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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 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Observation and simulation of the radial evolution and stream structure of solar wind turbulence

This paper presents a brief overview of the observed evolution in a variety of quantities describing the turbulent evolution of the interplanetary plasma and describes simulation results consistent with many features of the evolution. The turbulence is manifested through a dissipation at small scales in the inner heliosphere with a corresponding evolution in the breakpoint between a relatively flat and a Kolmogoroff spectrum; an evolution from kinetically to (slightly) magnetically dominated energy of the plasma fluctuations; a general decrease in the cross helicity or 'Alfvenicity'; changes in the anisotropy of the fluctuations; and the increasing predominance of quasi-pressure-balanced structures in the compressive component of the fluctuations. MHD simulations with shear layers either side of a central current sheet show that even in the absence of compressibility the lack of a mean field along the direction of the main flow in the current sheet leads to rapid nonlinear evolution and the observed characteristics of 'Elsasser spectra' of the fields in the inner heliosphere. Adding compressibility to the simulations does not greatly change the 'incompressive' quantities but leads in addition to observed correlations between a measure of compression and other quantities.

Roberts, D. A.↗
DOE OSTI2021

Asymmetric equilibrium core structures of pyramidal-II < c + a > dislocations in ten hexagonal-close-packed metals

The structures of pyramidal-II < c + a > dislocations, one of the most important defects in structural hexagonal-close-packed (HCP) metals, have not been fully characterized for many of the HCP metals in use today. Here, we employ ab initio informed phase-field dislocation dynamics to determine the minimum energy structure of pyramidal {1¯1¯22} < 11¯23 > dislocations in ten HCP metals, including Be, Co, Mg, Re, Ti, Zn, Cd, Hf, Y, and Zr. As input for the simulations, we calculate, using first-principles density functional theory, the {1¯1¯22} generalized stacking fault energy (GSFE) curves for all ten metals. From these calculations, it is found that magnetism in Co is necessary for achieving a local minimum in the GSFE curve. We observe in simulations that edge and screw character dislocations split into two partials separated by a low-energy intrinsic stacking fault. The splitting distance is shown to scale inversely with the local minimum energy normalized by the product of its shear modulus and Burgers vector. Interestingly, some HCP metals exhibit an asymmetric structure, with either unequal partial Burgers vectors or widths, in contrast to the symmetric configuration expected from linear elastic dislocation theory. We explain these structures by properties of the local maxima in their GSFE curves. Metals with larger degrees of elastic anisotropy result in dislocations with larger splitting distances than would be expected under the commonly used assumption of elastic isotropy. Furthermore, these findings on the sizes and asymmetry in the structures of pyramidal-II < c + a > dislocations are fundamental to understanding how these dislocations glide and interact or react with other defects when these metals are mechanically strained.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Sensitivity of the MnTe valence band to the orientation of magnetic moments

An effective model of the hexagonal (NiAs-structure) manganese telluride valence band in the vicinity of the A point of the Brillouin zone is derived. It is shown that whereas for the usual antiferromagnetic order (magnetic moments on the basal plane) band splitting at A is small, their out-of-plane rotation enhances the splitting dramatically (to about 0.5 eV). Here we propose extensions of recent experiments where such inversion of magnetocrystalline anisotropy has been observed in Li-doped MnTe to confirm this unusual sensitivity of a semiconductor band structure to magnetic order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Canted standing spin-wave modes of permalloy thin films observed by ferromagnetic resonance

