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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Effects of wind-tunnel disturbances on hypersonic boundary-layer transition. I, II.

An experimental investigation of the dependence on facility disturbance of transition on sharp cones has been made. In Part I, the sound radiated from the turbulent boundary layer on the walls of several facilities was measured with pressure transducers mounted flush with the cone surface. In Part II, a constant current hot-wire anemometer measured free-stream and cone shock layer disturbances in two hypersonic helium tunnels. Comparison of hot-wire results with surface pressure measurements in one of the helium tunnels indicate the latter data provide an accurate indication of the facility disturbance levels. Both studies show that transition Reynolds numbers correlate in terms of facility rms sound disturbance levels provided the laminar boundary layers on the models are similar.

Stainback, P. S.↗
DOE OSTI2022

A neural network‐enhanced reproducing kernel particle method for modeling strain localization

Abstract Modeling the localized intensive deformation in a damaged solid requires highly refined discretization for accurate prediction, which significantly increases the computational cost. Although adaptive model refinement can be employed for enhanced effectiveness, it is cumbersome for the traditional mesh‐based methods to perform while modeling the evolving localizations. In this work, neural network‐enhanced reproducing kernel particle method (NN‐RKPM) is proposed, where the location, orientation, and shape of the solution transition near a localization is automatically captured by the NN approximation via a block‐level neural network (NN) optimization. The weights and biases in the blocked parameterization network control the location and orientation of the localization. The designed basic four‐kernel NN block is capable of capturing a triple junction or a quadruple junction topological pattern, while more complicated localization topological patters are captured by the superposition of multiple four‐kernel NN blocks. The standard RK approximation is then utilized to approximate the smooth part of the solution, which permits a much coarser discretization than the high‐resolution discretization needed to capture sharp solution transitions with the conventional methods. A regularization of the NN approximation is additionally introduced for discretization‐independent material responses. The effectiveness of the proposed NN‐RKPM is verified by a series of numerical verifications.

Baek, Jonghyuk↗
DOE OSTI2019

Evaluating Large-Eddy Simulations of Boundary-Layer Cloud Response to Climate Change using MAGIC Observations

Over the cool waters off California, a persistent and extensive deck of low-lying clouds forms atop a shallow, turbulent layer of moist air extending 500-2000 meters above the ocean, above which the air is much warmer and drier. Its west edge varies with weather and season and is typically half way to Hawaii. This cloud deck and similar cloud decks elsewhere reflect sunlight back to space, cooling the underlying ocean and indeed the entire planet. They must be accurately represented by a climate model to correctly simulate our current climate or future global warming. Most climate models suggest these cloud decks will thin and break up in a warmer CO 2 -enriched climate, so extra sunlight would reach the ocean surface and amplify warming worldwide. But these models have difficulty accurately simulating the cloud thickness and extent because the turbulent cloud-forming air currents are much too small to be simulated by their coarse computational grid. In 2012-2013, the DOE Atmospheric Radiation Measurement MAGIC program instrumented a container ship transiting between California and Hawaii to observe clouds and rain, small particles (aerosols), and weather conditions. Data were obtained from 20 4-6 day cruises in each direction. This study tested whether a sophisticated ‘large-eddy simulation’ (LES) of the turbulent air motions that create the clouds in this region accurately predicts the variations in cloud properties seen across the different cruises. If so, that model is more believable for predicting cloud changes in this region in a future warmer climate. Our LES used a numerical mesh over a 6.4x6.4 km region with 50 m horizontal and 5 m vertical grid spacing to simulate the cloud-forming eddies and the sharp overlying transition between moist and dry air. To better compare with MAGIC observations, we configured our model to simulate a region that moved along with the ship. We used a weather forecast model to specify effects of the large-scale air flow. We compared our results with balloon ascents, a radar and other instruments pointed upward at the clouds, measurements of downward sunlight and infrared radiation, aerosols, and weather variables for all suitable cruise periods. Our LES was remarkably skillful in predicting the cloudiness variations measured by the ship. MAGIC also sampled mesoscale cells, regions of cloud thickening and thinning over distances of 10-50 km. An LES run over a larger region 100 km on a side shows these cells form spontaneously, enhanced by strong radiative heat loss at the cloud tops.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2018

