Thesis (MScIng)--University of Stellenbosch, 2005. / Bibliography. Also available via the Internet.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2005. / On t.p. "c" of Tc is subscript. Includes bibliographical references (leaves 80-90). Also available in electronic version.

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1994. / Vita. Abstract. Includes bibliographical references (leaves 71-73). Also available via the Internet.

Sm-Ba-Cu-O (SmBCO), which is a member of rare-earth barium cuprate [(RE)BCO] high-temperature superconductors, has significant potential for practical applications due to its higher critical transition temperature (Tc), higher critical current density (Jc), the so-called ‘peak effect’ characteristic at relatively high applied magnetic field and higher irreversibility field than that of the more established YBCO. The aim of this study is to investigate and overcome the obstacles in fabricating SmBCO bulk superconductors in air, to further improve their superconducting properties and, finally, to scale-up the fabrication of SmBCO single grains, therefore realising the engineering applications of this technologically important material. A modified seeding technique using an MgO-NdBCO generic seed accompanied by a buffer layer has been developed to process the SmBCO system to increase the success rate of growing single domain, bulk SmBCO superconductors in air via a top-seeded melt growth process using a conventional chamber furnace. Subsequently, the effects of doping in SmBCO bulk superconductors on the performance of SmBCO superconductors containing different dopants are discussed based on an analysis of their superconducting properties, including Tc and Jc, and on the microstructures of the samples. Furthermore, the scale-up of SmBCO bulk superconductors has been achieved by the addition of silver to the precursor powders and the introduction of a Y-123 layer beneath the SmBCO bulk pre-forms. Finally, trapped field measurements on successfully grown SmBCO bulk superconductors up to 41 mm in diameter have been performed and used to demonstrate significantly improved field trapping ability due to the optimization of the processing and composition of the SmBCO system. An Ag-SmBCO single grain of diameter 31 mm has achieved 1.033 T at 77 K, which is the highest value of trapped field reported worldwide for SmBCO samples of a similar size grown in air.

No description available.

No description available.

A series of modelling and experimental studies of the magnetic properties of bulk, single grain high temperature superconductors (HTS), such as RE-Ba-Cu-0 [(RE)BCO] where RE refers to a rare earth element, have been performed using a modelling framework developed in this study. This modelling framework can simulate the various magnetisation processes of bulk HTS in an attempt to understand the generation of relatively large magnetic fields by these materials for their potential use in a number of high field, 'quasi-permanent' magnet applications. Chapters I and 2 introduce bulk HTS based on their magnetic properties. More specifically, Chapter I describes the background for Type II bulk superconductors as a group of electrical materials whose various electromagnetic applications originate from their unique magnetic properties, and in particular from their ability to trap magnetic field by flux pinning ( often described by the Bean model) and their Jc-B (the Kim model) and E-J characteristics (the flux flow resistivity model and the power law model). Chapter 2 relates the properties of bulk superconductors, and especially those relating Jc, to the major research areas within bulk superconductivity of fabrication, magnetisation and modelling. State-of-the-art techniques for each area are discussed within the context of delivering effectively the full potential of Jc of a bulk superconductor in order to produce the maximum possible trapped field. Chapter 3 describes the modelling framework developed and used throughout this thesis, and presents three representative examples to demonstrate its capability in understanding the magnetic behaviour of bulk HTS during various magnetisation processes. The modelling framework solves Campbell's equation (which describes the force-displacement relation of magnetic flux lines) and the heat equation simultaneously using the finite element method (FEM) in the commercial software package FlexPDE. Two modes of simulation (timeindependent or time-dependent) are investigated, which are applicable to both static and pulsed field magnetisation (PFM) processes. This study extends significantly the research into PFM of bulk HTS, which has been limited generally to experimental techniques to date . Chapter 4 focuses on modelling the magnetisation of bulk HTS using split-coil arrangements, which serves as a comprehensive example of a direct application of the modelling framework developed. Split-coil arrangements are viewed as a preferred, but less understood, alternative to conventional solenoidal coils for practical magnetisation processes, and pa1ticularly for in situ PFM processes. Two major questions have been answered regarding the nature of split-coil magnetisation using the modelling framework developed: the geometrical conditions for designing an effective split-coil magnetisation ainngement have been established; the mechanisms of a split-coil magnetisation process, which consist of two distinct regimes of flux penetration, are understood. Both regimes are completely different from those observed in solenoidal-coil magnetisation processes. Finally, the association between the geometrical conditions and the mechanisms has been established Chapter 5 reports the development of a novel modelling-aided, non-destructive method of measuring Jc and the flux flow resistivity Pv (regarded as the key parameter of the flux flow resistivity model) in bulk HTS, which is generally considered impossible using common experimental techniques. This combination with experiment represents a second application of the modelling framework. The experimental part of this method involves magnetising a bulk HTS using a specific profile of external field, during which the induced voltage within the pick-up coil wound around the sample is measured and used to calculate key fields for the purposes of comparison. The modelling part of the method establishes good agreement between the measured and simulated fields using estimated values of Jc and Pv� Chapter 6 summarises all the research presented in this thesis from the perspective of the development and the application of the modelling framework for studying the magnetic properties of bulk HTS. It improves considerably the understanding of the mechanisms of magnetisation processes and the magnetic behaviour of these technologically important materials during their magnetisation. It also serves as a cost-effective tool for designing practical magnetisation arrangements and related processes in order to achieve the full capability of a bulk HTS effectively. Finally the framework plays a core potential role in the field of modelling-aided, non-destructive characterisation of the magnetic properties of (RE)BCO and other bulk superconductors.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

The thesis begins by providing an introduction to transformer theory. An ideal transformer is examined first, followed by full core transformer theory. The partial core transformer is then introduced and compared to the full core design. An introduction to superconductors is then presented where a simplified theory of superconductivity is given. High temperature superconductors are then examined including their physical structure, superconducting properties and the design of the superconducting wire. The early development of high temperature superconducting partial core transformers at the University of Canterbury is then examined. Early partial core development is discussed followed by some material testing at cryogenic temperatures. This work lead into the development of the first high temperature superconducting partial core transformer. This transformer failed during testing and an examination of the failure mechanisms is presented. The results of the failure investigation prompted an alternative winding insulation design which was implemented in a full core superconducting transformer. The modelling used to design a high temperature superconducting partial core transformer is then presented. Based upon the reverse design method, the modelling is used to determine the components of the Steinmetz equivalent transformer circuit. The modelling includes a combination of circuit theory and finite element analysis. An ac loss model for high temperature superconductors is also presented. A new 15 kVA, 230-230V high temperature superconducting partial core transformer was designed, built and tested. The windings are layer wound with first generation Bi2223 high temperature superconductor. The modelling was used to predict the performance of the transformer as well as the ac losses of the high temperature superconductor. A series of electrical tests were performed on the transformer including open circuit, short circuit, resistive load, overload, ac withstand voltage and fault ride through tests. The test results are compared with the model. The transformer was found to be 98.2% efficient at rated power with 2.86% voltage regulation.