Heating-Induced Performance Degradation of REBa2Cu3O7–x Coated Conductors: An Oxygen Out-Diffusion Scenario with Two Activation Energies

The degradation of the superconducting properties of REBa₂Cu₃O_7–x coated conductors when heated above 150 °C is a topic of concern for any REBa₂Cu₃O_7–x-based applications. This study makes it clear the role of thermally activated oxygen out-diffusion processes in the modification of the superconducting properties of commercial coated conductors heat treated in air. We point out that oxygen can diffuse out of the REBa₂Cu₃O_7–x layer along two main channels: one associated with the grain boundaries and a second one associated with the superconducting grains. The activation energy is lower for the oxygen out-diffusion events that take place at the grain boundaries, and this is the main cause of the degradation of the critical current at temperatures lower than ∼250 °C. More specifically, through a systematic investigation of the electrical and magnetic properties, we demonstrate that the decrease of the critical current upon heating the coated conductor is initially due to a loss in the transparency of the REBa₂Cu₃O_7–x grain boundaries, which determines a reduction of the effective superconductor cross section available for the superconducting current. The degradation of the intragrain properties becomes important only above ∼250 °C and manifests itself through a simultaneous variation of the critical current and of the critical temperature.