拡散法により作製した円柱状Bi系酸化物高温超伝導体の超伝導特性と組織

Abstract

Bi-2212 oxide superconductors have been synthesized by the diffusion reaction between a high melting point Sr-Ca-Cu oxide substrate and a low melting point Bi-Cu oxide coating layer. The substrate with Sr : Ca : Cu atomic ratio of 2 : 1 : 2 was pressed into a cylindrical rod by cold isostatic pressing, and then sintered. The coating layer with Bi : Cu ratio of 2 : 1 and Ag2O addition was coated around the surface of the substrate. The heat treatment was performed to produce the Bi-2212 diffusion layer at 820∼860°C. The Ag added to the coating layer enhances the diffusion reaction and precipitates on the surface of the specimen. The Bi-2212 diffusion phase, formed through the Bi-2201 phase, showed a dense and oriented structure with plate-like grains. The transport Ic and Jc of the diffusion layer, about 150 μm in thickness, exceeded 300 A and 2×108 A/m2 up to 25 K under self-field, respectively. The transport Jc decreased with temperature, being about 1×108 A/m2 at 35 K under 0.5 T. Post-annealing in the inert gas atmosphere improved Tc and Jc at higher temperatures. The contact resistivity of bulk ends of the specimen was in the order of 10−7 μΩ·m2 between 4.2 K and 65 K up to 3 T. The present Bi-2212 oxide cylinders with large transport Ic and Jc as well as low contact resistance may be attractive as current leads for cryocooler-cooled and conventional cryogen-cooled superconducting magnets.