An investigation of the influence of silver doping on the intergranular 'weak-link' properties of the superconducting system Y1Ba2Cu307-x.

Abstract

High-temperature superconducting materials have found considerable technological application and still have a largely unrealised potential. The key to unlocking this potential depends on a better understanding of their properties; in particular, the maximum 'critical current density' which these materials, in the form of wires, tapes, thin-films and bulk monolithic forms, are able to support for high-current applications. The 'critical current density' in a polycrystalline high-temperature oxide superconductor system is determined by a percolation process of the super current through a three dimensional grain-boundary network. Grain-boundaries in these systems behave as Josephson junction 'weak-links' and they severely limit the critical current density in the presence of even moderate self or applied magnetic fields. In the present work, isothermal quasi-static magnetisation measurements on the polycrystalline YIBa2Cu307-x system are presented and analysed. An effective granular penetration depth in conjunction with a critical state model, which includes an approximate treatment for the percolation process, is used to obtain many of the salient physical parameters of the grain-boundary Josephson junctions and of the three-dimensional grain boundary junction network. Determination of the temperature and magnetic field dependence of several of these parameters, in particular a magnetic field-independent critical current which depends on the micro structure of the grain-boundary junction network, allows for testing and verification of models of the weak-link and network behaviour. This treatment has been carried out specifically on various silver doped polycrystalline Y1Ba2Cu30 7-x specimens in order to determine and quantify the effects of silver doping. An improvement in the critical current density with silver doping is explained in terms of silver scavenging and ridding grain-boundaries of impurities, and a proximity effect where trace amounts of silver residing in the grain-boundaries decreases the normal resistance of the grain-boundary Josephson junction. The insight gained from silver doping experimentation led to a macroscopic investigation into the joining of large single-domain YIBa2Cu307-x specimens for large-scale applications.