THERMAL, SELF-FIELD, AND MICROSTRUCTURE EFFECTS FOR CRITICAL CURRENTSIN POLYCRYSTALLINE YBA2CU3O7-DELTA

A thermal model is used to calculate the temperature distribution alon g YBa2Cu3O7-delta 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 transp ort critical current measurements at 77 K. The zero field critical cur rent density J(c) is then shown to depend on the cross sectional area because of limitation by the self-induced magnetic field. Consistent w ith predictions from a critical state model based on the notion of a l ocal magnetic field in the ceramic; this effect is shown to be strongl y attenuated for porous or microcracked samples where the local curren t carrying capacity is reduced. Finally a correlation between the crit ical current density and the room temperature conductivity as a functi on of relative density is reported and discussed in terms of current d istribution within YBa2Cu3O7-delta ceramics. (C) 1997 Elsevier Science B.V.