Rules of non-superconducting phase particles on crack propagation in YBCO coated conductors fabricated by the IBAD-MOCVD

Abstract

Cracks caused by tensile stress in YBa2Cu3O7-x-coated conductors (YBCO-CCs) can cause irreversible degradation to their superconducting properties. Understanding the initiation and propagation modes of cracks can assist in preventing conductor failure and enhancing their mechanical properties in further. In this study, we used a chemical etching method and scanning electron microscopy to investigate crack morphology in the YBCO layer of conductors where the protective metal layer had been removed. For YBCO-CCs that experienced no deformation, many non-superconducting phase particles were observed and their grain size distribution corresponded to a Gaussian distribution. Energy dispersive x-ray spectroscopy identified these as Y-Cu-O particles. For the YBCO-CCs that experienced axial tension at 77 K, different propagation modes of cracks in the YBCO layer, including transgranular fracture, branching, deflection and pinning were observed for the first time. Statistical analysis demonstrated that transgranular fracture occurred in ∼ 95% of the crack modes. We analysed the reason for this phenomenon considering the thermal stresses stored inside and around the non-superconducting phase particles. The coefficient of thermal expansion of the Y2Cu2O5 particles is less than that of the YBCO superconducting matrix, and therefore, the hoop tensile stress generated near the boundary of the Y2Cu2O5 particle accelerates the bottom-up propagation of the crack. The other crack propagation modes such as crack branching, deflection, pinning and bridging in the YBCO layer can be considered mechanisms of blocking crack propagation that can increase the fracture toughness of the YBCO layer.