Influence of the maximum temperature during partial melt-processing of Bi-2212 thick films on microstructure and jc

The critical current density j(c) of Bi-2212 thick films (d = 60 mu m) depends on the maximum processing temperature T-max during the partial melting process. Processing at T-max = 893 degrees C leads to the highest j(c) of 3500 A/cm(2) (77 K, 0 T). A variation of T-max by +/-5 K from the optimum temperature leads to a drop of j(c) below 80% of the maximum value. Processing below the optimum T-max leads to insufficient consolidation. Partial melting at T-max = 904 degrees C leads to 5 vol% more residual peritectic phases with an average grain size about twice as large in the final microstructure compared to optimally processed samples. However, the drop of j(c) in samples processed at higher T-max is attributed to inhomogeneities in the microstructure. These areas have a reduced film thickness and consist of second phase grains, pores, and misaligned 2212 platelets. The more frequent formation of these inhomogeneities is attributed to inhomogeneous nucleation of the Bi-2212 platelets upon slow cooling at a rate of 5 K/h from the partially molten state. In optimally processed thick films the nucleation of the Bi-2212 platelets occurs regularly in the whole cross section of the film. If processed at higher T-max, fewer nucleation sites are present, leading to regions of high platelet density and regions containing no Bi-2212 grains. Due to the high capillary forces in the narrow gap between the Bi-2212 platelets, liquid is transported away from the platelet-free regions and stored between the Bi-2212 grains. Because of this phase separation, the solid peritectic grains persist in the areas where liquid was extracted. (C) 1998 Elsevier Science B.V.