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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 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Derivation of an applied nonlinear Schroedinger equation

We derive from first principles a mathematical physics model useful for understanding nonlinear optical propagation (including filamentation). All assumptions necessary for the development are clearly explained. We include the Kerr effect, Raman scattering, and ionization (as well as linear and nonlinear shock, diffraction and dispersion). We explain the phenomenological sub-models and each assumption required to arrive at a complete and consistent theoretical description. The development includes the relationship between shock and ionization and demonstrates why inclusion of Drude model impedance effects alters the nature of the shock operator. Unclassified Unlimited Release

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Study of beyond nearest-neighbor environment and intermediate-range order in a sodium aluminosilicate geopolymer using Reverse Monte Carlo modeling

A large scale three-dimensional structural model of a geopolymer with an approximate composition of NaAlSi 2 O 6 .5.5H 2 O obtained by Reverse Monte Carlo (RMC) modeling based on experimental high-energy x-ray diffraction is presented for the first time in order to obtain information regarding beyond nearest-neighbor environment and intermediate-range order (IRO). RMC model exhibits a three-dimensional network consisting of randomly cross-linked AlO 4 and SiO 4 tetrahedral units with uniformly distributed Na atoms and H 2 O molecules. The bond angle distributions, i.e. T-O-T, O-T-O and Na-O-Na angles, are somewhat distorted with lower values compared to analogous crystal systems. The origin of the first peak in the structure factor indicating IRO is investigated using the partial structure factors; Na-Na, Si-Na, Al-Si, O-H, and H-H atom pairs are found to be the main contributors. Ring size distribution analysis demonstrates that the structure mainly involves 6-, 7- and 8-membered rings. Finally, the coherence length of these IRO characteristics is ~10.3 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Average structure and local configuration of excess oxygen in UO2+x

Determination of the local configuration of interacting defects in a crystalline, periodic solid is problematic because defects typically do not have a long-range periodicity. Uranium dioxide, the primary fuel for fission reactors, exists in hyperstoichiometric form, UO2+x. Those excess oxygen atoms occur as interstitial defects, and these defects are not random but rather partially ordered. The widely-accepted model to date, the Willis cluster based on neutron diffraction, cannot be reconciled with the first-principles molecular dynamics simulations present here. We demonstrate that the Willis cluster is a fair representation of the numerical ratio of different interstitial O atoms; however, the model does not represent the actual local configuration. The simulations show that the average structure of UO2+x involves a combination of defect structures including split di-interstitial, di-interstitial, mono-interstitial, and the Willis cluster, and the latter is a transition state that provides for the fast diffusion of the defect cluster. The results provide new insights in differentiating the average structure from the local configuration of defects in a solid and the transport properties of UO2+x.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

High energy x-ray and neutron studies of disordered energy-related materials at extreme conditions

The fundamental scientific accomplishments are: (1) advances in a general description of the liquid state by employing structural models constrained by measurements to interpret experimental results and extend them to liquids in general, with special emphasis on (2) The structure of the high-temperature crystal and molten UO 2 and 3) water. Specifically, samples of UO 2 and water were probed using high-energy x-rays at the Advanced Photon Source. The high Z of UO 2 , and the 2-3mm diameter droplet shape of the molten sample, means that >100keV X-rays are required to minimize absorption and multiple scattering, which can distort the measured structure factor. A high flux of x-rays is also required to obtain sufficient statistical accuracy in short (a few seconds) measurement times. The scattered x-ray data were analyzed and pair distribution functions, extracted that characterize the local and long-range atomic structure of the material. The measurements of the hot UO 2 solid show a substantial increase in oxygen disorder and, upon melting, the average U-O coordination was found to decrease from 8 to 6.7±0.5. The research incorporated development of diffraction techniques, sample environment optimization and state-of-the-art simulation techniques. The symbiotic nature of the advances in simulation and experiment allowed for a more focused and informed development of future experiments, effective use of expensive beam time and generated new research agendas for the growing number of research groups, within the US and internationally, that focus on the structure of liquids. Molecular dynamics (MD) provided detailed information when combined with high-quality XN data including addressing key issues in liquids; the relationship between cooling path, structure and fictive temperature, and the trade-offs between network over connectedness in liquids containing low-coordination cations.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Probing electronic dead layers in homoepitaxial n-SrTiO 3 (001) films

