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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 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Vortex Dynamic Phases in Type II Superconducting Strips with Regular and Flattened Triangular Pinning Arrays

The dynamic behavior of vortices in type II superconducting infinite strips is simulated with two types of triangular pinning lattices: the regular one and a flattened triangular array that mimics the vortex lattice in a superconducting strip in the absence of a pinning array. The calculations were made at zero temperature and perpendicular magnetic field. The size effects are investigated for several strip widths maintaining the density and size of pinning centers unchanged. A driving force is applied in the infinite direction to analyze the depinning process and the different dynamic regimes. For the regular triangular pinning lattice, we found that there is a great richness of vortex dynamical phases because depending on strip width, part of the vortices may stabilize in different interstitial positions and the depinning process depends on the commensurability of each vortex chain. For the flattened triangular lattice, the system exhibits one ordered moving phase for narrower strips and two moving phases for the larger strip. As the transport force increases, vortices in the larger strip go from a disordered to an ordered phase. Moreover, the flattened lattice shows a much higher critical depinning force than the regular one due to commensurability effects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effects of Co Doping and Point Defect on the Ferromagnetism of ZnO

The effects of different doping proportions of Co doping and oxygen vacancies (V{sub O}) on the ferromagnetism of zinc oxide (ZnO) are controversial. To solve this problem, we investigated the effects of Co doping and point defects (V{sub O}, V{sub Zn}, H{sub i}, and Zn{sub i}) on the ferromagnetism of ZnO through first-principle calculations by using generalized gradient approximation + U (GGA + U) under density functional theory. Results show that the closer the distance between Co and vacancies (V{sub O} or V{sub Zn}), the lower the formation energy and the more stable the system will be. Co-doped ZnO with V{sub O} or V{sub Zn} exhibits long-range ordered ferromagnetism and Curie temperature above the room temperature. Especially, Co-doped ZnO with V{sub Zn} shows a half-metallic behavior, resulting in 100% conduction hole polarizability, which is highly beneficial for dilute magnetic semiconductors (DMSs) as a hole injection source. The ferromagnetism of Zn{sub 14}CoO{sub 16} is caused by the double exchange effects among the electrons of the O–2p, Co–3d, and Zn–3d orbits mediated by the hole carriers after complexes were formed by the Co doping and Zn vacancy. Similarly, the origin of ferromagnetism in Zn{sub 15}CoO{sub 15} is derived from the double exchange effects among the electrons of O–2p, Co–3d, and Zn–4s states mediated by the electron carriers after complexes were formed by the Co doping and O vacancy. In addition, the result shows that Co-doped ZnO with interstitial H (H{sub i}) or Zn (Zn{sub i}) is unfavorable for ferromagnetism and should be avoided experimentally.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Research on photoelectric properties of n-GaN (0001) surface with point defects via first-principles

In order to reveal the influence of n-type doping and point defects on the electronic and optical properties of GaN (0001) surface, the pristine and defective n-GaN (0001) surface models are established. The formation energy, work function, atomic structure, electronic and optical properties of these surfaces are discussed by first-principles calculations. The results show that the defects have the strong tendency to be close to the surface position, and the N interstitial defect (N{sub i}) and the Ga vacancy (V{sub Ga}) are easily formed. The work function of the surface with V{sub Ga} drops the greatest, and the conductivity of n-type increases. In addition, the absorption coefficient of the surface with various defects is smaller than that of the pristine n-type surface. The appropriate amount of Ga vacancies can increase the probability of electron emission, but it is not conducive to the absorption of photons.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Thermal Annealing Effect on Structure and Properties of Tungsten Surface Irradiated with High Fluence and Low Energy Alpha-Particles

The paper presents research into the surface morphology, helium cluster formation and mechanical properties of high-purity tungsten after the high fluence and low-energy helium ion implantation and subsequent annealing. Investigations are based on scanning electron and atomic force microscopy observations and thermal desorption measurements. Because of the local stresses appearing within the energy loss straggling of helium ions, atoms migrate at distances significantly increasing the ion projective range, and create mobile coalescence of helium atoms and immobile helium-vacancy clusters. The latter form helium bubbles in the energy loss straggling which cause the surface blistering. Local stresses induced by helium coalescence and helium-vacancy clusters beyond the energy loss straggling result in the material strengthening or hardness increase. Subsequent 600°С annealing provides the size growth of blisters on the metal surface irradiated with helium ions and high helium desorption due to the migration of mobile coalescence of helium atoms. With the increasing annealing temperature from 600 to 1000°С, the layer corresponding to the projective range of helium ions fully fractures due to the interstitial helium atoms releasing from immobile helium-vacancy clusters. After 1000°С annealing, the tungsten hardness returns to its initial value.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

