CRITICAL CURRENTS UP TO 71000A CM(-2) AT 77-K IN MELT-TEXTURED YBCO DOPED WITH BASNO3

We have performed critical current density measurements on melt textured YBa2Cu3O7 doped with BaSnO3, which exhibits transport values at 77 K as high as 7.1 x 10(4) A cm-2 in zero field and 1.1 x 10(4) A cm-2 at 20 T. A systematic study of this ceramic has been carried out using SEM and HREM observations in correlation with J(c) measurements. A textured microstructure is observed, characterized by rather regular striation corresponding to platelet-like 123 grains. Large 211 (green phase) inclusions of almost-equal-to 10 mum diameter are also observed, as well as smaller inclusions which correspond to BaSnO3 (< 1 mum) at the grain boundaries of the platelets or wrapped in the matrix. It has been found that transport J(c) data are distributed over a very wide range (2.5 x 10(3) 7.1 x 10(4) A cm-2). Nevertheless, magnetic J(c) measurements suggest that cracks of the order of micrometres may appear in some regions of the material leading to a dramatic decrease in J(c) for the corresponding sample. On the other hand, HREM observations demonstrate that extended defects such as intergrowth or disordering, as well as twin boundaries, cannot be considered to be the major factors for vortex pinning in the textured 123 material. The presence or absence of inhomogeneity on the nanoscale does not seem to influence the critical current density. Finally, it has been determined that pinning is more probably due to microstructure.