Microstructural investigation of current barriers in high temperature superconducting tapes and coated conductors

Microstructural barriers to supercurrent occur on many length scales in all high temperature oxide superconductors. Eliminating microstructural barriers is key to making these potentially valuable materials more favorable for commercial applications. In silver-sheathed Bi₂Sr₂CaCu ₂O_x (Bi-2212) tapes and multifilaments, the principal barriers on the scale of 10–100's of micrometers are bubbling, porosity, second phase particles, and poorly aligned grains. In state-of-the-art YBa₂ Cu₃O_x (YBCO) coated conductors, supercurrent barriers on the 0.1–100μm scale are grain boundaries. This thesis work clarifies the role of grain boundaries in the nickel substrate of RABiTS (Rolling Assisted Biaxially Textured Substrate) coated conductors. Plan-view SEM imaging, focused ion beam cutting, magneto-optical imaging and grain orientation mapping were used to determine barriers to supercurrent.; Experiments showed enhanced magnetic flux penetration, and hence reduced J_c, in the YBCO above nearly all nickel grain boundaries with misorientation angles (&thetas;) greater than 5°, independent of the rotation axis. Monochromatic backscattered electron Kikuchi pattern percolation maps imply there is a fully connected current path through the YBCO microstructure within the chosen tolerance angle criterion of the map. However, it is the grain boundary map that displays the constrictions of the current path. Therefore, grain boundary maps are better tools for illustrating supercurrent barriers than percolation maps.; Grain boundary maps and grain orientation maps were used to investigate how the texture of the substrate was transferred to the buffer layers and to the superconductor. Most grasp boundaries in the nickel were replicated in the buffer and superconductor layers with the same misorientation angle. Anisotropic growth and/or surface energy minimization may be responsible for the improvement in c-axis alignment in the YBCO over the buffer layer. However, the YBCO mosaic spread did not eliminate high angle grain boundaries, since >5° boundaries were still seen in YBCO grain boundary maps.; The results of this study on microstructural current barriers show that J_c improvements in RABiTS-type coated conductors require eliminating &thetas; > 5° boundaries in the nickel substrate.