Terence Wong

Effect of Solidification Conditions on Partial Melt Processed Bi2212/Ag Round Wire

In this study, we report the effect of cooling from the peak temperature reached during partial melt processing on the critical current for a Bi2212/Ag wire. A single-stage cooling approach is compared to two-stage cooling. For two-stage cooling, the first stage cooling rate and the cooling rate transition temperature were varied to investigate the effects of undercooling on the solidification behavior of the 2212 phase. Two-stage cooling results in higher Ic compared to single-stage cooling, and the cooling rate transition temperature was found to have a greater effect on Ic than the initial cooling rate.

The Effect of Filament Diameter on

In this study, we investigated the effect of filament size on Je for high filament count Bi2212 round wire conductors partial melt processed with Tmax between 882 and 890degC in pure oxygen. The filament size that produced maximum Je was between 12 and 15 mum depending on conductor design and Tmax. When the filament size was below 12 mum the filaments were unstable during partial melt processing leading to a break-up in the structure of the filament array and low Je. This effect is attributed the formation of interconnects between filaments which leads to coarsening driven by a reduction in interfacial surface energy.

{\rm J}_{\rm e}

High strength dispersion strengthened (DS) Ag/Al alloys with various Al content are studied as candidates for sheathing Bi2Sr2CaCu2O8+x (Bi2212) wire. The Ag/Al alloys are fabricated by powder metallurgy and internally oxidized in pure oxygen. The time and temperature of the internal oxidation heat treatment is varied to maximize the strength after undergoing the Bi2212 partial melt process (PMP). Vickers micro-hardness number (HVN), room temperature tensile behavior, optical and scanning electron microscopy, ion channeling contrast imaging using a focused ion beam and electrical resistivity measurements are used to characterize the alloys. An Ag/0.2wt%Mg (Ag/Mg) alloy is used for comparison. Results show that internal oxidation at 650?700? ? C for 4?h produces the highest HVN for the DS Ag/Al alloy; when oxidized at 675?? C for 4?h the HVN, yield strength and tensile strength of the DS Ag/Al are 50% higher than the corresponding values of Ag/Mg. Microstructural observations show that Al2O3 precipitates play the main role in strengthening the DS Ag/Al alloy. The alloy retains its fine grain structure and strength after PMP heat treatment.

in High Filament Count Bi2212/Ag Round Wire

High-strength high-elastic-modulus dispersion-strengthened (DS) silver aluminum alloys are studied for sheathing Bi2Sr2CaCu2O8 + x (Bi2212) round wire. DS is an effective method for producing a fine grain metallurgical structure that is resistant to softening during high-temperature heat treatment. Here, DS Ag/0.5-wt.% Al (AgAl) alloy sheet is produced using powder metallurgy and is compared with Ag/0.2-wt.% Mg (AgMg) alloy, which is currently the most common alloy used for Bi2212 wire. Room temperature (RT), 77- and 4.0-K tensile tests, Vickers microhardness, optical microscopy, field emission scanning electron microscopy, and electrical resistivity measurements are compared. Furthermore, Bi2212/AgMg and Bi2212/AgAl wires are produced and compared for short-sample and coil Ic (4.2 K; self-field). It is found that the AgAl solid wire shows high yield stress and ultimate tensile strength in the annealed condition at both RT and 4.0 K, as well as significant ductility at 4.0 K. Electrical transport measurements show that the Bi2212/AgAl wires perform as well or better than Bi2212/AgMg wires. Furthermore, no leakage is observed after partial melt processing (PMP) of Bi2212/AgAl spirals. After PMP, the Bi2212/AgAl wire not only has yield and tensile stresses slightly higher than those of the Bi2212/AgMg wire but also exhibits >; 2% elongation, which is several times higher than that of Bi2212/AgMg.

High strength oxide dispersion strengthened silver aluminum alloys optimized for Bi2Sr2CaCu2O8+x round wire

The use of a Ag-Ni composite sheath has been proposed as a means to improve the mechanical performance and reduce the cost of Bi-2212 conductor. However, Bi-2212 is highly reactive with metals other than Ag and the precious metals. Previous work has also shown that the Cu-oxide in Bi-2212 will react with oxides applied to the exterior of the Ag sheath. To investigate the feasibility of using a Ni reinforced composite sheath, a monocore tape was fabricated using a Ag sheathing tube reinforced with 44 volume percent Ni filaments. The tape was melt processed in oxygen and argon atmospheres and the structure of the Ni filaments and oxide core was examined. The Ni filaments were found to completely oxidize and incorporate Cu from the core. The oxide in the core was depleted in Cu as a result, which affected 2212 phase formation.