Single-Grain Bulk YBa{sub 2}Cu{sub 3}O{sub y} Superconductors Grown by Infiltration Growth Process Utilizing the YbBa{sub 2}Cu{sub 3}O{sub y} + Liquid Phase as a Liquid Source

The top-seeded infiltration growth (IG) processed YBa{sub 2}Cu{sub 3}O{sub y} (Y-123) has several advantages compared to the same material produced by other melt process techniques. There is usually no shrinkage, a limited number of pores, and an uniform secondary phase particle dispersion in the Y-123 matrix. These characteristics of the IG process are very attractive for several industrial applications. In this paper, we produced a large single-grain Y-123 by IG and top seeded melt-growth process. Utilizing commercial YbBa{sub 2}Cu{sub 3}O{sub y} (Yb-123) and a homemade Y{sub 2}BaCuO{sub 5} (Y-211), we produced bulk YBa{sub 2}Cu{sub 3}O{sub y} samples using Yb-123 + liquid (1:1) as a liquid source. Four facet lines on the top surfaces of the as-grown samples indicated a uniform growth from the seed up to sample edges. Magnetization measurements by SQUID magnetometer showed a sharp superconducting transition with T{sub c},{sub onset} at 92.1 K. The critical current density at 77 K and self-field was 39,000 A/cm{sup 2}. Trapped field results confirmed that single grain Y-123 samples were produced utilizing the above process. Our results prove that Yb-123 + liquid (1:1) as a liquid source is effective in producing single grain Y-123 material by IG process.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Progress in Critical Current Density (J{sub c}) in Sintered MgB{sub 2} Bulks

The present paper focuses on methods of further improving the flux pinning and critical current density of disk-shaped MgB{sub 2} bulk superconductors by adding excess Mg metal in combination with an optimum silver content and optimized processing conditions. Bulk MgB{sub 2} samples were produced by in situ solid-state reaction in Ar gas ambient using high purity commercial powders of Mg metal and 1.5 wt% carbon-coated amorphous B powders mixed in a fixed ratio of Mg/B = 1.1:2. Further, 4 wt% silver was added to improve flux pinning as well as mechanical performance of the bulk MgB{sub 2} material. The magnetization measurements confirmed a sharp superconducting transition with T{sub c,onset} at around 37 K, which is only by 1 K lower than in bulk MgB{sub 2} material produced without carbon-coated amorphous boron. The critical current density (J{sub c}) values significantly improved in the MgB{sub 2} material with 4 wt% of silver and 1.5 wt% of carbon-coated amorphous boron, sintered at 775 {sup ∘}C for 3 h. At 20 K, this sample showed J{sub c} at around 500 and 350 kA/cm{sup 2} in the self-field and 1 T, respectively, which makes it suitable for several industrial applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effect of Synthesis Temperature on Structural, Optical, and Magnetic Properties of ZnO Nanoparticles Synthesized by Combustion Method

ZnO nanoparticles at different temperatures were synthesized by using the combustion method. The effect of synthesis temperature on the properties of the nanoparticles was studied by using XRD, FESEM, EDS, TEM, photoluminescence (PL), Raman, diffuse reflectance spectra (DRS), and VSM characterization techniques. The XRD results reveal that the grown nanoparticles have a hexagonal wurtzite structure without any impurities and agreed with EDS results. FESEM and TEM micrographs show that the ZnO nanoparticles possess a spherical shape with agglomeration free, and the size increases with increase of synthesis temperature. From DRS studies, it was noticed that the band gap decreases with increase of synthesis temperature. In PL studies, blue peak at 465 nm may be due to defect-related transitions. A sharp intense peak in Raman spectra at 485 cm{sup −1} represents E{sub 2}H mode is a characteristic of a hexagonal wurtzite structure. The magnetic studies show that the magnetization decreases from 0.0172 to 0.0042 emu/g as the synthesis temperature increases from 400 to 550 °C. ZnO synthesized at 500 °C has a large squareness ratio. These materials are potential candidates for memory devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Epitaxial aluminum layer on antimonide heterostructures for exploring Josephson junction effects

