Relation between internal boundaries and critical current in textured YBa 2 Cu 3 O 7−δ : Transmission electron microscope observations

The local critical current (Ic) at 77 K measured at the mm scale in a 60 mm long sample of YBa2Cu3O7−δ prepared by a melting zone process is correlated to the microstructure. Lower values of Ic (<20 A) were obtained in a part of the sample which optical microscope examination showed to be generally polycrystalline. In contrast, the rest of the sample, consisting mostly of large textured domains, gave values for Ic of 120 A and above. Transmission electron microscope observations revealed that the textured domains contain internal boundaries. Depending on the scale of observation, the misorientation angles across the boundaries could vary from a few tenths of a degree up to a few degrees. This seems characteristic for boundaries in textured material which allow strong coupling of the superconducting current across themselves.