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Results for “polycrystalline material”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

High power, extended temperature range-capable, highly abuse overcharge and discharge tolerant rechargeable battery cell and pack

Provided are electrochemical secondary cells that exhibit excellent abuse tolerance, deep discharge and overcharge conditions including at extreme temperatures and remain robust and possess excellent performance. Cells as provided herein include: a cathode a polycrystalline cathode electrochemically active material including the formula Li1+xMO2+y, wherein −0.9≤x≤0.3, −0.3≤y≤0.3, and wherein M includes Ni at 80 atomic percent or higher relative to total M, the cathode electrochemically active material comprising a non-uniform distribution of Co; an anode including an anode electrochemically active material of the formula Li4+aTi5O12+b wherein −0.3≤a≤3.3, −0.3≤b≤0.3; and wherein the anode and the cathode each independently include a current collector substrate comprising aluminium.

Sahin, Kenan↗
DOE OSTI2020

Polycrystalline Thin-Film Research: Copper Indium Gallium Diselenide

Copper indium gallium diselenide (CIGS) solar cells are one of the primary focuses of research by the Thin Film Material Science and Processing Group. The group develops processes and materials related to thin-film polycrystalline photovoltaic (PV) devices as well as the equipment required for routine analysis of these devices and materials. We work closely with other groups in the Materials Science Center to achieve a deeper understanding of thin-film materials and devices.

capabilities↗
DOE OSTI2020

Enhancing Photocarrier Bulk Lifetime with Defect Engineering of Polycrystalline Passivated-Contact n-Cz Photovoltaic Devices

We study the photocarrier lifetime evolution of n-Cz Si material throughout the processing sequence for polycrystalline passivated contact devices. We show that a high temperature annealing pretreatment (known as Tabula Rasa) has a clear effect on enhancing bulk lifetimes of n-Cz Si. We further this development by integrating such defect engineering into the lower-temperature annealing of passivated contact. By applying oxidizing ambient gases during these anneals we report a photocarrier lifetime enhancement over an N 2 environment. This enhancement is exhibited in a 1-sun iV OC of 735 mV when annealed in O 2 rather than 708 mV in N 2.

14 SOLAR ENERGY↗
DOE OSTI2023

Machine learning based approach to predict ductile damage model parameters for polycrystalline metals

Damage models for ductile materials typically need to be parameterized, often with the appropriate parameters changing for a given material depending on the loading conditions. This can make parameterizing these models computationally expensive, since an inverse problem must be solved for each loading condition. Using standard inverse modeling techniques typically requires hundreds or thousands of high-fidelity computer simulations to estimate the optimal parameters. Additionally, the time of a human expert is required to set up the inverse model. Machine learning has recently emerged as an alternative approach to inverse modeling in these settings, where the machine learning model is trained in an offline manner and new parameters can be quickly generated on the fly, after training is complete. Here, this work utilizes such a workflow to enable the rapid parameterization of a ductile damage model called TEPLA with a machine learning inverse model. The machine learning model can efficiently estimate the model parameters much faster, as compared to previously employed methods, such as Bayesian calibration. The results demonstrate good accuracy on a synthetic test dataset and is validated against experimental data.

36 MATERIALS SCIENCE↗
DOE OSTI2023

An AI-driven microstructure optimization framework for elastic properties of titanium beyond cubic crystal systems

Materials design aims to identify the material features that provide optimal properties for various engineering applications, such as aerospace, automotive, and naval. One of the important but challenging problems for materials design is to discover multiple polycrystalline microstructures with optimal properties. This paper proposes an end-to-end artificial intelligence (AI)-driven microstructure optimization framework for elastic properties of materials. In this work, the microstructure is represented by the Orientation Distribution Function (ODF) that determines the volume densities of crystallographic orientations. The framework was evaluated on two crystal systems, cubic and hexagonal, for Titanium (Ti) in Joint Automated Repository for Various Integrated Simulations (JARVIS) database and is expected to be widely applicable for materials with multiple crystal systems. The proposed framework can discover multiple polycrystalline microstructures without compromising the optimal property values and saving significant computational time.

36 MATERIALS SCIENCE↗
NASA NTRS1973

Electrode materials for charge coupled devices

The use of polycrystalline silicon in charged coupled semiconductor devices is discussed. In particular, the electrical and optical properties of the material are characterized.

