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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 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

The role of cellular structure, non-equilibrium eutectic phases and precipitates on quasi-static strengthening mechanisms of as-built AlSi10Mg parts 3D printed via laser powder bed fusion

The quasi-static loading strengthening mechanism in the as-built state of an AlSi10Mg alloy 3D-printed via Laser Powder Bed Fusion (LPBF) was thoroughly identified and quantified using state-of-the-art electron microscopy and synchrotron X-ray diffraction techniques. The yield strength was comprehensively modelled through an in-depth characterization and quantification of the microstructural features as well as their effective volumes for strengthening. In particular, the non-equilibrium eutectic network was characterized with a two-fold structure: cell boundary particles as well as intracellular lamellar/fibrous networks, each consisting of discrete phases exhibiting a thru-thickness compositional gradient, a semi-coherent interface with the matrix and an abundance of crystal defects such as nano-sized sub-grains, microstrains and stacking faults. Further, these altogether made the eutectic phase the most potent contributor to the yield strength accounting for ~30–40% of the estimated value (i.e., cell boundary and eutectic network strengthening combined). Precipitates strengthening was identified as the second most potent mechanism via a shearing process only. Moreover, the presented methodology was able to capture the effect of LPBF processing variables on the individual strengthening contributions, e.g., the effect of a lower laser scanning speed on increasing the cell size as well as the mean precipitate size, which are shown to exhibit opposing impacts on strengthening.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Imaging Coherent Structural Dynamics with Ultrafast Electron Microscopy

Development of ultrafast electron and X-ray scattering methods has enabled direct routes to probing atomic-scale structural dynamics. This in turn has led to new insight into lattice responses to a variety of dynamic processes associated with phase transformations, electron-lattice correlations, and nanoscale mechanical motion, to name a few. Presently, the most wide-spread approaches involve using table-top, laser-driven electron-scattering chambers or femtosecond (fs) X-ray beamlines to probe dynamics in reciprocal space and, in many instances, over specimen regions that are large relative to unit-cell dimensions and discrete (atomic and nanoscale) structural and morphological defects (e.g., vacancies, line and screw dislocations, grain boundaries, interfaces, etc.). As a result, the transients monitored for relatively large-spot-size reciprocal-space measurements – typically some aspect of a Bragg reflection (intensity, position, width, shape) – may be comprised of a range of responses occurring within the probed region. To directly resolve the role of distinct morphologies and structural features on dynamics associated with the nucleation, emergence, evolution, and decay of coherent phenomena, one would ideally directly image the local, nanoscale behaviors and precisely correlate responses to the nature of the feature. Here, I will show how such effects can be directly probed with ultrafast electron microscopy (UEM). After a brief overview of the operating principles of UEM, I will discuss how we are using real-space imaging to directly visualize coherent structural dynamics in a variety of nanostructured and nanoscale materials.

47 OTHER INSTRUMENTATION↗
DOE OSTI2017

Beyond 11% efficient sulfide kesterite Cu 2 Zn x Cd 1– x SnS 4 solar cell: Effects of cadmium alloying

Here, kesterite Cu 2 ZnSnS 4 (CZTS) thin-film solar cells have drawn worldwide attention because of outstanding performance and earth-abundant constituents. However, problems such as coexistence of complex secondary phases, the band tailing issue, short minority lifetime, bulk defects, and undesirable band alignment at p-n interfaces need to be addressed for further efficiency improvement. In this regard, Cd alloying shows promise for dealing with some of these problems. In this work, a beyond 11% efficient Cd-alloyed CZTS solar cell is achieved, and the effects of Cd-alloying and mechanism underpinning the performance improvement have been investigated. The introduction of Cd can significantly reduce the band tailing issue, which is confirmed by the reduction in the difference between the photoluminescence peak and optical band gap (E g ) as well as decreased Urbach energy. The microstructure, minority lifetime, and electrical properties of CZTS absorber are greatly improved by Cd alloying. Further XPS analyses show that the partial Cd alloying slightly reduces the band gap of CZTS via elevating the valence band maximum of CZTS. This suggests that there are opportunities for further efficiency improvement by engineering the absorber and the associated interface with the buffer.

