Abstract
Measurements of the transport critical-current density (J c ), magnetization J c , and magnetoresistance in a number of bulk sintered samples of Y 1 Ba 2 Cu 3 O x from several different laboratories indicate that the transport J c is limited by weak-link regions between high J c regions. The weak-link J c has a Josephson character, decreasing by two orders of magnitude as the magnetic field is increased from 0.1 to 10 mT at 77 K. An examination of the grain-boundary region in Y 1 Ba 2 Cu 3 O x shows no observable impurities or second phases to the scale of the [001] lattice planes (∼12 Å). The effect of intrinsic conduction anisotropy is discussed. A current-transfer model is proposed in which weak conduction along the c axis plays a role in limiting J c at grain boundaries. Orienting the grains in the powder state during processing may result in enhanced transport J c in bulk conductors.