Non-collinear spin structures in materials that combine perpendicular and in-plane magnetic anisotropies are of great technological interest for microwave and spin wave-assisted magnetization switching. [Co/Pt] multilayers are well-known perpendicular anisotropy materials that have the potential to pin the magnetization of a soft magnetic layer, such as permalloy (Py), that has in-plane anisotropy, thereby forming a magnetic exchange spring. Here we report on multilayered [Co/Pt]/Pt/Py films, where an additional ultrathin Pt spacer has been included to control the coupling between the sub-units with in-plane and perpendicular magnetic anisotropy. Vector network analyser (VNA)-ferromagnetic resonance (FMR) measurements were made to obtain a complete picture of the resonant conditions, while the dynamical response of the sub-units was probed by synchrotron-based element- and phase selective x-ray detected FMR (XFMR). For all samples, only slight pinning of the dynamic magnetization of the Py by the [Co/Pt] was noted, and the FMR results were dominated by the 50 nm thick Py layer. Out-of-plane VNA-FMR maps reveal the presence of additional modes, e.g. a perpendicular standing spin-wave (PSSW) state. However, as the magnetic field is reduced below the saturation field, the PSSW state morphs continuously through a series of canted standing spin-wave (CSSW) states into a horizontal standing spin-wave (HSSW) state. The PSSW, CSSW and HSSW states are well described using a multilayer model of the Py film. The observation of CSSW modes is of particular relevance to microwave assisted magnetic recording, where microwave excitation stimulates precession of a soft layer canted out of plane by a pulsed magnetic field.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2026

Incommensurate Magnetism Drives Singular Angular Magnetoresistance in the Magnetic Weyl Semimetal CeAlGe

Here, we demonstrate that a multi-𝐤 incommensurate magnetic state in the Weyl semimetal CeAlGe gives rise to a singular angular magnetoresistance (SAMR), which is an electrical transport signature capable of detecting magnetic field direction with exceptional precision. In contrast, its sister compound CeAlSi is devoid of both multi-𝐤 order and SAMR. We reveal that both phenomena appear upon 57% Ge substitution in CeAlSi 1−𝑥⁢ Ge 𝑥 and coincide with electronic structure changes that soften the single-ion in-plane anisotropy and enhance Weyl-mediated magnetic interactions. These results unveil a remarkable connection between band topology, electronic transport, and collective magnetism in Weyl semimetals.

Yao, Xiaohan [Boston College, Chestnut Hill, MA (U↗
DOE OSTI2021

Elastic and thermodynamic properties of cerium-doped yttrium aluminum garnets

Cerium-doped yttrium aluminum garnets (Y 3-x Ce x Al 5 O 12 , Ce:YAGs) are promising yellow light-emitting components of solid-state white light-emitting diodes. Although there have been numerous studies examining the effects of Ce concentrations on the luminescent properties of Y 3-x Ce x Al 5 O 12 , the impacts of Ce dopant on the elastic and thermodynamic properties are not well understood. In this work, we used resonant ultrasound spectroscopy (RUS) to determine the effects of Ce doping (0.025, 0.1, 1 at. %) on the elastic and thermodynamic properties of Y 3-x Ce x Al 5 O 12 . The elastic moduli calculated via the Voigt–Reuss–Hill (VRH) method demonstrated that low Ce dopant concentrations (≤0.1 at. %) induced negligible effects on the elasticity of the YAG host matrix, while a high Ce concentration (1 at. %) yielded significant softening. RUS spectral analysis and SEM images suggested that the elastic softening originated from microstructural differences induced at higher Ce dopant concentrations. Additionally, we demonstrated an increase in elastic anisotropy at higher Ce concentrations, which further elucidated the correlations between structure and elasticity of Y 3-x Ce x Al 5 O 12 . Debye temperatures ($θ_D$), heat capacities ($C_p$), and thermal conductivities ($\mathit{κ}$) were calculated for Ce:YAGs through the relations of RUS-derived parameters (sound velocities, elastic moduli) and previously determined thermal expansion coefficients. Ce:YAG was found to have a significant reduction in $θ_D$, Cp, and $\mathit{κ}$ at Ce concentrations ≥1 at. %. Lastly, extrapolation of $C_p$ and $\mathit{κ}$ to higher temperatures allowed the modeling of thermal stress experienced by Y 3-x Ce x Al 5 O 12 disks up to 1073.15 K.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Coupling morphological and magnetic anisotropy for assembling tetragonal colloidal crystals

