Current barriers in Y-Ba-Cu-O coated conductors

Current barriers in high temperature superconductors occur on many length scales. Eliminating them is critical to making superconductors more favorable for commercial applications. In state-of-the-art YBa₂Cu₃O (YBCO) coated conductors, barriers to current in short lengths of tapes are grain boundaries. On the scale of several tens of micrometers, the role of grain boundaries in the nickel substrate was investigated using a combination of plan-view scanning electron microscopy (SEM), backscattered electron Kikuchi pattern (BEKP) analysis, magneto-optical (MO) imaging, and focused ion beam (FIB) analysis. The combined techniques showed enhanced magnetic flux penetration, and hence reduced critical current density (J_c), in the YBCO above nickel grain boundaries that have misorientation angles (θ) greater than 4° in pulsed laser deposited YBCO films and 5° in BaF₂-grown YBCO films. Additionally, BEKP percolation maps show that the YBCO layer is single-crystal-like below a percolation limit of a few degrees; the BEKP grain boundary maps displays the constrictions of the current path on the scale of 50-100 μm. On the sub-micrometer scale, the misorientation angles of grain boundaries in the YBCO layer were investigated using transmission electron microscopy imaging and diffraction analysis. The average and range of θ between neighboring YBCO islands above a single grain of the metal substrate were essentially the same in two samples with J _c values that differed by a factor of four. This suggests that defects on the macroscale, specifically the underlying nickel grain boundaries, may influence the J_c more than defects on the microscale. The results of this study suggest that that the way to improve the superconducting properties in the YBCO is to eliminate θ>4-5° boundaries in the nickel substrate or engineer the conductors in a way to increase the threshold angle