A Novel Shim-Assisted Resistance Spot Welding Process to Improve Weldability of Medium-Mn Transformation-Induced Plasticity Steel

Medium-Mn transformation-induced plasticity steels have great potential to significantly reduce vehicle weight and improve fuel economy due to their outstanding combination of high strength and excellent ductility. One bottleneck to the application is their poor weldability resulting from their high Mn contents. In this paper, three resistance spot welding set-ups, including no shim, an interstitial-free steel shim at the faying interface (shim-in) and shims against the electrodes (shim-out), were incorporated to investigate the weldability of Fe-7Mn-0.14C medium-Mn steel. Tensile-shear, cross-tension, and microhardness tests were used to evaluate the mechanical properties of the welds. Experimental results demonstrated that the failure mode of the welds transitioned from the interfacial fracture in the case of no shim to the desired nugget pull-out fracture in the shim-out set-up, resulting in dramatical improvements in both peak loads and their corresponding extensions during the tensile-shear and cross-tension tests. In contrast, the shim-in set-up made no improvement. What can contribute to such improvement was then discussed on the basis of observed morphologies and microstructures of welds.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Doping Effects on the Ductility of a Lightweight Refractory High-Entropy Alloy: Grain Boundary and Bulk Lattice Aspects

Doping elements in small amounts often segregate to grain boundaries (GBs) in alloys and can significantly impact mechanical properties and performance. Refractory high-entropy alloys (RHEAs) are known for their poor ductility, especially at low temperatures. Promoting GB cohesion through segregation can be an effective approach to mitigate embrittlement. Here, in this study, first-principles density functional theory (DFT) calculations were performed to examine the effects of important interstitial dopants (O, B, C, and N) and substitutional dopants (Cr, Y, La, and Ce) on the Σ5(310) [001] tilt GB of a lightweight RHEA Nb 32.5 Ti 27.5 Mo 22.5 Ta 12.5 Hf 2.5 Zr 2.5 . The DFT calculations reveal that certain dopants, such as B, C, Cr, N, and Y, exhibit favorable GB strengthening effects by improving bonding interactions with the bulk alloy. The impact of doping on the ductility parameter of the bulk lattice, defined as the ratio of surface energy to unstable stacking fault energy for the {110} <111> slip system, was also studied; and the results show that doping reduces the intrinsic ductility of the alloy, decreasing the D-parameter from 2.90 to 2.55, depending on the specific dopant. The present findings provide a foundational understanding at atomic level of the effect of representative dopants on mechanical properties of RHEAs and can be used to guide future alloy design for improved mechanical properties.

deformation charge density↗
DOE OSTI2022

Hydrogen induced dislocation core reconstruction in bcc tungsten

Dislocation, playing a crucial role in the plastic deformation of metals, can be significantly affected by introducing solute elements. Hydrogen (H) embrittlement is one such example, while the underlying mechanism for H affected dislocation structural stability and mobility remains unclear and the role of H has been controversial. Here, using first-principles calculations, we demonstrate that the effect of H on screw dislocation in bcc tungsten (W) is H concentration-dependent, signified by a H-induced transition of SD core structure. At low concentrations of H segregation, dislocation maintains the intrinsic easy-core structure, and H atoms are attached to the “periphery” of dislocation to enhance dislocation motion. In contrast, at high H concentrations, dislocation transforms into a hard-core, metal hydride-like structure, as H atoms become the “body” of dislocation to significantly reduce the dislocation mobility in W. Further, such local easy-to-hard transition can also be induced by the other solutes, including helium, carbon, nitrogen and oxygen. Our work sheds new light on the H-dislocation interactions in bcc W, having broad implications in the interstitial solute-related phenomena.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Crucial role of Reg I from acinar-like cell cluster touching with islets (ATLANTIS) on mitogenesis of beta cells in EMC virus-induced diabetic mice