Gajda, W. T. J.↗
NASA NTRS1982

A low-cost polysilicon process based on the synthesis and decomposition of dichlorosilane

Major process steps of a dichlorosilane based chemical vapor deposition (CVD) process for the production of polycrystalline silicon have been evaluated. While an economic analysis of the process indicates that it is not capable of meeting JPL/DOE price objectives ($14.00/kg in 1980 dollars), product price in the $19.00/kg to $25.00/kg range may be achieved. Product quality has been evaluated and ascertained to be comparable to semiconductor-grade polycrystalline silicon. Solar cells fabricated from the material are also equivalent to those fabricated from semiconductor-grade polycrystalline silicon.

Mccormick, J. R.↗
DOE OSTI2023

Imaging local luminescence variations in CdSe 0.1 Te 0.9 thin films grown by the colossal grain growth process

Clear visualization and understanding of luminescence properties of grain interiors and grain boundaries in polycrystalline thin-film photovoltaic materials are crucial to achieving high-performance solar cells. Luminescence-based measurements, for example, reveal sub-bandgap energetic disorder or impurity-related radiative transitions in these materials, and when performed in the absolute scale, they can show the extent of voltage-limiting non-radiative recombination. Here we use a hyperspectral imaging technique to investigate inter- and intra-grain photoluminescence variations in polycrystalline CdSe 0.1 Te 0.9 films made by the colossal grain growth process. We observe that grain boundaries show higher luminescence than grain interiors for all energies below the bandgap energy. Multiple distinct sub-bandgap defect transitions are detected and determined to be related to donor-acceptor pair impurities. A comparison of the excitation-intensity-dependent photoluminescence at the grain boundaries with grain interiors suggests that carrier concentration is higher at grain boundaries. This finding is supported by photoemission electron microscopy imaging of the films, showing approximately 70 meV upward energy shift in the band structure at the grain boundaries. Finally, we investigate a dramatic thermal quenching behavior related to the donor-acceptor pairs luminescence over a temperature range and use a rate-equation model to attribute it to the sudden redirection of the radiative recombination channels to non-radiative channels with increasing temperature.

14 SOLAR ENERGY↗
DOE OSTI2022

Compositional screening of Ce-doped (Gd,Lu,Y) 3 (Al,Ga) 5 O 12 ceramics prepared by quenching from melt and their luminescence properties

Quenching from the melt using an Optical Floating Zone furnace was investigated as a possible high-throughput preparation method in order to screen novel scintillating materials. To validate this method, polycrystalline rare-earth aluminum garnets and yttrium gallium aluminum garnets were synthesized and characterized by X-ray diffraction, photoluminescence and radioluminescence emission spectra as well as radioluminescence imaging and compared to the previously reported properties of those materials. In order to achieve rapid, but quantitative comparison of the materials, pellets fabricated from the synthesized powders were sintered; two different sintering conditions were investigated and compared. Therefore, a simplistic energy deposition and light absorption model for brightness of opaque samples under X-ray irradiation was developed to estimate the relative scintillation efficiency of the materials. Based on the results, Y 3 Al 5 O 12 , LuY 2 Al 5 O 12 and GdY 2 Al 5 O 12 seem to have the highest scintillation efficiency among the prepared samples.

36 MATERIALS SCIENCE↗
DOE OSTI2025

A structural analysis of ordered Cs3Sb films grown on single crystal graphene and silicon carbide substrates

Alkali antimonides are well established as high efficiency, low intrinsic emittance photocathodes for accelerators and photon detectors. However, conventionally grown alkali antimonide films are polycrystalline with surface disorder and roughness that can limit achievable beam brightness. Ordering the crystalline structure of alkali antimonides has the potential to deliver higher brightness electron beams by reducing surface disorder and enabling the engineering of material properties at the level of atomic layers. In this report, we demonstrate the growth of ordered Cs3Sb films on single crystal substrates 3C-SiC and graphene-coated 4H-SiC using pulsed laser deposition and conventional thermal evaporation growth techniques. The crystalline structures of the Cs3Sb films were examined using reflection high energy electron diffraction and x-ray diffraction diagnostics, while film thickness and roughness estimates were made using x-ray reflectivity. With these tools, we observed ordered domains in less than 10 nm thick films with quantum efficiencies greater than 1% at 530 nm. Moreover, we identify structural features such as Laue oscillations indicative of highly ordered films. We found that Cs3Sb films grew with flat, fiber-textured surfaces on 3C-SiC and with multiple ordered domains and sub-nanometer surface roughness on graphene-coated 4H-SiC under our growth conditions. We identify the crystallographic orientations of Cs3Sb grown on graphene-coated 4H-SiC substrates and discuss the significance of examining the crystal structure of these films for growing epitaxial heterostructures in future experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2024

