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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 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Vanadium Diboride (VB 2 ) Synthesized at High Pressure: Elastic, Mechanical, Electronic, and Magnetic Properties and Thermal Stability

Vanadium diboride (VB 2 ) with an AlB 2 -type structure has been synthesized at 8 GPa and 1700 K in a D-DIA-type multianvil apparatus. The obtained bulk modulus is B 0 = 262(2) GPa with fixed B' = 4.0 for VB 2 via high-pressure X-ray diffraction measurements. Meanwhile, VB 2 has also been demonstrated to possess a high Vickers hardness of 27.2 ± 1.5 GPa, a high thermal stability of 1410 K in air, among the highest for transition-metal borides, and an extremely low resistivity value (41 μΩ cm) at room temperature. Here, results from first-principles calculations regarding the mechanical and electronic properties of VB 2 are largely consistent with the experimental observations and further suggest that VB 2 possesses simultaneously the properties of a hard and refractory ceramic and those of an excellent electric conductor.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Magnetocaloric effect and the influence of pressure on magnetic properties of La-excess pseudo-binary alloys La{sub 1+δ}(Fe{sub 0.85}Si{sub 0.15}){sub 13}

The La-excess alloys La{sub 1+δ}(Fe{sub 0.85}Si{sub 0.15}){sub 13} (δ = 0.06 and 0.09) exhibit large magnetocaloric effect which has been attributed to the occurrence of itinerant-electron metamagnetic transition near the Curie temperature T{sub C}. The maximum entropy change −ΔS{sub m} was shown to be from 4.5 to 11.5 J/kg K for the applied field variation ΔH from 20 to 70 kOe, respectively. The estimated relative cooling power for ΔH = 70 kOe was 418 J/kg. The alloys show a typical NaZn{sub 13}-type cubic structure, featuring a doping-induced magnetovolume effect with the increase in T{sub C}. Under the applied pressure up to 2 GPa, the T{sub C} as deduced from resistance measurements decreased linearly, ΔT{sub C} = 113 (for δ = 0.06) and 111 K (for δ = 0.09), together with a corresponding decrease of resistivity, Δρ = 6.1 μΩ m at room temperature for both samples. At a low pressure, the effect of spontaneous magnetostriction on T{sub C} caused by applying the pressure appeared to have a similar magnitude to that of the negative magnetovolume effect caused by La-excess doping. In comparison with other stoichiometric La(Fe{sub 1−x}Si{sub x}){sub 13} compounds, the pressure in our case was shown to have a smaller influence on T{sub C}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Electronic structure, magnetic properties, and pairing tendencies of the copper-based honeycomb lattice Na 2 Cu 2 TeO 6

Spin-1/2 chains with alternating antiferromagnetic (AFM) and ferromagnetic (FM) couplings have attracted considerable interest due to the topological character of their spin excitations. Here, using density functional theory and density-matrix renormalization-group (DMRG) methods, we have systematically studied the dimerized chain system Na 2 Cu 2 TeO 6 with a d 9 electronic configuration. Near the Fermi level, in the nonmagnetic phase the dominant states are mainly contributed by the Cu 3d x 2 -y 2 orbitals highly hybridized with the O 2p orbitals, leading to an “effective” single-orbital low-energy model. By calculating the relevant hoping amplitudes, we explain the size and sign of the exchange interactions in Na 2 Cu 2 TeO 6 . In addition, a single-orbital Hubbard model is constructed for this dimerized chain system where the quantum fluctuations are taken into account. Both AFM and FM couplings (leading to an ↑|-|↓-|↓-↑| state) along the chain were found in our DMRG and Lanczos calculations, in agreement with density functional theory and neutron-scattering results. The hole pairing binding energy ΔE is predicted to be negative at Hubbard U~ 11eV, suggesting incipient pairing tendencies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