Evidence for anisotropy limitation on the transport critical- current in polycrystalline Y sub 1 Ba sub 2 Cu sub 3 O sub x.

Measurements of the transport critical-current density (J sub c), magnetization J sub c, and magnetoresistance in a number of bulk sintered samples of Y sub 1 Ba sub 2 Cu sub 3 O sub x from several different laboratories is dominated by "weak- link" regions between High-J sub c regions. The weak link J sub c has a tunneling character, decreasing by two orders of magnitude as magnetic field is increased from 0.1 mT to 10 mT, at 77 K. An examination of the grain boundary region in Y sub 1 Ba sub 2 Cu sub 3 O sub x shows no impurities, or non-stoichiometric regions on a scale of 5 angstroms. These microscopy data and recent reports of intrinsic anisotropy of J sub c in single crystals are consistent with a significant part of the weak-link phenomena in polycrystalline bulk Y sub 1 Ba sub 2 Cu sub 3 O sub x being caused by grains oriented with their weak conduction axis in the direction of current flow. Orienting the grains in the powder state during processing may result in high J sub c in bulk Y sub 1 Ba sub 2 Cu sub 3 O sub x.