Recently, we reported the presence of distinct cell clusters named acinar-like cell clusters touching Langerhans islets with thin interstitial surrounding (ATLANTIS) in human pancreas. A morphological study in humans demonstrated that ATLANTIS and islet cell clusters are found together in the microenvironment enclosed by a common basement membrane, and ATLANTIS releases vesicles containing Regenerating gene protein (REG Iα) to islet cell clusters. We examined 1) the presence or absence of ATLANTIS in homozygous Reg I (mouse homologue of human REG Iα) deficient (Reg I{sup −/-}) and wild-type mice, and 2) the possible role of ATLANTIS in the regeneration of beta cell clusters after encephalomyocarditis (EMC) virus (D-variant) infection in Reg I{sup −/-} and wild-type mice. ATLANTIS was found in both wild-type and Reg I{sup −/-} mice. In both groups, mean blood glucose increased transiently to greater than 14.0 mmol/L at 5 days after EMC virus infection and recovered to baseline at 12 days. At 12 days after EMC virus infection, lower BrdU labeling indices were observed in islet beta cells of Reg I{sup −/-} mice compared to wild-type mice. Beta cell volume 12 days after EMC virus infection in Reg I{sup −/-} mice did not differ from that of wild-type mice. These results suggest that Reg I, which is released from ATLANTIS to islet beta cell clusters, has a crucial role in beta cell regeneration in EMC virus-induced diabetes. The presence of mechanism(s) other than that mediated by Reg I in beta cell restoration after destruction by EMC virus was also suggested.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2024

Improved thermoelectric performance of α- and β- Cu 2 Se through suppression of hole density using extrinsic copper vacancies

Modulating Cu + ion disorder in Cu 2 Se can enable control over the polymorphism and the carrier density leading to enhanced thermoelectric properties for both α- and β-Cu 2 Se. Here we report that the incorporation of Cr 3+ into the Cu 2 Se crystal lattice facilitates the stabilization of α-Cu 2 Se at 300 K leading to a large (~140%) reduction in the carrier density both below and above the phase transition. This is attributed to the reduction in the density of intrinsic copper interstitials (Cu$^{•}_{i}$) within the Cu (2-δ-λ) (Cr$^{··}_{Cu}$)λ(V$^{'}_{Cu}$)δ(Cu$^{π}_{i}$) δ-2λ (h • ) δ-2λ Se crystal lattice. Such optimization of the carrier density led to a large (63%) increase in the thermopower and a drastic (46%) reduction in the total thermal conductivity for both α- and β-Cu 2 Se matrices. Consequently, a significant enhancement of the thermoelectric performance is observed in the entire temperature range from 300 K to 773 K. This results in high average ZT values for both α-Cu 2 Se (ZT ave = 0.60) and β-Cu 2 Se (ZT ave = 0.97), which paves the way for both near room temperature and high temperatures applications. Furthermore, this work provides a new approach to optimize the thermoelectric performance of Cu 2 Se-based materials by leveraging the interaction between mobile intrinsic Cu$^{·}_{i}$ and extrinsic V$^{'}_{Cu}$ to suppress the hole density.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Hydrogen Localization and Cluster Formation in α-Zr from First-Principles Investigations

We report alpha-zirconium (α-Zr) is a hydrogen atom host material used in nuclear energy and semiconducting applications due to Zr's excellent properties such as corrosion resistance, low thermal neutrons cross-section, and thermodynamically favorable interstitial sequestering of H atoms. Herein we use density functional theory simulations to systematically analyze the initial stages of H insertion in the Zr matrix, and determine optimal hydrogen localization, distribution, and clustering tendencies from a thermodynamics and kinetics perspective. Our results suggest that H occupation of tetrahedral sites is strongly favored, consistent with previous experimental and theoretical studies. Further, a cooperative effect in H localization is shown where H atoms favor clustering. The H pair is found to be the most stable by occupying tetrahedral sites with pairs at second nearest-neighbor positions (TTn2). Among the four H atom clusters, the most favorable configuration has two TTn2 pairs forming "box " or "zig-zag" shaped-cluster with the pairs separated by 3.03 or 3.25 Å, respectively. Nudged elastic band simulations suggest that the formation of TTn2 is facile and is limited by the small diffusion barrier of the monomer of less than 0.1 eV. Further, the formation routes for the 4H atom clusters are also presented. The current study can serve as a foundation to systematically model H atom clustering in metals and alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Fast ionic conduction achieved through the design and synthesis of ceramic heterointerfaces

