The effect of density on critical current and oxygen stoichiometry of YBa2Cu3Ox superconductors

Abstract

It has been observed that "the most important parameter of a superconductor... is the critical current density Jc" (ref. 1). In thin films or single crystals of YBa2Cu3Ox, Jc is high at ∼106A cm−2 but in bulk materials Jc is much lower, typically 102–103 A cm−2 (ref. 4). The sources of variation in Jc are important but vary. Chemical contamination such as SiO2 impurities is known to reduce Jc5 but even in the absence of such contaminants Jc 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). Jc 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 Jc was reported demonstrating the anisotropy6. In bulk materials Jc 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 Jc in sintered bulk ceramics that display no preferred orientation. We examine the relation between sintered density, oxygen content and Jc and show how Jc 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 Jc in anisotropic bulk ceramics.