Microstructure of RABiTS-type high-Tc superconductor coated conductors

The objective of this dissertation was to characterize quantitatively the microstructure of the high Tc superconductor and buffer layer materials in prototype superconducting coated conductor tapes with the goal of understanding the microstructure-property relationships that determine their suitability for large scale applications of high Tc superconductivity. The coated conductor materials were fabricated by the rolling-assisted biaxially-textured substrates (RABiTS) approach at Oak Ridge National Laboratory. YBa2Cu 3O7-delta (YBCO), a high Tc superconductor, was deposited on various combinations of oxide buffer layers on RABiT metal tape substrates. Scanning and transmission electron microscopy, x-ray diffraction, atomic force microscopy, and Auger electron spectroscopy were used in combination to study the microstructure in the superconductor and buffer layers. The electromagnetic properties of the samples were determined in collaborations with researchers in the UW Applied Superconductivity Center and Oak Ridge National Laboratory.; Buffer layers deposited by both physical vapor deposition and chemical solution deposition methods showed a wide range of microstructures and surface topographies. Electron-beam evaporated yttia stabilized-zirconia (YSZ) buffer layers were composed of loosely packed, slab-shaped columnar grains with rectangular cross sections and average dimensions of 10nm by 50nm by the film thickness. Magnetron sputtered YSZ had a smooth and featureless surface and a 50nm diameter cell structure defined by the threading dislocations. Solution deposited buffer layers were locally dense, but showed larger scale surface roughness of ∼50 nm in some cases. The YBCO films grown epitaxially on these buffer layers had similar microstructures and properties, however. With one exception, the YBCO layers possessed island structures. The average misorientation angles of the boundaries that defined the islands were largely between 2 and 4° in the high critical current density. The island size increased with increasing film thickness and as the YBCO/lattice parameter mismatch decreased. The grain size distributions were log-normal. These quantitative descriptions of the low-angle grain boundary network undoubtedly will contribute to the resolution of on going debate on their role in determining the critical current density of coated conductor-type high Tc superconducting tapes.