In-depth resistance analysis of REBCO tape joints with indium insert and solders

Abstract

Joints between REBCO (rare-earth barium copper oxide) tapes with low joint resistance are crucial for many superconducting applications. Joining REBCO tapes with indium insert (In-joint) is a promising joining method to fabricate low resistive joints at low temperatures (20-120°C). This study investigated the joining conditions of In-joints such as pickling, surface roughness, joining time, and temperature. The joint resistivity (product of joint resistance and joint area) was successfully reduced to 22-30 nΩcm2 at 77 K in self-field. The constitutive factors of the joint resistivity were analysed separately along with the crosssectional observations. In this study, the interface resistivity of the REBCO tape was measured as 8.5 nΩcm2 for one REBCO tape by the previously proposed method. The resistivity of the joining interface Cu/In was calculated as <3 nΩcm2 by subtracting the other resistivities from the entire joint resistivity. This result reveals the lower limit of the joint resistivity: The sum of the resistivity (nΩcm2) of indium (measurable by thickness), the resistivity of Cu/In (<3nΩcm2), and the interface resistivity of the REBCO tape (measurable beforehand). Furthermore, we demonstrated a lower and less varied joint resistivity of In-joints than those of the soldered joints.