EFFECT OF THE PHASE COMPOSITIONS AT THE FINAL STAGE OF HEAT-TREATMENTON THE CRITICAL-CURRENT DENSITY IN BI-2223 AG TAPES/

The effect of the phase compositions after the final stage of heat tre atment on the critical current density (J(c)) in Bi:2223/Ag tapes has been investigated. A high volume fraction of Bi:2223 has been consider ed the most critical factor in achieving a high J(c); however, it is e xtremely difficult to attain pure Bi:2223 phase via routine thermomech anical processing. Although a higher volume fraction of Bi:2223 phase can be attained using a higher processing temperature, it is always fo und to co-exist with Bi:2201 which resides at Bi:2223 colony boundarie s. As these possess a plate-like morphology, the Bi:2201 joins weak li nks in a conductible chain. Bi:2201 can be eliminated using an anneali ng step (800-825 degrees C) at the end of the last thermal cycle, resu lting in a significant improvement in J(c). At annealing temperatures below 800 degrees C, Bi:2223 decomposes into Bi:3221 and other cuprate s, which degrade J(c) further. Magnetization, AC susceptibility and re sistivity measurement results confirm that the Bi:2201 and Bi:3221 act as weak links.