THE EFFECT OF COOLING RATES ON TRANSPORT CURRENT PROPERTIES AND THE CRITICAL-TEMPERATURE OF AG-SHEATHED BSCCO-2223 SUPERCONDUCTING TAPES

The transport current properties of Ag-sheathed (Bi, Pb)(2)Sr2Ca2Cu3Oy BSCCO-2223 superconducting tapes were analysed as a function of cooli ng rate. Changing the cooling rate showed a pronounced effect on the t ransport current of the superconductor heat-treated at 810 degrees C i n a 7% O-2 atmosphere. Results indicated that, during fast cooling, th e thin layer of BSCCO adjacent to the Ag sheath is under compression. The compressive stress causes microcracking and affects the alignment and interconnectivity of 2223 grains in the thin layer of BSCCO next t o the silver sheath. This changes the current path through the high-cr itical-current-density region in the superconductor. A comparison betw een furnace-cooled tapes and slowly cooled tapes (10 degrees C h(-1) t o 780 degrees C and then 1 degrees C min(-1) to room temperature) show ed-that the latter attained two or three times higher I-c values. Base d on this observation, a cooling schedule that includes several interm ediate cooling steps is suggested. The effect of the lattice parameter on T-c was investigated. Behaviour analogous to that of the 2212 phas e was found, T-c was affected by changes in the c-axis of the 2223 pha se. However, the effect was not as pronounced as it was in the 2212 ph ase.