We combine state-of-the-art oxide epitaxial growth by hybrid molecular beam epitaxy with transport, x-ray photoemission, and surface diffraction, along with classical and first-principles quantum mechanical modeling to investigate the nuances of insulating layer formation in otherwise high-mobility homoepitaxial n-SrTiO 3 (001) films. Our analysis points to charge immobilization at the buried n-SrTiO 3 /undoped SrTiO 3 (001) interface as well as within the surface contamination layer resulting from air exposure as the drivers of electronic dead-layer formation. As Fermi level equilibration occurs at the surface and the buried interface, charge trapping reduces the sheet carrier density (n 2D ) and renders the n-STO film insulating if n 2D falls below the critical value for the metal-to-insulator transition.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Exploiting fast detectors to enter a new dimension in room-temperature crystallography

A departure from a linear or an exponential decay in the diffracting power of macromolecular crystals is observed and accounted for through consideration of a multi-state sequential model. A departure from a linear or an exponential intensity decay in the diffracting power of protein crystals as a function of absorbed dose is reported. The observation of a lag phase raises the possibility of collecting significantly more data from crystals held at room temperature before an intolerable intensity decay is reached. A simple model accounting for the form of the intensity decay is reintroduced and is applied for the first time to high frame-rate room-temperature data collection.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Evolution of microstructure and residual stress during annealing of austenitic and ferritic steels

In this work the recovery and recrystallization processes occurring in ferritic and austenitic steels were studied. To determine the evolution of residual stresses during material annealing the nonlinear sin{sup 2}ψ diffraction method was used and an important relaxation of the macrostresses as well as the microstresses was found in the cold rolled samples subjected to heat treatment. Such relaxation occurs at the beginning of recovery, when any changes of microstructure cannot be detected using other experimental techniques. Stress evolution in the annealed steel samples was correlated with the progress of recovery process, which significantly depends on the value of stacking fault energy. - Highlights: • X-ray diffraction was used to determine the first order and second order stresses. • Diffraction data were analyzed using scale transition elastoplastic models model. • Stress relaxation in annealed ferritic and austenitic steels was correlated with evolution of microstructure. • Influence of stacking fault energy on thermally induced processes was discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Study of titanate nanotubes by X-ray and electron diffraction and electron microscopy

The structure of titanate nanotubes (Ti-NTs) was studied by a combination of powder X-ray diffraction (PXRD), electron diffraction and high resolution transmission electron microscopy (HRTEM). Ti-NTs are prepared by hydrothermal treatment of TiO{sub 2} powder. The structure is identified by powder X-ray diffraction as the one based on the structure of H{sub 2}Ti{sub 2}O{sub 5}·H{sub 2}O phase. The same structure is obtained by projected potential from HRTEM through-focus image series. The structure is verified by simulated PXRD pattern with the aid of the Debye formula. The validity of the model is tested by computing Fourier transformation of a single nanotube which is proportional to measured electron diffraction intensities. A good agreement of this calculation with measured precession electron diffraction data is achieved. - Highlights: • Titanate nanotubes were prepared by hydrothermal method. • X-ray powder diffraction indicated their structure based on that of H{sub 2}Ti{sub 2}O{sub 5}·H{sub 2}O. • Structural model was created with the aid of high-resolution electron microscopy. • The model was verified with electron diffraction data. • X-ray powder diffraction pattern was calculated with the aid of the Debye formula.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Bulk texture evolution of nanolamellar Zr–Nb composites processed via accumulative roll bonding

