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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Anisotropy effects on the kinetics of colloidal crystallization and melting: comparison of spheres and ellipsoids

We use alternating current (AC) electric field assisted self-assembly to produce two-dimensional, millimeter scale arrays of ellipsoidal colloids and study the kinetics of their phase reconfiguration by means of confocal microscopy, light scattering, and computer simulation. We find that the kinetics of orientational and positional ordering can be manipulated by changing the shape of the colloids: ellipsoids with aspect ratio 2.0 melt into disordered structures 5.7 times faster compared to spheres. On the other hand, ellipsoids self-assemble into ordered crystals at a similar rate to spheres. Confocal microscopy is used to directly visualize defects in the self-assembled structures. Small-angle light scattering (SALS) quantifies the light diffraction response, which is sensitive to the kinetics of positional and orientational ordering in the self-assembled anisotropic structures. We find three different light diffraction patterns: a phase with high orientational order (with chain-like structure in real space), a phase with high positional and orientational order (characteristic of a close-packed structure), and a phase that is disordered in position but with intermediate orientational order. The large influence of aspect ratio on the kinetics of the positionally and orientationally ordered phase is explored through simulation; it is found that the number of particle degrees of freedom controls the difference between the melting rates of the ellipsoids and spheres. This research contributes to the understanding of reconfiguration kinetics and optical properties of colloidal crystals produced from anisotropic colloids.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructural study and simulation of intrinsic two-way shape memory behavior of functionally graded Ni-rich/NiTiCu thin film

Highlights: • Ni-rich/Ti-rich bi-layer thin film was deposited on Si by using DC magnetron sputtering. • The bi-layer exhibits the gradient structure and mechanical properties through the thickness. • The bi-layer shows intrinsic two-way shape memory effect due to functionally graded structure. • The intrinsic two-way shape memory effect was confirmed by finite element simulation. - Abstract: In this research, the gradient structure and mechanical properties of an austenitic/martensitic (Ni{sub 50.8}Ti/Ni{sub 45}TiCu{sub 5} (at.%)) bi-layer thin film was investigated. The bi-layer was deposited on a Si substrate using DC magnetron sputtering. After crystallization of the film at 773 K for 60 min, the microstructure and mechanical properties of the bi-layer thin film were characterized using X-ray diffraction (XRD), transmission electron microscopy (STEM and HRTEM) and nanoindentation, respectively. The diffraction pattern illustrated that the crystallized bi-layer was combined of martensitic and austenitic layers while secondary ion mass spectroscopy (SIMS) and high resolution transmission electron microscopy analysis demonstrated the existence of a compositional gradient through the thickness and residual strain in the interface of the bi-layer, respectively. Furthermore, the compositional gradient in the bi-layer led to gradual variations in the structure, hardness and Young's modulus through the thickness of the bi-layer. The intrinsic two-way shape memory effect due to its functionally graded structure and the induced stress was confirmed by experimental test and finite element simulation.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Atomic-scale probing of short-range order and its impact on electrochemical properties in cation-disordered oxide cathodes

Abstract Chemical short-range-order has been widely noticed to dictate the electrochemical properties of Li-excess cation-disordered rocksalt oxides, a class of cathode based on earth abundant elements for next-generation high-energy-density batteries. Existence of short-range-order is normally evidenced by a diffused intensity pattern in reciprocal space, however, derivation of local atomic arrangements of short-range-order in real space is hardly possible. Here, by a combination of aberration-corrected scanning transmission electron microscopy, electron diffraction, and cluster-expansion Monte Carlo simulations, we reveal the short-range-order is a convolution of three basic types: tetrahedron, octahedron, and cube. We discover that short-range-order directly correlates with Li percolation channels, which correspondingly affects Li transport behavior. We further demonstrate that short-range-order can be effectively manipulated by anion doping or post-synthesis thermal treatment, creating new avenues for tailoring the electrochemical properties. Our results provide fundamental insights for decoding the complex relationship between local chemical ordering and properties of crystalline compounds.

