Nb3Sn: macrostructure, microstructure, and propertycomparisons for bronze and internal Sn process strands

摘要

The variation in irreversibility field, B*(T), with temperature has been measured for Nb3Sn superconducting strands manufactured for ITER using vibrating sample and SQUID magnetometers. The high performance strands were developed for both high transport critical current density, Jc, and low hysteresis loss. Despite a wide variety of designs and components, the strands could be split into two distinctive groups, based on the extrapolated irreversibility fields, which lie about 10% lower than the upper critical field. “Bronze-process” strands exhibited consistently higher B*(T) (28 T to 31 T) compared with “internal Sn” process (24 T to 26 T) conductors. The intrinsic critical current density of the superconductor, Jc(sc), and the specific pinning force of the grain boundaries, Qgb, were evaluated using the measured Jc, and image analysis of the macro- and micro-structures. A bronze-processed Nb(-Ta)3Sn was found to have a higher Jc(sc) but lower Qgb than Nb3Sn produced from Nb filaments. This work shows that the maximization of Jc is both an intrinsic flux pinning issue and a quantitative issue of how much Nb3Sn can be put into the composite package. The results for the ITER strands are compared to a high Jc (but also high hysteresis loss) internal Sn strand. The high Jc strand had a much higher J c(sc) and Qgb than any of the ITER strands