Effect of final annealing temperature on critical current density of Ag/Bi-(Pb)-Sr-Ca-Cu-O tapes

A two-stage annealing procedure in the final thermal cycle has been tested for processing Ag/Bi:2223 tapes. The effect of various annealing temperatures in the final stage of heat treatment on phase transition, microstructure and critical current (I-c) was investigated for Ag/Bi:2223 tapes. It was found that annealing at higher temperatures (835-845 degrees C) leads to a relatively purer Bi:2223 phase but a small fraction of Bi:2201 always coexists with Bi:2223. Whereas annealing at lower temperatures (800-825 degrees C) results in conversion of Bi:2201 to Bi:2212 and possibly partial decomposition of Bi:2223. I-c was reduced to half for tapes annealed at 835-845 degrees C due to the presence of' Bi:2201, compared with tapes annealed at low temperatures. Scanning electron microscopy (SEM) studies Show that, although the amount of Bi:2201 is small, it resides along the Bi:2223 colony boundaries, blocking the current flow between colonies. The drastic weak link effect of Bi:2201 is attributable to the combination of the elongated platelike morphology and insulating property. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.