Significant enhancement of compositional and superconducting homogeneity in Ti rather than Ta-doped Nb3Sn

Nb3Sn wire is now very close to its final optimization, but despite its classical nature, a detailed understanding of the role of Ta and Ti doping in the A15 is not fully developed. Long thought to be essentially equivalent in their influence on H-c2, they were interchangeably applied. Here, we show that Ti produces significantly more homogeneous chemical and superconducting properties. Despite Ta-doped samples having a slightly higher T-c onset in zero-field, they always have a wider T-c-distribution. In particular, whereas the Ta-doped A15 has a T-c-distribution extending from 18 down to 5-6 K (the lowest expected T-c for the binary A15 phase), the Ti-doped samples have no A15 phase with T-c below similar to 12 K. The much narrower T-c distribution in the Ti-doped samples has a positive effect on their in-field T-c-distribution too, leading to an extrapolated mu H-0(c2)(0) 2 T larger than the Ta-doped wire. Ti-doping also appears to be very homogeneous even when the Sn content is reduced in order to inhibit breakdown of the diffusion barriers in very high J(c) conductors. The enhanced homogeneity of the Ti-doped samples appears to result from its assistance of rapid diffusion of Sn into the filaments and by its incorporation into the A15 phase interchangeably with Sn on the Sn sites of the A15 phase. (C) 2016 AIP Publishing LLC.