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

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The deformation behavior of the surrounded Cu stabilized YBCO coated conductor basedon the Hastelloy substrate and its influence on the critical current were preciselyinvestigated. The mechanical properties were assessed at room temperature and 77 K. Thegreatest contribution was brought by two metallic components of the Hastelloy substrateand Cu stabilized layers. The internal strain exerted on the superconductingYBCO layer was determined directly by using synchrotron radiation facilities. Thethermally induced residual strain with compressive component decreased duringthe 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. BeyondAff, the increasing rate of internal strain slowed down, suggesting brittle behavior, that is, theformation of micro-cracks. The applied strain dependence of the critical currentcould be divided into two regions. In the reversible region, the strain dependenceobeyed the intrinsic strain effect and was well expressed by the Ekin formula.Beyond the reversible limit, the critical current decreased rapidly with strain. Thedegradation 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%. Onthe other hand, the strain at the critical current maximum was observed to be0.035–0.012%. These facts suggest re-examining the hypothesis supposing that the criticalcurrent maximum appears at the force free strain in YBCO coated conductors.