14 SOLAR ENERGY↗
DOE OSTI2020

Intrinsic Ion Transport Properties of Block Copolymer Electrolytes

Knowledge of intrinsic properties is of central importance for materials design and assessing suitability for specific applications. Self-assembling block copolymer electrolytes (BCEs) are of great interest for applications in solid-state energy storage devices. A fundamental understanding of ion transport properties, however, is hindered by the difficulty in deconvoluting extrinsic factors, such as defects, from intrinsic factors, such as the presence of interfaces between the domains. Here, we quantify the intrinsic ion transport properties of a model BCE system consisting of poly(styrene-block-ethylene oxide) (SEO) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt using a generalizable strategy of depositing thin films on interdigitated electrodes and self-assembling fully connected parallel lamellar structures throughout the films. Comparison between conductivity in homopolymer poly(ethylene oxide) (PEO)-LiTFSI electrolytes and the analogous conducting material in SEO over a range of salt concentrations (r, molar ratio of lithium ion to ethylene oxide repeat units) and temperatures reveals that between 20% and 50% of the PEO in SEO is inactive. Using mean-field theory calculations of the domain structure and monomer concentration profiles at domain interfaces-both of which vary substantially with salt concentration-the fraction of inactive PEO in the SEO, as derived from conductivity measurements, can be quantitatively reconciled with the fraction of PEO that is mixed with greater than a few volume percent of polystyrene. Despite the detrimental interfacial effects for ion transport in BCEs, the intrinsic conductivity of the SEO studied here (ca. 10 -3 S/cm at 90 degrees C, r = 0.085) is an order of magnitude higher than reported values from bulk samples of similar molecular weight SEO (ca. 10 -4 S/cm at 90 degrees C, r = 0.085). Finally, this work provides motivation and methods for pursuing improved BCE chemical design, interfacial engineering, and processing.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Quantum dot based memory devices: Current status and future prospect by simulation perspective

We discuss the simulation of floating gate MOSFET memory performance consists of Si/Ge/Si quantum dots for electronics storage nodes with higk-κ material as the tunnel oxide. Heterostructure quantum dot was proposed to maintain the good memory performance without losing the long retention characteristic. As the results, retention time could be increase significantly without deteriorating the writing/erasing speed. By selected the dot size of 9–15nm and the tunnel oxide thickness of 2nm, the retention characteristic can reach up to 10{sup 9}s while the writing and erasing time at the order of 10{sup −6}s. By replace the SiO{sub 2} tunnel oxide with high-κ material such as HfO{sub 2}, ZrO{sub 2}, and Y{sub 2}O{sub 3} the leakage current due to the shrinkage of tunnel oxide thickness can be suppressed by the factor of 10 for the EOT lower than ∼1nm. Moreover, the charge-trapping that generated by the defect at Si substrate and higk-κ material interface are fully considered. We found that, the charge-trapping affects the retention time and memory operation performance. By increasing the charge-trapping depth and width, the memory operation time decrease significantly while the retention characteristic slightly improved.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Superior plastic flow stability of self-patterned carbide – amorphous ceramic nanostructures

Amorphous ceramics and carbides exhibit superb strength but poor plasticity. Here, we synthesized TiC-SiOC nanostructures with TiC-nanocarbides embedded in amorphous ceramic SiOC by co-sputtering followed by high-temperature annealing and/or irradiation. TiC-SiOC nanostructures exhibit high strength and good plastic flow stability even after heavy irradiation, 7 GPa at room temperature and 3.6 GPa at 700 ℃ with a uniform strain of about 10%∼18%. The uniform deformation is accommodated by the shearing of amorphous ceramic and the rotation of nanocarbides. Nanocarbides inhibit the propagation of shear banding in amorphous SiOC, and amorphous-crystal interfaces act as sinks to manage irradiation-induced defects.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Device Fabrication using Crystalline CdTe and CdTe Ternary Alloys Grown by MBE

We fabricated epitaxial CdTe:In/CdTe:As homojunction and CdZnTe/CdTe and CdMgTe/CdTe heterojunction devices grown on bulk CdTe substrates in order to study the fundamental device physics of CdTe solar cells. Selection of emitter-layer alloys was based on passivation studies using double heterostructures as well as band alignment. Initial results show significant device integration challenges, including low dopant activation, high resistivity substrates and the development of low-resistance contacts. To date, the highest open-circuit voltage is 715 mV in a CdZnTe/CdTe heterojunction following anneal, while the highest fill factor of 52% was attained in an annealed CdTe homojunction. In general, all currentvoltage measurements show high series resistance, capacitancevoltages measurements show variable doping, and quantum efficiency measurements show low collection. Ongoing work includes overcoming the high resistance in these devices and addressing other possible device limitations such as non-optimum junction depth, interface recombination, and reduced bulk lifetime due to structural defects.

