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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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17 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Undercooling studies on Nb-Pt and Nb-Si alloys using the 105 meter drop tube

Niobium-platinum samples of compositions ranging from 16 to 32 at. pct have been undercooled to as much as 540 K in the low gravity, containerless environment of a 105 meter drop tube. Undercooling was terminated in the Nb-Pt samples by the nucleation and growth of the Nb3Pt phase. In the 16-18 at. pct Pt samples, this resulted in samples which are completely Nb3Pt, in contrast to both the equilibrium phase diagram and the nonundercooled samples which formed with Nb dendrites and interdendritic Nb3Pt. Undercoolings for the Nb-Si samples were up to 670 K, which corresponds to 27 percent of the liquidus temperature or 80 percent of the estimated hypercooling limit. In the Nb-Si system, a coupled zone was identified as well as a metastable extension of the solubility limit of Si in Nb due to deep undercooling.

Robinson, M. B.↗
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.↗
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.↗
NASA NTRS1988

Supercooling effects in faceted eutectic Nb-Si alloys

The effect of melt supercooling on the microstructure of an Nb-58 at. pct Si alloy is investigated experimentally using an electromagnetic levitation apparatus. It is found that, starting with an alloy nominally of eutectic composition, nucleation of Nb5Si3 occurs in the supercooled liquid first. Upon further cooling, the remaining liquid continues to supercool until the second phase, NbSi2 is nucleated, which is commonly accompanied by rapid recalescence. The primary phase exibits a eutectoid-type decomposition. The observations are discussed with reference to the results of quantitative microstructural measurements, compositional and thermal analysis, and preliminary thermodynamic modeling of the phase diagram.

Gokhale, A. B.↗
DOE OSTI2022

Stability, metallicity, and magnetism in niobium silicide nanofilms

Modern superconducting qubits based on two-dimensional (2D) transmons typically involve the growth of Nb thin films on high-resistivity Si substrates. Since imperfections at the Nb-Si heterointerface have been implicated as a source of two-level systems that limit quantum coherence times, detailed characterization and understanding of niobium silicide interfacial layers are critical to improving superconducting qubit technology. While bulk binary intermetallic niobium silicide phases are well understood, the thermodynamic phase stability and properties of ultrathin niobium silicides, such as those found at the Nb-Si heterointerface in 2D transmons, have not yet been explored. Here, we report finite-sized effects for ultrathin niobium silicide films using density functional theory calculations and predict nanoscale stabilization of Nb 6 Si 5 over the bulk α-Nb 5 Si 3 phase. This result is consistent with our experimental observations of a niobium silicide interfacial layer between a sputtered Nb thin film and the underlying Si substrate. Furthermore, our calculations show that Nb 6 Si 5 nanofilms are nonmagnetic, making them superior to nanofilms of α-Nb 5 Si 3 that exhibit antiferromagnetic correlations detrimental to long coherence times in superconducting qubits. Furthermore, by providing atomic-scale insight into niobium silicide nanofilms, this paper can help guide ongoing efforts to optimize Nb-Si heterointerfaces for long coherence times in superconducting qubits.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Exploring the relationship between deposition method, microstructure, and performance of Nb/Si-based superconducting coplanar waveguide resonators

Superconducting quantum circuits (SQC) are one of the most promising hardware platforms for quantum computing, yet their performance is currently limited by the presence of various structural defects inside the circuit's structure. Despite impressive progress in the past decade, a precise understanding of the origin of these defects from various fabrication processes and their impact on coherence is still lacking. Here, in this study, we performed a comprehensive investigation on the microstructure, superconductivity, and resonator quality factor of Nb films deposited by high-power impulse magnetron sputtering (HiPIMS) and direct current (DC) magnetron sputtering. A suite of characterization techniques, including electron microscopy with spectroscopy, secondary ion mass spectrometry, magneto-optical microscopy, and pump-probe reflectivity spectroscopy is used. We reveal that niobium (Nb) resonators fabricated using HiPIMS exhibit a smaller average grain size, thicker surface oxide with larger thickness variations (rougher surface), and a thicker amorphous Nb/Si interface layer compared to samples deposited by DC sputtering. We identified that the amorphous Nb oxides (mainly located at the Nb surface and along the grain boundaries) and Nb-Si amorphous layers (at the Nb/Si interface) are major and potential sources of two-level system (TLS), while off-stochiometric oxides and suboxides of Nb close to the surface, crystalline defects (i.e., dislocations at grain boundary, point defects introduced during deposition) are main contributors of non-TLS sources. Our findings clarify the relationship between different defects and coherence loss mechanisms, highlighting the importance of material microstructure control on performance optimization in SQC.

