A NEW MODEL FOR THE FORMATION OF HIGH-T-C PHASE IN SUPERCONDUCTIVE (BI, PB)(2)SR2CA2CU3OX GLASS-CERAMICS

The formation mechanism of the high-T-c phase through the glass-cerami c route and the role of Pb on the formation of this phase have been in vestigated. It was found that a new compound with the chemical composi tion Pb2Sr3-xCaxCuOy (x=1.8) precipitates at around 550 degrees C. Thi s phase is stable up to 800 degrees C, where it begins to decompose, a nd at 850 degrees C it completely disappears. It was found that some p art of the released Pb diffuses into the 2212 phase leading to the for mation of Pb-containing 2212 phase, (Bi,Pb)(2)Sr2CaCu2Ox. On the other hand, an endothermic peak, probably arising from the melting of (Bi,P b)(2)Sr2CaCu2Ox phase or melting at grain boundaries containing Pb2+, was observed at 856 degrees 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 kinet ics 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).