Effect of filament twist and high resistive barrier on the properties of Ag-sheathed Bi2223 tape

Abstract

Although the introduction of filament twisting with high resistive barrier between the filaments is effective to reduce the AC loss, the Jc value of the tape was easily decreased by the filament twisting and insertion of high resistive barrier. This decrease in Jc is a problem to achieve low AC loss and high Ic tape. In this study, the effects of filament twist and high resistive barrier on the microstructure and Jc property were investigated to overcome the Jc reduction. The Bi2223 tapes with filament twist or oxide barrier were fabricated by powder-in-tube method. The Jc value of twist tape gradually decreased as decreasing the twist pitch, and the degree of Jc reduction was small in slim tape compared to wide tape. This reduction would attribute to the small actual cross sectional area of the filament perpendicular to the current direction because the filament has a tilt angle from the tape axis. On the other hand, the conversion ratio of Bi2223 became small as increasing the thickness of SZO barrier layer. Since the SZO barrier has less reactivity with 2223 filament, this suppression of the Bi2223 formation would not be caused by the formation of impurity phase but would be caused by the suppression of gas release from the filaments by oxide barrier. The conversion ratio of Bi2223 increased in the short sample where the gas could be released from the both end of the tape. These results suggest that the gas diffusion from filaments would be important for the formation of Bi2223. Hence, the use of slim tape for decreasing the tilt angle of the filament and keeping the diffusion path of gas from the filaments may improve the Jc value of twist and barrier tapes.