) is the predominant metal additive manufacturing technique that benefits from a significant body of academic study and industrial investment given its ability to create complex geometry parts. Despite LPBF’s widespread use, there still exists a need for process monitoring to ensure reliable part production and reduce post-build quality assessments. Towards this end, we develop and evaluate machine learning-based predictive models using height map-derived quality metrics for single tracks and the accompanying pyrometer and high-speed video camera data collected under a wide range of laser power and laser velocity settings. We extract physically intuitive low-level features representative of the meltpool dynamics from these sensing modalities and explore how these vary with the linear energy density. We find our Sequential Decision Analysis Neural Network (SeDANN) model – a scientific machine learning model that incorporates physical process insights – outperforms other purely data-driven black box models in both accuracy and speed. The general approach to data curation and adaptable nature of SeDANN’s scientifically informed architecture should benefit LPBF systems with an evolving suite of sensing modalities and post-build quality measurements.

Flow through electrodes are the core, active component in a number of electrochemical systems including fuel cells and flow batteries. Controlling fluid flow and mass transport is a core engineering challenge in these systems and has a dramatic impact on cell power, utilization, and efficiency. Most advanced electrode materials are composed of microscale, disordered porous media. The length scales of these materials allow for very large surface area but at the cost of low flow speeds, low surface mass transfer rates, high pressure drops, and added complexity and cost from the need to use external flow distribution systems like flowfields. The multiscale control over electrode architecture enabled by using additively manufactured electrodes could provide a potential resolution to these trade-offs. To elucidate the mass transport of these electrodes, we use resolved, continuum computation to analyze the fluid flow and species transport in direct ink-write printed, carbon aerogel electrodes. We determine the mass transfer correlations (Sh ~ Pea) across two different lattice aerogel geometries and validate these results against the experimentally manufactured and tested flow-through electrodes operated at limiting current. In contrast to conventional electrodes, the mesoscopic length scales in our electrodes lead to finite Reynolds numbers and we find an increase in the mass correlation exponent (a) as inertial effects become important. We analyze the local mass transfer coefficients in the electrode and correlate regions of enhanced mass transfer with secondary flows in the electrode. Our work provides a promising new pathway to increased mass transfer rates in flow-through electrodes. LLNL-ABS- 810719 This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Creation of pores and defects during laser powder bed fusion (LPBF) can lead to poor mechanical properties and thus must be minimized. Post-build inspection is required to ensure the printed parts contain acceptably low defect concentrations. These inspections are time consuming and costly, especially for large or complex parts. As a potential solution, in situ process monitoring can be used to detect the creation of defects, characterize local material behavior and predict expected component properties. However, the precise relationship between pore creation and in situ process monitoring still needs to be understood. In this work, high-speed infrared diode-based pyrometry and high-speed optical imaging signals were used to monitor LPBF printing of 446 stainless steel 316 L single tracks with varying laser power and velocity. Results indicate an increase in pyrometer signal and melt pool dimensions with increasing laser power and decreasing velocity in agreement with previous work. In addition, careful analysis of pyrometer signal reveals a distinct signature of the conduction-to-keyhole mode transition which was confirmed by metallography. Critically, pore defect initiation as characterized by ex situ X-ray radiography was correlated with in situ thermal monitoring signals to derive the probability of defect creation. Our results show that, in principle, a probabilistic prediction of pore formation can be achieved based on in situ high-speed pyrometry monitoring of the LPBF melt pool.

Compared to tedious, multi-step treatments for electroless gold plating of traditional thermoplastics, this communication describes a simpler three-step procedure for 3D printed crosslinked polyacrylate substrates. This allows for the synthesis of ultralight gold foam microlattice materials with great potential for architecture-sensitive applications in future energy, catalysis, and sensing.

The tough bulk of dentin in teeth supports enamel, creating cutting and grinding biostructures with superior failure resistance that is not fully understood. Synchrotron-based diffraction methods, utilizing micro- and nanofocused X-ray beams, reveal that the nm-sized mineral particles aligned with collagen are precompressed and that the residual strains vanish upon mild annealing. We show the link between the mineral nanoparticles and known damage propagation trajectories in dentin, suggesting a previously overlooked compression-mediated toughening mechanism.

