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

Abstract

For power applications, where high current carrying capabilities are required and therefore thick film and bulk material are called for, the Bi 2Sr 2Ca 1Cu 2O 8− 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 0 14 x 24 and sterically hindered diffusion and redistribution of melt originating from the highly anisotropic crystallizing Bi-2212 platelets. Therefore considerable amounts of residual 0 14 x 24 and “4413”, the mixed state of Bi-2212 and 2201, and residual melt are present in the microstructures at 850 °C. The subsequent annealing in oxygen leads to a limited dissolution of 0 14 x 24 phase and the conversion of “4413” to Bi-2212. Thermodynamic calculations of BiO 3/2–Sr 2/3Ca 1/3O–CuO cross-sections at 850 °C at different pO 2 by the CALPHAD method show that the stability range of 0 14 x 24 depends highly on the oxygen partial pressure ( pO 2). These results correlate perfectly with the experimental observations. By annealing at low pO 2 (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 pO 2 (1 bar). This increase is attributed to a gain of current carrying cross-section as a consequence of enhanced 0 14 x 24 dissolution.