Influence of copper volume fraction on tensile strain/stress tolerances of critical current in a copper-plated DyBCO-coated conductor

The influence of the volume fraction (Vf) of copper, plated at room temperature over a DyBa2Cu3O7-δ-coated conductor, on the tensile strain tolerance and stress tolerance of critical current at 77 K was studied over a wide range of copper Vf values. The copper plating exerts a tensile stress during cooling because copper has a higher coefficient of thermal expansion than the substrate conductor. Before application of tensile strain, the copper plated at room temperature yielded at 77 K when the copper Vf was lower than a critical value, and was in an elastic state at 77 K when the copper Vf was higher than the critical value. The strain tolerance of critical current increased with increasing copper Vf due to an increase in thermally induced compressive strain in the substrate tape. The stress tolerance of critical current decreased with increasing copper Vf because copper is softer than the substrate tape. These results, together with the trade-off between strain tolerance and stress tolerance (i.e., stress tolerance decreases with increasing strain tolerance), were analyzed by modeling. The results show that the restriction imposed by the trade-off, which limits the ability to simultaneously obtain a high strain tolerance and a high stress tolerance, can be relaxed by strengthening the copper. (author)