Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2006. / Includes bibliographical references (leaves 111-115). Also available in electronic version.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references. Also available in electronic version.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 79-82). Also available in electronic version.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (M.S.)--Oregon Graduate Center, 1987.

Philosophiae Doctor - PhD / Titanium oxynitride has attracted research interest for the fact that it is a bioactive non-toxic material. It is suitable for surface coating of biomaterials and in other applications such as anti-reflective coatings, while oxygen-rich titanium oxynitride has been applied in thin film resistors and photocatalysis. Two common types of titanium oxynitrides are TiOₓNᵧ. and TiO₂-ₓNᵧ. In this work, titanium mixed metals oxynitrides (Ti-TiO₂, Mg-TiO₂ and Mg-Sn-TiO₂) were synthesized for the first time using ball milling (BM) and annealing processes. Their structural, morphological, surface, optical, and gas sensing properties were studied in detail. Structural analyses showed that upon milling a pure TiO₂ phase, tetragonal to orthorhombic phase transformation occurred. However, when milling TiO₂ mixed with Mg, Sn and Ti no evidence of the transformation was observed. Furthermore, scanning electron microscopy, transmission electron microscopy and atomic force microscopy showed that the milling process promotes particle refinement. The gas sensing analyses also demonstrated that the sensing response of the TiO₂, Mg-TiO₂ and Mg-Sn-TiO₂ materials improved upon milling. Moreover, the Mg-TiO₂ showed improved sensing compared to pure TiO₂ due to incorporation of Mg, which might have resulted in a decrease of charge carrier concentration. The Mg-TiO₂ sensing materials showed fast response-recovery time of ~32 s and ~48 s, respectively, as well as high selectivity to NH₃ gas compared to other gases (H₂, and CH₄). In addition, the improved response observed for the milled samples is due to increased surface area and pore diameter, providing more active sites for the target gas and allowing more gas adsorption with an increase in point defects related to oxygen vacancies (Vo), which are the most favorable adsorption sites for oxygen species and thus can enhance the possibility of interaction with gas molecules. A combination of photoluminescence, x-ray photoelectron spectroscopy, vibrating sample magnetometer and sensing analyses demonstrated that a direct relation exists between the magnetization, sensing and the relative occupancy of the Vo present on the surface of TiO₂ nanoparticles. Therefore, based on these finding we conclude that the milling process promotes particle refinement, resulting in an increased BET surface and partial breaking of Ti–O bonds on the TiO₂ surface layer, which results in the formation of oxygen vacancies in the TiO₂ lattice, therefore anticipating improved sensing response. / National Research Foundation

Low dimensional semiconductors are promising materials with diverse range of applications in a variety of fields. Specifically, in recent times low dimensional oxide and sulfide based semiconductors are regarded as materials that can have potential applications in chemical gas sensor, optoelectronic devices and memristor. How ever, in some cases it is envisioned that appropriate doping as well as phase stabilization is important in enhancing their material properties. This work presents the synthesis, characterization and application of various (pristine and doped) quasi-one dimensional metal oxides (TiO2, VO2) and two-dimensional materials (CuO thin film, MoS2). Some practical protocols for stabilization of specific phases at ambient conditions via a new method of doping in VO2 nanostructures with aluminum, is demonstrated. Similarly, a temperature-doping level phase diagram for the free-standing nanostructures in the temperature range close to the ambient conditions was presented. TiO2 nanowire was doped during growth and electrical measurements on individual TiO2 single crystal nanowires indicate that light in visible range can induce electron-hole pair formation. Furthermore, gas sensing (CO, H2) measurements taken under visible light irradiation imply that photo-activated chemical oxidization on the surface of TiO2 nanowires occurs, which is responsible for the observed measurements. Further, the effect of self heating in some nanostructures was also explored. Since self-heating is a prospective power-efficient energy delivery channel to the conductometric chemical sensors that require elevated temperatures for their operation, the unprecedentedly low power consumption can be achieved via minimizing the heat dissipation in the optimized device architecture. By investigating the heat dissipation in these devices we show that the thermal, electrical and chemical properties of the self-heated semiconducting nanowires appear to be strongly coupled with each other at nanoscale. This opens up unique opportunity to fabricate low power nanoscopic sensing leading to an ultra-small and power efficient single nanostructure gas recognition system. The CuO film based lateral devices were fabricated and studied for its resistive switching behavior. A good, stable and reproducible threshold RS performance of CuO film was obtained by electrical measurement. Finally, the micro-flake MoS2 based FET photoelectronic device was fabricated (using mechanically exfoliated MoS2) and its electronic and photoelectronic properties were investigated. We show that though the FET mobility values of MoS2 microflake is in the average range, but the photo-responsivity is much higher compared to most of others similar sulfide based 2D layered materials.