25 ENERGY STORAGE↗
DOE OSTI2019

Evaluating the accuracy of common γ-Al 2 O 3 structure models by selected area electron diffraction from high-quality crystalline γ-Al 2 O 3

Single crystal and textured polycrystalline γ-Al 2 O 3 thin films were synthesized by oxidation of NiAl(110) in air at 850 °C for 1 and 2 hours, respectively, and used to evaluate the accuracy of two spinel-based and two nonspinel models by comparison of selected-area electron diffraction (SAED). The lattice interplanar distances derived from the polycrystalline SAED pattern most closely matched the cubic spinel γ-Al 2 O 3 model. The single-crystal SAED spot pattern reflected symmetry consistent with both the cubic spinel and tetragonal nonspinel models, yet, the Al cation distribution better matched the cubic spinel model based on the relative intensities of diffraction spots. Our work indicates that the traditional cubic spinel model is a more accurate model of γ-Al 2 O 3 than the other models considered. Furthermore, the spinel-based monoclinic model is more accurate than the monoclinic nonspinel model. The understanding of the relative accuracy of the different models is key for simulating γ-Al 2 O 3 containing systems and is of general interest for the metal oxide and ceramic communities.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Rapid 3D nanoscale coherent imaging via physics-aware deep learning

Phase retrieval, the problem of recovering lost phase information from measured intensity alone, is an inverse problem that is widely faced in various imaging modalities ranging from astronomy to nanoscale imaging. The current process of phase recovery is iterative in nature. As a result, the image formation is time consuming and computationally expensive, precluding real-time imaging. Here, we use 3D nanoscale X-ray imaging as a representative example to develop a deep learning model to address this phase retrieval problem. We introduce 3D-CDI-NN, a deep convolutional neural network and differential programing framework trained to predict 3D structure and strain, solely from input 3D X-ray coherent scattering data. Our networks are designed to be “physics-aware” in multiple aspects; in that the physics of the X-ray scattering process is explicitly enforced in the training of the network, and the training data are drawn from atomistic simulations that are representative of the physics of the material. We further refine the neural network prediction through a physics-based optimization procedure to enable maximum accuracy at lowest computational cost. 3D-CDI-NN can invert a 3D coherent diffraction pattern to real-space structure and strain hundreds of times faster than traditional iterative phase retrieval methods. Our integrated machine learning and differential programing solution to the phase retrieval problem is broadly applicable across inverse problems in other application areas.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Multi-Modal Ptychography: Recent Developments and Applications

Utilisation of partially coherent X-rays in Coherent X-ray Diffraction Imaging (CDI) and X-ray ptychography have brought intense research and developments for the past few years. Generally, data reconstructions convergence can be achieved by considering to incorporate partially coherent X-rays in ptychographic iterative reconstructions algorithms. Recently, exploration of both X-ray probe and sample modes have been studied and it was suggested that sufficient constraints are required to achieve accurate data reconstructions due to the mathematical inseparability of multiple sample modes that are superimposed into a single set of ptychographic intensity patterns. Here we review the multi-modal ptychography that can be used as a new emerging technique to investigate dynamical sample systems with partially coherent X-rays by recent experimental and simulated demonstrations.

47 OTHER INSTRUMENTATION↗
DOE OSTI2015

Ice formation on nitric acid coated dust particles: Laboratory and modeling studies

Changes in the ice nucleation characteristics of atmospherically relevant mineral dust particles due to nitric acid coating are not well understood. Further, the atmospheric implications of dust coating on ice-cloud properties under different assumptions of primary ice nucleation mechanisms are unknown. We investigated ice nucleation ability of Arizona test dust, illite, K-feldspar and quartz as a function of temperature (-25 to -30°C) and relative humidity with respect to water (75 to 110%). Particles were size selected at 250 nm and transported (bare or coated) to the ice nucleation chamber to determine the fraction of particles nucleating ice at various temperature and water saturation conditions. All dust nucleated ice at water-subsaturated conditions, but the coated particles showed a reduction in their ice nucleation ability compared to bare particles. However, at water-supersaturated conditions, we observed that bare and coated particles had nearly similar ice nucleation characteristics. X-ray diffraction patterns indicated that structural properties of bare dust particles modified after acid treatment. We found that lattice parameters were slightly different, but crystallite sizes of the coated particles were reduced compared to bare particles. Next, single-column model results show that simulated ice crystal number concentrations mostly depends upon fraction of particles that are coated, primary ice nucleation mechanisms, and the competition between ice nucleation mechanisms to nucleate ice. In general, we observed that coating modify the ice-cloud properties and the picture of ice and mixed-phase cloud evolution is complex when different primary ice nucleation mechanisms are competing for fixed water vapor mass.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2015

