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

Abstract

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 nΩ·cm2. The second group of joints underwent a series of thermal cyclings, upon which the initial resistivity range of 35‒49 nΩ·cm2 broadened to 25‒128 nΩ·cm2. 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.

Sintering Nano-Silver Paste by Resistive Joule Heating Process for 2G HTS Tape Joints

“…Figure 4a shows the I c retention percentage versus specific resistance of the different furnace heating and resistive Joule heating samples. Most studies had suggested that a good joint required low specific resistance (<1 µΩ•cm 2 ) [27][28][29] and high I c retention percentage (>80%) [27], shown in Figure 4a with four clearly distinguished blocks. The first block indicates property of high specific resistance (>1 µΩ•cm 2 ) due to insufficient sintering of the nano-silver paste, and shows the obvious feature that some un-sintered nano-silver particle remains dark gray instead of white after the process, as shown in Figure 4b, which was observed in the test heated at 180 • C (furnace heating).…”