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 thi n films from many-to-one epitaxy have been reviewed to relate the crit ical current density (J(c)) across a boundary, its energetics, its geo metric description and its nanoscopic structure. As there is much evid ence showing that not all high angle boundaries have detrimental effec ts 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. No t all high angle boundaries are the same. The various high-angle-but-l ow-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 m any-to-one epitaxy. The geometrical parameters can be useful to predic t to some extent the energetics of the boundaries and therefore, toget her with a thermodynamic criterion form a useful concept to explain wh y some boundaries are clean and some are preferred sites for precipita tion of extraneous phase(s). Translational boundaries, stacking faults , and twin boundaries are of low energies, atomically clean and have b een shown to support high J(c). A nanoscopically clean boundary is a n ecessary 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). Ex amples are the 45 degrees-boundaries from bi-epitaxy and the 90 degree s-boundaries at substrate etched steps, both utilized as weak-link jun ctions.