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Hunt, Andrew

Publications and source records attributed to Hunt, Andrew.

12 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Stable, predictable and training-free operation of superconducting Bi-2212 Rutherford cable racetrack coils at the wire current density of 1000 A/mm 2

High-temperature superconductors (HTS) could enable high-field magnets stronger than is possible with Nb-Ti and Nb 3 Sn, but two challenges have so far been the low engineering critical current density J E , especially in high-current cables, and the danger of quenches. Most HTS magnets made so far have been made out of REBCO coated conductor. Here we demonstrate stable, reliable and training-quench-free performance of Bi-2212 racetrack coils wound with a Rutherford cable fabricated from wires made with a new precursor powder. These round multifilamentary wires exhibited a record J E up to 950 A/mm 2 at 30 T at 4.2 K. These coils carried up to 8.6 kA while generating 3.5 T at 4.2 K at a J E of 1020 A/mm 2 . Different from the unpredictable training performance of Nb-Ti and Nb 3 Sn magnets, these Bi-2212 magnets showed no training quenches and entered the flux flow state in a stable manner before thermal runaway and quench occurred. Also different from Nb-Ti, Nb 3 Sn, and REBCO magnets for which localized thermal runaways occur at unpredictable locations, the quenches of Bi-2212 magnets consistently occurred in the high field regions over a long conductor length. These characteristics make quench detection simple, enabling safe protection, and suggest a new paradigm of constructing quench-predictable superconducting magnets from Bi-2212.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

High-Performance Bi-2212 Round Wires Made With Recent Powders

Multifilamentary Bi 2 Sr 2 CaCu 2 O x (Bi-2212) wire made by the powder-in-tube technique is the only high-temperature superconductor made in the round shape preferred by magnet builders. The critical current density (J C ) of Bi-2212 round wire was improved significantly by the development of overpressure heat treatment in the past few years. Bi-2212 wire is commercially available in multiple architectures and kilometer-long pieces and is a very promising conductor for very high-field NMR and accelerator magnets. Here, we studied the effects of precursor powder and heat treatment conditions on the superconducting properties and microstructure of recent Bi-2212 wires. Short samples of recent wire with optimized overpressure processing showed J C (4.2 K, 15 T) = 6640 A/mm 2 and J C (4.2 K, 30 T) = 4670 A/mm 2 , which correspond to engineering critical current densities J E (4.2 K, 15 T) = 1320 A/mm 2 and J E (4.2 K, 30 T) = 930 A/mm 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2018

Ultrafast Spectral Dynamics of CsPb(Br x Cl 1– x ) 3 Mixed-Halide Nanocrystals

In this study we investigated the spectral dynamics of cesium lead mixed-halide, CsPb(Br x Cl 1– x ) 3 perovskite nanocrystals probed with complementary spectral techniques: time-resolved photoluminescence and transient absorption spectroscopy. Mixed-halide perovskite nanocrystals were synthesized via a hot-injection method followed by anion exchange reactions. Our results show that increased Cl content in perovskite nanocrystals (a) diminished the photoluminescence quantum yield and gave rise to rapid radiative recombination of carriers; (b) resulted in rapid thermalization of hot carriers and low carrier temperatures, which suggests weaker hot-phonon bottleneck and Burstein–Moss effects; (c) decreased the bandgap renormalization energy, which suggests high exciton binding energy and poor charge extraction in Cl substituted perovskite nanocrystals; and (d) increased the number of carriers undergoing Auger losses, where Auger processes dominate over trap-assisted recombination. Furthermore, these findings provide a generalized framework to guide researchers as to when mixed-halide perovskite nanocrystals would be useful for optoelectronic technologies and when they would be detrimental to device performance.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Microscopic vertical orientation of nano-interspaced graphene architectures in deposit films as electrodes for enhanced supercapacitor performance

