Effect of Post-Annealing on the Phase Transformation and Critical Current Density of (Bi, Pb)-2223/AgAu Tapes

During the sintering process of Ag-alloy sheathed Bi-2223 tapes, post-annealing (PA) played an important role on the final current capacity by forming a Pb-rich phase named (Pb,Bi)₃Sr₂Ca₂CuO_x (3221) and healing the cracks. Due to the different oxygen penetration rate between AgAu alloy and pure Ag, which is traditionally used in other Bi-2223 tapes, the influence of PA parameters on the 3221 phase content, microstructures, and current capacity of AgAu tapes should be systematically studied. In this study, PA processes with different temperatures of 770–800 °C have been performed on fully reacted 37-filamentary (Bi, Pb)-2223/AgAu tapes in the atmosphere of 7.5% O₂ balanced with Ar. It is found that the 3221 phase appeared after PA with its amount first increasing then decreasing with increasing PA temperature. The critical temperature T_c and peak temperature (T_p) obtained with AC susceptibility measurement both increased with increasing PA temperature, and reached the maximum value at 780 °C then decreased with increasing PA temperature. Due to the improvement of T_c and intergrain connectivity, the critical current density (J_c) increased by 24% after PA at 780 °C. Meanwhile, the 3221 phase formation mechanism has also been systematically discussed by varying the amount of remnant liquid in tapes in HT1 process.