Lithium (Li) chloride and iron oxychloride (FeOCl), typically nonconductive, were combined to form a [Li 1+δ Cl] δ+ /[FeOCl] δ- heterointerface composite material (LFH), achieving ionic conductivities of >1 mS cm -1 . Analysis techniques (scanning transmission electron microscopy [STEM] and electron energy-loss spectroscopy [EELS]) indicated that the microstructure of LFH consisted of an amorphous LiCl-based shell surrounding a crystalline FeOCl-based core. Electrochemical measurements alongside solid-state 6,7 Li nuclear magnetic resonance (NMR) and molecular dynamic simulations revealed Li+ as the sole conductive species, with a diffusion barrier of ~0.25 eV. X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) results further supported interstitial Li + diffusion at the heterointerface and within the LiCl phase, made possible by the heterointerface. Despite susceptibility to electronic conductivity, iron’s defects and multivalency (Fe³⁺, Fe²⁺) enable the Fe–O–Cl framework to accept Cl - , facilitating Li⁺ ionic conduction. Finally, a prototype solid-state cell (showing 97% Coulombic efficiency) demonstrated the viability of this heterointerface design for applications in energy storage.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Role of chalcogen vacancies and hydrogen in the optical and electrical properties of bulk transition-metal dichalcogenides

Abstract Like in any other semiconductor, point defects in transition-metal dichalcogenides (TMDs) are expected to strongly impact their electronic and optical properties. However, identifying defects in these layered two-dimensional materials has been quite challenging with controversial conclusions despite the extensive literature in the past decade. Using first-principles calculations, we revisit the role of chalcogen vacancies and hydrogen impurity in bulk TMDs, reporting formation energies and thermodynamic and optical transition levels. We show that the S vacancy can explain recently observed cathodoluminescence spectra of MoS 2 flakes and predict similar optical levels in the other TMDs. In the case of the H impurity, we find it more stable sitting on an interstitial site in the Mo plane, acting as a shallow donor, and possibly explaining the often observed n-type conductivity in some TMDs. We also predict the frequencies of the local vibration modes for the H impurity, aiding its identification through Raman or infrared spectroscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Crystal structures and formation mechanisms of boron-rich tungsten borides

Boron-rich tungsten borides tend to adopt mechanically unfavorable layered structures but experimentally exhibit excellent mechanical properties rivalling traditional superhard solids. Unravelling the contraindicated structure-property relationship, however, has been impeded by their structural ambiguities because of the difficulty in probing boron and atomic deficiency of these borides. Here, we study crystal structures of boron-rich tungsten borides WB 3+x and WB 2+x by neutron diffraction based on high-quality samples prepared by a high-pressure method, leading to definitive structural resolutions for both borides with unique compositions of WB 5.14 and WB 2.34 . Combined with theoretical calculations, their structural stability is revealed to be closely related to atomic deficiency, which is governed by the valence-band filling with an optimal valence-electron concentration of ~10 per cell. The presence of interstitial boron trimers at the vacant W : 2b sites in WB 5.14 alters the crystal symmetry, making the Wyckoff 2d site more favorably occupied by W, rather than the 2c site, as previously misassigned. Here, the staggered planar boron layers and wrinkled boron bonding in WB 2.34 are identified to be crucial for stabilizing its structure. These findings unveil the longstanding structural mysteries of boron-rich tungsten borides and offer powerful insights for rational design of borides by defect chemistry.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Primary intrinsic defects and their charge transition levels in β–Ga 2 O 3

A steady-state photocapacitance (SSPC) setup directly connected to the beamline of a MeV ion implanter is utilized to study primary intrinsic defects in β – Ga 2 O 3 generated by He implantation at cryogenic temperatures (120 K). At low temperatures, the migration of defects is suppressed, and hence the generation of primary intrinsic defects is expected to prevail. SSPC measurements reveal defect-related optical transitions in halide vapor-phase epitaxy (HVPE) -grown β – Ga 2 O 3 thin films with onset energies at 1.3 ( T 1 ) , 1.7 ( T 2 ) , 1.9 ( T 3 ) , 2.6 ( T 4 ) , 3.7 ( T 5 ) , and 4.2 eV ( T 6 ) . T 2 , T 4 , T 5 , and T 6 were observed in as-received HVPE-grown β – Ga 2 O 3 thin films, whereby T 2 is only sporadically observed. The introduction rates for T 3 , T 4 , as well as T 6 indicate an origin related to primary intrinsic defects. Notably, T 1 and T 3 are only observed after He implantation at cryogenic temperatures. Hybrid-functional calculations were performed to estimate the optical absorption cross-section spectra for the gallium ( Ga i ) and oxygen ( O i ) interstitials as well as the corresponding vacancies ( V Ga and V O , respectively), and compared with the measured onsets for optical absorption found by SSPC measurements. Indeed, we propose T 3 to be associated with Ga i ( + / + 2 ) and/or V GaI ( - 3 / - 2 ) , while T 4 is suggested to be related to V O K ( 0 / + ) ( K = I , II , III ) and/or V GaII ( - 3 / - 2 ) . Additionally, several further charge-state transition levels associated with V GaI and V GaII may contribute to T 4 and T 6 . We further studied the kinetics of the defects created with He implantation by exposing the sample to room temperature. The kinetics observed for T 3 and T 4 further support the proposed assignments of the corresponding defect signatures.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Temperature dependence of vacancy/self-interstitial recombination volumes in copper

