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8 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on Nb3Si by Materials Project

Nb3Si is Uranium Silicide structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Nb is bonded to eight equivalent Nb and four equivalent Si atoms to form NbNb8Si4 cuboctahedra that share corners with twelve equivalent NbNb8Si4 cuboctahedra, edges with eight equivalent SiNb12 cuboctahedra, edges with sixteen equivalent NbNb8Si4 cuboctahedra, faces with four equivalent SiNb12 cuboctahedra, and faces with fourteen equivalent NbNb8Si4 cuboctahedra. All Nb–Nb bond lengths are 2.89 Å. All Nb–Si bond lengths are 2.89 Å. Si is bonded to twelve equivalent Nb atoms to form SiNb12 cuboctahedra that share corners with twelve equivalent SiNb12 cuboctahedra, edges with twenty-four equivalent NbNb8Si4 cuboctahedra, faces with six equivalent SiNb12 cuboctahedra, and faces with twelve equivalent NbNb8Si4 cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Nb3Si by Materials Project

Nb3Si crystallizes in the cubic Pm-3n space group. The structure is three-dimensional. Nb is bonded in a 6-coordinate geometry to two equivalent Nb and four equivalent Si atoms. Both Nb–Nb bond lengths are 2.56 Å. All Nb–Si bond lengths are 2.87 Å. Si is bonded to twelve equivalent Nb atoms to form a mixture of face and edge-sharing SiNb12 cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Nb3Si by Materials Project

Nb3Si crystallizes in the tetragonal P4_2/n space group. The structure is three-dimensional. there are three inequivalent Nb sites. In the first Nb site, Nb is bonded in a 2-coordinate geometry to two equivalent Si atoms. There are one shorter (2.62 Å) and one longer (2.66 Å) Nb–Si bond lengths. In the second Nb site, Nb is bonded in a 4-coordinate geometry to four equivalent Si atoms. There are a spread of Nb–Si bond distances ranging from 2.60–2.72 Å. In the third Nb site, Nb is bonded in a 2-coordinate geometry to three equivalent Si atoms. There are a spread of Nb–Si bond distances ranging from 2.59–2.87 Å. Si is bonded in a 9-coordinate geometry to nine Nb atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Nb3Si by Materials Project

Nb3Si crystallizes in the orthorhombic Cmmm space group. The structure is three-dimensional. there are two inequivalent Nb sites. In the first Nb site, Nb is bonded in a distorted L-shaped geometry to two equivalent Nb and two equivalent Si atoms. Both Nb–Nb bond lengths are 2.90 Å. Both Nb–Si bond lengths are 2.60 Å. In the second Nb site, Nb is bonded in a distorted body-centered cubic geometry to eight Nb atoms. All Nb–Nb bond lengths are 2.91 Å. Si is bonded in a distorted square co-planar geometry to four equivalent Nb atoms.

36 MATERIALS SCIENCE↗
NASA NTRS1991

Containerless processing and rapid solidification of Nb-Si alloys of hypereutectic composition

A combination of bulk undercooling in an electromagnetic levitation apparatus and splat quenching between two copper plates is used to process Nb-Si alloys in order to maximize rapid solidification conditions and minimize the effects of recalescence, with emphasis on the solidification of characteristics of alloys in the 21 to 27 at. pct Si range of composition. SEM and TEM as well as X-ray diffraction are used to characterize the microstructures of the processed samples. In the range of compositions studied, the splat-quenched drops always formed the tetragonal Nb3Si phase directly from the liquid. Drops solidified in the coil were characterized by the presence of the primary intermetallic Nb5Si3 and the absence of both peritectic Nb3Si and the equilibrium eutectic. In these cases, a metastable alpha-Nb + beta-Nb5Si3 eutectic formed. The results are discussed in terms of possible metastable configurations of the Nb-Si phase diagram as well as concepts of nucleation and growth kinetics applied to the Nb3Si and Nb5Si3 intermetallics.

Hofmeister, W. H.↗
DOE OSTI2024

Data Science Enabled Enabled Discovery of Superconductors (Final Progress Report)

This Final Technical Report describes efforts by 4 PIs at the University of Florida (Peter Hirschfeld, Richard Hennig, Greg Stewart and James Hamlin), over the period September 2019-August 2023, to use data science and machine learning techniques to discover new conventional superconductors. The PIs constructed a discovery loop with two theorists and two experimentalists to: develop algorithms to machine learn descriptors correlating strongly with the critical temperature Tc (PI's Peter Hirschfeld, UF Physics and Richard Hennig, UF Materials Science and En), synthesize and measure properties of promising materials, and feed back the knowledge gained into the prediction algorithm. This work was motivated by the theoretical prediction and experimental discovery of high-pressure, high-pressure hydride superconductors, and to find ways to recreate the high critical temperatures in these systems at ambient pressure. Highlights from the grant include: 1) a new equation for Tc in terms of moments of the electron-phonon spectral function, improving on the so-called Allen-Dynes equation (1975); 2) study of the metastable A15 superconductor Nb3Si, formed under explosive compression at ~1000GPa to determine the kinetic barrier to the ground state structure; 3) the development of ultra-fast machine-learned atomic potentials for molecular dynamics, and 4) the discovery of superconductivity at 19K in WB2 arising from metastable defect structures in the crystal.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1983

Experiments with phase transitions at very high pressure

Diamond cells were constructed for use to 1 Mbar. A refrigerator for cooling diamond cells was adapted for studies between 15 and 300 K. A cryostat for superconductivity studies between 1.5 to 300 K was constructed. Optical equipment was constructed for fluorescence, transmission, and reflectance studies. X-ray equipment was adapted for use with diamond cells. Experimental techniques were developed for X-ray diffraction studies using synchrotron radiation. AC susceptibility techniques were developed for detecting superconducting transitions. The following materials were studied: compressed solidified gases (Xe, Ar), semiconductors (Ge, Si, GaAs), superconductors (Nb3Ge, Nb3Si, Nb3As, CuCl), molecular crystals (I).

Spain, I. L.↗
NASA NTRS1991

Containerless processing and rapid solidification of Nb-Si alloys in the niobium-rich eutectic range

Containerless processing and rapid solidification techniques were used to process Nb-Si alloys in the Nb-rich eutectic range. Electromagnetically levitated drops were melted and subsequently splat-quenched from different temperatures. A variety of eutectic morphologies was obtained as a function of the degree of superheating or undercooling of the drops prior to splatting. Metallic glass was observed only in drops quenched from above the melting temperature. Microstructures of splats deeply undercooled prior to quenching were very fine and uniform. These results are discussed in terms of classic nucleation theory concepts and the expected heat evolution at different regions of the splat during the rapid quenching process. The locations of the coupled-zone boundaries for the alpha-Nb + Nb3Si eutectic are also suggested.

Hofmeister, W. H.↗