Modelling and experimental studies of the magnetic properties of bulk high temperature superconductors
University of Cambridge
Department of Engineering
Doctor of Philosophy (PhD)
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Xu, Z. (2013). Modelling and experimental studies of the magnetic properties of bulk high temperature superconductors (Doctoral thesis). https://doi.org/10.17863/CAM.11734
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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.
This record's DOI: https://doi.org/10.17863/CAM.11734
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