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

Abstract

Due to the limited available piece length of YBCO coated conductors (i.e., tapes or wires) and the different requirements for magnetic field, joint is inevitable for manufacturing high temperature superconducting magnets. In this work, a sintering nano silver process was developed and used to connect YBCO tapes stabilized by silver layer by low temperature and short time sintering of Ag nanoparticle paste. The thermodynamic characteristics of Ag nanoparticle paste were explored by a TG/DSC setup. The effect of sintering temperature, mechanical pressure and lapped length on microstructures and electrical properties of joint 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 contribute to increasing the critical current Ic of joint. With increasing the sintering temperature, Ic of joint is close to that of the virgin tape and joint resistance experiences a small fluctuation, but joint connectivity is enhanced. As the temperature reaches 205℃, Ic decreases to 84% of the virgin tape and joint resistance increases obviously. In addition, axial tension strength at room temperature has been improved with the increase of mechanical pressure, while the resistance has not demonstrated distinct variation. Considering electromechanical properties, the optimal joining process is determined as sintering at 180℃ and 30 MPa for 10 min. The joint by this technology possesses closely connected interface and well sintered nano silver microstructure with pores. By further extending lapped length, YBCO joint resistivity as low as ~10.56 nΩ.cm2 has been obtained, which is around a quarter of that of soldering joint, and the process is much easier than that of silver diffusion joint.

Performance characteristics of REBCO coated conductor joints fabricated by flux-free hybrid welding

“…Recently, a low-temperature sintering of nano-Ag paste was proposed for joining CC tapes, which combines the benefits of soldering and Ag diffusion methods. By utilizing optimal conditions of sintering temperature of 180 • C and pressure of 30 MPa for 10 min, the minimum obtained R sj was about 10.56 nΩ cm 2 [33]. These methods exhibit distinct advantages over the lengthy and intricate process of superconducting and Ag diffusion joints.…”

Development of a multi-pancake HTS magnet operated in persistent current mode based on superconducting joint of REBCO coated conductors

“…However, the connection issue of CCs is still inevitable due to the limited length of a single CC and the possibility of local critical current (I c ) reduction. Various methods have been developed for joining CCs in magnet applications, such as solder joint [6,7], ultrasonic joint [8][9][10], diffusion joint [11][12][13][14], and superconducting joint [15][16][17][18][19][20][21]. It should be noted that joint resistance cannot be avoided except when superconducting joints are used.…”