We use molecular dynamics to calculate rate coefficients for recombination of vacancies with self-interstitial atoms (SIAs) in Cu at temperatures from 300 to 700 K. From these results, we calculate vacancy/SIA recombination volumes and find that they decrease from around 290 (where is one atomic volume) at 300 K to 160 at 500 K and above. By counting the number of distinct pathways by which a stable SIA may migrate to a site of spontaneous recombination with a nearby vacancy, we find a lower bound estimate of 168 for the recombination volume. We furthermore rationalize its temperature dependence based on differences between the activation energies for recombination and SIA migration. Furthermore, our work sheds light on the fundamental nature of the recombination volume and provides information that may be incorporated into multiscale models of radiation response in solids.

36 MATERIALS SCIENCE↗
DOE OSTI2019

XAFS Spectroscopy Study of Microstructure and Electronic Structure of Heterosystems Containing Si/GeMn Quantum Dots

Using X-ray absorption near edge structure spectroscopy, extended X-ray absorption fine structure spectroscopy, atomic force microscopy, and Rutherford backscattering spectroscopy, the features of the microstructure and elemental composition of Si/GeMn magnetic systems obtained by molecular beam epitaxy and containing quantum dots are studied. Intense mixing of Ge and Si atoms is found in all samples. The degree of mixing (diffusion) correlates with the conditions of synthesis of Si/GeMn samples. For these systems, direct contacts of germanium atoms with manganese atoms are characterized and the presence of interstitial manganese with tetrahedral coordination and substitution of manganese for germanium and silicon in the lattice sites is found. The presence of stoichiometric phases Ge{sub 8}Mn{sub 11}, Ge{sub 3}Mn{sub 5} is not detected. The correlations of the Ge, Si, and Mn coordination numbers in the Ge environment are determined both with the Mn flux value (evaporator temperature) and with the temperature at which quantum dots are grown, as well as with other synthesis conditions. The manganese concentration in the samples is determined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Hyperfine Interaction and Shockley–Read–Hall Recombination in Semiconductors

Experimental and theoretical studies on optical orientation and spin-dependent recombination in a semiconductor in a magnetic field under the normal incidence of circularly polarized radiation onto the sample surface are reviewed. The experiments were carried out on GaAs{sub 1 –x}N{sub x} solid solutions, in which Ga{sup 2+} interstitial displacement defects play the role of deep paramagnetic centers responsible for spin-dependent recombination. It is established that, in the investigated materials, the hyperfine interaction of a localized electron with one nucleus of the paramagnetic center remains strong even at room temperature. The theory is compared with an experiment conducted in the steady-state excitation mode and under two-pulse pump-probe conditions. An analytical formula for spin beats in a magnetic field is derived.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Formation of Nanosized Surface Layers of 03Kh17N12M2 Stainless Steel by Implantation of N{sup +} Ions

Accumulation of nitrogen in nanosized surface layers of 03Kh17N12M2 stainless steel has been detected upon N{sup +} ion implantation up to 14 at % together with metal nitrides, mainly of chromium nitride CrN and interstitial solid solution. It has been demonstrated that N{sup +} ion implantation accompanied by preliminary irradiation by Ar{sup +} and O{sup +} ions decreases maximum nitrogen concentration by at least two times. It is assumed that this is stipulated by segregation to surface layers of iron atoms upon irradiation by Ar{sup +} ions as well as formation of chromium oxide Cr{sub 2}O{sub 3} and chromium hydroxide CrOOH upon irradiation by O{sup +} ions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