Percolative current flow in high-J_c, polycrystalline high-T_c superconductors

In this article, experiments designed to ascertain the percolative nature of current flow in high critical-current density (J_c) polycrystalline superconductors are reviewed. A direct correlation between the grain-orientation texture with current transport in high-J_c TlB₂Ca₂Cu₃O_x thick films and Bi₂Sr₂Ca₂Cu₃O_x powder-in-tube tapes is obtained. Magnetooptical visualization of the current flow in the same regions confirms the percolative nature of current flow. Furthermore, numerical modeling of the current flow based on the observed grain boundary character distribution and the effects of the grain boundary misorientation angle on J_c was found to predict the percolative nature of current flow. These studies indicate that in order to increase the J_c further, increased fractions of small-angle boundaries are required and, hence, provide a research direction for the next generation of high-temperature superconducting wire.