The effect of the maximum processing temperature on the microstructure and electrical properties of melt processed Ag-sheathed Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ tape

摘要

The critical current density (J/sub c/) is very sensitive to the maximum temperature (T/sub m/) used to melt process Ag-sheathed Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ tapes. In this study we have found that the optimum T/sub m/ was 894-896/spl deg/C and that variations of /spl plusmn/2/spl deg/C strongly decreased J/sub 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/sub m/. In addition, the room temperature electrical resistivity of the core was lowest for the tapes with the maximum J/sub c/ at 4.2 K. A formation of macropores was observed in tapes processed at T/sub 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/sub 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/sub c/ depends on T/sub m/ primarily because the effective cross-section of the core depends on T/sub m/.