A STUDY OF THE COMPETITIVE REACTIONS OCCURRING DURING THE HEAT-TREATMENT OF SILVER-SHEATHED BI1.8PB0.4SR2.0CA2.2CU3.0OY AEROSOL SPRAY-PYROLYSIS POWDER

The phase conversion reactions that occur when a fine, micron-size powder of overall composition Bi1.8Pb0.4Sr2.0Ca2.2Cu3.0Oy (2223) is heat treated in a silver tube have been studied. We find that non-superconducting phases, particularly alkaline earth cuprates, grow more readily than the desired 2223 phase, because the activation energy for their growth (similar to 300 kJ/mole in air) is about a factor of five less than for the Bi2Sr2CaCu2O(x) (2212) to 2223 conversion (similar to 1.5 MJ/mole). A reduced oxygen partial pressure (7.5% O-2/Ar) is found to favor the 2223 phase and CuO. The reaction to 2223 occurs preferentially at the Ag interface; continued growth occurs preferentially from the initial 2223 interface plates. The study shows that an aligned 2223 microstructure requires a smooth Ag-oxide interface and very close attention to thermal processing.