ELECTRICAL TRANSPORT ACROSS GRAIN-BOUNDARIES IN BICRYSTALS OF YBA2CU3O7-DELTA

The superconducting properties of about 20 bulk-scale flux-grown bicrystals of YBa2Cu3O7-delta have been measured in order to deduce the nature of the coupling across their grain boundaries. Bicrystals with two types of misorientation relationship were studied: (i) theta [001] (C-type) and (ii) 90-degrees [010] plus an additional rotation about the a(I) parallel-to c(II) axis (P-type). A general trend for a transition from flux pinning (FP) to Josephson Junction (JJ) to resistive (R) behavior was observed as the misorientation angle increased. However, FP character was observed well into the high-angle regime, in particular, for P-type bicrystals, and JJ behavior was observed at as low as 10-degrees [001]. The J(ct) values of two bicrystals were increased by a second oxygen anneal. In one case (28-degrees [001] boundary), extended oxygenation produced a change from JJ to mixed JJ/FP behavior, markedly raising the transport critical current density (J(ct)) at all fields. HRTEM has been performed on two of the same bicrystals which were electromagnetically characterized. A thin second phase layer unobservable by conventional TEM was observed in a resistive boundary, whereas no such phase was detected at a JJ boundary. We conclude that the character of the boundary cannot be predicted either from its misorientation angle or the magnitude of its J(c).