14 SOLAR ENERGY↗
DOE OSTI2016

Identifying local structural states in atomic imaging by computer vision

Abstract The availability of atomically resolved imaging modalities enables an unprecedented view into the local structural states of materials, which manifest themselves by deviations from the fundamental assumptions of periodicity and symmetry. Consequently, approaches that aim to extract these local structural states from atomic imaging data with minimal assumptions regarding the average crystallographic configuration of a material are indispensable to advances in structural and chemical investigations of materials. Here, we present an approach to identify and classify local structural states that is rooted in computer vision. This approach introduces a definition of a structural state that is composed of both local and nonlocal information extracted from atomically resolved images, and is wholly untethered from the familiar concepts of symmetry and periodicity. Instead, this approach relies on computer vision techniques such as feature detection, and concepts such as scale invariance. We present the fundamental aspects of local structural state extraction and classification by application to simulated scanning transmission electron microscopy images, and analyze the robustness of this approach in the presence of common instrumental factors such as noise, limited spatial resolution, and weak contrast. Finally, we apply this computer vision-based approach for the unsupervised detection and classification of local structural states in an experimental electron micrograph of a complex oxides interface, and a scanning tunneling micrograph of a defect-engineered multilayer graphene surface.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Final Report: Rational Design of Wide Band Gap Buffer Layers for High-Efficiency Thin-Film Photovoltaics

The main objective of this project is to enable rational design of wide band gap buffer layer materials for CIGS thin-film PV by building understanding of the correlation of atomic-scale defects in the buffer layer and at the buffer/absorber interface with device electrical properties. Optimized wide band gap buffers are needed to reduce efficiency loss from parasitic absorption in the buffer. The approach uses first-principles materials simulations coupled with nanoscale analytical electron microscopy as well as device electrical characterization. Materials and devices are produced by an industrial partner in a manufacturing line to maximize relevance, with the goal of enabling R&D of new buffer layer compositions or deposition processes to push device efficiencies above 21%. Cadmium sulfide (CdS) is the reference material for analysis, as the prototypical high-performing buffer material.

14 SOLAR ENERGY↗
DOE OSTI2012

Improved cladding nano-structured materials with self-repairing capabilities

When designing nuclear reactors or the materials that go into them, one of the key challenges is finding materials that can withstand an outrageously extreme environment. In addition to constant bombardment by radiation, reactor materials may be subjected to extremes in temperature, physical stress, and corrosive conditions. A limitation in fuel burnup is and usage of the nuclear fuel material is related to the structural material radiation damage, that makes the fuel be removed with low-burnup and immobilized in the waste storage pools. The advanced burnup brings cladding material embitterment due to radiation damage effects corroborated with corrosion effects makes the fuel pellet life shorter. The novel nano-clustered structured sintered material may mitigate simultaneously the radiation damage and corrosion effects driving to more robust structural materials that may make the nuclear reactor safer and more reliable. The development of nano-clustered sinter alloys provides new avenues for further examination of the role of grain boundaries and engineered material interfaces in self-healing of radiation-induced defects driving to the design of highly radiation-tolerant materials for the next generation of nuclear energy applications. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2024

Manipulating topological properties in Bi 2 Se 3 / BiSe /transition metal dichalcogenide heterostructures with interface charge transfer

