Thermal Cycling of (RE)BCO-Based Superconducting Tapes Joined by Lead-Free Solders

We prepared overlap soldered joints of high-temperature superconducting tapes, using various materials and preparation conditions. In order to select the joints with optimal performance, we correlated their electrical properties (derived from current-voltage curves) with the microstructure of the respective joint cross-section by scanning electron microscopy. With the first group of joints, we focused on the effect of used materials on joint resistivity and critical current, and we found that the dominant role was played by the quality of the internal interfaces of the superconducting tape. Initial joint resistivities ranged in the first group from 41 to 341 n omega center dot cm(2). The second group of joints underwent a series of thermal cyclings, upon which the initial resistivity range of 35-49 n omega center dot cm(2) broadened to 25-128 n omega center dot cm(2). After the total of 135 thermal cycles, three out of four joints showed no signs of significant degradation. Within the limit of 100 thermal cycles, reliable soldered joints can be thus prepared, with normalized resistivity not exceeding 1.4 and with normalized critical current above the value of 0.85.

Automated summary

Overlap soldered joints of high-temperature superconducting tapes were prepared using various materials and preparation conditions, with the quality of internal interfaces playing a dominant role in joint resistivity and critical current. Reliable joints can be prepared within the limit of 100 thermal cycles, with normalized resistivity not exceeding 1.4 and normalized critical current above 0.85.