The high pressure structure and equation of state of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) up to 20 GPa: X-ray diffraction measurements and first principles molecular dynamics simulations

Recent theoretical studies of 2,6-diamino-3,5-dinitropyrazine-1-oxide (C{sub 4}H{sub 4}N{sub 6}O{sub 5} Lawrence Livermore Molecule No. 105, LLM-105) report unreacted high pressure equations of state that include several structural phase transitions, between 8 and 50 GPa, while one published experimental study reports equation of state (EOS) data up to a pressure of 6 GPa with no observed transition. Here we report the results of a synchrotron-based X-ray diffraction study and also ambient temperature isobaric-isothermal atomistic molecular dynamics simulations of LLM-105 up to 20 GPa. We find that the ambient pressure phase remains stable up to 20 GPa; there is no indication of a pressure induced phase transition. We do find a prominent decrease in b-axis compressibility starting at approximately 13 GPa and attribute the stiffening to a critical length where inter-sheet distance becomes similar to the intermolecular distance within individual sheets. The ambient temperature isothermal equation of state was determined through refinements of measured X-ray diffraction patterns. The pressure-volume data were fit using various EOS models to yield bulk moduli with corresponding pressure derivatives. We find very good agreement between the experimental and theoretically derived EOS.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

In situ TEM observation of alpha-particle induced annealing of radiation damage in Durango apatite

A major issue in thermochronology and U-Th-Pb dating is the effect of radiation damage, created by α-recoils from α-decay events, on the diffusion of radiogenic elements (e.g., He and Pb) in host mineral. Up until now, thermal events have been considered as the only source of energy for the recovery of radiation-damage. However, irradiation, such as from the α-particle of the α-decay event, can itself induce damage recovery. Quantification of radiation-induced recovery caused by α-particles during α-decay events has not been possible, as the recovery process at the atomic-scale has been difficult to observe. Here we present details of the dynamics of the amorphous-to-crystalline transition process during α-particle irradiations using in situ transmission electron microscopy (TEM) and consecutive ion-irradiations: 1 MeV Kr 2+ (simulating α-recoil damage), followed by 400 keV He+ (simulating α-particle annealing). Upon the He + irradiation, partial recrystallization of the original, fully-amorphous Durango apatite was clearly evident and quantified based on the gradual appearance of new crystalline domains in TEM images and new diffraction maxima in selected area electron diffraction patterns. Thus, α-particle induced annealing occurs and must be considered in models of α-decay event damage and its effect on the diffusion of radiogenic elements in geochronology and thermochronology.

58 GEOSCIENCES↗
DOE OSTI2018

Atomistic determination of the surface structure of Cu 2 O(111): experiment and theory

Cuprous oxide (Cu 2 O) is a promising catalyst for several important reactions. However, the atomic structures of defective Cu 2 O surfaces, which critically affect the catalytic properties both thermodynamically and kinetically, are not unambiguously characterized. High-resolution scanning tunneling microscopy (STM), combined with density functional theory (DFT) calculations and STM simulations, has been used to determine the atomic structure of the (111) surface of a Cu 2 O bulk crystal. The single crystal surface, processed by ultrahigh vacuum cleaning and oxygen annealing, shows a (1 × 1) periodicity in the low-energy electron diffraction pattern. The pristine (defect-free) Cu 2 O(111) surface exhibits a lattice of protrusions with hexagonal symmetry under STM, which is attributed to the dangling bonds of the coordinatively unsaturated copper (Cu U ) atoms on the surface. Two types of surface atomic defects are also identified, including the Cu U vacancy and the oxygen-vacancy-induced local surface restructuring. The electronic structure of this surface measured by $dI/dV$ spectroscopy shows an energy band gap of ~1.6–2.1 eV. Consistent with $dI/dV$ measurements, DFT calculations identified surface states within the electronic band gap arising from the Cu ions on the surface. Our results provide a clear picture of the pristine and defective Cu 2 O(111) surface structure in addition to the formation mechanism of the reconstructed surface, paving the way toward studying the site-dependent reactivity of this surface.