In this study, we present results of our recent work of epitaxially-grown aluminum (epi-Al) on antimonide heterostructures, where the epi-Al thin film is grown at either room temperature or below zero °C. A sharp superconducting transition at T ∼ 1.3 K is observed in these epi-Al films. We further show that supercurrent states are realized in Josephson junctions fabricated in the epi-Al/antimonide heterostructures with mobility μ ∼ 1.0 × 10 6 cm 2 /Vs. These results clearly demonstrate we have achieved growing high-quality epi-Al/antimonide heterostructures, a promising platform for the exploration of Josephson junction effects for quantum information science and microelectronics applications.

Antimonide heterostructures↗
DOE OSTI2018

Nanoalloy catalysts inside fuel cells: An atomic-level perspective on the functionality by combined in operando x-ray spectroscopy and total scattering

We introduce an experimental approach for structural characterization of catalysts for fuel cells combining synchrotron x-ray spectroscopy and total scattering. The approach allows probing catalysts inside operating fuel cells with atomic-level precision (~ 0.02 Å) and element specificity (~ 2–3 at%) in both time (~ 1 min) and space (~ μm) resolved manner. The approach is demonstrated on exemplary Pd-Sn and Pt-Ni-Cu nanoalloy catalysts for the oxygen reduction reaction (ORR) deposited on the cathode of an operating proton exchange membrane fuel cell. In operando x-ray data show that under operating conditions, the catalyst particles can undergo specific structural changes, ranging from sub-Å atomic fluctuations and sharp nanophase transitions to a gradual strain relaxation and growth, which inflict significant losses in their ORR activity. Though triggered electrochemically, the changes are not driven solely by differences in the reduction potential and surface energy of the metallic species constituting the nanoalloys but also by the formation energy of competing nanophases, mismatch between the size of individual atomic species and their ability to interdiffuse fast in search of energetically favorable configurations. Given their complexity, the changes are difficult to predict and so the resulting ORR losses remain difficult to limit. We show that in operando knowledge of the structural evolution of nanoalloy catalysts helps create strategies for improving their activity and stability. In particular, we show that shaping Pd-Sn nanoalloys rich in Pd as cubes reduces the interdiffusion of atoms at their surface and so makes them better catalysts for ORR in fuel cells in comparison to other Pd-Sn nanoalloys. In addition, we demonstrate that the approach introduced here can provide knowledge of other major factors affecting the performance of fuel cells such as operating temperature and the overall catalyst utilization, in particular the evolution of elemental and mass distribution of catalyst particles over the cells’ cathode. Last but not least, we discuss how in operando x-ray spectroscopy and total x-ray scattering can bridge the knowledge gap between the widely used in situ SAXS, EXAFS and monocrystal surface XRD techniques for structural characterization of nanoalloy catalysts explored for energy related applications.

25 ENERGY STORAGE↗
DOE OSTI2017

Magnetic field devices for neutron spin transport and manipulation in precise neutron spin rotation measurements

The neutron spin is a critical degree of freedom for many precision measurements using low-energy neutrons. Fundamental symmetries and interactions can be studied using polarized neutrons. Parity-violation (PV) in the hadronic weak interaction and the search for exotic forces that depend on the relative spin and velocity, are two questions of fundamental physics that can be studied via the neutron spin rotations that arise from the interaction of polarized cold neutrons and unpolarized matter. The Neutron Spin Rotation (NSR) collaboration developed a neutron polarimeter, capable of determining neutron spin rotations of the order of 10 -7 rad per meter of traversed material. This paper describes two key components of the NSR apparatus, responsible for the transport and manipulation of the spin of the neutrons before and after the target region, which is surrounded by magnetic shielding and where residual magnetic fields need to be below 100 μG. These magnetic field devices, called input and output coils, provide the magnetic field for adiabatic transport of the neutron spin in the regions outside the magnetic shielding while producing a sharp nonadiabatic transition of the neutron spin when entering/exiting the low-magnetic-field region. In addition, the coils are self contained, forcing the return magnetic flux into a compact region of space to minimize fringe fields outside. The design of the input and output coils is based on the magnetic scalar potential method.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2025

