A new model for the formation of high-T _c phase in superconductive (Bi, Pb)₂Sr₂Ca₂Cu₃O _x glass-ceramics

Abstract

The formation mechanism of the high-T _c phase through the glass-ceramic route and the role of Pb on the formation of this phase have been investigated. It was found that a new compound with the chemical composition Pb₂Sr_{3 − x} Ca _x CuO _y (x = 1.8) precipitates at around 550‡C. This phase is stable up to 800‡C, where it begins to decompose, and at 850 ‡C it completely disappears. It was found that some part of the released Pb diffuses into the 2212 phase leading to the formation of Pb-containing 2212 phase, (Bi, Pb)₂Sr₂CaCu₂O _x . On the other hand, an endothermic peak, probably arising from the melting of (Bi, Pb)₂Sr₂CaCu₂O _x phase or melting at grain boundaries containing Pb²⁺, was observed at 856‡C only in Pb-containing samples that were heat treated. The liquid phase attributed to the endothermic peak may enhance the formation of high-T _c phase (2223 phase). The growth kinetics for the high-T _c phase were analysed using the Johnson-Mehl-Avrami equation; the results indicate that the growth of the high-T _c phase is controlled by a diffusion process and the activation energy for its formation in the initial stage (shorter than 96 h) is 576 ± 45 kJ mol⁻¹.