Critical current limiting factors in post annealed (Bi,Pb)2Sr2Ca2Cu3Ox tapes

Processing (Bi,Pb)₂Sr₂Ca₂Cu₃O_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_c, critical current density J_c and flux pinning characteristic field H_p. The midpoint T_c was raised from 103 to 109 K by post anneal, while H_p was increased from 126 to 183 mT. Post anneal at about 790°C precipitated Pb from Bi-2223 phase to form a Pb-rich phase, thus the microstructure looked worse but T_c, J_c and H_p were raised. J_c was raised as the Bi-2212 phase signature in the T_c trace decreased. The most important observation is that raising T_c and minimizing residual Bi-2212 are decisive factors for increasing J_c at 77 K in Bi-2223 tapes.