Low-Resistance and Strong-Adhesion Soldering of Second-Generation High-Temperature Superconductor Tapes Within a Short Time

An effective soldering process of second-generation coated conductors (CCs) with low joint resistance was presented by using home-made equipment in the present study. The benefit of preprocessing, i.e., polishing, was proved with respect to the reduced joint resistances based on the voltage-current curves. It is revealed that the joint resistance of 10
^-8
Ω with an overlapping length as short as 2 cm can be achieved, while the joint resistances would further decrease with increasing the overlapping area, in agreement with Ohm's law. In the case of the overlapping length of 8 cm, the order of nΩ can be achieved for the resistances of solder joints. The heating time in joint fabrication process can be shortened to 1 min without negative effects on joint resistance, speeding up the soldering process largely. The effectiveness was evidenced further by the hard degradation in Ic through the joint part of the studied tapes and the enhanced adhesion between solder layer and Cu layer according to the cross-section observation. Temperature dependence of the joint specimen resistances suggested that the lattice constant of superconducting layer after jointing may be preferable to attain a higher critical temperature than that of original tape. The fragmentation of the superconducting layer and decohesion of the interface between hastelloy/buffer and ceramic layers, not the Cu-solder interface for the joint by external force, implied that the valid soldering was achieved using the present process.