The effect of density on critical current and oxygen stoichiometry of YBa₂Cu₃O_x superconductors

Abstract

It has been observed that "the most important parameter of a superconductor... is the critical current density J_c" (ref. 1). In thin films or single crystals of YBa₂Cu₃O_x, J_c is high at ∼10⁶A cm⁻² but in bulk materials J_c is much lower, typically 10²–10³ A cm⁻² (ref. 4). The sources of variation in J_c are important but vary. Chemical contamination such as SiO₂ impurities is known to reduce J_c⁵ but even in the absence of such contaminants J_c varies widely. Some groups have experienced difficulty in maint-aining high (1,000 A cm^-2 in bulk ceramic) current densities and sometimes even zero resistance at 77 K at high densities of the ceramic (>6.2 g cm⁻³). J_c is known to be anisotropic being greater in the a–b planes and least in the c-axis direction. In thin films a thirty-fold difference in J_c was reported demonstrating the anisotropy⁶. In bulk materials J_c has been increased to 7.5 x 10³ A cm⁻² by melt textured growth of the ceramic in order to induce preferred orientation⁷. Here we report the factors controlling J_c in sintered bulk ceramics that display no preferred orientation. We examine the relation between sintered density, oxygen content and J_c and show how J_c is strongly dependent on both the volume fraction of solid and on the oxygen stoichiometry, both being strongly influenced by the ceramic density. The problem we address is how to optimize the solid connectivity and simultaneously maintain a high oxygen content thereby maximizing J_c in anisotropic bulk ceramics.