Electromechanical Properties of Lapped Joints of Y0.5Gd0.5Ba2Cu3O7-z Coated Conductors Joined with Lead-free Solders

Due to the limited available piece length of Y0.5Gd0.5Ba2Cu3O7-z (YGdBCO) coated conductors (CCs), joints are inevitable for manufacturing high temperature superconducting (HTS) devices. The stable operation of HTS devices is largely determined by the quality of joints, because the electromechanical performance of joints is usually lower than that of original tapes. In this work, lead-free Sn42Bi58 solder was applied to make the lap joints for YGdBCO CCs. Compared with conventional Sn60Pb40 solder, lead-free Sn42Bi58 solder is environmentally friendly and soldering operation can be carried out at lower temperatures below 150 degrees C because the melting point is about 40 degrees C lower, thereby further reducing the CCs deterioration of property in joining process. The influence of loading pressure, pressurization speed and lapped length on the critical current, resistance and n values of the YGdBCO joints were investigated by measurement of voltage-current curve under self-field at liquid nitrogen temperature. Results show that by optimizing soldering technique, a 25 cm-long joint with quite low resistance of 4.35 similar to 5.58 n Omega and comparable critical current to virgin CCs can be repeatedly achieved under a pressure of 12.5 MPa and a pressurization speed of 50 N/s. The mechanical behavior of the joints under axial tension was studied and the critical axial tension force of single CCs and joint portion is 213 and 212 N, respectively. The above results show that compared with traditional soldering techniques, the robustness and reproducibility of the soldering joints with low resistance and high tension performance can be significantly improved by the joining technique based on this lead-free Sn42Bi58 solder, which offers another promising choice for joints manufacturing in the large-scale applications of YGdBCO CCs.