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

Point group symmetry of cadmium arsenide thin films determined by convergent beam electron diffraction

Cadmium arsenide (Cd 3 As 2 ) is one of the first materials to be discovered to belong to the class of three-dimensional topological semimetals. Reported room-temperature crystal structures of Cd 3 As 2 differ subtly in the way the Cd vacancies are arranged within its antifluorite-derived structure, which determines if an inversion center is present and if Cd 3 As 2 is a Dirac or Weyl semimetal. Here, we apply convergent beam electron diffraction (CBED) to determine the point group of Cd 3 As 2 thin films grown by molecular-beam epitaxy. Using CBED patterns from multiple zone axes, high-angle annular dark-field images acquired in scanning transmission electron microscopy, and Bloch wave simulations, we show that Cd 3 As 2 belongs to the tetragonal 4/mmm point group, which is centrosymmetric. In conclusion, the results show that CBED can distinguish very subtle differences in the crystal structure of a topological semimetal, a capability that will be useful for designing materials and thin film heterostructures with topological states that depend on the presence of certain crystal symmetries.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Operando neutron diffraction reveals mechanisms for controlled strain evolution in 3D printing

Abstract Residual stresses affect the performance and reliability of most manufactured goods and are prevalent in casting, welding, and additive manufacturing (AM, 3D printing). Residual stresses are associated with plastic strain gradients accrued due to transient thermal stress. Complex thermal conditions in AM produce similarly complex residual stress patterns. However, measuring real-time effects of processing on stress evolution is not possible with conventional techniques. Here we use operando neutron diffraction to characterize transient phase transformations and lattice strain evolution during AM of a low-temperature transformation steel. Combining diffraction, infrared and simulation data reveals that elastic and plastic strain distributions are controlled by motion of the face-centered cubic and body-centered cubic phase boundary. Our results provide a new pathway to design residual stress states and property distributions within additively manufactured components. These findings will enable control of residual stress distributions for advantages such as improved fatigue life or resistance to stress-corrosion cracking.

36 MATERIALS SCIENCE↗
DOE OSTI2018

X-Ray Study of the Superstructure in Heavily Doped Porous Indium Phosphide

An indium-phosphide InP sample subjected to the pore-generation procedure and then doped with S atoms is studied by the methods of X-ray diffraction analysis (XRD) and small-angle X-ray scattering (SAXS) (with CuK{sub α1}-radiation). The XRD data demonstrate that the sample consists of (coherent) aligned homogeneous components. A point detector is used to obtain, in the anomalous transmission mode by Borrmann, a set of SAXS curves at sample positions varied by azimuthal rotations. The SAXS data are used to simulate a 2D SAXS pattern for the sample under study, which makes it possible to determine the long-distance translation symmetry and, consequently, the presence of a superstructure. The interplanar distances in the superstructure in the directions (110) and (1 0) of the InP lattice are found to be ~260 and 450 nm, respectively. The symmetry group of the superstructure is determined as C{sub 2v} in the (001) plane of the sample lattice.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Spatial Signal Detection Using Continuous Shrinkage Priors

Motivated by the problem of detecting changes in two-dimensional X-ray diffraction data, we propose a Bayesian spatial model for sparse signal detection in image data. Our model places considerable mass near zero and has heavy tails to reflect the prior belief that the image signal is zero for most pixels and large for an important subset. We show that the spatial prior places mass on nearby locations simultaneously being zero, and also allows for nearby locations to simultaneously be large signals. The form of the prior also facilitates efficient computing for large images. We conduct a simulation study to evaluate the properties of the proposed prior and show that it outperforms other spatial models. As a result, we apply our method in the analysis of X-ray diffraction data from a two-dimensional area detector to detect changes in the pattern when the material is exposed to an electric field.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2024

