A theoretically based critical-state model for granular high- Tc superconductors applied to YBCO

Abstract

A critical-state model, based on a critical current density dependence on internal field J c ( H), which has been modeled for a random 3D network of weak-link Josephson junctions assumming spatial distribution of current in an individual grain- boundary junction (GBJ), is presented. The model is used in an analysis of isothermal magnetisation and pulse transport critical current measurements on sintered 123-YBCO superconducting material in the low-field “Josephson” regime. The analysis yields self-consistent agreement with experiment for both types of measurement. It also gives predictions for the superconducting parameters of the inter- and intra-granular material and yields some physical insights into the behaviour of the junctions. In particular, there exists a field-independent component of the inter-granular critical current which depends on the GBJ microstructure. In the present specimen material it generally increases with decreasing temperature but shows a marked increase at about 60 K corresponding to the onset of superconductivity in the 60 K oxygen-deficient phase in the GBJ.