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 textur ed 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 a t 20 T. A systematic study of this ceramic has been carried out using SEM and HREM observations in correlation with J(c) measurements. A tex tured microstructure is observed, characterized by rather regular stri ation corresponding to platelet-like 123 grains. Large 211 (green phas e) inclusions of almost-equal-to 10 mum diameter are also observed, as well as smaller inclusions which correspond to BaSnO3 (< 1 mum) at th e grain boundaries of the platelets or wrapped in the matrix. It has b een found that transport J(c) data are distributed over a very wide ra nge (2.5 x 10(3) 7.1 x 10(4) A cm-2). Nevertheless, magnetic J(c) meas urements suggest that cracks of the order of micrometres may appear in some regions of the material leading to a dramatic decrease in J(c) f or the corresponding sample. On the other hand, HREM observations demo nstrate that extended defects such as intergrowth or disordering, as w ell as twin boundaries, cannot be considered to be the major factors f or vortex pinning in the textured 123 material. The presence or absenc e of inhomogeneity on the nanoscale does not seem to influence the cri tical current density. Finally, it has been determined that pinning is more probably due to microstructure.