Morphological and magnetic anisotropy can be combined in colloidal assembly to create unconventional secondary structures. We show here that magnetite nanorods interact along a critical angle, depending on their aspect ratios and assemble into body-centered tetragonal colloidal crystals. Under a magnetic field, size-dependent attractive and repulsive domains develop on the ends and center of the nanorods, respectively. Our joint experiment-computational multiscale study demonstrates the presence of a critical angle in the attractive domain, which defines the equilibrium bonding states of interacting rods and leads to the formation of non–close-packed yet hard-contact tetragonal crystals. Small-angle x-ray scattering measurement attributes the perfect tetragonal phase to the slow assembly kinetics. The crystals exhibit brilliant structural colors, which can be actively tuned by changing the magnetic field direction. These highly ordered frameworks and well-defined three-dimensional nanochannels may offer new opportunities for manipulating nanoscale chemical transformation, mass transportation, and wave propagation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1982

Solar flare shocks in interplanetary space and solar flare particle events

Data on particle events resulting from three solar flares are examined to determine the effect of shocks on the population of protons at energies not less than 30 MeV. The arrival of the shock is found to mark the start of the decay phase of the particle event, and changes in particle anisotropy are found to be related to features in the shock structure. Two distinct modes of particle propagation are found to exist: a direct component, and a second component which has a time scale of days and is seen in two of the flares which also produced interplanetary shock. In addition, a model is described in which the protons are accelerated by the shocks, and it is shown to be in agreement with observations.

Evenson, P.↗
NASA NTRS1983

Spatial Fluctuations in the Diffuse Cosmic X-Ray Background

The bright, essentially isotropic, X-ray sky flux above 2 keV yields information on the universe at large distances. However, a definitive understanding of the origin of the flux is lacking. Some fraction of the total flux is contributed by active galactic nuclei and clusters of galaxies, but less than one percent of the total is contributed by the or approximately 3 keV band resolved sources, which is the band where the sky flux is directly observed. Parametric models of AGN (quasar) luminosity function evolution are examined. Most constraints are by the total sky flux. The acceptability of particular models hinges on assumptions currently not directly testable. The comparison with the Einstein Observatory 1 to keV low flux source counts is hampered by spectral uncertainties. A tentative measurement of a large scale dipole anisotropy is consistent with the velocity and direction derived from the dipole in the microwave background. The impact of the X-ray anisotropy limits for other scales on studies of large-scale structure in the universe is sketched. Models of the origins of the X-ray sky flux are reviewed, and future observational programs outlined.

Shafer, R. A.↗
NASA NTRS1989

ASTROS: A multidisciplinary automated structural design tool

ASTROS (Automated Structural Optimization System) is a finite-element-based multidisciplinary structural optimization procedure developed under Air Force sponsorship to perform automated preliminary structural design. The design task is the determination of the structural sizes that provide an optimal structure while satisfying numerous constraints from many disciplines. In addition to its automated design features, ASTROS provides a general transient and frequency response capability, as well as a special feature to perform a transient analysis of a vehicle subjected to a nuclear blast. The motivation for the development of a single multidisciplinary design tool is that such a tool can provide improved structural designs in less time than is currently needed. The role of such a tool is even more apparent as modern materials come into widespread use. Balancing conflicting requirements for the structure's strength and stiffness while exploiting the benefits of material anisotropy is perhaps an impossible task without assistance from an automated design tool. Finally, the use of a single tool can bring the design task into better focus among design team members, thereby improving their insight into the overall task.