36 MATERIALS SCIENCE↗
NASA NTRS1988

Rapid solidification of Nb-base alloys

New Nb-base alloys are of interest for aerospace structural applications at high temperatures, viz, 800 to 1650 C. Fundamental information regarding the effects of rapid solidification in achieving greatly refined microstructures, extended solid solubility, suppression of embrittling equilibrium phases, and formation of new phases is desired in a number of Nb-X alloys. The microstructures and selected properties of Nb-Si and other Nb-base alloys are presented for materials both rapidly quenched from the equilibrium liquidus and rapidly solidified following deep supercooling. Electromagnetic levitation was used to achieve melting and supercooling in a containerless inert gas environment. A variety of solidification conditions were employed including splatting or drop casting of supercooled samples. The morphology and composition of phases formed are discussed in terms of both solidification history and bulk composition.

Gokhale, A. B.↗
NASA NTRS1990

Containerless processing using electromagnetic levitation

The theory and practice of containerless processing via electromagnetic (EM) levitation is reviewed briefly. The use of EM levitation for the processing of alloys is described with particular emphasis on the bulk melt supercooling phenomenon in a containerless environment. The various effects associated with rapid solidification via bulk melt supercooling are discussed with examples of Nb-Si alloys. It is suggested that a detailed analysis of such effects can be utilized to select the potentially most promising alloys for future space-based processing.

Gokhale, A. B.↗
NASA NTRS2019

Low-Loss Microstrip Transmission Line Fabricated with Improved Liftoff Process

The µ-Spec integrated spectrometer operating at ~500 GHz, employs thin film superconducting Nb microstrip transmission lines deposited directly on a thin (450 nm) single-crystal silicon dielectric. This single-crystal silicon layer is chosen as the dielectric layer due to its low intrinsic loss, with the goal of achieving both high- efficiency and precise phase control in a compact spectrometer architecture. To avoid roughening or etching through the thin single-crystal silicon dielectric a liftoff technique was developed for patterning these microstrip transmission lines and ground plane structures. This two- layer liftoff process was designed for use with sputter deposition and resulted in a US patent. Although this original technique provided precise control of linewidth, results of initial prototype spectrometer devices and separate diagnostic co-planer waveguide resonator devices showed that unexpected loss was being introduced due to the lift-off process. This extra loss was believed to be due to the “tails” (thin tapered regions) at the edge of the metal traces resulting from the sputtering process, as well as an amorphous oxide layer at the Nb-Si interface. We have since demonstrated an improved lift-off technique, which provides a clean metal-Si interface and removes the loss-inducing tails by a two-step selective etching method. This results in a decrease in microwave loss by more than an order of magnitude when measured in co-planar waveguide microwave resonator structures. We present these microwave test results and also SEM and TEM images of the microstrip interfaces and edge profiles before and after application of the improved process.

Hess, Larry A.↗
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 Nb5Si3 by Materials Project

Nb5Si3 crystallizes in the tetragonal I4/mcm space group. The structure is three-dimensional. there are two inequivalent Nb+2.40+ sites. In the first Nb+2.40+ site, Nb+2.40+ is bonded to six Si4- atoms to form NbSi6 octahedra that share corners with six equivalent NbSi6 octahedra, corners with sixteen equivalent NbSi5 trigonal bipyramids, and faces with eight equivalent NbSi5 trigonal bipyramids. The corner-sharing octahedra tilt angles range from 0–52°. There are four shorter (2.60 Å) and two longer (2.99 Å) Nb–Si bond lengths. In the second Nb+2.40+ site, Nb+2.40+ is bonded to five Si4- atoms to form NbSi5 trigonal bipyramids that share corners with four equivalent NbSi6 octahedra, corners with twelve equivalent NbSi5 trigonal bipyramids, edges with seven equivalent NbSi5 trigonal bipyramids, faces with two equivalent NbSi6 octahedra, and a faceface with one NbSi5 trigonal bipyramid. The corner-sharing octahedra tilt angles range from 43–64°. There are a spread of Nb–Si bond distances ranging from 2.61–2.74 Å. There are two inequivalent Si4- sites. In the first Si4- site, Si4- is bonded in a 10-coordinate geometry to ten Nb+2.40+ atoms. In the second Si4- site, Si4- is bonded in a 9-coordinate geometry to eight Nb+2.40+ and one Si4- atom. The Si–Si bond length is 2.41 Å.