Thesis (Masters Diploma (Electrical Engineering)--Cape Technikon, Cape Town,1991 / A detailed study into the development of a three phase motor drive, inverter and microprocessor controller using a scalar control method. No mathematical modelling of the system was done as the drive was built around available technology. The inverter circuit is of a Vo~tage source inverter configuration whicp uses MOSFETs switching at a base frequency of between 1.2 KHz and 2 KHz. Provision has been made for speed control and dynamic braking for special applications, since the drive is not going to be put into a specific application as yet, it was felt that only a basic control should be implemented and space should be left for special requests from prospective customers. The pulses for the inverter are generated from the HEF 4752 I.e. under the control of the micro processor thus giving the processor full control over the inverter and allowing it to change almost any parameter at any time. Although the report might seem to cover a lot of unimportant ground it is imperative that the reader is supplied with the back-ground information in order to understand where A.e. drives failed in the past and where A.e. drives are heading in the future. As well as where this drive seeks to use available technology to the best advantage.

M.Ing. (Electrical and Electronic Engineering) / This thesis covers the development of the Mosmatrix, a high speed, high power switch which is implemented with an array of mosfet switching elements. The switching performance of the Mosmatrix proves that is is possible to employ existing semiconductor technology to switch pulses of 1,5 Joule, several hundred Ampere at several kilovolt, in the microsecond and sub-microsecond regime. The switch has demonstrated rates of current rise in the order of lOkA per J.1s during tumon without the use of tum-on snubbers (magnetic assist) . The rate of current fall during tum-off has been of the same magnitude. No other switch has demonstrated this level of repetitive current interrupt ability. The work covers the properties, switching requirements anc' switching performance of mosfet switching elements as well as the development of an isolated drive circuit.

Mixed metal oxides are ionic compounds containing at least two metal ions within an oxide structure. The literature contains a plethora of examples of mixed metal oxides on the bulk scale, which have been well characterised, however, mixed metal oxides on the nanoscale are far less well understood. The work presented here investigates the Bi2O3-CeO2-Nb2O5 mixed oxide system and characterises the resulting nanoparticles and crystal structures. Although the parent oxides are well known and much work has previously been done in analysing their crystal structures, combinations of these oxides have not been well characterised, especially on the nanoscale. Using high resolution electron microscopy (HRTEM), powder X-ray diffraction (PXRD), electron dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) as analytical tools, the structures of the nanoparticles in this system have been explored. As each of the parent oxides possess useful properties, which have been utilised in industrial applications such as electrolyte components in solid oxide fuel cells and as catalysts in a range of chemical reactions, it was hypothesised that if all three metal ions could be contained in one particle they could show novel and interesting characteristics. It was proposed that due to the more relaxed crystal structure in nanoparticles, the solid solubility of the metal ions should be increased, and a solid solution of ions would form. This work presents results showing the synthesis of binary and ternary oxides in the nano-form within the Bi2O3-CeO2-Nb2O5 system, including quantitative analysis of these particles. Secondly, and most importantly, it presents the first successful synthesis of quaternary oxide nanoparticles containing bismuth, cerium and niobium using the low temperature resin-gel method. Finally, the work attempts to explain how and why the ions are ordered in a given arrangement, with bismuth showing a preference for surface site occupation, as shown by XPS data, and describes some preliminary computational results which corroborate the experimental data.