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Results for “surface anisotropy”

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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Modeling brittle fracture due to anisotropic thermal expansion in polycrystalline materials

Here, this work investigated brittle fracture of polycrystalline materials due to thermal stresses arising from anisotropic thermal expansion. We used phase-field fracture simulations with the properties of alpha-uranium (α-U) and assumed a linear elastic mechanical response. Three-dimensional simulations were used to predict fracture for various conditions and crystallographic textures. We found that fracture was more pronounced during cooling than during heating because the anisotropy increased with temperature. We also found that the total crack surface area increased with increasing average misorientation, while the net shape change of the material decreased with increasing misorientation. Two-dimensional simulations in which one crystallographic coefficient of thermal expansion (CTE) was set to zero indicated that the expansion behavior in the crystallographic direction with the smallest CTE was the primary cause of fracture.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Atom Probe Tomography Analysis of Mica

Laser-assisted atom probe tomography (APT) is a relatively new, powerful technique for sub-nanometric mineral and biomineral analysis. However, the laser-assisted APT analysis of highly anisotropic and chemically diverse minerals, such as phyllosilicates, may prove especially challenging due to the complex interaction between the crystal structure and the laser pulse upon applying a high electric field. Micas are a representative group of nonswelling clay minerals of relevance to a number of scientific and technological fields. In this study, a Mg-rich biotite was analyzed by APT to generate preliminary data on nonisotropic minerals and to investigate the effect of the crystallographic orientation on mica chemical composition and structure estimation. Here, the difference in results obtained for specimens extracted from the (001) and ( hk 0) mica surfaces indicate the importance of both experimental parameters and the crystallography. Anisotropy of mica has a strong influence on the physicochemical properties of the mineral during field evaporation and the interpretation of APT data. The promising results obtained in the present study open the way to future innovative APT applications on mica and clay minerals and contribute to the general discussion on the challenges for the analysis of geomaterials by atom probe tomography.

3D reconstructions↗
DOE OSTI2025

Curvature Induced Modifications of Chirality and Magnetic Configuration in Perpendicular Films

Designing curvature in three-dimensional (3D) magnetic nanostructures enables controlled manipulation of local energy landscapes, allowing for the modification of noncollinear spin textures relevant for next-generation spintronic devices. In this study, we experimentally investigate 3D magnetization textures in a Co/Pd multilayer film, exhibiting strong perpendicular magnetic anisotropy (PMA), deposited onto curved Cu nanowire meshes with diameters as small as 50 nm and lengths of several microns. Utilizing magnetic soft X-ray nanotomography, we achieve reconstructions of 3D magnetic domain patterns at approximately 30 nm spatial resolution. This approach provides detailed information on both the orientation and magnitude of magnetization within the film. Our results reveal that interfacial anisotropy in the Co/Pd multilayers drives the magnetization toward the local surface normal. In contrast to typical labyrinth domains observed in planar films, the presence of curved nanowires significantly alters the domain structure, with domains preferentially aligning along the nanowire axis in close proximity, while adopting random orientations farther away. We report direct experimental observation of a curvature-induced Dzyaloshinskii-Moriya interaction (DMI), which is quantified to be approximately one-third of the intrinsic DMI in Co/Pd stacks. The curvature induced DMI enhances stability of Néel-type domain walls. These experimental observations are further supported by micromagnetic simulations. Altogether, our findings demonstrate that introducing curvature into magnetic nanostructures provides a powerful strategy for tailoring complex magnetic behaviors, paving the way for the design of advanced 3D racetrack memory and neuromorphic computing devices.

Raftrey, David↗
NASA NTRS1986

Cometary spectroscopy

The spectroscopic and morphological characteristics of comets over a wide range of heliocentric distances are investigated as they may suggest or constrain models of cometary formation environments and evolution. This program also contributes to the understanding of the general comet population with which spacecraft data will be compared. Long slit spectra and direct images of all observable comets (M2<18) are obtained on a monthly basis with either a UV sensitive microchannel plate or a red sensitive CCD spectrograph/camera. Scale lengths of the principal emission of OH, NH, CN, C3, C2, NH2 and OI different comets can be compared. The direct images are used for studies of dust anisotropy which can provide data on the spin vector and gross surface morphology.

Larson, S.↗
NASA NTRS1995

Reionization and the cosmic microwave background in an open universe

If the universe was reionized at high reshift (z greater than or approximately equal to 30) or never recombined, then photon-electron scattering can erase fluctuations in the cosmic microwave background at scales less than or approximately equal to 1 deg. Peculiar motion at the surface of last scattering will then have given rise to new anisotropy at the 1 min level through the Vishniac effect. Here the observed fluctuations in galaxy counts are extrapolated to high redshifts using linear theory, and the expected anisotropy is computed. The predicted level of anisotropies is a function of Omega(sub 0) and the ratio of the density in ionized baryons to the critical density and is shown to depend strongly on the large- and small-scale power. It is not possible to make general statements about the viability of all reionized models based on current observations, but it is possible to rule out specific models for structure formation, particularly those with high baryonic content or small-scale power. The induced fluctuations are shown to scale with cosmological parameters and optical depth.

Persi, Fred M.↗
NASA NTRS2003

Intracellular stress tomography reveals stress focusing and structural anisotropy in cytoskeleton of living cells

We describe a novel synchronous detection approach to map the transmission of mechanical stresses within the cytoplasm of an adherent cell. Using fluorescent protein-labeled mitochondria or cytoskeletal components as fiducial markers, we measured displacements and computed stresses in the cytoskeleton of a living cell plated on extracellular matrix molecules that arise in response to a small, external localized oscillatory load applied to transmembrane receptors on the apical cell surface. Induced synchronous displacements, stresses, and phase lags were found to be concentrated at sites quite remote from the localized load and were modulated by the preexisting tensile stress (prestress) in the cytoskeleton. Stresses applied at the apical surface also resulted in displacements of focal adhesion sites at the cell base. Cytoskeletal anisotropy was revealed by differential phase lags in X vs. Y directions. Displacements and stresses in the cytoskeleton of a cell plated on poly-L-lysine decayed quickly and were not concentrated at remote sites. These data indicate that mechanical forces are transferred across discrete cytoskeletal elements over long distances through the cytoplasm in the living adherent cell.

NASA Discipline Cell Biology↗
DOE OSTI2022

MOCVD Surface Preparation of V-Groove Si for III-V Growth

V-groove nanopatterning of Si substrates has recently demonstrated promise for achieving high-quality III-V-on-Si epitaxy while providing a lower-cost processing route than chemo-mechanical polishing to produce epi-ready planar wafers. A key factor in determining the crystalline quality of III-V buffer layers is the Si surface structure and its chemical composition. Unlike planar Si surfaces, the surfaces of V-grooves prior to growth have not been studied in detail. Here, we study the surface of V-groove Si prepared for GaP nucleation via X-ray photoelectron spectroscopy and low-energy electron diffraction. We identify several pretreatments, using both 830 and 1000 annealing under an As background pressure, as being suitable for deoxidizing and cleaning the V-groove Si surface. The V-groove Si was found to behave similarly to reference Si(0 0 1) and Si(1 1 1) planar samples, demonstrating that in situ techniques such as reflection anisotropy spectroscopy can be used on reference samples to infer the state of the V-groove surface, and indicating that the extensive research on planar Si surfaces can be directly applied to V-grooves.

III-V epitaxy↗
DOE OSTI2026

Tuning Anisotropic Optical Properties of Inorganic and Hybrid Organic–Inorganic MXenes via Topochemical Surface Modification

Surface groups are central to the properties of MXenes, yet their role in optical anisotropy remains largely unexplored. Here, we use a topochemical route to synthesize single crystals of stacked Ti 3 C 2 Cl 2 and hybrid organic–inorganic MXenes (h-MXenes) with lateral sizes of 38–75 μm, rotational registry, and tunable interlayer spacing. Solid-state NMR spectroscopy shows that topochemical substitution generates mixed amido, imido, and hydride surface motifs, which modify the electronic structure of the Ti 3 C 2 inorganic core. Imaging spectroscopic ellipsometry with micron-scale spatial resolution enables reconstruction of the complex dielectric tensor of individual multilayer crystals. Ti 3 C 2 Cl 2 exhibits a type-II hyperbolicity above 930 nm, whereas h-MXenes do not display hyperbolicity within the measured 300–1700 nm window, instead showing reduced in-plane conductivity, suppressed out-of-plane light absorption, and a chain-length-dependent blue shift of a near-infrared absorption feature. These results demonstrate topochemical surface modification as a direct handle for engineering MXenes as surface-programmable optical media.

