COOLING RATE EFFECTS ON THE MICROSTRUCTURE, CRITICAL-CURRENT DENSITY, AND T-C TRANSITION OF ONE-POWDER AND 2-POWDER BSCCO-2223 AG-SHEATHED TAPES

An important variable controlling the critical current density (J(c)) of Ag-sheathed BSCCO-2223 tapes is the degree of phase purity of the reacted tapes, Most correlations between J(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(c) (77 K, 0 T) of so-called ''one-powder'' tapes can be raised by as much as 50% (from similar to 8,000 A/cm(2) to 12,000 A/cm(2)) by slow cooling in 7.5% O-2 at 0.05 degrees C/min, even though large 2212 grains are usually seen in the slowly cooled microstructure. However, the higher J(c) of the slow-cooled state does correlate with a sharper T-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(c).