Evidence for weak link and anisotropy limitations on the transport critical current in bulk polycrystalline Y\u3csub\u3e1\u3c/sub\u3eBa\u3csub\u3e2\u3c/sub\u3eCu\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e χ \u3c/sub\u3e

Measurements of the transport critical-current density (Jc), magnetization Jc, and magnetoresistance in a number of bulk sintered samples of Y1Ba2Cu3O χ from several different laboratories indicate that the transport Jc is limited by weak-link regions between high Jc regions. The weak-link Jc 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 Y1Ba2Cu3O χ 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 Jc at grain boundaries. Orienting the grains in the powder state during processing may result in enhanced transport Jc in bulk conductors

The possible occurrence of multi-phase behavior in the high T sub c superconductor YBa sub 2 Cu sub 3 O sub x

We report here the results obtained for powder diffraction studies on four YBa{sub 2}Cu{sub 3}O{sub x} samples in which a model with two orthorhombic phases was used to obtain an improved agreement. The implications of this structural model on the systematic variation of {Tc} with oxygen content will be discussed. 28 refs., 3 figs., 2 tabs