Local Interface Effects Modulate Global Charge Order and Optical Properties of 1T–TaS2/1H–WSe2 Heterostructures

1T-TaS2 is a layered charge density wave (CDW) crystal exhibiting sharp phase transitions and associated resistance changes. These resistance steps could be exploited for information storage, underscoring the importance of controlling and tuning the CDW states. Given the importance of out-of-plane interactions in 1T-TaS2, modulating interlayer interactions by heterostructuring is a promising method for tailoring CDW phase transitions. In this work, we investigate the optical and electronic properties of heterostructures comprising 1T-TaS2 and monolayer 1H-WSe2. By systematically varying the thickness of 1T-TaS2 and its azimuthal alignment with 1H-WSe2, we find that intrinsic moiré strain and interfacial charge transfer introduce CDW disorder in 1T-TaS2 and modify the CDW ordering temperature. Furthermore, our studies reveal that the interlayer alignment impacts the exciton dynamics in 1H-WSe2, indicating that heterostructuring can concurrently tailor the electronic phases in 1T-TaS2 and the optical properties of 1H-WSe2. This work presents a promising approach for engineering the optoelectronic behavior of heterostructures that integrate CDW materials and semiconductors.

charge density wave↗
DOE OSTI2025

Chromium versus Aluminum: Impact of Nickel Alloy Composition and Interfacial Kinetics on High-Temperature Passivating Oxide Formation

High-temperature corrosion resistance depends critically on the formation of a passivating surface oxide, which is highly sensitive to alloy composition and structure. Such details often elude experimental investigation, and simplified analytical models fail to provide a truly chemical view of passivating oxide evolution. Here, we explicitly compare the fundamental chemistry of Cr and Al as prototypical passivating elements in Ni alloys by directly simulating competing reaction and diffusion processes within the oxide film using kinetic Monte Carlo and density functional theory. We find that the origin and expression of passivating behavior during early-stage thermal oxidation are qualitatively different between the two alloy systems. Ni–Cr alloys feature a sudden onset of passivation associated with a sharp phase transition upon Cr enrichment that directly couples oxidation kinetics to phase transformation behavior. In contrast, Ni–Al alloys display more continuous oxide phase variation with Al enrichment, ultimately resulting in a lower composition threshold for passivation and a thinner passivating layer. In addition, we elucidate the nonobvious role of metal exchange within the alloy near the oxide boundary, which fundamentally alters film composition and passivating behavior. Furthermore, our results have key implications for engineering improved corrosion-resistant alloys, both in terms of compositional variation and processing.

Alloys↗
DOE OSTI2016

Superconductivity in the Narrow Gap Semiconductor RbBi 11/3 Te 6

Superconductivity was discovered in the layered compound RbBi 11/3 Te 6 , featuring Bi vacancies and a narrow band gap of 0.25(2) eV at room temperature. In addition, a sharp superconducting transition at similar to 3.2 K was observed in polycrystalline ingots. The superconducting volume fraction of oriented single crystals is almost 100%, confirming bulk superconductivity. Systematic Se and Sb substitutions in RbBi 11/3-y Sb y Se x Te 6-x , revealed a dependence of the superconducting transition on composition that can increase the T c up to similar to 10%. The RbBi 11/3 Te 6 system is the first member of the new homologous series Rb[Bi 2n+11/3 Te 3n+6 ] with infinite Bi 2 Te 3 -like layers. Lastly, the large degree of chemical tunability of the electronic structure of the homology via doping and/or substitution gives rise to a new family of superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Electric field control of magnetization direction across the antiferromagnetic to ferromagnetic transition