X-ray linear dichroic tomography of crystallographic and topological defects

Abstract The functionality of materials is determined by their composition 1–4 and microstructure, that is, the distribution and orientation of crystalline grains, grain boundaries and the defects within them 5,6 . Until now, characterization techniques that map the distribution of grains, their orientation and the presence of defects have been limited to surface investigations, to spatial resolutions of a few hundred nanometres or to systems of thickness around 100 nm, thus requiring destructive sample preparation for measurements and preventing the study of system-representative volumes or the investigation of materials under operational conditions 7–15 . Here we present X-ray linear dichroic orientation tomography (XL-DOT), a quantitative, non-invasive technique that allows for an intragranular and intergranular characterization of extended polycrystalline and non-crystalline 16 materials in three dimensions. We present the detailed characterization of a polycrystalline sample of vanadium pentoxide (V 2 O 5 ), a key catalyst in the production of sulfuric acid 17 . We determine the nanoscale composition, microstructure and crystal orientation throughout the polycrystalline sample with 73 nm spatial resolution. We identify and characterize grains, as well as twist, tilt and twin grain boundaries. We further observe the creation and annihilation of topological defects promoted by the presence of volume crystallographic defects. The non-destructive and spectroscopic nature of our method opens the door to operando combined chemical and microstructural investigations 11,18 of functional materials, including energy, mechanical and quantum materials.

Science & Technology - Other Topics↗
NASA NTRS1986

Scaling attenuation data characterizes changes in material microstructure

Ultrasonic attenuation was measured for polycrystalline samples of nickel and copper with various grain-size distributions produced by heat treatment. Attenuation as a function of frequency was determined for a sample having a known mean grain diameter D. It was found that, once this function is determined, it can be scaled to determine the mean grain size of other samples of the same material with different mean grain diameters. These results were obtained using broadband pulse-echo ultrasound in the 25 to 100 MHz frequency range. The results suggest an ultrasonic, nondestructive approach for verifying heat treatment of metals.

Generazio, E. R.↗
DOE OSTI2025

Arsenic activation and compensation in single crystal CdTe bilayers

In state-of-the art polycrystalline CdTe photovoltaics, group-V dopant activation is about 2%. Low activation can create electronic defects and lead to recombination and band tail losses. To develop methods to overcome this limitation, dopant activation was systematically investigated using molecular beam epitaxy (MBE) grown single crystal bilayers of As-doped CdTe on undoped CdTe. Results suggest multiple paths for improved As-activation in polycrystalline CdTe-based devices. It was found that the carrier concentration in this MBE material saturated at ∼3 × 1016 cm−3, with high levels (>50%) of As-activation possible. High activation could be achieved with a post-growth activation temperature of ∼450 °C, when the initial doping level was below the saturation level. However, at typical polycrystalline As incorporation levels (>5 × 1016 cm−3), the excess As is inactive or compensating, requiring elevated temperatures (500–600 °C) to achieve high activation. Oxygen in the annealing ambient was detrimental, while the effect of CdCl2 in the ambient is more case-dependent. A 575 °C activation anneal was combined with a 450 °C CdCl2 treatment to better understand the implications for polycrystalline CdTe. Interestingly, on highly doped samples, processes ending with a high temperature step displayed high activation, while those ending at 450 °C significantly reduced the carrier concentration (with or without CdCl2 in the ambient). Low activation can be restored with another high temperature anneal, allowing reproducible toggling between high and low activation based on the final temperature. Photoluminescence revealed the presence of donor–acceptor pairs in the low activation state that appear to be associated with a compensating defect.

14 SOLAR ENERGY↗
NASA NTRS2005

Inorganic Photovoltaics Materials and Devices: Past, Present, and Future

This report describes recent aspects of advanced inorganic materials for photovoltaics or solar cell applications. Specific materials examined will be high-efficiency silicon, gallium arsenide and related materials, and thin-film materials, particularly amorphous silicon and (polycrystalline) copper indium selenide. Some of the advanced concepts discussed include multi-junction III-V (and thin-film) devices, utilization of nanotechnology, specifically quantum dots, low-temperature chemical processing, polymer substrates for lightweight and low-cost solar arrays, concentrator cells, and integrated power devices. While many of these technologies will eventually be used for utility and consumer applications, their genesis can be traced back to challenging problems related to power generation for aerospace and defense. Because this overview of inorganic materials is included in a monogram focused on organic photovoltaics, fundamental issues and metrics common to all solar cell devices (and arrays) will be addressed.