We report that it was recently demonstrated that bulk two-phase 50/50 Zr–Nb nanolayered composites with 90 nm individual layers can be fabricated from an initial coarse-layered composite with 1 mm layers via the severe plastic deformation process of accumulative roll bonding. During the deformation, the Zr phase retained its hcp crystal structure and the Zr–Nb interface remained sharp. Here we use a combination of neutron diffraction and dislocation-based polycrystal plasticity constitutive modeling to assess the evolution of texture and deformation mechanisms over a four order-of-magnitude range in layer thickness. The phase textures in the nanocomposite strongly deviate from that of Zr or Nb rolled in monolithic form, becoming highly peaked and intense. The model suggests that texture development in the Nb phase is associated with multiple slip and contributions from both {1 1 2} <1 1 0> slip and {1 1 0} <1 1 0> slip. Lastly, in the Zr phase the model suggests that the texture develops due to a predominance of prismatic and basal slip.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Decoding crystallography from high-resolution electron imaging and diffraction datasets with deep learning

While machine learning has been making enormous strides in many technical areas, it is still massively underused in transmission electron microscopy. To address this, a convolutional neural network model was developed for reliable classification of crystal structures from small numbers of electron images and diffraction patterns with no preferred orientation. Diffraction data containing 571,340 individual crystals divided among seven families, 32 genera, and 230 space groups were used to train the network. Despite the highly imbalanced dataset, the network narrows down the space groups to the top two with over 70% confidence in the worst case and up to 95% in the common cases. As examples, we benchmarked against alloys to two-dimensional materials to cross-validate our deep-learning model against high-resolution transmission electron images and diffraction patterns. We present this result both as a research tool and deep-learning application for diffraction analysis.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2021

Probabilistic Deep Learning Approach to Automate the Interpretation of Multi-phase Diffraction Spectra

Autonomous synthesis and characterization of inorganic materials requires the automatic and accurate analysis of X-ray diffraction spectra. For this task, we designed a probabilistic deep learning algorithm to identify complex multi-phase mixtures. At the core of this algorithm lies an ensemble convolutional neural network trained on simulated diffraction spectra, which are systematically augmented with physics-informed perturbations to account for artifacts that can arise during experimental sample preparation and synthesis. In this work, larger perturbations associated with off-stoichiometry are also captured by supplementing the training set with hypothetical solid solutions. Spectra containing mixtures of materials are analyzed with a newly developed branching algorithm that utilizes the probabilistic nature of the neural network to explore suspected mixtures and identify the set of phases that maximize confidence in the prediction. Our model is benchmarked on simulated and experimentally measured diffraction spectra, showing exceptional performance with accuracies exceeding those given by previously reported methods based on profile matching and deep learning. We envision that the algorithm presented here may be integrated in experimental workflows to facilitate the high-throughput and autonomous discovery of inorganic materials.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Absence of Sum Frequency Generation in Support of Orthorhombic Symmetry of α-Chitin

The native polymorphic structures of chitin, namely α- and β-chitin, were studied with X-ray diffraction (XRD), infrared and Raman spectroscopies, and sum-frequency generation (SFG) vibrational spectroscopy. The currently presented model of α-chitin crystal based on X-ray and electron diffraction study has an antiparallel chain arrangement imposed by the orthorhombic symmetry in contrast to parallel chain arrangements in β-chitin and native cellulose; yet, the natural occurrence of antiparallel chain arrangement for α-chitin is controversial to widely accepted biosynthetic mechanisms for structural polysaccharides such as chitin and cellulose, where chains are successively polymerized and crystallized in a unidirectional way. We compared the SFG signals among samples with similar crystallinity, morphology, and textures but differing in allomorphs. The SFG signal from α-chitin was ~40-fold weaker compared to β-chitin having a similar lateral dimension. The strong SFG signals arising from β-chitin can be explained by a net polar ordering of parallel packed chains. These results strongly supports the antiparallel chain orientation and high symmetry (orthorhombic) of α-chitin suggested by diffraction analysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Changes in microstructure characteristics of cement paste on carbonation

