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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 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Dilute carbon in H3S under pressure

Abstract Recently, room temperature superconductivity was measured in a carbonaceous sulfur hydride material whose identity remains unknown. Herein, first-principles calculations are performed to provide a chemical basis for structural candidates derived by doping H 3 S with low levels of carbon. Pressure stabilizes unusual bonding configurations about the carbon atoms, which can be six-fold coordinated as CH 6 entities within the cubic H 3 S framework, or four-fold coordinated as methane intercalated into the H-S lattice, with or without an additional hydrogen in the framework. The doping breaks degenerate bands, lowering the density of states at the Fermi level ( N F ), and localizing electrons in C-H bonds. Low levels of CH 4 doping do not increase N F to values as high as those calculated for $$Im\bar{3}m$$ I m 3 ¯ m -H 3 S, but they can yield a larger logarithmic average phonon frequency, and an electron–phonon coupling parameter comparable to that of R 3 m -H 3 S. The implications of carbon doping on the superconducting properties are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Probing the magnetic profile of diluted magnetic semiconductors using polarized neutron reflectivity

Room temperature ferromagnetism has been observed in the Cu doped ZnO films deposited under an oxygen partial pressure of 10 -3 and 10 -5 torr on Pt (200 nm)/Ti (45 nm)/Si (001) substrates using pulsed laser deposition. Due to the deposition at relatively high temperature (873 K), Cu and Ti atoms diffuse to the surface and interface, which significantly affects the magnetic properties. Depth sensitive polarized neutron reflectometry method provides the details of the composition and magnetization profiles and shows that an accumulation of Cu on the surface leads to an increase in the magnetization near the surface. Our results reveal that the presence of the copper at Zn sites induces ferromagnetism at room temperature, confirming intrinsic ferromagnetism.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Tuning the bandgap of Cs 2 AgBiBr 6 through dilute tin alloying

The promise of lead halide hybrid perovskites for optoelectronic applications makes finding less-toxic alternatives a priority. The double perovskite Cs 2 AgBiBr 6 ( 1 ) represents one such alternative, offering long carrier lifetimes and greater stability under ambient conditions. However, the large and indirect 1.95 eV bandgap hinders its potential as a solar absorber. Here we report that alloying crystals of 1 with up to 1 atom% Sn results in a bandgap reduction of up to ca. 0.5 eV while maintaining low toxicity. Crystals can be alloyed with up to 1 atom% Sn and the predominant substitution pathway appears to be a ~2 : 1 substitution of Sn 2+ and Sn 4+ for Ag + and Bi 3+ , respectively, with Ag + vacancies providing charge compensation. Spincoated films of 1 accommodate a higher Sn loading, up to 4 atom% Sn, where we see mostly Sn 2+ substitution for both Ag + and Bi 3+ . Density functional theory (DFT) calculations ascribe the bandgap redshift to the introduction of Sn impurity bands below the conduction band minimum of the host lattice. Using optical absorption spectroscopy, photothermal deflection spectroscopy, X-ray absorption spectroscopy, 119 Sn NMR, redox titration, single-crystal and powder X-ray diffraction, multiple elemental analysis and imaging techniques, and DFT calculations, we provide a detailed analysis of the Sn content and oxidation state, dominant substitution sites, and charge-compensating defects in Sn-alloyed Cs 2 AgBiBr 6 ( 1:Sn ) crystals and films. An understanding of heterovalent alloying in halide double perovskites opens the door to a wider breadth of potential alloying agents for manipulating their band structures in a predictable manner.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Robust battery interphases from dilute fluorinated cations

Controlling solid electrolyte interphase (SEI) in batteries is crucial for their efficient cycling. Herein, we demonstrate an approach to enable robust battery performance that does not rely on high fractions of fluorinated species in electrolytes, thus substantially decreasing the environmental footprint and cost of high-energy batteries. In this approach, we use very low fractions of readily reducible fluorinated cations in electrolyte (~0.1 wt%) and employ electrostatic attraction to generate a substantial population of these cations at the anode surface. As a result, we can form a robust fluorine-rich SEI that allows for dendrite-free deposition of dense Li and stable cycling of Li-metal full cells with high-voltage cathodes. Our approach represents a general strategy for delivering desired chemical species to battery anodes through electrostatic attraction while using minute amounts of additive.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Ferromagnetic resonance studies of Fe thin films with dilute heavy rare-earth impurities

