A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. / The properties-of systems which undergo a metal-insulator (MI) transition are currently being extensively studied. Both the transport and thermodynamic properties of these MI systems show interesting behaviour, particularly near the MI transition. A complete theory to describe MI systems does not yet exist. In the present work the focus is on the thermodynamic properties of MI systems, in particular on the specific heat and susceptibility. The thermodynamic properties in the absence of a magnetic field are now well understood, with models such as the two-fluid model giving a good account of the behaviour. In finite magnetic fields the thermodynamic properties are even more interesting and varied. It is the aim of the present work to develop and test models to explain the effects of applying magnetic fields to MI systems. The focus is mainly on phosphorous doped silicon, and the results are gratifying. The theory should, however, also apply to other similar MI systems. / Andrew Chakane 2018

No description available.

This thesis details the design, fabrication and measurement of an integrated optical Bragg grating filter, operating at a free space wavelength of 1532 nm, based on silicon-on-insulator (SOI) ridge waveguide. Grating-based integrated devices can interact with optical signals in photonic integrated circuits (PIC) in such a way as to selectively transmit, reflect or detect the signals that are resonant with these devices. Channel filters can access one channel of a wavelength division multiplexed signal without disturbing the other channels and are therefore important elements in WDM communications. Resonator filters are attractive candidates because they can potentially realize the narrowest linewidth for a given device size. Device models for this kind of device are developed by using the MATLAB programming language. Coupled mode theory (CMT) for filters, and the effective index method (EIM) which reduces a three dimensional (3D) analysis into two dimensions is used as modeling theoretical background. Computer modeling identifies the effect of device structure on the performance of the devices, and is also used to predict the output characteristics of this kind of device. This provides an understanding of device physics and operation, and a basis for comparison with experimental results. A common fabrication sequence for integrated optical Bragg grating filters based on SOI ridge waveguides is designed, developed and demonstrated. This includes the photomask for optical ridged waveguide, interferometic lithography for grating pattern and high accuracy RIE etching. This work demonstrates Bragg grating as a technology for realizing PIC in SOI material system, and presents the technology required to design, fabricate, characterize, and model these integrated devices. / Thesis / Master of Applied Science (MASc)

Composite insulators are widely being used in power industry to alternate traditional porcelain-based insulators for their advantages, including better pollution performance, low maintenance cost, light weight, compact line design. However, due to the short application history and experience, the degradation of composite insulators in natural environment is a big concern for the power utilities. The knowledge on the degradation of composite insulators is being studied world wide. The methods to assess the working conditions of composite insulators are being studied and created. In Queensland University of Technology (QUT), the approach based on chemical analysis methods was first developed. The work in this thesis based on the previous research work is focused on correlating electrical characteristics with chemical analysis results of the composite insulators and physical observations results. First,the electrical characteristics of composite insulators were presented and analysed, including leakage current, cumulative current, peaks of leakage current, the statistic results of the leakage current. Among them, the characteristics of leakage current were mainly studied. The shape of waveforms was found to relate to the degree of discharge activities of the composite insulators. The waveforms analysed by FFT revealed that the odd harmonic components became obvious during the discharge activities. The correlations between the electrical characteristics of composite insulators and chemical analysis results showed that the composition of composite insulators plays significant roles in terms of electrical performance. The oxidation index (O.I.) and the ester/ketone ratio (E/K) differentiated the different degradation reasons of the composite insulators in the test conditions. Finally, the thesis presents one approach, which aims to assess the surface conditions of composite insulators in an easy manner and in short time.

No description available.

Metal-insulator-semiconductor switches (MISS), in which the T denotes some form of thin semi-insulating layer and the semiconductor part consists of a pn junction, are part of the general class of regenerative switching devices which includes the thyristor. The switching behaviour of the MISS derives from the ability of the MIS junction to exhibit current gain and to exist in two modes, deep depletion and inversion. In this thesis, a general model for the regenerative switching is proposed after investigating the properties of the MIS junction both theoretically and experimentally. Results from MIS diodes with tunnelling-thickness oxide Mayers indicate that interface states play a dominant role in their electrical behaviour and that the uniformity of the oxide is poor, giving rise to a large spread in the current-voltage characteristics. Subsequently, the epitaxial form of the MISS device is investigated and in particular the importance of isolation of the pn junction. It is concluded that spreading effects set a practical lower limit to the device dimensions, making the epitaxial form unsuitable for microelectronic applications. An alternative semi-insulator, 'silicon-rich oxide' (SRO) is introduced as an optional I-layer with possibly greater integrity than tunnel oxide. MIS diodes formed with SRO are shown to have very similar properties to tunnelling diodes. Large area devices fabricated using this material are surprisingly discovered to exhibit stable negative differential resistance (NDR). Although this discovery at first appears to be contrary to normal circuit stability criteria and to the regenerative feedback model itself, both of these points are resolved. It is shown that the frequency of oscillation of an unstable device is controlled by the external circuit. Then it is proposed that if this frequency is greater than the maximum frequency of operation of the regenerative mechanism, stable NDR is observed. In the final chapter, alternative lateral MISS structures which should overcome the geometrical limitations of epitaxial devices are discussed.

