RELATION BETWEEN INTERNAL BOUNDARIES AND CRITICAL-CURRENT IN TEXTUREDYBA2CU3O7-DELTA - TRANSMISSION ELECTRON-MICROSCOPE OBSERVATIONS

The local critical current (I-c) at 77 K measured at the mm scale in a 60 mm long sample of YBa2Cu3O7-delta prepared by a melting zone proce ss is correlated to the microstructure. Lower values of I-c (<20 A) we re obtained in a part of the sample which optical microscope examinati on showed to be generally polycrystalline. In contrast, the rest of th e sample, consisting mostly of large textured domains, gave values for I-c of 120 A and above. Transmission electron microscope observations revealed that the textured domains contain internal boundaries. Depen ding on the scale of observation, the misorientation angles across the boundaries could vary from a few tenths of a degree up to a few degre es. This seems characteristic for boundaries in textured material whic h allow strong coupling of the superconducting current across themselv es.