Dispersion-Strengthened Silver Alumina for Sheathing

Ag/Al alloys with various Al content (0.50 wt%, 0.75 wt%, 1.00 wt%, and 1.25 wt%) are made by powder metallurgy and used as the outer sheath material for Bi2Sr2CaCu2O8 + x (Bi2212)/Ag/AgAl multifilamentary round wires (RW). Bi2212/Ag/AgAl RW microstructural, mechanical and electrical properties are studied in various conditions, including as-drawn, after internal oxidation, and after partial melt processing (PMP). The results are compared with the behavior of a Bi2212/Ag/Ag0.20Mg wire of the same geometry. The grains in as-drawn Ag/Al alloys are found to be ~25% smaller than those in the corresponding Ag/0.20 wt%Mg, but after PMP, the Ag/Al and Ag/0.20 wt%Mg grain sizes are comparable. Tensile tests show that Bi2212/Ag/AgAl green wires have yield strength (YS) of ~115 MPa, nearly 65% higher than that of Bi2212/Ag/Ag0.20Mg. After PMP, the Bi2212/Ag/AgAl YS is about 35% greater than that of Bi2212/Ag/Ag0.20Mg. Furthermore, Bi2212/Ag/AgAl wires exhibit higher ultimate tensile strength and modulus and twice the elongation-to-failure. Atomic resolution high-angle annular dark-field scanning transmission electron microscopy, high resolution transmission electron microscopy and energy dispersive spectroscopy demonstrate the formation of nanosize MgO and Al2O3 precipitates via internal oxidation. Large spherical MgO precipitates are observed on the Ag grain boundaries of Ag/0.20 wt%Mg alloy, whereas the Al2O3 precipitates are distributed homogenously in the dispersion-strengthened (DS) Ag/Al alloy. Furthermore, it is found that less Cu diffused from the Bi2212 filaments in the Bi2212/Ag/Ag0.75Al wire during PMP than from the filaments in the Bi2212/Ag/Ag0.20Mg wire. These results show that DS Ag/Al alloy is a strong candidate for improved Bi2212 wire.

\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{8 + {x}}

Titanium doping has been successfully employed to increase high field Jc in internal tin (IT) type conductors. Titanium doping is achieved by placing Nb-47Ti rods within the filament array. Doping occurs during reaction heat treatment as a result of titanium diffusing from the Nb-47Ti source into the niobium filaments. We have investigated doping of internal-tin-tube (ITT) type conductors using Nb-47Ti rods placed in holes drilled into the niobium annulus of each filament. The titanium distribution at various stages during reaction heat treatment was investigated by SEM/EDS and the A15 grain boundaries were examined with Scanning Auger Microanalysis. We observed that there is minimal titanium diffusion into the niobium until the Nb3Sn reaction front reaches the Nb-47Ti source, and that there is a loss of titanium to the residual filament core and the core/annulus interface. It was determined that the primary route for titanium diffusion is through the A15 grain boundaries.

Multifilamentary Wire

We investigated the fabrication of ex‐situ processed MgB2 round wire by hot extrusion. Composite ex‐situ MgB2 monocore billets were fabricated with a composite sheath consisting of either an AISI 304 or AISI 430 stainless steel clad with copper. The stainless steel was intended to serve as reinforcement for the powder core and maintain mechanical compatibility between the MgB2 core and the copper cladding. The composite billets were extruded at temperatures between 593 and 900 °C at a reduction ratio of 5.2 (81% RA). The billets having the 304 SS barrier cracked during extrusion, while the billets with 430 SS were successfully extruded. Several additional monocore billets with 430 SS/Cu sheath were extruded at 650 °C and reduction ratio of 5.0 (80% RA). The extruded rods were cold drawn, restacked in a copper tube in an 18 filament array, and subsequently cold drawn to 2 mm diameter. A 61 filament restack billet was also assembled and hot extruded at 538 °C and R = 5.0. The results demonstrate that hot ex...