Heterostructures of topological insulator Bi 2 Se 3 on transition metal dichalcogenides (TMDCs) offer a new materials platform for studying novel quantum states by exploiting the interplay among topological orders, charge orders and magnetic orders. Here, the diverse interface attributes, such as material combination, charge re-arrangement, defect and strain, can be utilized to manipulate the quantum properties of this class of materials. Recent experiments of Bi 2 Se 3 /NbSe 2 heterostructures show signatures of strong Rashba band splitting due to the presence of a BiSe buffer layer, but the atomic level mechanism is not fully understood. We conduct first-principles studies of the Bi 2 Se3/BiSe/TMDC heterostructures with five different TMDC substrates (1T phase VSe 2 , MoSe 2 , TiSe 2 , and 2H phase NbSe 2 , MoSe 2 ). We find significant charge transfer at both BiSe/TMDC and Bi 2 Se 3 /BiSe interfaces driven by the work function difference, which stabilizes the BiSe layer as an electron donor and creates interface dipole. The electric field of the interface dipole breaks the inversion symmetry in the Bi 2 Se 3 layer, leading to the giant Rashba band splitting in two quintuple layers and the recovery of the Dirac point in three quintuple layers, with the latter otherwise only occurring in thicker samples with at least six Bi 2 Se 3 quintuple layers. Besides, we find that strain can significantly affect the charge transfer at the interfaces. Our study presents a promising avenue for tuning topological properties in heterostructures of two-dimensional materials, with potential applications in quantum devices.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Domain matched epitaxial growth of (111) Ba{sub 0.5}Sr{sub 0.5}TiO{sub 3} thin films on (0001) Al{sub 2}O{sub 3} with ZnO buffer layer

Epitaxial (111) Ba{sub 0.5}Sr{sub 0.5}TiO{sub 3} (BST) thin films have been grown by pulsed laser deposition on (0001) Al{sub 2}O{sub 3} substrate with ZnO as buffer layer. The x-ray ω-2θ, Φ-scan and reciprocal space mapping indicate epitaxial nature of BST thin films. The domain matched epitaxial growth of BST thin films over ZnO buffer layer was confirmed using Fourier filtered high resolution transmission electron microscope images of the film-buffer interface. The incorporation of ZnO buffer layer effectively suppressed the lattice mismatch and promoted domain matched epitaxial growth of BST thin films. Coplanar inter digital capacitors fabricated on epitaxial (111) BST thin films show significantly improved tunable performance over polycrystalline thin films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Interplay between Facets and Defects during the Dissociative and Molecular Adsorption of Water on Metal Oxide Surfaces

Both surface terminations and defects play a central role in determining how water interacts with metal oxides, thereby setting important properties of the interface that govern reactivity such as the type and distribution of hydroxyl groups. However, the interconnections between facets and defects remain poorly understood, limiting the usefulness of conventional notions such as that hydroxylation is controlled by metal cation exposure at the surface. Here, using hematite (α-Fe 2 O 3 ) as a model system, we show how oxygen vacancies overwhelm surface cation-dependent hydroxylation behavior. Synchrotron-based ambient-pressure X-ray photoelectron spectroscopy was used to monitor the adsorption of molecular water and its dissociation to form hydroxyl groups in situ on (001), (012), or (104) facet-engineered hematite nanoparticles. Supported by density functional theory calculations of the respective surface energies and oxygen vacancy formation energies, the findings show how oxygen vacancies are more prone to form on higher energy facets and induce surface hydroxylation at extremely low relative humidity values of 5 x10 - 5 %. Further, when these vacancies are eliminated, the extent of surface hydroxylation across the facets is as expected from the areal density of exposed iron cations at the surface. These findings help answer fundamental questions about the nature of reducible metal oxide-water interfaces in natural and technological settings and lay the groundwork for rational design of improved oxide-based catalysts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Ab Initio Investigation of Charge Trapping Across the Crystalline- Si -Amorphous- Si O 2 Interface

