Microstructure and transport behavior of grain boundaries in YBa2Cu3Oy: A comparison between thin films and bulk bicrystals

The microstructure and transport properties of grain boundaries in thin film and bulk bicrystals of YBa2Cu3O7-δ have been studied in detail. In this paper, we will concentrate on the unique features of grain boundaries in dual-seeded bulk bicrystals, particularly in comparison with grain boundaries in thin film bicrystals. The structures of these two types of grain boundaries can be very different: thin film boundaries typically exhibit meandering and impurity phases that extend through the thickness of the film while the bulk boundaries exhibit long, straight facets that can be relatively free of impurity phases. While the meandering configurations of grain boundaries in thin films has led to inconsistencies in transport results, the planar grain boundaries found in bulk samples provide an excellent opportunity to study the grain boundary transport behavior in a simpler geometry. Furthermore, the fabrication of these bulk bicrystals is flexible and reproducible so that boundaries of virtually any misorientation may be prepared. As an example of this versatility, we have systematically measured the transport critical current density as a function of misorientation angle in bulk bicrystals and compare these results with those measured in thin film bicrystals.