Current distributions of grain boundaries in differently processed melt-textured YBa2Cu3O7

The local current density distribution of grain boundaries in differently p rocessed melt-textured YBa2Cu3O7- delta (YBCO) is investigated by a magneto -optical technique and inversion of Biot-Savart's law. By correlating the l ocal current density with the microstructure revealed by a scanning electro n microscope, we are able to distinguish the depression of the critical, cu rrent density j(c) at clean grain boundaries (GBs) from that occurring at n ormal phases and cracks at and in the vicinity of GBs. The GBs are formed b y different multi-seeding melt-growth processes using a non-stoichiometric precursor which is optimized with respect to the bulk critical current. In order to obtain GBs with extended regions free of normal phases and with la rge critical current densities the amount of liquid phase and the density o f Y2BaCuO5 particles has to be controlled. The properties of GBs can be tai lored by both controlling the amount of liquid phase and the seed distance. The related changes in the growth and microstructure are analysed and we s how how to grow extended GBs almost free of normal phases with optimized cr itical current densities in multiple-seeded melt-growth YBCO.