Low-resistance joints for YBCO-coated conductors with Ag nanoparticle paste

Abstract

Abstract Due to the limited available piece length of YBCO-coated conductors (i.e. tapes or wires) and the different requirements for magnetic field, joints are inevitable for manufacturing high-temperature superconducting magnets. In this study, a sintering nano-silver (Ag) process was developed and used to connect YBCO tapes stabilized by anAg layer with low-temperature and short-time sintering of Ag nanoparticle (NP) paste. The thermodynamic characteristics of Ag NP paste were explored using a TG/DSC setup. The effects of sintering temperature, mechanical pressure and lapped length on microstructures and electrical properties of joints were comprehensively investigated. It is found that the pre-volatilization of low-boiling-point solvent in the paste is beneficial to improve the densification of sintered structure, thus contributing to increasing the critical current I c of the joint. With increasing sintering temperature, the I c of the joint will be close to that of the virgin tape, and the joint resistance experiences small fluctuations, but joint connectivity is enhanced. As the temperature reaches 205 °C, I c decreases to 84% of the virgin tape, and joint resistance increases obviously. In addition, the axial tension strength at room temperature is improved with the increase in mechanical pressure, while the resistance does not demonstrate distinct variation. Considering the electromechanical properties, the optimal joining process is determined as sintering at 180 °C and 30 MPa for 10 min. The joint with this technology possesses a closely connected interface and a well-sintered nano-Ag microstructure with pores. By further extending the lapped length, a YBCO joint resistivity as low as ∼10.56 nΩ cm2 is obtained, which is around a quarter of that of the soldering joint, and the process is much easier than that of the Ag diffusion joint.