Theory of the critical current density in YBa2Cu3O7 ceramics

One of the stumbling blocks in the practical application of the high (Tc) ceramic materials is their low critical current densities as compared to typical values for single crystals and epitaxial thin films. It is generally believed that the predominate factor limiting the critical current is the weak link behavior of the superconducting current at grain boundaries. Grain alignment has been used to obtain favorable orientations of the grains and/or obtain favorable grain boundaries. The case is studied where the c-axes of the grains are aligned, in a common direction. This type of alignment is obtained by a variety of techniques, such as the application of a uniaxial pressure during processing or magnetic alignment of powder particles. In these samples the relative orientation of the two grains at grain boundaries involves a rotation about the c-axis. Conduction across these boundaries would be within the ab-plane, which has a larger coherence length than that in c-axis.