Ph.D. / The use of cutting edge computer simulation tools such as Computational Fluid Dynamics (CFD) enables engineers to design, analyse and predict how effective and efficient a new design concept will be long before the plant or system is actually built. Although CFD software codes have progressed in vast leaps and bounds over the past ten to fifteen years, their application can still be found to be limited when complex systems have to be simulated. In such cases it is often possible for engineers to adapt or modify the software codes in order to cater for a specific need. This research is based on such a problem where the mere application of a standard CFD code was not sufficient to simulate the complexities and interactions which are found when analysing the performance and effectiveness of an Air Cooled Condenser (ACC). In the case of a mechanical draught ACC the multitude of fans which are employed to feed the cooling system with sufficient cooling air, require special treatment when attempting to simulate them using a standard CFD code. Most codes cater for detailed simulation of a rotating fan, however such simulation techniques require numerical meshes in excess of 40 000 computational cells per fan. In the case of an ACC such as the one at Matimba power station in South Africa, 288 fans would have to be simulated which would require a numerical mesh in excess of 11 million computational cells. Although this size of numerical problem could possibly be solved on some of the worlds fastest computers, it would not provide a practical engineering solution to the problem. The ACC at Matimba power station is the largest of its kind in the world. However, poor availability resulting from high sensitivity to changing ambient conditions such as high wind speeds, high ambient temperatures and changing wind directions prompted an urgent need for detailed simulation of the entire ACC system. As will be shown in the literature study, some attempts have been made to simulate air cooled condensers or parts thereof, however two main factors constantly limited the accuracy and usability of CFD codes for this application, (i) the interactive simulation of the fans with prevailing ambient conditions and (ii) the interaction between the performance of the ACC and the response of the turbine, which is thermodynamically coupled to the ACC via the steam pressure and temperature in the steam ducts. The abovementioned factors have in most cases restricted simulations to steady-state solutions and have also required tremendous computational and human effort resulting from the complexities surrounding the treatment of the high number of fans which need to be simulated. This research study submits a unique new integrated approach to simulation of a complex system such as an ACC, including its multitude of fans together with the complex interaction between the entire ACC system and the changing ambient conditions. A fan simulation sub-model was developed and tested and good agreement was achieved with the fan design data. Further sub models were developed in order to simulate the interaction between the ACC and the turbine generator. A test case was simulated and final results were compared to on site measurements achieving good agreement with physical test data and unit operating data.

The spatiotemporal evolution of temperature of leaves-on-branchlet carbon based electric double layer capacitors (EDLCs) under imposed constant current was studied using a continuum thermal model. The hot spot aggregated at the tips of graphene petals (GPs), particularly at the high concave surface, at the beginning of the charging step. As the charging proceeded, the overall temperature rose continuously, and the temperature distribution was likely uniform throughout the graphene petals due to an increasingly uniform distribution of ions on GPs surfaces. To elucidate the effects of electrode geometry on the change of temperature, several simple two-dimensional structures were also simulated in the charging step. Concave and planar structures contributed to high temperature change, while a convex structure tended to alleviate the hot spot. An insight into geometric effects on the thermal behavior may lead engineers to develop a new class of nanomaterials for supercapacitors.

No description available.

