Microstructural Studies of the Effect of Heat-Treatment on Bi,Pb-2223 Films Prepared by RF Sputtering

The uniform single phase Bi,Pb-2223 films are needed in order to acquire fundamental superconductivity data for the development of Bi,Pb-2223 wires. However, Bi,Pb-2223 films that we previously reported had the non-uniform microstructure containing several other phases. In this study we investigated the microstructural evolution during the heat treatment and processing route to fabricate uniform Bi,Pb-2223 films elucidated. An as-grown film had a small amount of Bi-2223 phase and a large amount of Ca-Sr-Cu-O phase. C-axes of some Bi-2223 crystals are not aligned along the normal direction of the STO substrate. In the primary stage of heat treatment, the formation of Bi,Pb-2223 phase proceeds. In the second stage of heat treatment, although the flat Cu-O layers in the Bi,Pb-2223 phase are formed by liquid-phase reactions, the grain coarsening of Ca-Sr-Cu-O phase also occurs. From the observation results, the fabrication process should be optimized based on the following points: uniform dispersion of penetration (diffusion) paths of Pb atoms into the film; short duration of heat treatment to avoid phase separation of Bi,Pb-2223 and coarsening of Ca-Sr-Cu-O phase.