The dependence of microstructure and superconducting phase formation on post-sintering cool-rate of BiCaSrCu₂O_x ceramic

Ceramic samples of nominal composition BiCaSrCu₂O_x have been fabricated via mixed-oxide and spray-dried routes. The former method has consistently yielded samples of higher mechanical density (5.20 cf 4.04 g cm^-3). In an attempt to optimise the formation of the higher-temperature superconducting phase (T_c approximately 105 K), samples produced by both methods have been subjected to a range of post-sintering cool-rates. AC diamagnetic susceptibility measurements to 4.2 K indicate that this process parameter determines critically the superconducting properties of resultant samples. In particular, slow cool-rates, or equivalently long sintering times, are essential if complete volume superconductivity is to be achieved. Microstructural analysis reveals that a 'flaky' powder morphology yields samples of good mechanical strength. Evidence of the higher-T_c phase has only been detected in low density samples, suggesting that the oxygen content of the lattice plays a significant role in the formation of this phase. A distinct acicular morphology for the higher-T_c phase has been observed compared with a flaky grain geometry characteristic of the 85 K phase. The acicular microstructure is significantly less sensitive to the application of magnetic fields than the latter. It is suggested that the difficulties experienced in producing single phase material which exhibits superconductivity above 100 K in this system could be attributed to a variation in stoichiometry within a single-crystal lattice.