Degradation of YBCO coated conductors caused by over-current pulses

YBCO tapes are expected to be used in future high temperature superconductor (HTS) applications because they have excellent J_c characteristics at high temperatures and in high magnetic fields. For application to electric power devices such as transmission cables, transformers, and fault current limiters, the YBCO tapes might be subjected to a short-circuit fault current that is 10-30 times the normal operating current. In a worst-case scenario, YBCO tapes may degrade and burn. Therefore, it is important to clarify the mechanism of the degradation caused by an over-current pulse. This paper describes the experimental results of the degradation of the YBCO tapes through a series of over-current pulse tests. We focussed on the degradation temperature and carried out the experiments with different bending strains. Measurements were performed as a function of the amplitude of an over-current pulse for an operating temperature of 80 K (Gifford-McMahon cryocooler was adopted) in a self-field. We also examined a tape after the experiments with magneto-optical (MO) imaging, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron diffraction patterns.