Accurate microscopic description of the charge-trapping process from semiconductor to defects in the dielectric-oxide layer is of paramount importance for understanding many microelectronic devices such as complementary metal-oxide-semiconductor (CMOS) transistors, as well as electrochemical reactions. Unfortunately, most current microscopic descriptions of such processes are based on empirical models with parameters fitted to experimental device performance results or simplified approximations like the Wentzel-Kramers-Brillouin (WKB) method. Some critical questions are still unanswered, including: What controls the charge-hopping rate, the coupling strength between the defect level to semiconductor level, or the energy difference? How does the hopping rate decay with defect-semiconductor distance? What is the fluctuation of the defect level, especially in amorphous dielectrics? Many of these questions can be answered by ab initio calculations. However, to date, there are few ab initio studies for this problem mainly due to technical challenges from atomic-structure construction to large-system calculations. Here, using the latest advances in calculation methods and codes, we study the carrier-trapping problem using density-functional theory (DFT) based on the Heyd-Scuseria-Ernzerhof (HSE) exchange correlation functional. The valence bond random-switching method is used to construct the crystalline-Si-amorphous-SiO2 (c-Si/a-SiO2) interfacial atomic structure, and the HSE yields a band offset that agrees well with experiments. The hopping rate is calculated with the Marcus theory, and the hopping-rate dependences on the gate potential and defect distances are revealed, as well as the range of fluctuation results from amorphous structural variation. We also analyze the result with the simple WKB model and find a major difference in the description of the coupling constant decay with the defect-semiconductor distance. Our results provide the ab initio simulation insights for this important carrier-trapping process for device operation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Unipolar resistive switching in metal oxide/organic semiconductor non-volatile memories as a critical phenomenon

Diodes incorporating a bilayer of an organic semiconductor and a wide bandgap metal oxide can show unipolar, non-volatile memory behavior after electroforming. The prolonged bias voltage stress induces defects in the metal oxide with an areal density exceeding 10{sup 17 }m{sup −2}. We explain the electrical bistability by the coexistence of two thermodynamically stable phases at the interface between an organic semiconductor and metal oxide. One phase contains mainly ionized defects and has a low work function, while the other phase has mainly neutral defects and a high work function. In the diodes, domains of the phase with a low work function constitute current filaments. The phase composition and critical temperature are derived from a 2D Ising model as a function of chemical potential. The model predicts filamentary conduction exhibiting a negative differential resistance and nonvolatile memory behavior. The model is expected to be generally applicable to any bilayer system that shows unipolar resistive switching.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Displacement Threshold Energy and Recovery in an Al-Ti Nanolayered System with Intrinsic Point Defect Partitioning

A method is established and validated using molecular dynamics (MD) to determine the displacement threshold energies as Ed in nanolayered, multilayered systems of dissimilar metals. The method is applied to specifically oriented nanolayered films of Al-Ti where the crystal structure and interface orientations are varied in atomic models and Ed is calculated. Methods for defect detection are developed and discussed based on prior research in the literature and based on specific crystallographic directions available in the nanolayered systems. These are compared and contrasted to similar calculations in corresponding bulk materials, including fcc Al, fcc Ti, hcp Al, and hcp Ti. In all cases, the calculated Ed in the multilayers are intermediate to the corresponding bulk values but exhibit some important directionality. In the nanolayer, defect detection demonstrated systematic differences in the behavior of Ed in each layer. Importantly, collision cascade damage exhibits significant defect partitioning within the Al and Ti layers that is hypothesized to be an intrinsic property of dissimilar nanolayered systems. This type of partitioning could be partly responsible for observed asymmetric radiation damage responses in many multilayered systems. In addition, a pseudo-random direction was introduced to approximate the average Ed without performing numerous simulations with random directions.

Al-Ti↗
DOE OSTI2015

Structural properties of Ge on SrTiO{sub 3} (001) surface and Ge/SrTiO{sub 3} interface

Germanium−perovskite oxide heterostructures have a strong potential for next-generation low-voltage and low-leakage metal-oxide semiconductor field-effect transistors. We investigated the atomic structure and electronic properties of Ge on perfect and defective (001) SrTiO{sub 3} by first-principle calculations. The specific adsorption sites at the initial growth stage and the atomic structure of Ge on the SrTiO{sub 3} (001) substrate have been systematically investigated. The surface grand potential was calculated and compared as a function of the relative chemical potential. The complete surface phase diagram was presented. The energetically favorable interfaces were pointed out among the atomic arrangements of the Ge/SrTiO{sub 3} (001) interfaces. The atomic structure and electronic properties of the intrinsic point defects were calculated and analyzed for the Ge/SrTiO{sub 3} (001) interfaces.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