Novel Pb-Free Superconducting Joint Between NbTi and Nb3Sn Wires Using High-Temperature-Tolerable Superconducting Nb–3Hf Intermedia

There was no other method but to use Pb-alloy solder intermedia to establish superconducting joints between NbTi and Nb
₃
Sn wires beyond 0.5 T. However, Pb is environmentally hazardous. In this context, new alternative joint methods without using these substances were strongly desired in applications such as high-field NMR systems. Recently, we have developed a novel superconducting joint method using high-temperature-tolerable (HTT) superconducting Nb-alloy intermedia that enable us to establish the superconducting joint between NbTi and Nb
₃
Sn wires by a combination of chemical and mechanical bonding processes. In this work, the joint was surely confirmed to have ultra-low resistance by current decay measurements. The yield rate of successful superconducting joint between Nb
₃
Sn and HTT Nb-alloy was almost 100% and the scattering of
I
_c
value was small. Meanwhile, the joint between NbTi and HTT Nb-alloy showed large scattering of
I
_c
. We observed NbTi filament cracks in mechanically-pressed joints between NbTi and HTT Nb-alloy, using a high resolution X-ray CT device with micro X-ray source. The filament cracking could account for the large scattering of
I
_c
in the joints.