Enhanced residual secondary phase dissolution by atmosphere control in Bi-2212 superconductors

For power applications, where high current carrying capabilities are required and therefore thick film and bulk material are called for, the Bi2Sr2Ca1Cu2O8-d (Bi-2212) compound has evolved as one of the most promising. During partial melt processing the crystallization from the partial melt state upon cooling is incomplete, due to the stability of 014x24 and sterically hindered diffusion and redistribution of melt originating from the highly anisotropic crystallizing Bi-2212 platelets. Therefore considerable amounts of residual 014x24 and ''4413'', the mixed state of Bi-2212 and 2201, and residual melt are present in the microstructures at 850 deg. C. The subsequent annealing in oxygen leads to a limited dissolution of 014x24 phase and the conversion of ''4413'' to Bi-2212. Thermodynamic calculations of BiO3/2-Sr2/3Ca1/3O-CuO cross-sections at 850 deg. C at different pO2 by the CALPHAD method show that the stability rAe of 014x24 depends highly on the oxygen partial pressure (pO2). These results correlate perfectly with the experimental observations. By annealing at low pO2 (0.1 bar) after crystallization, the amount of residual phases is reduced by more than 50% and the critical current density of the fully processed material increases by 65% compared to material annealed at high pO2 (1 bar). This increase is attributed to a gain of current carrying cross-section as a consequence of enhanced 014x24 dissolution