The main objective of the research was the fabrication and characterization of MOS/MIS capacitors with ZnO as the insulating layer. Comparison with the already well known behavior of MOS/MIS capacitors with SiO2 as insulator was used to facilitate determination of the ZnO characteristics. Moreover, thermal annealing of the samples led to increased understanding of the role of defects on the dielectric properties of the ZnO layers in the MOS/MIS devices. Hall-effect transport measurements and x-ray diffraction (XRD) spectroscopy are used to analyze the structure and electronic surface characteristics of the ZnO insulator. Capacitance-voltage (C-V) measurements are used to understand the effect of surface interface charges and fixed oxide charges in the MOS/MIS (metal-oxide (insulator)-semiconductor) capacitor. The results of the Hall-effect measurement will reveal several things; the sheet resistance, carrier concentration, and mobility as well as confirm the type of silicon used. The optical spectrophotometry measurement confirmed the band gap of 3.2 eV for ZnO. The x-ray diffraction data confirmed a (002) orientation polycrystalline wurtzite ZnO structure. Initial capacitance-voltage measurement of SiO2 and ZnO revealed that the capacitance was larger for SiO2 than for ZnO. This study also explores the impact of thermal annealing on the performance of the ZnO capacitors. Hall-effect measurements are used to evaluate the influence of thermal annealing on the resistivity, carrier concentration and mobility as a function of annealing temperature. ZnO is an n-type semiconductor; this n-type conductivity is due to deviations from the stoichiometry as a result of oxygen vacancies and interstitial zinc. After ZnO samples were annealed at different temperatures, the Hall-effect measurements were performed. After thermal annealing, the mobility increased significantly by two orders of magnitude, but both the carrier concentration and the sheet density decreased. A threshold voltage (turn-on) of -1V was observed for the ZnO sample annealed at 980oC. ZnO is very versatile material with the potential for use in field effect transistors, solar cells, sensors, surface acoustic wave devices and photodiodes due to the high conductivity and high transmittance in the visible part of the spectrum. ZnO as an insulator works through analytical solutions, but not necessarily through this investigation. The difference in oxide thickness during rf magentron sputtering change the capacitance for ZnO making it lower. For n-type substrates it appears that the capacitance after annealing was higher than the capacitance before annealing. After annealing, a stretched out capacitance-voltage curve indicates the presence of trapped oxide charges and an unsmoothed surface. A high resistivity material could be used for some devices. However, typically low resistivity materials are used. After ZnO samples were annealed (unetched) at different temperatures, the Hall-effect were performed and the mobility increased significantly by two orders of magnitude, but the sheet density decreased along with the carrier concentration. The only sample that appears to come to a high frequency C-V in equilibrium is the ZnO sample annealed at 980oC. The depletion region was distinguishable and the transition point (threshold voltage) was found to be at -1 V.

No description available.

The major objectives of this study were to examine electrical conduction properties of BaTiO3-based multilayer ceramic ( MLC ) capacitors in order to gain a better understanding of the conduction transport mechanisms inside the devices. The experiments involved mainly leakage current versus time measurements under both low temperature-low voltage stress and high temperature-high voltage stress. It was established that leakage current conduction in a MLC capacitor under temperature-voltage stress can be divided into three different conduction regions due to different mechanisms. Those regions are polarization current, DC conduction current and degradation current. The polarization current decreases with time as a power law relation, i.e. Ic(t) t-m where the exponent value m is strongly dependent on the type of capacitor and temperature, but is only weakly dependent on the applied voltage. It has been proposed that two degradation models ( a charge carrier concentration model and a reduction of grain boundary barrier height model ) can explain the degradation behavior for the Z5U devices tested. Degradation measurements indicate that the lifetime for Z5U capacitors can be described by Minford's expression. However, these models account only partly for X7R degradation. X7R behavior, is characterized by an early power law time dependence, followed by exponential voltage dependence. The most probable conduction transport mechanism in X7R capacitors is small polaron hopping, while grain boundary transmission may be the predominant conduction transport mechanism in Z5U capacitors. / Master of Science

The self noise output of switched-capacitor (SC) filters was examined. The two main noise sources in SC filters are switch transistors and the operational amplifiers. These noise sources were modeled as switch thermal white noise and op-amp input white noise. The existence of direct and sample-and-held noise outputs were demonstrated. The direct noise, resulting from direct circuit path, was shown to be relatively small: While the sample-and-held noise, arising from the direct noise samples, dominates the total noise output because of the aliasing effect of direct white noise. Z-domain analysis was performed on the Z-domain noise models of SC filters to evaluate the sample-and-held noise. A SC integrator, a SC biquad and a SC 7th order filter were analyzed. The predicted noise outputs of the last two filters were compared with their experimental results and good correlation between the two results has been obtained. / M.S.