Neill, D. J.↗
NASA NTRS2024

Anisotropy of Density Fluctuations in the Solar Wind at 1 au

A well-known property of solar wind plasma turbulence is the observed anisotropy of the autocorrelations, or equivalently the spectra, of velocity and magnetic field fluctuations. Here we explore the related but apparently not well-studied issue of the anisotropy of plasma density fluctuations in the energy-containing and inertial ranges of solar wind turbulence. Using 10 yr (1998–2008) of in situ data from the Advanced Composition Explorer mission, we find that for all but the fastest wind category, the density correlation scale is slightly larger in directions quasi-parallel to the large-scale mean magnetic field as compared to quasi-perpendicular directions. The correlation scale in fast wind is consistent with isotropic. The anisotropy as a function of the level of correlation is also explored. We find at small correlation levels, i.e., at energy-containing scales and larger, the density fluctuations are close to isotropy for fast wind, and slightly favor more rapid decorrelation in perpendicular directions for slow and medium winds. At relatively smaller (inertial range) scales where the correlation values are larger, the sense of anisotropy is reversed in all speed ranges, implying a more "slablike" structure, especially prominent in the fast wind samples. We contrast this finding with published results on velocity and magnetic field correlations.

Interplanetary turbulence↗
DOE OSTI2023

Anisotropic magnetism and electronic structure of trigonal EuAl 2 Ge 2 single crystals

Understanding the interplay between magnetic and electronic degrees of freedom is of profound recent interest in different Eu-based magnetic topological materials. Here, we studied the magnetic and electronic properties of the layered Zintl-phase compound EuAl 2 Ge 2 crystallizing in the trigonal CaAl 2 Si 2 -type structure. We report zero-field neutron diffraction, temperature T - and magnetic-field H-dependent magnetic susceptibility χ(T, H), isothermal magnetization M(T, H), heat capacity C p (T, H), and electrical resistivity ρ(T, H) measurements, together with T-dependent angle-resolved photoemission spectroscopy (ARPES) measurements complemented with first-principle calculations. EuAl 2 Ge 2 undergoes second-order A-type antiferromagnetic (AFM) ordering below T N = 27.5(5) K, with the Eu moments (Eu 2+ , S = 7/2) aligned ferromagnetically in the ab plane while these layers are stacked antiferromagnetically along the c axis. The critical fields at which all moments become parallel to the field are 37.5(5) and 52.5(5) kOe for H ∥ ab and H ∥ c, respectively. The H = 0 magnetic structure consists of trigonal AFM domains associated with ab-plane magnetic anisotropy and a field-induced reorientation of the Eu spins in the domains is also evident at T = 2 K below the critical field H c1 = 2.5(1) kOe. The ρ(T) measurements reveal metallic behavior transforming into a slight resistivity increase on cooling towards T N . A pronounced loss of spin-disorder scattering is observed below T N . The ARPES results show that EuAl 2 Ge 2 is metallic both above and below T N , and the Fermi surface is anisotropic with two hole pockets at the zone center and one small electron pocket at each M point. In the AFM phase, we directly observe folded bands in ARPES due to the doubling of the magnetic unit cell along the c axis with an enhancement of quasiparticle weight due to the complex change in the coupling between the magnetic moments and itinerant electrons on cooling below T N . The observed electronic structure is well reproduced by first-principle calculations, which also predict the presence of nontrivial electronic states near the Fermi level in the AFM phase with Z 2 topological numbers 1;(000)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1979

Tidal interactions in the expanding universe - The formation of prolate systems

The study estimates the magnitude of the anisotropy that can be tidally induced in neighboring initially spherical protostructures, be they protogalaxies, protoclusters, or even uncollapsed density enhancements in the large-scale structure of the universe. It is shown that the linear analysis of tidal interactions developed by Peebles (1969) predicts that the anisotropy energy of a perturbation grows to first order in a small dimensionless parameter, whereas the net angular momentum acquired is of second order. A simple model is presented for the growth of anisotropy by tidal interactions during the nonlinear stage of the development of perturbations. A possible observational test is described of the alignment predicted by the model between the orientations of large-scale perturbations and the positions of neighboring density enhancements.