Electric-field-induced magnetic switching can lead to a new paradigm of ultra-low power nonvolatile magnetoelectric random access memory (MeRAM). To date the realization of MeRAM relies primarily on ferromagnetic (FM) based heterostructures which exhibit low voltage-controlled magnetic anisotropy (VCMA) efficiency. On the other hand, manipulation of magnetism in antiferromagnetic (AFM) based nanojunctions by purely electric field means (rather than E-field induced strain) remains unexplored thus far. Ab initio electronic structure calculations reveal that the VCMA of ultrathin FeRh/MgO bilayers exhibits distinct linear or nonlinear behavior across the AFM to FM metamagnetic transition depending on the Fe- or Rh-interface termination. We predict that the AFM Fe-terminated phase undergoes an E-field magnetization switching with large VCMA efficiency and a spin reorientation across the metamagnetic transition. In sharp contrast, while the Rh-terminated interface exhibits large out-ofplane (in-plane) MA in the FM (AFM) phase, its magnetization is more rigid to external E-field. These findings demonstrate that manipulation of the AFM Néel-order magnetization direction via purely E-field means can pave the way toward ultra-low energy AFM-based MeRAM devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Shell-mediated control of surface chemistry of highly stoichiometric magnetite nanoparticles

Magnetite (Fe 3 O 4 ) nanoparticles are one of the most studied nanomaterials for different nanotechnological and biomedical applications. However, Fe 3 O 4 nanomaterials gradually oxidize to maghemite (γ-Fe 2 O 3 ) under conventional environmental conditions leading to changes in their functional properties that determine their performance in many applications. Here we propose a novel strategy to control the surface chemistry of monodisperse 12 nm magnetite nanoparticles by means of a 3 nm-thick Zn-ferrite epitaxial coating in core/shell nanostructures. We have carried out a combined Mössbauer spectroscopy, dc magnetometry, X-ray photoelectron spectroscopy and spatially resolved electron energy loss spectroscopy study on iron oxide and Fe 3 O 4 /Zn 0.6 Fe 2.4 O 4 core/shell nanoparticles aged under ambient conditions for 6 months. Our results reveal that while the aged iron oxide nanoparticles consist of a mixture of γ-Fe 2 O 3 and Fe 3 O 4 , the Zn-ferrite-coating preserves a highly stoichiometric Fe 3 O 4 core. Therefore, the aged core/shell nanoparticles present a sharp Verwey transition, an increased saturation magnetization and the possibility of tuning the effective anisotropy through exchange-coupling at the core/shell interface. The inhibition of the oxidation of the Fe 3 O 4 cores can be accounted for in terms of the chemical nature of the shell layer and an epitaxial crystal symmetry matching between the core and the shell.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2012

Electric field assisted sputtering of Fe{sub 3}O{sub 4} thin films and reduction in anti-phase boundaries

Fe{sub 3}O{sub 4} thin films have been prepared by a new approach of in-situ electric field assisted sputtering. Raman shift, {rho}(T), and M(T) measurements reveal a reduction in anti-phase boundaries (APBs), which is explained by the electric field induced surface diffusion favouring [111] oriented growth with lesser defects. Early magnetization saturation (4 kOe) and its higher value (441 emu/cm{sup 3}), low electron phonon coupling constant (0.48), sharp Verwey transition {Delta}T{sub V} = 12 K, Arrhenius law of conduction via delocalized states above T{sub V} and lower value of localization radius (1.5 nm) below T{sub V} compared to that observed in un-assisted films confirmed that the electric field assisted grown films are nearly free from anti-phase boundaries.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Near-ideal strength in metal nanotubes revealed by atomistic simulations