The structure and magnetic properties of Fe{sub 1-x}Gd(Tb){sub x} films prepared by sputtering have been investigated using x-ray diffraction, vibrating sample magnetometer, and ferromagnetic resonance. Magnetic measurements show that the saturation magnetization decreases as the increase of Gd(Tb) content, which agrees with that Gd(Tb) atomic moments align antiparallel with Fe moments. Theoretical fittings of the angular dependence of ferromagnetic resonance field yield the second- and forth-order perpendicular anisotropy constants K{sub perpendicular1} and K{sub perpendicular2}, which show a spin reorientation with increasing Gd(Tb) doping. The intrinsic contribution of ferromagnetic resonance linewidth as a result of Gilbert damping is separated from the extrinsic contribution because of the inhomogeneity of the films. It is found that the additions of Gd additives do not significantly improve the damping constant, whereas the Tb dopants increase the damping constant from 0.04 without Tb to 0.11 with 19% Tb, thus a remarkable enhancement of 175%.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Magnetism of dilute Co(Hf) and Co(Pt) nanoclusters

An investigation of the magnetic properties of Co-rich nanoparticles alloyed with a small fraction of Pt and Hf is presented. Co(Hf) and Co(Pt) nanoparticles with less than 15 at% of dopants were produced using a cluster-deposition method. The nanoparticles have sizes of less than 10 nm and show improved magnetic properties upon doping. Maximum coercivities of 900 Oe (at 300 K) and 2000 Oe (at 10 K) were observed for Co nanoparticles alloyed with 14.1 at% of Hf. Doped nanoparticles also exhibit high anisotropies, such as K-1 = 9.98 Mergs/cm(3) (14.1 at% of Hf) and K-1 = 8.24 Mergs/cm(3) (9.5 at% of Pt), as compared to Co nanoparticles (K-1 = 6.21 Mergs/cm(3)). (C) 2012 American Institute of Physics. [doi:10.1063/1.3678582]

Balamurugan, B↗
DOE OSTI2012

Enhanced room temperature electronic and thermoelectric properties of the dilute bismuthide InGaBiAs

We report room temperature electronic and thermoelectric properties of Si-doped In{sub 0.52}Ga{sub 0.48}Bi{sub y}As{sub 1-y} with varying Bi concentrations. These films were grown epitaxially on a semi-insulating InP substrate by molecular beam epitaxy. We show that low Bi concentrations are optimal in improving the conductivity, Seebeck coefficient, and thermoelectric power factor, possibly due to the surfactant effects of bismuth. We observed a reduction in thermal conductivity with increasing Bi concentration, which is expected because of alloy scattering. We report a peak ZT of 0.23 at 300 K.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Magnetic instability in a dilute circular rarefaction wave

The growth of magnetic fields in the density gradient of a rarefaction wave has been observed in simulations and in laboratory experiments. The thermal anisotropy of the electrons, which gives rise to the magnetic instability, is maintained by the ambipolar electric field. This simple mechanism could be important for the magnetic field amplification in astrophysical jets or in the interstellar medium ahead of supernova remnant shocks. The acceleration of protons and the generation of a magnetic field by the rarefaction wave, which is fed by an expanding circular plasma cloud, is examined here in form of a 2D particle-in-cell simulation. The core of the plasma cloud is modeled by immobile charges, and the mobile protons form a small ring close to the cloud's surface. The number density of mobile protons is thus less than that of the electrons. The protons of the rarefaction wave are accelerated to 1/10 of the electron thermal speed, and the acceleration results in a thermal anisotropy of the electron distribution in the entire plasma cloud. The instability in the rarefaction wave is outrun by a TM wave, which grows in the dense core distribution, and its magnetic field expands into the rarefaction wave. This expansion drives a secondary TE wave.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