Ye. High et al., A STUDY OF THE COMPETITIVE REACTIONS OCCURRING DURING THE HEAT-TREATMENT OF SILVER-SHEATHED BI1.8PB0.4SR2.0CA2.2CU3.0OY AEROSOL SPRAY-PYROLYSIS POWDER, Physica. C, Superconductivity, 220(1-2), 1994, pp. 81-92
The phase conversion reactions that occur when a fine, micron-size pow
der of overall composition Bi1.8Pb0.4Sr2.0Ca2.2Cu3.0Oy (2223) is heat
treated in a silver tube have been studied. We find that non-supercond
ucting 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 preferentia
lly at the Ag interface; continued growth occurs preferentially from t
he initial 2223 interface plates. The study shows that an aligned 2223
microstructure requires a smooth Ag-oxide interface and very close at
tention to thermal processing.