Hiroaki Kumakura

Superconducting High-Tc Oxide/Metal Matrix Composites Produced by Internal Oxidation of Unidirectionally Solidified Ag-Yb-Ba-Cu Alloys

Superconducting high-Tc oxide/metal matrix composites were produced by internal oxidation of unidirectionally solidified Ag-Yb-Ba-Cu alloys. The critical temperature of an internally oxidized Ag-23at.%(Yb1Ba2Cu3) alloy is strongly influenced by the unidirectional solidification processing conditions and the temperature sequence of the subsequent heat treatment. Samples produced with a slow growth rate had higher Tc’s than samples produced with a fast growth rate when given the same oxidization heat treatment. This is interpreted as an annealing effect associated with the slow growth rate, since the Tc’s of the high growth rate samples were increased by an oxygen-free anneal prior to the oxidization heat treatment. To prevent sample melting, and the resulting loss of the solidification structure, the initial heat treatment temperature should be less than or equal to 650°C. To consistently obtain a material with a zero resistance Tc of about 85K and a narrow transition, ATC less than 5K, the final heat treatment temperature should be at 875°C or above.

Fabrication and Properties of Flexible Tapes of High-Tc Bi-(Pb)-Sr-Ca-Cu-O Superconductors

The discovery of high-Tc oxide superconductors, especially of the Ba2YCu3O7, Bi-Sr-Ca-Cu-O2 and Tl-Ba-Ca-Cu-O3 compounds, which show superconductivity above liquid nitrogen temperature, has had great impact not only basic physics but also on the field of superconductor application. The application of a compound superconductor to a superconducting magnet requires the winding of tape or wire conductors with sufficient flexibility into a solenoid. Among the various superconducting compounds having intrinsic brittleness, Nb3Sn and V3Ga compounds with A15 structure have been successfully fabricated into a tape or wire form by the so-called ‘bronze process’ which utilizes the solid state reaction. The oxide high-Tc superconductors are also intrinsically brittle, and special techniques must be developed for making tape or wire conductors. The studies on stress dependence of the high-Tc oxide superconductors are also practical interest, because the superconductor must withstand various stresses caused by coil winding, Lorentz force etc. Standard ceramic processes of mixing powders and organic formulations are applicable for preparing of high-Tc oxide superconductor tape or wire, as demonstrated for Ba2YCu3O7 compound.4,5 However, these tapes or wires became rather brittle after the final sintering process.

Preparation and Properties of Pb-Doped Bi-Sr-Ca-Cu-O Superconductors

Bi-Sr-Ca-Cu-O superconductors with a Pb addition were prepared by a conventional solid state reaction. Early in the sample heat treatment a large volume fraction of low-Tc phase (Bi2Sr2Ca1Cu2Ox) exists. With further sintering the volume fraction of the high-Tc phase (Bi2Sr2Ca2Cu3Ox) gradually increases. A zero resistance transition temperature of 110K was achieved for samples with a nominal composition of Bi0.7Pb0.3Sr1Ca1Cu1.8Ox and a sintering treatment of 845°C for 300 h in air. The critical current density, Jc, of the sample at 77K in zero magnetic field is 210 A/cm2, which is much larger than that of Pb-free sample. A pellet compaction process to densify and align the grains of high-T phase during an interrupted sintering treatment has been shown to be an effective method to increase Jc. A J greater than 1100 A/cm2 can be easily achieved by this process. A new mode of modulation with a wave length of about 50 A is seen in the Pb modified material in addition to the conventional modulation observed in the Pb-free system. Periodicities of the new, as well as, conventional modulation modes become less regular in the Pb-doped system.