Development of Bismuth-Based High-T_c Superconducting Wires

Abstract

(Bi, Pb)₂Sr₂Ca₂Cu₃O_X (Bi2223) superconducting wires (T_c∼110 K) is a promising candidate of high-T_c superconducting wire which can be expected for practical use. We have proceeded with its research of Ag-sheathed wire using Bi2223 superconductor and has developed semi-mass producing long wires (in the kilometer class) with high J_c. And for application, magnet, power transformer and power cable have been developed. At present we improve the performance of Bi2223 wires and have proceeded with cable project in the real scale.
But it is still necessary for practical use to improve the performance of Bi2223 wires. In this paper, we concentrated on the sintering process and applied controlled over pressure sintering method. The sintering process has a problem which the mass density increased on rolling process decreases during sintering. Since decreased density causes the formation of pores and a decrease of supercurrent between Bi2223 grains which grow during sintering, it is considered to be one of the major causes of the performance of Bi2223 wires. Controlled over pressure sintering is a method which applies gas pressure to Bi2223 wires during sintering because of suppression of decreasing mass density.
As the results of short samples, controlled over pressure sintering method improved Bi2223 phase homogeneity, misalignment angle, connectivity, critical current density and mechanical property. Furthermore, when cooled in liquid nitrogen and warmed up to room temperature, the wires may often balloon up by vaporization of liquid nitrogen entered in the pores of filaments through the defects of the sheath. However controlled over pressure sintering removes pores in filaments and it is confirmed that wires were not ballooned after cooled in liquid nitrogen under 1 MPa for 24H so far.
To apply controlled over pressure sintering to long length Bi2223 wires, we developed the large furnace which kilometer-scaled Bi2223 wires can be sintered. The performance of long length wires is almost equal to that of short samples.