High-temperature reactors make use of tri-structural coated fuel particles as basic fuel components. These TRISO particles consist of fissionable uranium dioxide fuel kernels, about 0.5 mm in diameter, with each kernel individually encased in four distinct coating layers, starting with a porous carbon buffer, then an inner pyrolytic carbon (IPyC) layer, followed by a layer of ceramic silicon carbide (SiC) and finally an outer pyrolytic carbon layer (OPyC). Collectively, the coating layers provide the primary barrier that prevents release of fission products generated during burn up in the UO2 fuel kernel. It is crucial to understand how the fission products contained within the fuel interact with the coating layers and how they are distributed within the fuel. The first step commonly performed to obtain the information on distribution is removal of the coating layers. The purpose of this study was to investigate the possible use of wet chemical etching techniques with the aim of removing the coating layers of ZrO2 coated fuel particles in a controlled way and to establish experimental parameters for controlled dissolution of irradiated fuel particles. Stepwise dissolution of coated fuel particle coating layers, containing zirconia kernels has been investigated by chemical etching experiments with acidic solutions of different mixtures. The heating methods used include heating by conventional methods, hot plates and a muffle furnace, a reflux-heating system and microwave-assisted digestion. The etching mixtures were prepared from a number of oxidizing acids and other dehydrating agents. The capability of each reagent to etch the layer completely and in a controlled manner was examined. On etching the first layer, the OPyC, the reflux heating method gave the best results in removing the layer, its advantage being that the reaction can be carried out at temperatures of about 130 ºC for a long time without the loss of the acid. The experimental results demonstrated that a mixture composed of equal amounts of concentrated nitric and sulfuric acid mixed with chromium trioxide dissolves the OPyC layer completely. The most favourable experimental conditions for removal of OPyC from a single coated fuel particle were identified and found to depend on the etching solution composition and etching temperature. Light microscopy yielded first-hand information on the surface features of the samples. It allowed fast comparison of etched and untreated sample features. The outer surface of particles prior to chemical etching of the outer pyrolytic carbon layer appeared black in colour with an even surface compared to the etched surfaces which appeared to have an uneven metallic grey, shiny texture. The scanning electron microscope (SEM) examination of the chemically treated outer carbon layer samples gave information on the microstructure and it demonstrated that the outer pyrolytic carbon layer could be readily removed using a solution of HNO3/H2SO4/CrO3, leaving the exposed SiC layer. Complete removal of the layer was confirmed by energy dispersive X-ray spectroscopic (EDS) analysis of the particle surface. For etching the second layer, the silicon carbide layer, microwave-assisted chemical etching was the only heating technique found to be useful. However, experimental results demonstrated that this method has limited ability to digest the sample completely. Also common chemical etchants were found to be ineffective for dissolving this layer. Only fluoride containing substances showed the potential to etch the layer. The results show that a mixture consisting of equal amounts of concentrated hydrofluoric and nitric acid under microwave heating at 200 ºC yielded partial removal of the coating and localized attack of the underlying coating layers. The SEM analyses at different intervals of etching showed: partial removal of the layer, attack of the underlying layers and, in some instances, that attack started at grain boundaries and progressed to the intra-granular features. The SEM results provide evidence that etching of the silicon carbide layer is strongly influenced by its microstructure. From these findings, it is concluded that etching of the silicon carbide under the investigated experimental conditions yields undesirable results and that it does not provide complete removal of the layer. This method has the potential to etch the layer to some extent but has limitations. Copyright / Dissertation (MSc)--University of Pretoria, 2013. / Chemical Engineering / unrestricted

DC to DC step-down applications with high input voltage and high voltage conversion ratio operational requirements, such as photovoltaic battery chargers, are subject to high conduction losses, high switching losses and substantial reverse-recovery losses when minority carrier principle diodes are used. The recent introduction of silicon carbide diodes with high breakdown voltages has made possible the elimination of reverse-recovery losses at high voltage levels and as such has sparked interest in their use due to the potential efficiency improvements. This report presents the results of a comprehensive analysis on the use of silicon carbide diodes and their counterparts, ultrafast silicon diodes, in conventional buck converters and isolated current-fed buck converters in high input voltage and high voltage conversion ratio step-down applications. The analysis illustrates both theoretically, with the use of steady-state average models, and experimentally the substantial efficiency benefits of the use of reverse-recovery free silicon carbide diodes in the conventional buck converter and the small but significant improvement in the efficiency of the isolated current-fed buck converter. The improvements of the conventional buck converter paired with silicon carbide diodes are shown to be significant enough to grant the variant the most efficient position for power levels below 1 kW. In addition, the four variants are categorized based on their cost and performance; therefore, providing engineers with a convenient guide to aid their selection of the appropriate converter depending on the operational requirements. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

