Ta, Ti and Hf effects on Nb3Sn high-field performance: temperature-dependent dopant occupancy and failure of Kramer extrapolation

Abstract

The increasing demand for improving the high-field (16–22 T) performance of Nb3Sn conductors requires a better understanding of the properties of modern wires much closer to irreversibility field, HIrr. In this study we investigated the impact of Ta, Ti and Hf doping on the high-field pinning properties, the upper critical field, Hc2, and HIrr. We found that the pinning force curves of commercial Ti and Ta doped wires at different temperatures do not scale and that the Kramer extrapolation from low field data, typically used by magnet designers to estimate high-field critical current density and magnet operational margins, is not reliable and significantly overestimates the actual HIrr. In contrast, new laboratory scale conductors made with Nb–Ta–Hf alloy have improved high-field Jc performance and, despite contributions by both grain boundary and point defect pinning mechanisms, have more predictable high-field behavior. Using Extended x-ray Absorption Fine Structure spectroscopy, EXAFS, we found that for the commercial Ta and Ti doped conductors, the Ta site occupancy in the A15 structure gradually changes with the heat treatment temperature whereas Ti is always located on the Nb site with clear consequences for Hc2. This work reveals the still limited understanding of what determines Hc2, HIrr and the high-field Jc performance of Nb3Sn and the complexity of optimizing these conductors so that they can reach their full potential for high-field applications.