The electrical characteristics, including degradation, of high K (≅ 500) barium titanate based thick film capacitors were studied. It was found that a gold conductor made from Au metallo-organic paste is not compatible with the porous high K dielectric material. The leakage current of a thick film capacitor made from Ag/Pd thick film conducting paste and high K dielectric has shown ohmic and space charge limited current behavior with a 3/2 power voltage dependence. Voltage independence of thermal activation energy and time dependence of a leakage current have been observed. A healing effect by reversal of bias polarity and humidity effect on leakage current have also been studied. The 3/2 power voltage dependence can be attributed to point emission from surface indentations or clusters of Ag particles at the electrode. Two possible degradation mechanisms based either on simple vacancy movement or a reduced grain boundary potential barrier height are suggested. It has been found that an electronic conduction model based on reduced grain boundary barrier height is more reasonable than that of simple vacancy induced current model. / M.S.

Pure, single phase stoichiometric Pb(Fe_0.5Nb_0.5)O₃ (PFN) powders were successfully formed by molten salt synthesis using mixture of NaCl and KCI salts. Lower temperatures and shorter times (0.5 hour at 800°C) were needed for single phase PFN formation from molten salts relative to those required for solid-state methods (4 hours at 1000°C). A systematic study indicating the effects of process parameters, such as temperature, time, and amount of flux with respect to starting oxides, on the PFN formation mechanism and its resulting powder characteristics is reported. The particle size increased with increasing synthesis temperature, the rate of increase is greatest above 900°C which is close to the melting point of lead oxide. PFN powders formed by molten salt synthesis were cuboidal, and were free from agglomerates. The sinterability, microstructure, and dielectric properties of these powders have been studied for the pure form and with the presence of lead oxide or lithium carbonate. The dielectric properties were sensitive to as-sintered density, the type of additive and the amount of additive. For pure PFN, the highest valve of dielectric constant is 12,270 at 1MHz, which is sintered at 1100°C for 13 hours. Ceramics sintered with lead oxide additive exhibited inferior dielectric properties although lead oxide served as a sintering aid to increase the as-sintered density. The dielectric properties of PFN with lithium carbonate sintered at 1000°C were attractive: the dielectric constant was increased to 14,000 at 1MHz and the D.C. conductivity was reduced. This reduction in the D.C. conductivity was interpreted in terms of the substitution of lithium for iron. / Master of Science

The development of efficient, high-performance materials for electrical energy storage and conversion applications has become a must to meet an ever-increasing need for electrical energy. Among devices developed for this purpose, capacitors have been used for pulsed power applications that require large power density with millisecond-scale charge and discharge. However, conventional polymeric films, which possess high breakdown strength, are limited due to low permittivity and hence compromise the energy storage capability of capacitors. In order to develop high energy density dielectric materials for pulsed power applications, two hurdles must be overcome: 1) the appropriate selection of materials that possess not only large permittivity but also high breakdown strength, 2) the optimization of material processing to improve morphology of dielectric films to minimize loss during energy extraction process. This thesis will present the development of novel dielectric material, with emphasis on the optimization of material and thin film processing toward improved morphology as ways to achieve high energy density at the material level. After first two chapters of introduction and experimental details, Chapter 3 will demonstrate the improvement of nanocomposite morphology via processing optimization and study its effect on the energy storage characteristics of nanocomposites thereof. Chapter 4 will investigate dielectric sol-gel materials containing dipolar cyano side groups, which are relatively a new class of material for pulsed power applications. Finally, Chapter 5 will discuss the effect of tunneling barrier layer on sol-gel films to mitigate charge carrier injection and associated conduction and breakdown phenomena, which would be significantly detrimental to the energy storage performance of dielectric sol-gel films.