Thesis (Ph.D. (Natural and Agricultural Science)) -- University of Pretoria, 2009. / Includes bibliographical references.

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

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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.

The electrical resistivities of typical AWG20-Class3 and AWG18-Class27 Ni-coated Cu wires were monitored at 400 ºC for times up to 5500 hours; the resistivities increased by 6.9% and 2.3%, respectively. Microstructural analysis of the thermally aged wires revealed evidence of Ni-Cu interdiffusion. Diffusion experiments were performed on Ni-Cu metal foils in the range 400 to 600 ºC; Ni-Cu compositional profiles across the Ni-Cu interface were collected by energy dispersive X-ray spectrometry. Ni-Cu interdiffusivities determined by the Boltzmann-Matano method were typically 2.5×10-17 m2s-1;calculated activation energies for Ni-Cu interdiffusion were between 79.4 and 89.8 kJ•mol-1. Analysis of the available Ni-Cu interdiffusion data suggested a dependence on grain size of the Cu foils used. A concentric-circle, diffusion-resistivity model was developed. Using the experimentally determined Ni-Cu interdiffusion data, it was possible to accurately predict the resistivity of a Ni-coated Cu wire at 400 ºC as a function of time. It is predicted that the resistivity of the AWG20-Class3 wire would increase by 10% after annealing for 48,000 hours at 400 ºC; in contrast, heating an AWG18-Class27 wire for a much longer time of 140,000 hours would incur the same increase in its resistivity. Low temperature co-fired ceramics (LTCC) with a formulation of 11ZnO-10MoO3 (NSZM) were prepared with additions of 0.5 to 2.0 wt% B2O3 via the mixed oxide route. The NSZM samples were sintered at 850-950ºC to over 96% of theoretical density with co-existence of both ZnMoO4 and Zn3Mo2O9 phases. With increasing the addition of B2O3 to NSZM the relative permittivity, dielectric strength and thermal conductivity increased. NSZM prepared with 1.0 wt% B2O3 exhibited a relative permittivity of 11.1, dielectric strength of 17.6 kV•mm-1, linear thermal expansion of 4.7 ppm•K-1and thermal conductivity of 1.3 W•m-1•K-1. The LTCC material is a possible candidate for insulating applications because of its low dielectric constant and adequate dielectric strength. LTCC insulation films were applied to Ni disc substrates by dip coating; the suspensions contained 5 to 20 vol% NSZM ceramic powders, 1.0 wt% B2O3, a polyvinyl butyral (PVB) based binder system, plus solvents and organic additives. A microstructural study of the LTCC films revealed that the insulation thickness varied from 4.3 to 47.3 µm with the ceramic content of starting suspension. The dielectric strength of these films was in the range 24.2 to 43.7 kV•mm-1. These results showed that dip coating is a promising method for applying the LTCC insulation to Ni-based metal substrates. LTCC-insulated wires were manufactured by withdrawing Ni-coated Cu conductors from the suspension, containing 15 vol% ceramic powders, followed by co-firing at 500 ºC. The LTCC-coated wire exhibited an insulation thickness of 40.3 µm and a breakdown voltage of 798 V. These results suggest that the LTCC-coated wire is a possible candidate for use in high temperature machine windings.

The objective of this study was to determine the effect of aging on the tenderness and palatability of beef from the semitendinosus muscle of the round. Tenderness may be affected by the length of the aging period and the aging temperature. Steaks from the semitendinosus muscle were stored for 0, 3, 6, 9, 12, and 24 hr at two temperatures, 43.3°C and 2.2°C. Ultra high temperature (UHT) pasteurization was accomplished by subjecting the meat to 1100°C for 20 seconds. UHT pasteurization denatures surface proteins, destroys vegetative pathogens, and eliminates some spoilage organisms from the surface of the meat. UHT -treated steaks were cooked in a microwave oven to an internal temperature of 71.1°C. Two cores were taken from the center of each cooked steak. Shear values (lb) were determined on the cores using a Warner Bratzler shear. The results of the study showed that the UHT-treated steaks that were stored at 43.3°C were significantly more tender than those that were refrigerated at 2.2°C regardless of the length of the aging period up to 24 hr. Additional tenderness at 2.2°C when the aging period exceeds 24 hr may be possible. Total plate count (TPC) of raw and UHT-treated steaks was determined using the standard plate count method. The average TPC for the raw steaks was significantly higher than the UHT-treated steaks. The TPC was not significantly different between the UHT- treated steaks that were aged and those that were not aged. A trained panel was used for sensory evaluation to evaluate the moistness, tenderness, and flavor of the steaks using a 9-point hedonic scale. Sensory scores of the UHT-treated steaks revealed that steaks stored at 43.3°C had significantly more moisture and were more tender than those stored at 2.2°C. The panel noted more spoiled flavor among the steaks stored at 43.3°C than 2.2°C. Steaks stored at 2.2°C received significantly higher flavor scores than those stored at 43.3°C.

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Currently, the design of high temperature mechanical components is limited by material performance at elevated temperatures. Rocket nozzle materials, for example, need to survive exhaust gas temperatures up to 3000 ºC under high stresses for short periods of time. Additionally, one of the current challenges in hypersonic flight is the development of materials that will withstand the leading edge temperatures which exceed 2700 ºC. In these severe environments, the characterization of materials’ creep properties is essential. Conventional creep testing methods are limited to 1700°C. Using ESL, a group of researchers at the University of Massachusetts Amherst developed a non-contact creep method, which is not subject to such temperature limits. Using the non-contact method a spherical sample is rotated rapidly, and the driving load is applied by centripetal acceleration, which causes deformation. During previous creep tests, a laser supplied both the heating and driving rotational force to the sample. Since the rotation is controlled by the photon pressure emitted from the heating laser, the applied stress is coupled to the testing temperature. By developing an independent rotation control, non-contact creep tests could be conducted on a wider range of materials. A specialized high-speed induction motor was developed for use in high-temperature creep tests. In addition to creep behavior, the understanding of thermophysical properties is important for the emerging class of high temperature material. Using a previously developed method, non-contact density measurements were taken concurrently on the same materials as X-ray diffraction measurements. Over 35 materials were successfully processed including, aluminum, copper, hafnium, palladium, nickel, titanium and zirconium based alloys. Besides contributing to high temperature material databases, density measurements provide an understanding of solidus formation and short-range order in the liquid state.