This study investigates the influence of carbonation on the microstructure of cement paste cast with Ordinary Portland Cement (OPC), fly ash based Portland Pozzolana Cement (PPC) and Limestone Calcined Clay Cement (LC{sup 3}) using X-ray diffraction (XRD), thermal analysis (TGA), scanning electron microscope (SEM) and mercury intrusion porosimeter (MIP). A comparison is made between samples exposed to 3% carbon dioxide concentration and those exposed to natural CO{sub 2} concentrations. It is observed that the products formed on carbonation are similar in both the exposure conditions. Distinct rims corresponding to decalcified C-S-H are observed around clinker grains in the carbonated samples. Coarsening of pore structure is observed on carbonation in all the cements and an increase in the total porosity is observed in blended cements. Thermodynamic modeling indicates irrespective of the type of cement the total solid volume is reduced on complete carbonation. The volume change occurring on carbonation obtained from experimental results and thermodynamic modeling are compared.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Synthesis, structural and electrical properties of a new cobalt arsenate NaCo{sub 2}As{sub 3}O{sub 10}

The title compound sodium dicobalt triarsenate NaCo{sub 2}As{sub 3}O{sub 10} has been synthesized by solid state reaction. Crystal structure and electrical properties were studied by X-ray diffraction and complex impedance spectroscopy, respectively. The obtained structural model is supported by charge-distribution (CHARDI) analysis and bond-valence-sum (BVS) validations tools. The structure (triclinic, P−1, a=6.800 (8) Å, b=7.816 (9) Å, c=8.718 (3) Å, α=108.03 (2)°, β=108.48 (3)°, γ=100.11 (2)°) can be described as a three-dimensional framework resulted from corner-connection between cobalt metallic chains running along [−110] and As{sub 3}O{sub 10} groups; the negative charge is balanced by Na{sup +} ions which house the two tunnels parallel to a and b axes. Ball milling was used as mechanical means to reduce the particles sizes of the synthesized powder. At the optimal sintering temperature of 650 °C, 85% of the relative density was obtained. The conductivity measurements show that NaCo{sub 2}As{sub 3}O{sub 10} is a cationic conductor with an activation energy of 0.48 eV and a conductivity of σ=1.2×10{sup −5} S cm{sup −1} at 310 °C. The BVS model is extended to simulate the ionic migration pathways of alkali cations in the anionic framework. - Graphical abstract: 1D pathways link Na atoms along the a-axis with bond valence mismatch |ΔV(Na)|=0.64 v.u. - Highlights: • A new single crystal NaCo{sub 2}As{sub 3}O{sub 10} was grown by solid state reaction and its structure determined by single-crystal X-ray diffraction. • The purity of the powder sample was verified by Rietveld refinement. • The CIS measurements were optimized and the obtained spectra were fitted by electrical equivalent circuits. • The conduction pathways for Na{sup +} cations are simulated by means of the bond valence sum model.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Quantifying irradiation defects in zirconium alloys: a comparison between transmission electron microscopy and whole-pattern diffraction line-profile analysis - 2016-0089

