超電導V<SUB>3</SUB>Ga拡散線材の高磁界特性の改善について

Abstract

Effects of the heat treatment on the high-field performance in the diffusion-processed superconducting V3Ga have been investigated. Vanadium, V-5 at%Al, V-6 at%Ga and V-0.2 at%Zr substrate tapes were coated with gallium, electrically plated with copper, and finally heat-treated to form V3Ga layers. Ic’s at fields near Hc2 have been found to depend mainly on the layer thickness and Hc2 of V3Ga. The two-stage reaction process, which consists of a first reaction at a higher temperature to form enough thick V3Ga layers and a subsequent second reaction at a lower temperature to achieve high values of Tc and Hc2, has been found to improve the Ic in high magnetic fields. The second reaction leads to the increases in Tc by 0.4 K and in Hc2 by 1 T. The Hc2 enhancement can be interpreted only by the increase in Tc, since −[dHc2⁄dT]Tc remains unchanged through the second reaction. The increase in Tc has been discussed in relation to the long range order parameter. Generally, the variation in Hc2 with fabrication conditions such as substrate composition, gallium coating, copper plating and heat treatment seems to depend directly on the variation in Tc. This is due probably to the fact that Hc2 of V3Ga is strongly limited by the paramagnetic effect. Aluminum and gallium alloying to the substrate tape effectively enhances the growth rate of V3Ga layer. An overall Jc of 1.7×108 A/m2 has been obtained at 4.2 K and 20 T for the aluminum alloyed specimen treated at 973 K for 180 ks and subsequently at 873 K for more than 360 ks.