Effect of Zn Addition on NbSn Layer Formation in the Nb/Cu-Sn-Ti Diffusion Reaction

Enhancing the characteristics of Nb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>Sn superconducting wire is essential for the development of magnets for fusion reactors like ITER and DEMO. It has been established that Ti-doping has a significant effect on enhancing the upper critical magnetic field (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B_c2</i>). Ti is generally doped into Nb or Sn-alloys in practical Nb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>Sn superconducting wires, and it would be preferable to add Ti to the Sn side from a manufacturing perspective. However, Ti-doping on Sn sites could form some undesirable stable compounds layers at the interface with Nb as a diffusion barrier for Sn during the Nb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>Sn layer formation. It is challenging to find a new reaction route that destabilizes these compound layers in the Nb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>Sn formation process, which is expected to cause a dramatic improvement in Sn diffusion when Ti is doped to the Sn core. Nevertheless, there are few studies that fundamentally investigate this aspect. Studies have reported that Zn is effective for promoting Nb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>Sn layer formations. Therefore, in this study, specific diffusion couples of Nb/Cu/Sn were fabricated with different combinations of Ti and Zn doping to Cu and Sn and their diffusion reaction behaviors in Nb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>Sn layer formation were investigated. Besides, anticipating a potential grain refinement effect, an Mg and Zn co-doped sample was additionally fabricated. However, the grain refinement by Mg was not seen in the present wire configuration. The effect of Zn addition on promoting the Nb<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>Sn layer formation appeared at 650 °C/150 h HT, while it was not visible well at 685 °C/100 h HT.