Hepp, Aloysius F.↗
DOE OSTI2022

Defect control in the Heisenberg-Kitaev candidate material NaRuO 2

The combination of geometric frustration, extended hopping, spin-orbit coupling, and a disordered magnetic ground state make NaRuO 2 an attractive Heisenberg-Kitaev candidate material. Historically, NaRuO 2 has been a challenging material to produce, even in polycrystalline form. Here we present synthetic efforts that identify a propensity for Na Ru defects to form in NaRuO 2 , revealing a full solid-solution between NaRuO 2 and disordered Na 2 RuO 3 . We report the synthesis of alloys along the Na 3+x Ru 3–x O 6 solid solution and characterize changes in the bulk magnetization and electron transport as a function of Na loading. Furthermore, our results highlight the importance of stoichiometry control in NaRuO 2 when investigating and interpreting this material’s physical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Atomic step disorder on polycrystalline surfaces leads to spatially inhomogeneous work functions

Structural disorder causes materials’ surface electronic properties, e.g., work function (ϕ), to vary spatially, yet it is challenging to prove exact causal relationships to underlying ensemble disorder, e.g., roughness or granularity. For polycrystalline Pt, nanoscale resolution photoemission threshold mapping reveals a spatially varying ϕ=5.70±0.03 eV over a distribution of (111) vicinal grain surfaces prepared by sputter deposition and annealing. With regard to field emission and related phenomena, e.g., vacuum arc initiation, a salient feature of the ϕ distribution is that it is skewed with a long tail to values down to 5.4 eV, i.e., far below the mean, which is exponentially impactful to field emission via the Fowler–Nordheim relation. We show that the ϕ spatial variation and distribution can be explained by ensemble variations of granular tilts and surface slopes via a Smoluchowski smoothing model wherein local ϕ variations result from spatially varying densities of electric dipole moments, intrinsic to atomic steps, that locally modify ϕ. Atomic step-terrace structure is confirmed with scanning tunneling microscopy (STM) at several locations on our surfaces, and prior works showed STM evidence for atomic step dipoles at various metal surfaces. From our model, we find an atomic step edge dipole μ=0.12 D/edge atom, which is comparable to values reported in studies that utilized other methods and materials. Our results elucidate a connection between macroscopic ϕ and the nanostructure that may contribute to the spread of reported ϕ for Pt and other surfaces and may be useful toward more complete descriptions of polycrystalline metals in the models of field emission and other related vacuum electronics phenomena, e.g., arc initiation.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Single Crystalline GeSe Van Der Waals Ribbons With Uniform Layer Stacking, High Carrier Mobility, and Adjustable Edge Morphology

Abstract Performance of the group IV monochalcogenide GeSe in solar cells, electronic, and optoelectronic devices is expected to improve when high‐quality single crystalline material is used rather than polycrystalline films. Crystalline flakes represent an attractive alternative to bulk single crystals as their synthesis may be developed to be scalable, faster, and with higher overall yield. However, large – and especially large and thin – single crystal flakes are notoriously hard to synthesize. Here it is demonstrated that vapor‐liquid‐solid growth combined with direct lateral vapor‐solid incorporation produces high‐quality single crystalline GeSe ribbons with tens of micrometers size and controllable thickness. Electron microscopy shows that the ribbons exhibit perfect equilibrium (AB) van der Waals stacking order without extended defects across the entire thickness, in contrast to the conventional case of substrate‐supported flakes where material is added via layer‐by‐layer nucleation and growth on the basal plane. Electrical measurements show anisotropic transport and a high Hall mobility of 85 cm 2 V −1 s −1 , on par with the best single crystals to date. Growth from mixed GeSe and SnSe vapors, finally, yields ribbons with unchanged structure and composition but with jagged edges, promising for applications that rely on ample chemically active edge sites, such as catalysis or photocatalysis.

99 GENERAL AND MISCELLANEOUS↗
NASA NTRS1981

Evaluation of silicon-on-ceramic material for low-cost solar cells

The silicon-on-ceramic (SOC) process produces thin layers of silicon on inexpensive ceramic substrates. The layers are produced by unidirectional solidification of molten silicon, and are polycrystalline. Solar cells made from SOC material have performed with a conversion efficiency of 10.5%. In connection with the objective to improve the efficiency of SOC solar cells, an investigation was conducted to assess the relative importance of grain boundaries and impurities. In order to separate the two effects, light-beam-induced current (LBIC) measurements of the short-circuit current were made using a micron-sized light beam from a monochromator. The LBIC measurements are closely related to solar cell performance since the technique identifies the various spatial and spectral contributions to solar cell current. On the basis of the obtained results, it appears that structural defects are more important than chemical defects in limiting the performance of present SOC solar cells.

Zook, J. D.↗