An experimental investigation of the weak-link problem in granular high-Tc superconductors.
Grain boundary weak-link behaviour in superconductors is investigated using a critical state model (CSM), which has been specifically formulated for the applied field range o ~ Ha ~ Hc1g , where Hclg is the intra-granular lower critical field. The CSM includes an expression for the inter-granular critical current density, Jc(H, T), that has been derived from a percolation model for a random 3D network of weak-links in which the current Bowing through the individual grain boundary junctions is assumed to be spatially random. This expression, namely: Jc(H,T) = Jco[exp(-H/Ho)+b], where JCO ) Ho , and b are characteristic weak-link parameters, includes a field-dependent component and a field-independent component both of which are microstructurally sensiti ve. Calculated magnetisation behaviour, obtained from the critical state model, are fitted to experimental isothermal D.C magnetisation data, j\1(Ha) , for 0 ~ Ha ~ Hclg , 71.5K~ T ~ Tc, obtained from the two control specimens, namely: Y IBa2Cu307-x and EUIBa2Cu307-x , x ~ I, and from a third specimen, which is a mixture of the two control compounds. In the CSM fits, the temperature dependent characteristic parameters Jco , Ho, and bare treated as free fitting parameters. These free fitting parameters are subsequently fitted to specific theoretical models for Jca(T), Ho(T), and b(T) and a comparison is made between the control specimens and mixed specimen parameters to establish the effect of mixing on the grain boundary weak-link behaviour. Jca and b are found to be significantly larger in the mixed specimen as compared with either of the two control specimens.