TEM studies of interfacial microstructures in YBa₂Cu ₃O₇ coated conductors

A conductor based on the YBa₂Cu₃O_y (Y-123) superconductor requires a high degree of biaxial texture in the superconducting layer. To produce useful lengths of these so-called coated conductors, processes were developed that create near single-crystal conditions on or in polycrystalline metal substrates on the kilometer length scale. A coated conductor is typically comprised of a textured template, intermediate buffer layers, a Y-123 superconducting layer, and a noble metal cap layer. The biaxial texture that is present in the template must be transferred into the buffer layers and Y-123 films during epitaxial growth of the oxide films. Each process used to build the coated conductor creates an interface (parallel to the substrate) along which defects and/or interfacial reactions may result. These defects can be additive and propagate through the entire film structure to affect the growth and properties of the superconducting film. Furthermore, the short coherence length and strong dependence of Y-123 on grain-to-grain misorientations dictate that the grain boundaries (perpendicular to film) in this layer must be clean and of low misorientation. In this work, TEM investigations of the various microstructures that are found along these interfaces are discussed. The role of the substrates, buffer layers, and growth processes of the oxide layers are discussed in terms of the resulting microstructures that occur in the coated conductor.