Phase transformation and liquid phase conversion during the final processing of Bi-2223 /Ag PIT tapes and their influence on critical current density

The phase transformations during the final processing step of Bi-2223/Ag PIT tapes and its effect on the critical current density have been investigated. In order to determine the phase composition during the final cooling process, tapes were quenched at temperatures between 700 degrees C and 845 degrees C at the end of the two-step process. On cooling, the liquid phase was converted to either an amorphous phase, superconducting or impurity phases, depending on the cooling step. The optimal annealing temperature to convert the liquid to 2223 was found to be 825 degrees C, At annealing temperatures higher than 825 degrees C, the liquid phase was converted to 2212, 2223 and amorphous phase on cooling. Between 825 degrees C and 800 degrees C, if any liquid phase was present in the tape, it converted to 2212 and other phases. 223 decomposed into 3221 and other phases below 800 degrees C. The composition of phases formed during cooling had a significant effect on grain connectivity and flux pinning in the tapes. The residual liquid phase was largely responsible for weak links, substantially reducing J(c). (C) 1999 Elsevier Science B.V. All rights reserved.