RAPID PHASE-FORMATION IN B-2223-AG SUPERCONDUCTING TAPES

Parameters that promote rapid formation of aligned high-T(c) phase in the high-temperature superconductor (Bi, Pb)2Sr2Ca2Cu3O(x) (Bi-2223) p owder in tube superconducting tapes have been studied. A rapid phase f ormation was achieved by two means. Firstly, frequent mechanical press ings during the early stages of the heat treatment were applied. The p ressings counteract the expansion that occurs during sintering, increa se grain connectivity and reduce diffusion distances. Secondly, the fr action of process time within the thermodynamically stable region of t he Bi-2223 phase was maximized by ramping of the oxygen partial pressu re during heating and cooling. In addition, the starting phase composi tion of the superconductor powder was found to have a critical influen ce on the reaction speed. A complex phase composition gave the fastest formation rate of Bi-2223 phase. Within 30 h of processing time, crit ical current densities exceeding 12 kA cm-2 (1 muV cm-1, 77 K, 0 T) we re obtained compared to 2-4 kA cm-2 without the above measures. Also, the effect of variations in oxygen partial pressure, pressing schedule s, partial melting and a two-component starting powder based on (Bi, P b)2Sr2CaCu2Ox were investigated for this process.