Effect of filament twist on the structure and properties of Ag-sheathed Bi2223 tape

Abstract

Ag-sheathed Bi2223 tapes show good performances and are ready for many prototype applications. However, drastic reduction of AC loss is strongly required for practical AC power applications. To reduce the AC loss under AC external magnetic field, the introduction of filament twisting with high resistive barrier is effective, but the filament twisting tends to decrease the J c value. This decrease in J c is a problem to balance low AC loss with high I c value. In this study, the effect of filament twisting on the microstructure and J c property was investigated to overcome the J c reduction. The Bi2223 tapes sheathed with Ag–Cu alloy were fabricated with a standard PIT method. Although the formation of Bi2223 phase was not affected by filament twisting from the XRD analysis and SEM observation, the J c values of the twisted tapes decreased about 30% compared to that of non-twisted tape. In the case of the same twist pitch, the sample rolled under large pass reduction ratio showed relatively high J c value. This high J c would be caused by the high aspect ratio of the filament of those tapes, in which the grain alignment along the filament interface would be well. On the other hand, a reason for the J c reduction of twist tapes is proposed to be the misorientation at grain boundaries of Bi2223 grains in the transference region near the edges of the tape, where the filaments move up and down in the thickness direction.