Optimization of the Direct Diffusion Bonding Process Using Cu Stabilizing Layers of the REBCO Coated Conductors

Abstract

Low resistivity joint technique for high-temperature superconducting tapes is one of the most indispensable techniques for practical applications. In the present study, we propose solid phase joining technique to make direct joint of REBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> (REBCO, RE:rare earth elements) tapes, as an alternative to conventional solder joining technique. In this solid phase joining technique, we directly join the copper stabilizing layer and get rid of the soldering material, so that we can expect reduction of joint resistivity. The important points for direct joining technique are to remove the oxide layer on the metal surface and to make the joint surface adhere to each other. It means that, after removing the oxide film on the surface, it is necessary to have an optimum pressure so that sufficient adhesion can be obtained while maintaining a clean surface. First, we examined the type and concentration of the acid used to remove the oxide film. Next, the joining pressure, joining atmosphere, and joining time were optimized, while the joining temperature was fixed at 200°C. Finally, we have succeeded in joining REBCO tape with this technique. The measured resistivity at liquid nitrogen temperature was about 50-80 nΩcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .