Enhancement mechanism of uniaxial tensile electro-mechanical behaviors in second-generation high-temperature superconducting tapes

The second-generation high-temperature superconducting (2 G-HTS) tapes are subjected to complex stress/strain in practical applications. For the complex operation conditions, lamination techniques are widely used to improve the robustness of 2 G-HTS tapes. However, lamination makes the 2 G-HTS tape structure more complicated, which can significantly affect their electro-mechanical behaviors. In this work, uniaxial monotonic tensile tests were performed to obtain the stress-strain relationship and the strain dependence of critical current (I_c(ε)) in liquid nitrogen conditions. Two types of laminated 2 G-HTS tapes (copper-laminated and stainless steel-laminated) were compared with copper-plated 2 G-HTS tapes. The results showed that the irreversible tensile strains of Cu-laminated and SS-laminated tapes were much higher than that of Cu-plated tapes, which increased by ∼35 % and ∼60 %, respectively. High consistency between the experimental and theoretical I_c values in 2 G-HTS tapes under tensile strain was verified by the I_c(ε) model. Based on the residual strain (ε_res) calculation of composite multilayer structures, we confirmed that it was mainly due to the increase of the compressive residual strain of superconducting layers. Further, tensile fatigue tests also confirmed the significant improvement of the tensile fatigue electro-mechanical behaviors in the laminated 2 G-HTS tapes. Finally, 2D X-ray and SEM cooperative analysis were used to characterize the tape microstructure before and after fatigue, and the correlation between micro-defects and non-destructive imaging features was established. It was found that the internal solder layer and its interface were seriously damaged to release the stress. This work provides a strategy to improve the electro-mechanical behaviors of 2 G-HTS tapes by adjusting the composite structure to change residual strain.