Thermal, self-field, and microstructure effects for critical currents in polycrystalline YBa₂Cu₃O_7-δ

A thermal model is used to calculate the temperature distribution along YBa₂Cu₃O_7-d bars in liquid nitrogen for the case of heat sources situated at the ends. The predictions allow an appropriate choice of experimental conditions which avoid thermal effects influencing transport critical current measurements at 77 K. The zero field critical current density J_c is then shown to depend on the cross sectional area because of limitation by the self-induced magnetic field. Consistent with predictions from a critical state model based on the notion of a local magnetic field in the ceramic; this effect is shown to be strongly attenuated for porous or microcracked samples where the local current carrying capacity is reduced. Finally a correlation between the critical current density and the room temperature conductivity as a function of relative density is reported and discussed in terms of current distribution within YBa₂Cu₃O_7-d ceramics. © 1997 Elsevier Science B.V.