Stress Tolerance and Fracture Mechanism of Solder Joint of YBCO Coated Conductors

YBCO coated conductors have been expected to be applied to superconducting magnetic energy storage (SMES) due to high critical current density under high magnetic field and possibility of reducing cooling cost. Solder joints are essential to fabricate a high T_c superconducting coil for SMES system which requires long length of coated conductors. Not only low joint resistance but sufficient mechanical strength is needed, since conductors are exposed to large electromagnetic force generated by large transport current and high magnetic field. In the present study, influence of tensile load on transport property through the joint was investigated. The solder joint with sufficiently low resistance of 5.3 nOmega was attained before loading. Such joint can carry the load up to 650 N without substantial degradation. For further applied load, degradation attributed to fracture at the edge of the conductor is firstly observed. Overall fracture is caused by delamination at the interface between YBCO and CeO₂ . As a result, importance of interfacial strength between the superconducting and buffer layer is revealed to realize both low joint resistance and mechanical strength for solder joint.