Biomineralization in organisms is strictly regulated, and therefore, chemical compositions as well as crystal structures of the minerals are species specific. During the embryonic development, sea urchin larvae produce a calcite endoskeleton (spicules) that contains about 5% of Mg. For sea urchins and other organisms, it is assumed that Mg is important for the process of biomineralization and for the mechanical properties of the resulting biomineral. To study the influence of Mg on skeletal growth and on biomineral structure and composition, sea urchin larvae spicules were chosen as an in vivo test system. For this purpose, the Mg/Ca ratio was modified in the artificial seawater medium wherein sea urchin larvae were growing. It was shown that Mg deficiency during larval development caused morphology defects of the larvae and of their calcite spicules. The Mg distribution within the larvae skeleton was analyzed and found to be homogenous. An in vivo reduction of the Mg content influenced the mechanical performance of larval spicules (Young’s modulus and hardness). The investigations of larvae exposed to reduced Mg conditions highlight the important role Mg plays for sea urchin larvae development, biomineralization process and the resulting biomineral. The sea urchin larvae are presented as an ideal model to study different effects on larval development and morphology, especially on the biomineral properties.

Dentin in teeth is a bone-like nanocomposite built of carbonated hydroxyapatite (cHAP) mineral particles, protein and water that does not remodel nor heal. It is assumed to be excellently adapted for decades of mechanical function, due to the interplay between its constituents. Using samples of human origin, we combine heat treatments with synchrotron X-ray diffraction, second-harmonic generation microscopy, Raman spectroscopy, and phase contrast-enhanced nano-tomography to study the water-assisted functional coupling of the biocomposite components. Across roots we find a gradual reduction in the c-lattice parameter of the cHAP nano-crystals, from 6.894 Å externally down to 6.885 Å in deeper tooth regions. Here, approaching the pulp, tissue is formed at later stages of tooth development. In all regions, a compressive strain of ~0.3 % is observed upon drying by mild heating (125 ºC). Dehydration results in a substantial increase in the averaged microstrain fluctuations in the mineral nanoparticles. The mineral crystallite platelet lengths fall off from ~36 nm externally to ~26 nm closer to the pulp. Our results suggest that both morphology and mineral-collagen coupling allow mineral nano-particles in dentin to sustain rather large stresses of 300 MPa, far exceeding mastication stresses, and that such loads are sustained through durable collagen fibril contractions.

Tooth dentine and other bone-like materials contain carbonated hydroxyapatite nanoparticles within a network of collagen fibrils. It is widely assumed that the elastic properties of biogenic hydroxyapatites are identical to those of geological apatite. By applying hydrostatic pressure and by in situ measurements of the a- and c- lattice parameters using high energy X-ray diffraction, we characterize the anisotropic deformability of the mineral in the crowns and roots of teeth. The collected data allowed us to calculate the bulk modulus and to derive precise estimates of Young׳s moduli and Poisson׳s ratios of the biogenic mineral particles. The results show that the dentine apatite particles are about 20% less stiff than geological and synthetic apatites and that the mineral has an average bulk modulus K=82.7 GPa. A 5% anisotropy is observed in the derived values of Young׳s moduli, with E11≈91 GPa and E33≈96 GPa, indicating that the nanoparticles are only slightly stiffer along their long axis. Poisson׳s ratio spans ν≈0.30–0.35, as expected. Our findings suggest that the carbonated nanoparticles of biogenic apatite are significantly softer than previously thought and that their elastic properties can be considered to be nearly isotropic.

Monolithic porous bulk materials have many promising applications ranging from energy storage and catalysis to high energy density physics. High resolution additive manufacturing techniques, such as direct laser writing via two photon polymerization (DLW-TPP), now enable the fabrication of highly porous microlattices with deterministic morphology control. In this work, DLW-TPP is used to print millimeter-sized foam reservoirs (down to 0.06 g cm−3) with tailored density-gradient profiles, where density is varied by over an order of magnitude (for instance from 0.6 to 0.06 g cm−3) along a length of