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

Current barriers in high temperature superconductors occur on many length s cales. Eliminating them is critical to making superconductors more favorabl e for commercial applications. In state-of-the-art YBa2Cu3O (YBCO) coated c onductors, barriers to current in short lengths of tapes are grain boundari es. On the scale of several tens of micrometers, the role of grain boundari es in the nickel substrate was investigated using a combination of plan-vie w scanning electron microscopy (SEM), Backscattered Electron Kikuchi Patter n (BEKP) analysis, magneto-optical (MO) imaging, and Focused Ion Beam (FIB) analysis, The combined techniques showed enhanced magnetic flux penetratio n, and hence reduced critical current density (J(c)), in the YBCO above nic kel grain boundaries that have misorientation angles (theta) greater than 4 degrees in pulsed laser deposited YBCO films and 5 degrees in BaF2-grown Y BCO films. Additionally, BEKP percolation maps show that the YBCO layer is single-crystal-like below a percolation limit of a few degrees; the BEKP gr ain boundary maps displays the constrictions of the current path on the sca le of 50-100 mum, On the sub-micrometer scale, the misorientation angles of grain boundaries in the YBCO layer were investigated using transmission el ectron microscopy imaging and diffraction analysis. The average and range o f theta between neighboring YBCO islands above a single grain of the metal substrate were essentially the same in two samples with J(c) values that di ffered by a factor of four. This suggests that defects on the macroscale, s pecifically 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 e liminate theta > 4-5 degrees boundaries in the nickel substrate or engineer the conductors in a way to increase the threshold angle.