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 Nb5Si3 by Materials Project

Nb5Si3 crystallizes in the hexagonal P6_3/mcm space group. The structure is three-dimensional. there are two inequivalent Nb+2.40+ sites. In the first Nb+2.40+ site, Nb+2.40+ is bonded to five equivalent Si4- atoms to form distorted NbSi5 trigonal bipyramids that share corners with eight equivalent NbSi6 octahedra, corners with eight equivalent NbSi5 trigonal bipyramids, edges with six equivalent NbSi5 trigonal bipyramids, and faces with four equivalent NbSi6 octahedra. The corner-sharing octahedra tilt angles range from 41–55°. There are a spread of Nb–Si bond distances ranging from 2.61–2.86 Å. In the second Nb+2.40+ site, Nb+2.40+ is bonded to six equivalent Si4- atoms to form distorted NbSi6 octahedra that share corners with six equivalent NbSi6 octahedra, corners with twelve equivalent NbSi5 trigonal bipyramids, edges with three equivalent NbSi6 octahedra, faces with two equivalent NbSi6 octahedra, and faces with six equivalent NbSi5 trigonal bipyramids. The corner-sharing octahedral tilt angles are 35°. All Nb–Si bond lengths are 2.69 Å. Si4- is bonded in a 9-coordinate geometry to nine Nb+2.40+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Nb5Si3 by Materials Project

Nb5Si3 crystallizes in the tetragonal I4/mcm space group. The structure is three-dimensional. there are two inequivalent Nb+2.40+ sites. In the first Nb+2.40+ site, Nb+2.40+ is bonded to six Si4- atoms to form distorted NbSi6 pentagonal pyramids that share corners with fifteen equivalent NbSi6 pentagonal pyramids, corners with four equivalent NbSi4 tetrahedra, edges with three equivalent NbSi6 pentagonal pyramids, edges with two equivalent NbSi4 tetrahedra, and faces with seven equivalent NbSi6 pentagonal pyramids. There are a spread of Nb–Si bond distances ranging from 2.66–2.95 Å. In the second Nb+2.40+ site, Nb+2.40+ is bonded to four equivalent Si4- atoms to form NbSi4 tetrahedra that share corners with sixteen equivalent NbSi6 pentagonal pyramids, edges with eight equivalent NbSi6 pentagonal pyramids, and edges with two equivalent NbSi4 tetrahedra. All Nb–Si bond lengths are 2.67 Å. There are two inequivalent Si4- sites. In the first Si4- site, Si4- is bonded in a 10-coordinate geometry to ten Nb+2.40+ atoms. In the second Si4- site, Si4- is bonded in a 10-coordinate geometry to eight equivalent Nb+2.40+ and two equivalent Si4- atoms. Both Si–Si bond lengths are 2.56 Å.

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↗
DOE OSTI2020

Materials Data on Nb3Si2 by Materials Project

Nb3Si2 crystallizes in the tetragonal P4/mbm space group. The structure is three-dimensional. there are two inequivalent Nb+2.67+ sites. In the first Nb+2.67+ site, Nb+2.67+ is bonded in a square co-planar geometry to four equivalent Si4- atoms. All Nb–Si bond lengths are 2.66 Å. In the second Nb+2.67+ site, Nb+2.67+ is bonded in a distorted hexagonal planar geometry to six equivalent Si4- atoms. There are two shorter (2.60 Å) and four longer (2.70 Å) Nb–Si bond lengths. Si4- is bonded in a 9-coordinate geometry to eight Nb+2.67+ and one Si4- atom. The Si–Si bond length is 2.44 Å.

36 MATERIALS SCIENCE↗