Hybrid materials↗
DOE OSTI2023

Orientation-dependent etching of silicon by fluorine molecules: A quantum chemistry computational study

Anisotropic etching is a widely used process in semiconductor manufacturing, in particular, for micro- and nanoscale texturing of silicon surfaces for black silicon production. The typical process of plasma-assisted etching uses energetic ions to remove materials in the vertical direction, creating anisotropic etch profiles. Plasmaless anisotropic etching, considered here, is a less common process that does not use ions and plasma. The anisotropy is caused by the unequal etching rates of different crystal planes; the etching process, thus, proceeds in a preferred direction. In this paper, we have performed quantum chemistry modeling of gas-surface reactions involved in the etching of silicon surfaces by molecular fluorine. The results confirm that orientation-dependent etch rates are the reason for anisotropy. The modeling of F 2 dissociative chemisorption on F-terminated silicon surfaces shows that Si–Si bond breaking is slow for the Si(111) surface, while it is fast for Si(100) and Si(110) surfaces. Both Si(100) and Si(110) surfaces incorporate a larger number of fluorine atoms resulting in Si–Si bonds having a larger amount of positive charge, which lowers the reaction barrier of F 2 dissociative chemisorption, yielding a higher etch rate for Si(100) and Si(110) surfaces compared to Si(111) surfaces. Molecular dynamics modeling of the same reactions has shown that the chosen reactive bond order potential does not accurately reproduce the lower reaction barriers for F 2 dissociative chemisorption on Si(100) and Si(100) surfaces. Furthermore, reparameterization is necessary to model the anisotropic etching process that occurs at lower temperatures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Cross slip of extended dislocations in face-centered cubic metals through phase-field modeling

Cross slip is a dislocation mechanism that significantly impacts the mechanical behavior of engineering alloys. Here, in this work, we advance a 3D phase-field dislocation dynamics (PFDD) mesoscale technique to simulate cross slip across a broad range of face-centered cubic (FCC) metals. The formulation incorporates elastic anisotropy, an FCC numerical grid, and a high-fidelity representation of the entire γ -surface from density functional theory for eight FCC metals and no adjustable parameters or rules. The relaxed core structures under zero stress for all metals are predicted to extend in plane. The analytical model for stacking fault width agrees well with the PFDD result under the assumption of elastic isotropy but overestimates it under elastic anisotropy, when the degree of anisotropy is large. The dynamic simulations are designed to elucidate the material parameters that influence the propensity for cross slip. Whether cross slip occurs under a non-Schmid stress or to bypass a hard obstacle, the critical stress to cross slip scales strongly with the anisotropic energy coefficient for a screw dislocation.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Efficient high-fidelity TRISO statistical failure analysis using Bison: Applications to AGR-2 irradiation testing

The ability of tri-structural isotropic (TRISO) fuel to contain fission products is largely dictated by the quality of the manufacturing process, since most of the fission product release is expected to occur due to coating layer failure in a small number of particles containing defects. The Bison fuel performance code has capabilities to predict failure in individual particles, accounting for the presence of defects, and to apply statistical analysis methods to compute the probability of failure in a set of fuel particles. Bison has recently undergone significant development both to improve its physical representations of fuel particle behavior and to improve the efficiency of its statistical failure calculations. Physical model improvements include new capabilities to account for the pressure generated by fission gases on inner pyrolytic carbon (IPyC) crack surfaces and to use local material coordinate orientation to accurately incorporate the anisotropy in the material properties in aspherical particles. To improve statistical modeling efficiency, a direct integration approach which involves directly integrating the failure probability function associated with statistically varying parameters has been developed. The direct integration approach is much more efficient than the Monte Carlo (MC) schemes commonly employed, and allows Bison to directly run high-dimensional fuel performance models, which improves the accuracy of failure probability calculations. Finally, a set of benchmark problems is considered here to compare the MC and direct integration approaches, and a statistical failure analysis of compacts in the Advanced Gas Reactor (AGR)-2 experiments is performed using the direct integration approach.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Magnetic Vortex and Hyperthermia Suppression in Multigrain Iron Oxide Nanorings

Single-crystal iron oxide nanorings have been proposed as a promising candidate for magnetic hyperthermia application because of their unique shape-induced vortex-domain structure, which supports good colloidal stability and enhanced magnetic properties. However, the synthesis of single crystalline iron oxide has proven to be challenging. In this article, we showed that chemically synthesized multigrain magnetite nanorings disfavor a shape-induced magnetic vortex-domain structure. Our results indicate that the multigrain Fe 3 O 4 nanorings with an average outer diameter of ~110 nm and an inner to outer diameter ratio of ~0.5 do not show a shape-induced vortex-domain structure, which was observed in the single-crystal Fe 3 O 4 nanorings of similar dimensions. At 300 Ks, multigrain magnetite nanorings showed an effective anisotropy field of 440 Oe, which can be attributed to its high surface area and intraparticle interaction. Both calorimetric and AC loop measurements showed a moderate inductive heating efficiency of multigrain magnetite nanorings of ~300 W/g at 800 Oe. Our results shed light on the magnetic ground states of chemically synthesized multigrain Fe 3 O 4 nanorings.

