Thesis (Ph.D.)--University of Wollongong, 2004. / Typescript. Includes bibliographical references.

Thesis (M.Eng.)--University of Wollongong, 2004. / Typescript. Includes bibliographical references: leaf 104-108.

Thesis (M.S.)--State University of New York at Binghamton, Department of Physics, Applied Physics and Astronomy, 2009. / Includes bibliographical references.

The work described in this thesis consists of an investigation into the behaviour of the critical current density (Jc) of several different high temperature superconductors (HTSCs) as a function of temperature and applied magnetic field. The focus of this research has been to investigate the discrepancies which generally exist between magnetic and transport measurements on HTSCs. In order to do this a number of systems were selected with different weak link strengths, overall alignment and pinning characteristics. Systematic studies were carried out on these systems using both transport and magnetic techniques. The results obtained were compared to obtain a coherent picture of the relation between critical currents in HTSCs, their granularity and structure, and how this accounts for the differences in magnetic and transport measurements. The Jc of granular bulk sintered YBCO was measured as a function of magnetic field and orientation, and attempts made to fit the results obtained to theory. The results obtained indicate that hysteresis of Jc with field cannot be explained by flux trapping alone. Jc measurements were carried out on the 'hub-and-spoke' (H-S) grains of melt-processed YBCO thick films. These indicated that within each H-S grain the current is constrained to radial paths through the centre of the grain. This has major implications for the analysis of any measurements carried out on these samples, as well as for applications, as only a small fraction of the sample carries the applied current. Silver-clad tapes of Tl:1223 and Tl:2223 were measured in different temperatures, applied magnetic fields and orientations. The variation of Jc along the tape length was also measured to provide an estimate of their homogeneity. It was found that the processing route used in the production of these tapes had not aligned the superconductor within them, and also that the tape properties were not homogenous along their length. This probably arises from the crystal structure of the thallium materials. The Jcs of melt-processed thick films of Bi:2212 on silver substrates were measured as a function of temperature and applied magnetic field, and compared with results obtained from magnetic measurements obtained from a VSM. Attempts were made to fit these results to theory. The results obtained show clear scaling behaviour of Jc with T at constant B and imply that the variation in the form of the Jc versus T curves is due simply to the suppression of Tc with increasing applied field.

Current transfer behaviour in anisotropic superconducting bodies is the central topic of this thesis and focuses on the effect that the nonlinearity of the electric field dependence upon the local current density value and anisotropy have on the nature of current transport. The main motivation for this work was the desire for a better understanding of the conceptually difficult behaviour of current transport in superconducting bodies and examines current transfer quantitatively for a number of important problems on the macroscopic and microscopic scale. This behaviour is examined both experimentally and using computer models. The successful development of a powerful, robust and adaptable numerical model for analysing the complex current transfer behaviour has been the primary aim of this work. The range of parameters appropriate to macroscopic models of the Bi-2212 CRT system has been experimentally examined using a specifically constructed apparatus for the measurement of current transport characteristics. A study of the self-field properties of the Bi-2212 CRT material using a new experimental technique and mathematical analysis is presented and has allowed the importance of the self-field effect in the numerical model to be assessed. An essential requirement for the practical application of high current superconducting devices is the development of low resistance current contacts. The research presented examines this macroscopic current transfer problem and aims to explain experimentally observed current transfer characteristics at high applied currents. Existing models cannot explain these characteristics. Current transfer on the microscopic scale is also examined. Models of current transfer have been developed from descriptions of specific microstructures that are thought to characterise the microstructure of Bi-2223 and Bi-2212 silver-sheathed tapes. This thesis specifically presents modelling of current transfer between c-axis, low-angle c-axis and edge-on c-axis tilt oriented grain interfaces; the principal current transfer paths between individual current elements of the microstructural models of current flow in polycrystalline HTSs.

Thesis (Ph.D.)--University of Wollongong, 2006. / Typescript. Includes bibliographical references: leaf 199-216.