Growth features and intergranular connectivity of melt processed YBCO

Large grain melt processed Y-Ba-Cu-O (YBCO) samples have been prepared by seeded and unseeded growth techniques. The current carrying ability within individual grains and across grain boundaries has been investigated and correlated with features in the microstructure of samples fabricated by both techniques. The development of an inhomogeneous, cell-like growth microstructure in seeded samples at distances = 4 mm from the seed is related to a Saturation in inclusion density and volume proportion of Y2BaCuO5 (Y211) particles. A significant decrease in critical current density is associated with this change over a wide temperature range. The resistance of high angle c-axis grain boundaries is observed to depend critically on the magnitude of the injection current whereas the intragranular resistance is not influenced significantly by this variable. Jc of a grain boundary fabricated by unseeded melt growth is estimated to be less than 100 A cm(-2) at 77 K in zero applied field, which is more than two orders of magnitude lower than the intragranular Jc. Field screening measurements suggest that low angle grain boundaries do not form weak links between grains in modest magnetic fields and hence do not present a significant barrier to current flow. (C) 1998 Elsevier Science Ltd. All rights reserved.