36 MATERIALS SCIENCE↗
NASA NTRS1975

Apollo 12, 15, and 16 lunar surface magnetometer experiment data analysis

The polarization of magnetometer signals detected at the Apollo 15 Hadley site by the lunar surface magnetometer has been studied to determine the source of the signal anisotropy which is observed and caused by the polarization. Instrument and data chain malfunction (cross-talk) seems ruled out. The source appears real and apparently connected with the Imbrium basin using reasonable inferences regarding the electromagnetic structure of the Moon. A theory is developed using moons with holes and conducting caps where the Imbrium basin is; results of calculations are consistent, though not unique, in specifying an anomaly in the electrical conductivity underlying Mare Imbrium. Distinct differences are noted from plasma sheet and diamagnetic cavity transfer functions, but the lobes appear, as for all other data, not to be vacuum for study of the moon. A discussion is given of problems connected with transfer of data, software, and theoretical programs from NASA Ames Research Center to the University of Arizona, and a summary is given of the conversion from IBM to CDC formats.

Sonett, C. P.↗
NASA NTRS1984

Orientation and Morphology Effects in Rapid Silicon Sheet Solidification

Radial growth anisotropies and equilibrium forms of point nucleated, dislocation free silicon sheets spreading horizontally on the free surface of a silicon melt were measured for (100), (110), (111), and (112) sheet planes. The growth process was recorded. Qualitative Wulff surface free energy polar plots were deduced from the equilibrium shapes for each sheet plane. Predicted geometries for the tip shape of unidirectional, dislocation free, horizontally grown sheets growing in various directions within the planes were analyzed. Polycrystalline sheets and dendrite propagation were analyzed. For dendrites, growth rates on the order of 2.5 m/min and growth rate anisotropies of 25 are measured.

Ciszek, T. F.↗
NASA NTRS1984

The soft X-ray background as a supernova blast wave viewed from inside: Solar abundance models

A model of the soft X-ray background is presented in which the Sun is assumed to be inside an active supernova blast wave. The blast wave evolves in a preexisting cavity. The broad band surface brightnesses is explained by such a blast wave with an explosion energy of E sub approx. 5 x 10 to the 50th power ergs and radius 80 to 100 pc, using solar abundances. An approach to treating the problem of large anisotropies in the ambient medium is also explored, accommodating the observed anticorrelation between the soft X-ray surface brightness and the 21 cm column density. It is found that only for post shock temperatures below 10 6 K a shock propagating into a density enhancement will be dimmer than a similar shock in a lower density region.

Edgar, R. J.↗
NASA NTRS1986

The soft X-ray background as a supernova blast wave viewed from inside - Solar abundance models

A model of the soft X-ray background is presented in which the sun is assumed to be inside an active supernova blast wave. The blast wave evolves in a preexisting cavity. The broad band surface brightnesses is explained by such a blast wave with an explosion energy of E sub approximately 5 x 10 to the 50th power ergs and radius 80 to 100 pc, using solar abundances. An approach to treating the problem of large anisotropies in the ambient medium is also explored, accommodating the observed anticorrelation between the soft X-ray surface brightness and the 21 cm column density. It is found that only for post shock temperatures below 10 to the 6 power K a shock propagating into a density enhancement will be dimmer than a similar shock in a lower density region.

Edgar, R. J.↗
NASA NTRS1992

COBE anisotropy from supercluster gas

It is suggested that the microwave background anisotropy detected by the COBE DMR might be dominated not by the direct gravitational effect of primordial fluctuations in the last scattering surface, but by scattering off of moving electrons in optically thin, nearby superclusters. Hot diffuse clouds of ionized gas created during supercluster collapse produce Sunyaev-Zel'dovich and Doppler background anisotropy whose properties may closely mimic those of primordial anisotropy in current data. Strategies for and difficulties in separating the effects are discussed, based on the anisotropy spectrum, autocorrelation, correlation with galaxy catalogs, X-ray emission, and integrated spectral distortions.

Hogan, Craig J.↗