Critical role of phase transformation during processing of Ag/Bi:2223 tapes

Phase transformation during the final stage of Ag/2223 tape processing has been investigated through quenching and normal cooling in a specially-designed two-step sintering process. It was found that the phase assemblage in the final tape was determined by the equilibrium composition at the sintering temperature and also from any phase transformation which occurred on cooling. A two-stage sintering procedure in the final thermal cycle was found to be effective in transforming the liquid (or amorphous phase) and residual 2201 into 2212 and 2223. However, the annealing temperature during the final step was critical to the final phase assemblage. The optimal annealing temperature of the second step in the two-step process was around 825 degrees C, where all the low T-c phases and impurities were at a minimum. Annealing at temperatures below 810 degrees C, resulted in a substantial increase in 3221, and a lower J(c). Annealing above 825 degrees C led to a large 2212 fraction with a small amount of 2201 because of insufficient time needed to convert these phases into 2223 on normal cooling.