Wong Yuen-fai, Dick. / Includes bibliographical references / Thesis (M.Ph.)--Chinese University of Hong Kong, 1985

Wong Chung Ki. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references (leaves 93-94). / Text in English; abstracts in English and Chinese. / Wong Chung Ki. / Abstract --- p.i / Acknowledgement --- p.iii / Contents --- p.iv / List of Figures --- p.viii / List of Tables --- p.xii / Chapter Chapter 1. --- Introduction --- p.1 / Chapter 1.1 --- Motivation --- p.1 / Chapter 1.2 --- Double exchange model --- p.3 / Chapter 1.3 --- This project --- p.5 / Chapter Chapter 2. --- The generalized double exchange model --- p.6 / Chapter 2.1 --- Assumptions of the model --- p.6 / Chapter 2.2 --- Transfer process of the mobile electrons --- p.7 / Chapter 2.3 --- Double exchange model --- p.9 / Chapter 2.4 --- The generalized double exchange model --- p.10 / Chapter 2.5 --- Previous results on the model --- p.13 / Chapter 2.5.1 --- Solution for two localized spins --- p.13 / Chapter 2.5.2 --- Ground state configuration for one doping electron and the electron concentration at half fill --- p.13 / Chapter 2.5.3 --- The planar spiral state and the canted ferromagnetic state --- p.14 / Chapter Chapter 3. --- Ground state calculations --- p.16 / Chapter 3.1 --- Minimization of ground state energy --- p.17 / Chapter 3.2 --- Energy and spin configuration --- p.18 / Chapter 3.3 --- Occupation number of mobile electrons --- p.19 / Chapter 3.4 --- Rotated occupation number of mobile electrons --- p.19 / Chapter Chapter 4. --- One-dimensional ground state results --- p.21 / Chapter 4.1 --- Zero superexchange coupling --- p.21 / Chapter 4.1.1 --- Total energy of the system --- p.21 / Chapter 4.1.2 --- Spin configuration and nearest neighbor spin-spin correlation --- p.23 / Chapter 4.1.3 --- Occupation number --- p.29 / Chapter 4.2 --- Antiferromagnetic superexchange coupling --- p.34 / Chapter 4.2.1 --- Total energy of the system --- p.35 / Chapter 4.2.2 --- Spin configuration and nearest neighbor spin-spin correlation --- p.36 / Chapter 4.2.3 --- Occupation number --- p.39 / Chapter 4.3 --- Discussions on the one-dimensional results --- p.43 / Chapter Chapter 5. --- Two-dimensional ground state results --- p.46 / Chapter 5.1 --- Zero superexchange coupling --- p.46 / Chapter 5.1.1 --- Total energy of the system --- p.46 / Chapter 5.1.2 --- Localized spin configuration --- p.47 / Chapter 5.1.3 --- Occupation number --- p.51 / Chapter 5.2 --- Nonzero superexchange coupling --- p.54 / Chapter 5.2.1 --- Total energy of the system --- p.54 / Chapter 5.2.2 --- Spin configuration --- p.54 / Chapter 5.2.3 --- Occupation number --- p.57 / Chapter 5.3 --- Discussions --- p.59 / Chapter Chapter 6. --- Finite temperature calculations on the model --- p.62 / Chapter 6.1 --- Quantum Monte Carlo simulation --- p.62 / Chapter 6.2 --- Heat capacity and magnetic susceptibility --- p.63 / Chapter 6.3 --- Localized spin-spin correlation --- p.64 / Chapter Chapter 7. --- Results at finite temperature --- p.65 / Chapter 7.1 --- Two-dimensional results without superexchange interaction --- p.65 / Chapter 7.1.1 --- Fourier transform of the spin-spin correlation --- p.65 / Chapter 7.1.2 --- Occupation number --- p.68 / Chapter 7.2 --- Two-dimensional results with nonzero superexchange interaction --- p.73 / Chapter 7.2.1 --- Fourier transform of the spin-spin correlation --- p.73 / Chapter 7.2.2 --- Occupation number --- p.74 / Chapter 7.3 --- Three-dimensional results with anisotropic exchange interaction --- p.77 / Chapter 7.3.1 --- Fourier transform of the spin-spin correlation --- p.77 / Chapter 7.3.2 --- Occupation number --- p.79 / Chapter 7.3.3 --- Magnetization --- p.81 / Chapter 7.3.4 --- Heat capacity --- p.81 / Chapter 7.3.5 --- Magnetic susceptibility --- p.82 / Chapter 7.3.6 --- Discussions --- p.83 / Chapter Chapter 8. --- Conclusions --- p.85 / Chapter Appendix A. --- Details on Euler rotation of the localized spin --- p.87 / Chapter Appendix B. --- Details on the calculation of the occupation number --- p.89 / Chapter B.1 --- Occupation number --- p.89 / Chapter B.2 --- Rotated occupation number --- p.90 / Chapter Appendix C. --- Fourier transform of the spin-spin correlation at T≠0 --- p.92 / Bibliography

No description available.

No description available.