This paper reported a novel two-step process to fabricate high-performance supercapacitor films that contain microscale domains of nano-interspaced, re-stacked graphene sheets oriented perpendicular to the surface of current collector substrate, i.e., carbon fiber paper. In the two-step process, we first used ligand molecules to modify the surface of graphene oxide (GO) sheets and manipulate the interspacing between the re-stacked GO sheets. The ligand-modified GOs, i.e., m-GOs, were then reduced to obtain more conductive graphene (m-rGO), where X-ray diffraction measurement results indicated well-controlled interlayer spacing between the restacked m-rGO sheets up to 1 nm. The typical lateral dimension of the restacked m-rGO sheets were ~40 µm. Then, electrical field was introduced during m-rGO slurry deposition process to induce the vertical orientation of the m-rGO sheets/stacks in the film deposit. The direct current electrical field induced the orientation of the domains of m-rGO stacks along the direction perpendicular to the surface of deposit film, i.e., direction of electric field. Also, the applied electric field increased the interlayer spacing further, which should enhance the diffusion and accessibility of electrolyte ions. As compared with the traditionally deposited “control” films, the field-processed film deposits that contain oriented structure of graphene sheets/stacks have shown up to ~1.6 times higher values in capacitance (430 F/g at 0.5 A/g) and ~67% reduction in equivalent series resistance. Finally, the approach of using electric field to tailor the microscopic architecture of graphene-based deposit films is effective to fabricate film electrodes for high performance supercapacitors.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Nano-micro-structured Graphene Film Electrodes for Enhanced Performance of Supercapacitors

The graphene structures at either nanoscale and microscale are important to the film electrode performance. At nanoscale, a molecular engineering approach (i.e., molecular ligand grafting) was used to control the interspacing between restacked graphene sheets. At microscale, this work applied an external field to align the graphene sheets along the field direction during the film deposition from liquid slurry of molecularly engineered graphene. Five different ligand molecules were effectively grafted on the surfaces of well-dispersed graphene oxides (GOs) in liquid suspension and chemically reduced to GOs (rGOs), which then were modified by grafted molecules to form (m-rGOs). XRD data shows well controlled nanometer sized interlayer spacings between the re-stacked sheets of m-rGOs. Then, we investigated the effects of ligand grafting on the deposit film structure. Their baseline electrochemical cell performance was evaluated by different aqueous electrolytes (e.g., H2SO4 vs. KOH) for the following external field alignment. Initially, the m-rGO film electrodes exhibited higher capacitance performance with H2SO4 electrolyte, compared with KOH. Also, the external field has rendered the graphene sheets oriented velrtically to the film surface, resulting in enhanced capacitance due to enhanced diffusion and accessibility of electrolyte ions into the graphene film structure. Aligning graphene nanosheets in film deposits via external field could be a novel versatile fabrication technique to tailor microscopic architecture of graphene coatings for high performance supercapacitor.

Jang, Gyoung Gug↗
NASA NTRS2005

Update on High-Temperature Coils for Electromagnets

A report revisits the subject matter of "High-Temperature Coils for Electromagnets" (LEW-17164), NASA Tech Briefs, Vol. 26, No. 8, (August 2002) page 38. To recapitulate: Wires have been developed for use in electromagnets that operate at high temperatures. The starting material for a wire of this type can be either a nickel-clad, ceramic-insulated copper wire or a bare silver wire. The wire is covered by electrical-insulation material that is intended to withstand operating temperatures in the range from 800 to 1,300 F (.430 to .700 C): The starting wire is either primarily wrapped with S-glass as an insulating material or else covered with another insulating material wrapped in S-glass prior to the winding process. A ceramic binding agent is applied as a slurry during the winding process to provide further insulating capability. The turns are pre-bent during winding to prevent damage to the insulation. The coil is then heated to convert the binder into ceramic. The instant report mostly reiterates the prior information and presents some additional information on the application of the ceramic binding agent and the incorporation of high-temperature wire into the windings.

Kascak, Albert F.↗
NASA NTRS2002

Fail Safe, High Temperature Magnetic Bearings

This paper contributes to the magnetic bearing literature in two distinct areas: high temperature and redundant actuation. Design considerations and test results are given for the first published combined 538 C (1000 F) high speed rotating test performance of a magnetic bearing. Secondly, a significant extension of the flux isolation based, redundant actuator control algorithm is proposed to eliminate the prior deficiency of changing position stiffness after failure. The benefit of the novel extension was not experimentally demonstrated due to a high active stiffness requirement. In addition, test results are given for actuator failure tests at 399 C (750 F), 12,500 rpm. Finally, simulation results are presented confirming the experimental data and validating the redundant control algorithm.

Minihan, Thomas↗
NASA NTRS1995

An efficient software implementation of a variable rate modem

This paper describes the software implementation of the baseband portion of a variable rate modem. The modem can handle arbitrary symbol rates with a fixed input/output sample rate. This approach reduces hardware complexity related to external clock generation circuitry, offers complete flexibility in the selection of symbol rates, and conveniently accommodates symbol timing and symbol rate corrections.

Mantha, Ramesh↗