Press-Welding Between High-Temperature Superconducting Tape Conductors With Low-Temperature Solders Including Indium

The onsite technology of simple and quickly-fabricated low-resistive joints is required to install the high-temperature superconducting (HTS) DC feeder cable for railway systems. We have proposed applying press-welding with indium to the cable joints, which is solid-phase bonding at room temperature (RT) or with low-temperature heat treatment below the melting point of indium. The feeder cables with joints must be laid at RT, i.e., not only joint performance at 77 K, but also mechanical behavior at RT is crucial. In this study, we fabricated tape-to-tape joint samples using copper-alloy-laminated Bi-2223 tapes and copper-stabilized Rare-earth Barium Copper Oxide (REBCO) coated conductors with indium (In), tin (Sn), In52Sn48, and layered structures of In/Sn/In without any heating. The joint with In showed the minimum joint resistivity at 77 K among the joints with the selected inserted metals and less influenced by the joint pressures over 10 MPa. For the other inserted metals, 100 MPa joint pressure was needed to obtain low joint resistance, but they were still higher than that in the case of In. Additionally, the joint fabrication partially failed even with 100 MPa joint pressure in the case of Sn. The creep testing at RT for the 20-mm-long joint showed joints with In were broken at ∼100 s after applying a tensile force of 200 N whereas the joints with Sn and In-plated Sn kept the joint structure over ∼29 hours and until ∼13 hours, respectively. The joints with indium started to deform at ∼3 hours, but kept the joint structure until ∼39 hours when the tensile force decreased to 100 N or joint length increased to 40 mm. As a conclusion, tensile force at RT must be kept low enough to avoid creep deformation for joints with In or further development is necessary for the other inserted materials.