Binney, J.↗
DOE OSTI2025

Subvoxel Control of Fiber Orientation via Multidirectional Shearing in 3D Printing

Anisotropy, the characteristic of materials exhibiting different properties based on their direction, is widespread in nature. Conventional manufacturing techniques often fall short in recreating such complex anisotropy. While 3D printing allows for precise fiber deposition in anisotropic composites, previous studies have only achieved bulk reorientation of fibers at the voxel level in two dimensions. This limits the replication of localized 3D anisotropies found in natural materials. To address this, a novel 3D printing technique is presented that enables subvoxel control of fiber orientation in all three directions using multidirectional shearing via nozzle rotation and inclination. The fiber orientation control in these experimental tests is driven by a numerical model, enabling a fully digital approach to program microstructures within a strand. This programmability is demonstrated through mechanical and thermal tests, illustrating a localized and controllable response to external stimuli. Achieving such complex anisotropy holds potential across several fields, including wearables and biomedical implants, lightweight composite structures, and energy storage technologies such as batteries and supercapacitors.

3D printing↗
NASA NTRS1996

The Sensitivity of Large-Eddy Simulation to Local and Nonlocal Drag Coefficients at the Lower Boundary

It was found that the homogeneity of the surface drag coefficient plays an important role in the large scale structure of turbulence in large-eddy simulation of the convective atmospheric boundary layer. Particularly when a ground surface temperature was specified, large horizontal anisotropies occurred when the drag coefficient depended upon local velocities and heat fluxes. This was due to the formation of streamwise roll structures in the boundary layer. In reality, these structures have been found to form when shear is approximately balanced by buoyancy. The present cases, however, were highly convective. The formation was caused by particularly low values of the drag coefficient at the entrance to thermal plume structures.

Schowalter, D. G.↗
NASA NTRS2004

Anisotropy and Alfvenicity of hourly fluctuations in the fast polar solar wind

Ulysses magnetic and plasma data obtained in the fast polar solar wind are used to study the nature of fluctuations in the frequency range between the turbulent inertial range and the lower-frequency range dominated by quasi-static structures originating at the Sun. For daily variations of hourly averages of the magnetic field components the anisotropy of the fluctuations is less than in the higher-frequency range, and the minimum varianced direction loses its alignment with the magnetic field in favor of a more radial direction as the solar distance increases.

Solar wind Alfvenicity waves anisotropy Ulysses↗
DOE OSTI2021

Quantum spin state transitions in the spin-1 equilateral triangular lattice antiferromagnet Na 2 BaNi(PO 4 ) 2

We report we have grown single crystals of Na 2 BaNi(PO 4 ) 2 , a spin-1 equilateral triangular lattice antiferromagnet (ETLAF), and performed magnetic susceptibility, specific heat, and thermal conductivity measurements at ultralow temperatures. The main results are as follows: (i) At zero magnetic field, Na 2 BaNi(PO 4 ) 2 exhibits a magnetic ordering at 430 mK with a weak ferromagnetic moment along the $\textit{c}$ axis. This suggests a canted 120° spin structure, which is in a plane including the crystallographic $\textit{c}$ axis due to the existence of an easy-axis anisotropy and ferromagnetically stacked along the $\textit{c}$ axis. (ii) With increasing field along the $\textit{c}$ axis, a 1/3 magnetization plateau is observed, which means that the canted 120° spin structure is transformed into an up-up-down (UUD) spin structure. With even higher fields, the UUD phase further evolves into possible V and V' phases. (iii) With increasing field along the $\textit{a}$ axis, the canted 120° spin structure is possibly transformed into an umbrella phase and a V phase. Therefore Na 2 BaNi(PO 4 ) 2 is a rare example of a spin-1 ETLAF with single-crystalline form that exhibits easy-axis spin anisotropy and a series of quantum spin state transitions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