The effect of the maximum processing temperature on the microstructure and electrical properties of melt processed Ag-sheathed Bi2Sr2CaCu2Ox tape

The critical current density (J(c)) is very sensitive to the maximum temperature (T-m) used to melt process Ag-sheathed Bi2Sr2CaCu2Ox, tapes, In this study we have found that the optimum T-m, was 894-896 degrees C and that variations of +/-2 degrees C strongly decreased J(c), We found that the density of the oxide core and its Vickers hardness displayed a maximum in tapes that had been processed at the optimum T-m. In addition, the room temperature electrical resistivity of the core was lowest for the tapes with the maximum J(c) at 4.2 K. A formation of macropores was observed in tapes processed at T-m above the optimum one, MO imaging and SEM observation showed that the magnetic flux penetrates more easily into tape areas containing macropores. Together these observations show that T-m exerts a very powerful effect on the macroscopic density of the oxide core, this controlling the connectivity and the effective cross-section of the oxide core. Thus J(c) depends on T-m primarily because the effective cross-section of the core depends on T-m.