Cooling rate effects on the microstructure, critical current density, and T/sub c/ transition of one- and two-powder BSCCO-2223 Ag-sheathed tapes

An important variable controlling the critical current density (J/sub c/) of Ag-sheathed BSCCO-2223 tapes is the degree of phase purity of the reacted tapes. Most correlations between J/sub c/ and microstructure show that it is highly desirable to reduce the amount of non-superconducting second phases to as low a level as practical. In recent studies of the influence of cooling rate after the final reaction, we find contradictions to this general rule. The J/sub c/ (77 K, 0 T) of so-called "one-powder" tapes can be raised by as much as 50% (from /spl sim/8000 A/cm/sup 2/ to 12000 A/cm/sup 2/) by slow cooling in 7.5%O/sub 2/ at 0.05/spl deg/C/min, even though large 2212 grains are usually seen in the slowly cooled microstructure. However, the higher J/sub c/ of the slow-cooled state does correlate with a sharper T/sub c/ transition. Experiments with "two-powder" tapes have produced similar results. These apparently anomalous results emphasize the important role played by the connectivity of the polycrystalline core in determining J/sub c/.>