Segregation and dissolution reactions of the 2223 phase in the Bi, Pb-Sr-Ca-Cu-O system on annealing in air

Thermal behavior of the Pb-substituted Bi-2223 phase was examined by characterizing samples annealed at 500°C to 850°C in air and also those subsequently reheated at 850°C in air by XRD, SEM and magnetic measurements. Very interesting reactions occurred, which were segregation of precipitates on annealing below 830°C and dissolution of the precipitates on subsequent reheating at 850°C into the 2223 phase. The precipitates were identified as Sr_2.5Bi_0.5Pb₃Ca₂CuO₂. The precipitation and the dissolution were clearly confirmed by morphological observations of a fixed portion of a sample before and after heat treatments. These results strongly suggest that the 2223 phase changes its composition reversibly between the relatively Pb-rich composition stabilized above 850°C and relatively Pb-poor ones stabilized in essentially the same structure below 830°C. However, T_c remained almost constant at 113 K.