Influence of Hf addition on the superconducting properties of Nb₃Sn

Nb ₃ Sn 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 Nb ₃ Sn. 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 Nb ₃ Sn. Hf addition was found to refine the Nb ₃ Sn average grain size from 135 nm to 100 nm. Generally, ternary additives could enhance the upper critical field B _c2 of Nb ₃ Sn by introducing disorders in the Nb ₃ Sn 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 Nb ₃ Sn.