Intrinsic limits ofJ_cin composite metallic/Bi(Pb)-2223 multifilamentary tapes with twisted filaments

Abstract

The intrinsic limits of Jc in composite Bi(Pb)-2223 multifilamentary tapes due to the reduction of the effective cross section and the enlargement of the twisted filaments have been analysed. A simple model of the twisted filaments as elliptic helices grouped in concentric shells have been used to derived analytical results for the intrinsic variation of Jc with the twist pitch, tp. In tapes with high thickness to width aspect ratios and large tp values, this model predicts a linear dependence of Jc with 1/tp, when the shape and superconducting filling factor of the tapes are constant. The Jc(tp) values depend on a set of parameters that define the helices, which may be derived from the analysis of the morphology of the filaments inside the tapes. To assess the model we have compared the predictions with the results here reported on Ag and Ag0.94Au0.06 alloy sheathed tapes with 19 and 37 twisted filaments, respectively. In tapes with high Jc, we have found a good agreement between the variation of Jc with tp experimentally measured and the intrinsic limits imposed by the filament geometry.

Torsion tests of filamentary Nb/CuSn and BSCCO/Ag composite wires

“…On the other hand, the extensive twisting (short twist pitch) of composite wire can lead to some filament damage and to apparent transport current capability reduction. For the short t p values needed to decouple the filaments, there are always important reductions of J c [4][5][6]. The decrease of the tape's J c with the twist pitch may be partially ascribed to intrinsic reductions of the critical current, I c , due to the current flow differences associated with the morphology of twisted filaments.…”

“…The main intrinsic effects are (i) decrease of the effective filament cross-section, i.e. of the area perpendicular to the superconducting current flow, and (ii) elongation of the filaments, which will shorten the tape length needed for the appearance of 1 µV cm −1 electric fields [5].…”