Internal residual strain and critical current maximum of a surrounded Cu stabilized YBCO coated conductor

The deformation behavior of the surrounded Cu stabilized YBCO coated conductor based on the Hastelloy substrate and its influence on the critical current were precisely investigated. The mechanical properties were assessed at room temperature and 77 K. The greatest contribution was brought by two metallic components of the Hastelloy substrate and Cu stabilized layers. The internal strain exerted on the superconducting YBCO layer was determined directly by using synchrotron radiation facilities. The thermally induced residual strain with compressive component decreased during the tensile loading and changed to a tensile component at the force free strain (Aff), at which the internal stress becomes zero in the YBCO layer. Beyond Aff, the increasing rate of internal strain slowed down, suggesting brittle behavior, that is, the formation of micro-cracks. The applied strain dependence of the critical current could be divided into two regions. In the reversible region, the strain dependence obeyed the intrinsic strain effect and was well expressed by the Ekin formula. Beyond the reversible limit, the critical current decreased rapidly with strain. The degradation is suggested to be attributed to the formation of cracks in the YBCO layer. The force free strain evaluated from the mechanical properties was 0.26%. On the other hand, the strain at the critical current maximum was observed to be 0.035–0.012%. These facts suggest re-examining the hypothesis supposing that the critical current maximum appears at the force free strain in YBCO coated conductors.