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 Nb3Sn 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 Nb3Sn 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 Nb3Sn 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.