EFFECT OF SINTERING TEMPERATURE AND COOLING RATE ON MICROSTRUCTURE, PHASE-FORMATION, AND CRITICAL-CURRENT DENSITY OF AG-SHEATHED BI1.8PB0.4SR2CA2CU3OX SUPERCONDUCTING TAPES

Silver-sheathed Bi-Pb-Sr-Ca-Cu-O (2223) superconducting tapes (with a starting composition of Bi1.8Pb0.4SnCa1Cu2O8, calcium cuprate, and CuO ) were fabricated by the powder-in-tube technique. The tapes were sint ered at various temperatures to optimize the formation of Bi1.8Pb0.4Sr 2Ca2Cu3O10 phase within the tape. The results show that sintering with in the temperature range of 815-825 degrees C can produce tapes with h igh critical current density (J(c)). The J(c) of samples sintered at t he higher temperature of 825 degrees C, where more liquid is present, depended markedly on the rate at which tapes were cooled from the sint ering temperature; samples sintered at lower temperatures did not exhi bit such a cooling-rate effect. The optimum combination of phase purit y and microstructure that yielded an average transport J(c) of greater than or equal to 2.5 x 10(4) A/cm(2) was obtained when the tapes were sintered at 825 degrees C for 150 h and cooled at a rate of 25 degree s C/h from the sintering temperature. Quenching studies indicate that the Bi-2223 phase becomes unstable below 700 degrees C during slow coo ling. This result may have important implications for processing Bi-Sr -Ca-Cu-O tapes with high J(c). Addition of 15 vol.% Ag flakes to the m onolithic core exerted no significant effect on J(c).