Critical current limiting factors in post annealed (Bi; Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tapes

Abstract

Processing (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ (Bi-2223) tape by the oxide-powder-in-tube technique normally includes two heat treatments and one intermediate rolling. A three-step heat treatment was applied for the second heat treatment (HT2). Experimental procedures were designed for understanding the roles of each processing step in HT2 and the post anneal. It was found that post annealing improved the critical temperature T/sub c/, critical current density J/sub c/ and flux pinning characteristic field H/sub p/. The midpoint T/sub c/ was raised from 103 to 109 K by post anneal, while H/sub p/ was increased from 126 to 183 mT. Post anneal at about 790/spl deg/C precipitated Pb from Bi-2223 phase to form a Pb-rich phase, thus the microstructure looked worse but T/sub c/, J/sub c/ and H/sub p/ were raised. J/sub c/ was raised as the Bi-2212 phase signature in the T/sub c/ trace decreased. The most important observation is that raising T/sub c/ and minimizing residual Bi-2212 are decisive factors for increasing J/sub c/ at 77 K in Bi-2223 tapes.