Here we report extraordinary mechanical properties revealed by atomistic simulations in metal nanotubes with hollow interior that have been long overlooked. Particularly, the yield strength in [1 1 1] Au nanotubes is found to be up to 60% higher than the corresponding solid Au nanowire, which approaches the theoretical ideal strength in Au. Furthermore, a remarkable transition from sharp to smooth yielding is observed in Au nanotubes with decreasing wall thickness. The ultrahigh tensile strength in [1 1 1] Au nanotube might originate from the repulsive image force exerted by the interior surface against dislocation nucleation from the outer surface.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Local network structure of a-SiC:H and its correlation with dielectric function

The microscopic disordered structures of hydrogenated amorphous silicon carbide (a-Si{sub 1−x}C{sub x}:H) layers with different carbon contents have been determined based on the correlations between the dielectric function in the ultraviolet/visible region and the local bonding states studied by high-sensitivity infrared attenuated total reflection spectroscopy. We find that the microscopic structure of the a-Si{sub 1−x}C{sub x}:H layers fabricated by plasma-enhanced chemical vapor deposition shows a sharp structural transition at a boundary of x = 6.3 at. %. In the regime of x ≤ 6.3 at. %, (i) the amplitude of the a-SiC:H dielectric function reduces and (ii) the SiH{sub 2} content increases drastically with x, even though most of the C atoms are introduced into the tetrahedral sites without bonding with H. In the regime of x > 6.3 at. %, on the other hand, (i) the amplitude of the dielectric function reduces further and (ii) the concentration of the sp{sup 3} CH{sub n} (n = 2,3) groups increases. Moreover, we obtained the direct evidence that the sp{sup 2} C bonding state in the a-SiC matrix exists in the configuration of C = CH{sub 2} and the generation of the graphite-like C = CH{sub 2} unit suppresses the band gap widening significantly. At high C contents of x > 6.3 at. %, the a-SiC:H layers show quite porous structures due to the formation of microvoids terminated with the SiH{sub 2}/CH{sub n} groups. By taking the SiH{sub 2}/CH{sub n} microvoid generation in the network and the high-energy shift of the dielectric function by the local bonding states into account, the a-SiC:H dielectric function model has been established. From the analysis using this model, we have confirmed that the a-SiC:H optical properties in the ultraviolet/visible region are determined almost completely by the local network structures.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Ellipsometry characterization of polycrystalline ZnO layers with the modeling of carrier concentration gradient: Effects of grain boundary, humidity, and surface texture

Spectroscopic ellipsometry (SE) has been applied to study the effects of grain boundary, humidity, and surface texture on the carrier transport properties of Al-doped ZnO layers fabricated by dc and rf magnetron sputtering. In the SE analysis, the variation in the free carrier absorption toward the growth direction, induced by the ZnO grain growth on foreign substrates, has been modeled explicitly by adopting a multilayer model in which the optical carrier concentration (N{sub opt}) varies continuously with a constant optical mobility (μ{sub opt}). The effect of the grain boundary has been studied by comparing μ{sub opt} with Hall mobility (μ{sub Hall}). The change in μ{sub Hall}/μ{sub opt} indicates a sharp structural transition of the ZnO polycrystalline layer at a thickness of d ∼ 500 nm, which correlates very well with the structure confirmed by transmission electron microscopy. In particular, below the transition thickness, the formation of the high density grain boundary leads to the reduction in the μ{sub Hall}/μ{sub opt} ratio as well as N{sub opt}. As a result, we find that the thickness dependence of the carrier transport properties is almost completely governed by the grain boundary formation. On the other hand, when the ZnO layer is exposed to wet air at 85 °C, μ{sub Hall} reduces drastically with a minor variation of μ{sub opt} due to the enhanced grain boundary scattering. We have also characterized textured ZnO:Al layers prepared by HCl wet etching by SE. The analysis revealed that the near-surface carrier concentration increases slightly after the etching. We demonstrate that the SE technique can be applied to distinguish various rough textured structures (size ∼ 1 μm) of the ZnO layers prepared by the HCl etching.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