Irradiation-induced dislocations significantly affect the mechanical properties of zirconium alloys, altering slip and influencing creep and growth. Thus, the quantitative characterization of irradiation defects as a function of fluence, cold work, and/or thermal treatments is important for models that attempt to predict their impact on properties. Whole-pattern diffraction line-profile analysis (DLPA) is a well-established modern tool for microstructure characterization based on first-principle physical models for dislocation density measurements in plastically deformed materials. However, applying these DLPA methods directly to irradiated materials yields higher than expected dislocation density values compared with historical transmission electron microscopy (TEM) measurements and past line-broadening analysis studies calibrated to TEM observations. In an effort to understand these differences, a new microstructural model was developed for DLPA to specifically address dislocation structures consisting of elliptical - and -component loops. To compare the refined DLPA method with TEM measurements, high-resolution neutron diffraction patterns on nonirradiated and irradiated Zr-2.5Nb samples were collected with the Neutron Powder Diffractometer instrument at the Los Alamos Neutron Science Center and were evaluated. High-resolution TEM measurements were performed at the Reactor Materials Testing Laboratory, Queen's University, for comparison with the DLPA results. The capabilities and inherent uncertainties of both the refined DLPA and TEM methods are compared and discussed in detail. We show that the differences between the density values provided by DLPA and TEM are inherent to the methods and can be reconciled with the interpretation of the data. (authors)

36 MATERIALS SCIENCE↗
DOE OSTI2016

Formation, stability and crystal structure of mullite-type Al{sub 6−x}B{sub x}O{sub 9}

Mullite-type Al{sub 6−x}B{sub x}O{sub 9} compounds were studied by means of powder diffraction and spectroscopic methods. The backbones of this structure are chains of edge-connected AlO{sub 6} octahedra crosslinked by AlO- and BO-polyhedra. Rietveld refinements show that the a and b lattice parameters can be well resolved, thus representing an orthorhombic metric. A continuous decrease of the lattice parameters most pronounced in c-direction indicates a solid solution for Al{sub 6−x}B{sub x}O{sub 9} with 1.09≤x≤2. A preference of boron in 3-fold coordination is confirmed by {sup 11}B MAS NMR spectroscopy and Fourier calculations based on neutron diffraction data collected at 4 K. Distance Least Squares modeling was performed to simulate a local geometry avoiding long B-O distances linking two octahedral chains by planar BO{sub 3} groups yielding split positions for the oxygen atoms and a strong distortion in the octahedral chains. The lattice thermal expansion was calculated using the Grüneisen first-order equation of state Debye-Einstein-Anharmonicity model. - Graphical abstract: Local distortion induced by boron linking the octahedral chains. - Highlights: • Decreasing lattice parameters indicate a solid solution for Al{sub 6−x}B{sub x}O{sub 9} (1.09≤x≤2). • B-atoms induce a local distortion of neighboring AlO{sub 6} octahedra. • A preference of boron in BO{sub 3} coordination is confirmed by {sup 11}B MAS NMR spectroscopy. • An optimized structural model for Al{sub 6−x}B{sub x}O{sub 9} is presented.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Biomolecular Solvation Structure Revealed by Molecular Dynamics Simulations

To compare ordered water positions from experiment with those from molecular dynamics (MD) simulations, a number of MD models of water structure in crystalline endoglucanase were calculated. The starting MD model was derived from a joint X-ray and neutron diffraction crystal structure, enabling the use of experimentally assigned protonation states. Simulations were performed in the crystalline state, using a periodic 2 × 2 × 2 supercell with explicit solvent. Water X-ray and neutron scattering density maps were computed from MD trajectories using standard macromolecular crystallography methods. In one set of simulations, harmonic restraints were applied to bias the protein structure toward the crystal structure. For these simulations, the recall of crystallographic waters using strong peaks in the MD water electron density was very good, and there also was substantial visual agreement between the boomerang-like wings of the neutron scattering density and the crystalline water hydrogen positions. An unrestrained simulation also was performed. For this simulation, the recall of crystallographic waters was much lower. For both restrained and unrestrained simulations, the strongest water density peaks were associated with crystallographic waters. Overall, the results demonstrate that it is now possible to recover crystallographic water structure using restrained MD simulations but that it is not yet reasonable to expect unrestrained MD simulations to do the same. Further development and generalization of MD water models for force-field development, macromolecular crystallography, and medicinal chemistry applications is now warranted. In particular, the combination of room-temperature crystallography, neutron diffraction, and crystalline MD simulations promises to substantially advance modeling of biomolecular solvation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