Critical current degradation of commercial REBCO coated conductors under thermomechanical loads

The process to obtain a superconducting joint between Coated Conductors (CCs) involves the simultaneous application of temperature and transverse compressive pressure to promote the joining of the two adjacent RE BCO layers. We performed experiments to simulate this procedure by subjecting samples from commercial CCs to different combinations of pressure, and temperature. The objective was to understand the effects of the thermomechanical cycles the critical current (I-c) and, therefore, determine the upper limit for the achievable current in superconducting joint between CCs. We observed a reduction of the I c across the investigated parameter range that is accelerated at higher temperatures and pressures. For instance, the average reduction of I c measured at 77 K in self-field varied from 45% to 90% when increasing the pressure for samples heated at 820 degrees C, as compared to samples heated at the same temperature without applied pressure. As a complement to electrical transport measurements, we carried out STEM and EDX investigations to study the relation between the degradation of I (c) and alterations in the microstructure of RE BCO. These analyses indicate that the concurrent application of temperature and pressure accelerates the decomposition of the RE BCO phase. The EDX data suggests that the decomposition products could correspond with a peritectic reaction of the RE BCO occurring at lower temperatures, accelerated by the presence of an external pressure. This early decomposition occurs in localized areas of the RE BCO layer, constricting the available cross-section for current flow and thereby diminishing the critical current.