Multiple origins of extra electron diffractions in fcc metals

Diffuse intensities in the electron diffraction patterns of concentrated face-centered cubic solid solutions have been widely attributed to chemical short-range order, although this connection has been recently questioned. This article explores the many nonordering origins of commonly reported features using a combination of experimental electron microscopy and multislice diffraction simulations, which suggest that diffuse intensities largely represent thermal and static displacement scattering. A number of observations may reflect additional contributions from planar defects, surface terminations incommensurate with bulk periodicity, or weaker dynamical effects.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Modeling of Phase Transitions of Graphites to Diamond-Like Phases

The method of the density functional theory is used to study structural transformations between graphites and diamond-like phases. The calculations have been carried out in two approximations: a local density approximation and a generalized gradient approximation. It is found that the phase transitions of hexagonal graphene layers to a cubic diamond and diamond-like phases must occur at uniaxial compressions of ~57–71 GPa, whereas some diamond-like phases can be obtained from tetragonal graphene layers at significantly lower pressures of 32–52 GPa. The X-ray diffraction patterns have been calculated for the phase transition of graphite I4{sub 1}/amd to tetragonal LA10 phase that takes place at the minimum pressure that can be used for experimental identification of these compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

GaAs/Ge crystals grown on Si substrates patterned down to the micron scale

Monolithic integration of III-V compounds into high density Si integrated circuits is a key technological challenge for the next generation of optoelectronic devices. In this work, we report on the metal organic vapor phase epitaxy growth of strain-free GaAs crystals on Si substrates patterned down to the micron scale. The differences in thermal expansion coefficient and lattice parameter are adapted by a 2-μm-thick intermediate Ge layer grown by low-energy plasma enhanced chemical vapor deposition. The GaAs crystals evolve during growth towards a pyramidal shape, with lateral facets composed of (111) planes and an apex formed by (137) and (001) surfaces. The influence of the anisotropic GaAs growth kinetics on the final morphology is highlighted by means of scanning and transmission electron microscopy measurements. The effect of the Si pattern geometry, substrate orientation, and crystal aspect ratio on the GaAs structural properties was investigated by means of high resolution X-ray diffraction. The thermal strain relaxation process of GaAs crystals with different aspect ratio is discussed within the framework of linear elasticity theory by Finite Element Method simulations based on realistic geometries extracted from cross-sectional scanning electron microscopy images.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Structure and morphology evolution of silica-modified pseudoboehmite aerogels during heat treatment

Silica-modified pseudoboehmite aerogels (0, 10, 20 at% of Si) were prepared by sol–gel method followed by supercritical drying. The phase transformations, changes in structure and morphology upon calcination were thoroughly investigated by advanced X-Ray diffraction (XRD) techniques and high-resolution transmission electron microscopy (HRTEM). Obtained pseudoboehmite samples had specific nanostructure: ultrathin two-dimensional (2D) crystallites were loosely packed. The silica dopant drastically enhanced the crystallite anisotropy. Thus, the aerogel with Al:Si atomic ratio of 9:1 consisted of the pseudoboehmite nanosheets with thickness of one unit cell (average dimensions of 14.0×1.2×14.5 nm). The specific nanostructure caused remarkable features of experimental XRD patterns, including anisotropic peak broadening and appearance of forbidden reflection. Direct simulation of XRD patterns with using the Debye Scattering Equation allowed the size and morphology of pseudoboehmite crystallites to be determined. The silica addition strongly delayed formation of γ-alumina and further phase transformations upon calcinaton. Thermal stability of alumina was suggested to be affected by the particle morphology inherited from the pseudoboehmite precursor. - Graphical abstract: Pseudoboehmite samples had specific nanostructure: ultrathin two-dimensional (2D) crystallites were loosely packed. - Highlights: • Silica-doped boehmites were prepared by sol–gel method with supercritical drying. • Ultrathin two-dimensional crystallites of pseudoboehmite were obtained. • Changes in structure and morphology upon calcination were studied. • Simulation of XRD patterns was performed with use of the Debye Scattering Equation. • Thermal stability of alumina depended on morphology inherited from pseudoboehmite.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