Influence of Hf Addition on the Superconducting Properties of Nb₃Sn

Nb3Sn strands are widely used to make magnets above 10 T. At present, one major effort to further broaden their application is enhancing the non-Cu critical current density J(c,non-Cu) to meet the requirements of large scientific devices such as nuclear fusion and accelerator engineering. Element doping is an effective way to improve the properties of Nb3Sn. In this work, Nb-1at.%Hf alloy along with pure Nb were utilized to manufacture monofilament Tube-type sub-elements to study the influence of Hf addition on the superconducting properties of Nb3Sn. Hf addition was found to refine the Nb3Sn average grain size from 135 nm to 100 nm. Generally, ternary additives could enhance the upper critical field B-c2 of Nb3Sn by introducing disorders in the Nb3Sn lattice. However, no promotion effect on B-c2 was found for Hf addition. Flux pinning behavior obtained from the magnetization measurements indicates that grain boundary pinning dominates. This suggests that no second-phase particles that provide point pinning were generated during the heat treatment. In addition, it was found that Hf addition has a weak impact on the superconducting critical temperature of Nb3Sn.