Effect of precursor materials on I_c and I_c-H behaviors of Ag-clad (Bi,Pb)-2223 tapes

Thirty seven filamentary Bi-2223 Ag sheathed tapes have been fabricated by the PIT method using precursors which were prepared by the two-powder process. The different types of the precursors are the following: (a) a mixture of 2212 and CaCuO₂ (powder A), (b) a mixture of 2201 and CaCuO₂ (powder B). A nearly pure 2223 phase can be obtained after anneals at 818 °C in 8.5% O₂ in less than 40 h and 60 h for powder A and powder B, respectively. However, a small amount of impurity phases, especially the 2201 phase, still appears in the oxide core for tape B (fabricated from powder B) using a conventional thermomechanical process. An additional low-temperature annealing procedure at the end of the last thermal cycle was used to convert any residual 2201 in tape B into 2212, thereby improving the grain connectivity and overall I_c of tape B dramatically. But, this additional annealing stage had no distinct effect on the phase evolution and the critical current of the tape A (fabricated from powder A). Using both types of precursor mixtures, we obtain tapes with high phase purity, well-aligned grains, and reproducible critical current densities. The maximum I_c values (77 K, 0 T) of fully processed tapes with 0.20 mm thickness are 52 A and 42 A for powder A and powder B respectively.