Formation and microstructure of (Bi,Pb)-2223 phase in the diffusion process

Abstract

Bi-2223 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,Pb)–Cu oxide coating layer. The substrate with Sr:Ca:Cu atom ratio of 1.9:2.1:2.5 was pressed into cylindrical rods with 4 mm diameter or rectangular bars with 3 mm×3 mm×15 mm sizes, and then sintered. The coating layer with Bi:Pb:Cu ratio of 1.8:0.4:1 was coated around the substrate rods and bars. The heat treatment was performed to produce the (Bi,Pb)-2223 diffusion layer at 840 °C for different time. The Bi-2223 phase in the diffusion process is transformed from the Bi-2212 phase through the Bi-2201 phase. The Ag 2O added to the coating layer does not enhance the (Bi,Pb)-2223 diffusion reaction. The cylindrical sample is favorable for formation of (Bi,Pb)-2223 diffusion layer as compared with the rectangular sample. The optimum heat treatment to produce the (Bi,Pb)-2223 diffusion layer is 840 °C for 144 h.