NATURE OF GRAIN-BOUNDARIES AS RELATED TO CRITICAL CURRENTS IN SUPERCONDUCTING YBA2CU3O7-X

A representative assortment of grain boundaries and planar defects in YBa2Cu3O7-x (YBCO) from the literature together with boundaries in thin films from many-to-one epitaxy have been reviewed to relate the critical current density (J(c)) across a boundary, its energetics, its geometric description and its nanoscopic structure. As there is much evidence showing that not all high angle boundaries have detrimental effects on J(c) transport and exhibit weak-link behavior, the relevance of different types of boundaries as related to their nanoscopic structure and finally to the transboundary critical currents are emphasized. Not all high angle boundaries are the same. The various high-angle-but-low-energy boundaries can sometimes support high critical currents, for example: Sigma 3/90 degrees boundaries, Sigma 5, Sigma 17, off-Sigma 13, 45 degrees [001] tilt boundaries found in YBCO films grown under many-to-one epitaxy. The geometrical parameters can be useful to predict to some extent the energetics of the boundaries and therefore, together with a thermodynamic criterion form a useful concept to explain why some boundaries are clean and some are preferred sites for precipitation of extraneous phase(s). Translational boundaries, stacking faults, and twin boundaries are of low energies, atomically clean and have been shown to support high J(c). A nanoscopically clean boundary is a necessary but not a sufficient condition for a high transboundary J(c). In addition to the geometrical parameters, the processing conditions and other extrinsic factors can also affect the transboundary J(c). Examples are the 45 degrees-boundaries from bi-epitaxy and the 90 degrees-boundaries at substrate etched steps, both utilized as weak-link junctions.