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 c haracterizing samples annealed at 500 degrees C to 850 degrees C in ai r and also those subsequently reheated at 850 degrees C in air by XRD, SEM and magnetic measurements. Very interesting reactions occurred, w hich were segregation of precipitates on annealing below 830 degrees C and dissolution of the precipitates on subsequent reheating at 850 de grees C into the 2223 phase. The precipitates were identified as Sr2.5 Bi0.5Pb3Ca2CuOz. The precipitation and the dissolution were clearly co nfirmed by morphological observations of a fixed portion of a sample b efore and after heat treatments. These results strongly suggest that t he 2223 phase changes its composition reversibly between the relativel y Pb-rich composition stabilized above 850 degrees C and relatively Pb -poor ones stabilized in essentially the same structure below 830 degr ees C. However, T-c remained almost constant at 113K.