Loophole-free probing of thermal degradation mechanism in high temperature superconducting tapes

Heating of the REBa2Cu3O7-δ (REBCO) layer is a proximate trigger for degradation in high temperature superconducting tapes. Yet, a loophole-free characterization for probing the degradation mechanism remains unavailable. Here, we introduce in-plane grazing incidence X-ray diffractometry to directly observe the orthorhombic-to-tetragonal phase transition behaviors in the superconducting layer of heat-treated tapes. Based on the obtained diffraction patterns, oxygen out-diffusion is confirmed to be the origin of thermal degradation in REBCO-based tapes. During thermal degradation, the degradation rate of the tapes varies linearly with the lattice parameter difference between the a-axis and b-axis, whereas the proportions of in-plane a- and b-axis components in the biaxially textured REBCO layer remain unchanged. These pattern-derived values can serve as inherent indicators when commercial tapes undergo thermal quench events. In addition, the precisely determined electronic and chemical structures of heat-treated REBCO offer indirect evidence for the phase transition. These critical findings provide valuable insights into the thermal degradation mechanism of superconducting tapes, paving the way for the reliable fabrication and stable operation of superconducting devices based on REBCO coated conductors.