Effect of oxygen partial pressure on processing conditions and phase transformation in Ag/Bi-2223 tapes

The effect of oxygen partial pressure on processing parameters and phase transformation in Ag/Bi-2223 tapes was investigated using a two-step annealing process at the final thermal cycle of the powder-in-tube process. It was found that the temperature window for processing was widened and the volume fraction of Bi-2223 in the tape increased under low oxygen partial pressure. Using an atmosphere of N-2-7.5% O-2, nearly pure Bi-2223 phase was obtained by a proper control of multi-step annealing temperatures. The maximum J(c) and the highest Bi-2223 volume fraction appeared at T-1 = T-2 = 830 degreesC and T-3 = 785-805 degreesC in an N-2-7.5% O-2 atmosphere. The samples annealed in N-2-7.5% O-2 retained their J(c) values well over a wide annealing temperature range of 785-805 degreesC with only 10% deviation from the highest value. In contrast, for the same level of J(c) retention using conventional methods, the temperature window would be narrowed down to a half of this range. These results demonstrate that heat treatments in an N-2-7.5% O-2 atmosphere are advantageous for processing Bi-2223/Ag tapes on a large scale, as there would be much less demand on temperature control. (C) 2001 Elsevier Science B.V. All rights reserved.