Influence of precursor calcination parameters on the critical current density of Bi-2212 superconductors

Partial melt-processed Bi2Sr2Ca1Cu2O8+δ (Bi-2212) materials have low critical current densities (jc) when made from precursors where melting occurred during the calcination. In this paper, the origin of melt formation during calcination of a stoichiometric Bi:2, Sr:2, Ca:1, Cu:2 mixture of oxides and carbonates was identified and the consequences of this melt on the homogeneity of the calcination product and of fully processed Bi-2212 material studied. It was found that low oxygen partial pressure (pO2) atmospheres can build up inside the precursor particle network during the decomposition of the carbonates. This causes melt formation below 750 °C during calcination leading to phase separation and inhomogeneities in the calcined powder. The consequences of these inhomogeneities in the precursor on the jc of fully processed material and their microstructures was studied. In order to prevent melting below the peritectic decomposition temperature in the green bodies, the calcined precursors should be carbonate free but not yet fully converted to Bi-2212 in order to obtain maximum jc in partial melt-processed material. A calcined precursor that yields components of high jc shows maximal homogeneity on short length scales (<10 μm) just before it peritectically decomposes.