Ag-sheathed Bi2223 long tapes have been extensively used for various applications thus far particularly after the development of DI-BSCCO in 2004. DI-BSCCO is fabricated by two step sintering processes, 1st sintering under ambient pressure (PO2 ~ 8 kPa, ~840°C, ~100 h) to form the Bi2223 phase and 2nd sintering under high gas pressure (Ptotal = 30 MPa, PO2 ~ 8 kPa, ~840°C, ~100 h) after flat rolling for densification and controlling grain orientation. The overpressure sintering suppresses both generation of voids and degradation of c-axis grain orientation due to grain growth of Bi2223 . Typical Ic of DI-BSCCO is 200 A at 77 K in self field . On the other hand, sintering under low PO2 (< 5 kPa) was found to promote generation of Bi2223 from the precursor in our previous study. Therefore, high Ic tape fabrication process could be designed by only 1st sintering under over pressure and moderately low PO2 (< 5 kPa). Based on these backgrounds, effect of over pressure in 1st sintering process on phase formation and superconducting properties of Bi2223 tapes have been investigated to develop high Ic Bi2223 tapes in a short fabrication time.
Through the systematic studies on 1st sintering as functions of temperature, time, Ptotal and PO2, the phase formation rate of Bi2223 phase was found to monotonically decrease with an increase of Ptotal. An increase in tapes thickness after sintering is smaller for tapes sintered under over pressure than that of tapes sintered under ambient pressure. These indicate that phase formation of Bi2223 somehow suppressed in a highly dense oxide filament, while the densification of filaments is preferable to achieve high Ic. In addition, c-axis lengths of tapes sintered under over pressure are apparently shorter than the standard tapes, suggesting promotion of substitutions of (Ca, Bi) and Pb for the Sr site and Bi site, respectively. Critical current properties of the tapes will be reported including the effect of post-annealing to control cation composition as well as carrier doping state.
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