Comparison of heat assisted lap joints of high-temperature superconducting tapes with inserted indium foils

Easy and simple fabrication of a lap joint of high-temperature superconducting (HTS) tapes is desirable for various HTS applications such as power cables and current feeders. We have developed mechanical joints with a low-temperature heat treatment at around 370 K where indium foils are inserted between joint surfaces. Though the joint showed a good performance, it took over 10 min to fabricate one joint and ideally the fabrication time should be further shortened. In this study, we proposed to use ultrasonic welding (UW) to assist a heat generation at the joint for lap joints with indium foil, which can be completed within 1 s. We prepared various lap joint samples of HTS tapes: REBCO (REBa ₂ Cu ₃ O ₇ , RE: rare-earth) or BSCCO (Bi ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ ) tapes were pressed together with an indium foil using a UW machine or pressing machine with cartridge heaters. Then, the current-voltage curve was evaluated at 77 K and the joint resistance characteristics were discussed depending on the joint methods and HTS tapes. The samples with copper-stabilized REBCO tapes all showed joint resistivities of 31.6-35.6 nΩcm ² without critical current (I _C ) degradation for both joint methods. The samples with silver-sheathed BSCCO tapes showed joint resistivities of 18.6-46.5 nΩcm ² for both joint methods; however, some of those fabricated by the UW showed I _C degradation. The degradation was able to be prevented by optimizing UW conditions.