M.Sc. (Physics) / Amorhous silicon is one of the most promising materials for large area solar cells for terestrial photovoltaic applications. Unfortunately these cells suffer from two serious problems: the efficiencies drop when laboratory processes are scaled up and the cells degrade after some exposure to sunlight. The exact causes of these two problems are still unknown. In this project some aspects of the above two problems where investigated. The drop in efficiency due to scaling up of laboratory processes can be ascribed to macroscopic inhomogeneities in the film. An investigation was done by changing the chamber geometry and gas flow pattern to establish empirical conditions to obtain films with maximum macroscopic homogeneity. It was found that a uniform electric field above the substrate was the most important factor determining the macroscopic homogeneity of the film. The hydronamic gas flow pattern was of secondary importance. Some techniques to obtain a uniform electric field has been devised. The photo-degradation was investigated by illuminating films of o-Si.H with simulated sunlight for different lenghts of time. The change in the electrical and optical properties of the intrinsic films were determined as function of total photon flux. No change in the optical properties could be detected. The effect of the photo-degradation manifests itself in a drop in the the dark conductivity and photoconductivity. The observed phenomena is explained in terms of photo-induced deep levels in the gap. The Fermi level shifts to the middle of the gap due to these defect states, causing a drop in the free carrier concentration and conductivity. These defect levels increase the absorptiom coefficient in the long wavelength region, but they also decrease the lifetime of the photo-generated carriers. The photo-induced defects were investigated with the CPM-technique. A large part of this project involved the construction and commissioning of the CPM-apparatus. It was found that the light introduced two types of defects at energies 0.5 eV and 0.75 eV below the conduction band edge. The concentration of the defects increases with illumination, but saturates after about 24 hours of illumination. The defects could almost completely be annealed at ISOaC. The photo-degradation of o-Si.H solar cells is ascribed to the reduction in the carrier lifetimes of photo-generated carriers due to recombination at these defect centers.

Advances in space and nuclear technologies are limited by the capabilities of the conventional silicon (Si) electronics. Hence, there is a need to explore materials beyond Si with enhanced properties to operate in extreme environments. In this regards, silicon carbide (4H-SiC), a wide bandgap semiconductor, provides suitable solutions. In this thesis, radiation effects of 4H-SiC bipolar devices, circuits and dielectrics for SiC are investigated under various radiation types. We have demonstrated for the first time the radiation hardness of 4H-SiC logic circuits exposed to extremely high doses (332 Mrad) of gamma radiation and protons. Comparisons with previously available literature show that our 4H-SiC bipolar junction transistor (BJT) is 2 orders of magnitude more tolerant under gamma radiation to existing Si-technology. 4H-SiC devices and circuits irradiated with 3 MeV protons show about one order of magnitude higher tolerance in comparison to Si. Numerical simulations of the device showed that the ionization is most influential in the degradation process by introducing interface states and oxide charges that lower the current gain. Due to the gain reduction of the BJT, the voltage reference of the logic circuit has been affected and this, in turn, degrades the voltage transfer characteristics of the OR-NOR gates. One of the key advantages of 4H-SiC over other wide bandgap materials is the possibility to thermally grow silicon oxide (SiO2) and process device in line with advanced silicon technology. However, there are still questions about the reliability of SiC/SiO2 interface under high power, high temperature and radiation rich environments. In this regard, aluminium oxide (Al2O3), a chemically and thermally stable dielectric, has been investigated. It has been shown that the surface cleaning treatment prior to deposition of a dielectric layer together with the post dielectric annealing has a crucial effect on interface and oxide quality. We have demonstrated a new method to evaluate the interface between dielectric/4H-SiC utilizing an optical free carrier absorption technique to quantitative measure the charge carrier trapping dynamics. The radiation hardness of Al2O3/4H-SiC is demonstrated and the data suggests that Al2O3 is better choice of dielectric for devices in radiation rich applications. / <p>QC 20170119</p>

O comportamento dos materiais cerâmicos quanto à resistência ao choque térmico é um tema de grande interesse, devido às aplicações em que a confiabilidade frente a variações bruscas de temperatura é necessária. Neste trabalho foi estudado como a variação na proporção dos aditivos Y2O3:Al2O3 e diferentes parâmetros no processamento do carbeto de silício sinterizado via fase líquida como, tipo e temperatura de sinterização, podem influenciar na resistência ao choque térmico deste material. As misturas foram preparadas com 90%SiC+10%Y2O3:Al2O3 em mol, variando as proporções molares dos óxidos entre 2:1 e 1:4, com e sem prévia reação dos aditivos. As misturas foram compactadas e sinterizadas em forno resistivo de grafite nas temperaturas de 1750°C, 1850°C e 1950°C e, por prensagem a quente, a 1750°C e 1850°C, sendo avaliadas quanto à densificação. Após análise dos resultados preliminares, a sinterização sem pressão e as misturas com proporções 1:3 e 1:4 de Y2O3:Al2O3 previamente reagidos foram selecionadas para o estudo da resistência ao choque térmico. Os ciclos térmicos foram realizados com aquecimento em temperaturas de 600°C, 750°C e 900°C e resfriamento brusco em água em temperatura ambiente. A avaliação das amostras quanto à resistência ao choque térmico, feita por meio da determinação de módulo de elasticidade, porosidade, resistência à flexão e por análise microestrutural de trincas. As amostras sinterizadas na temperatura de 1950°C são as que apresentam o melhor desempenho em relação à resistência ao choque térmico, enquanto a variação na proporção Y2O3:Al2O3 de 1:3 para 1:4 não altera significativamente esta propriedade. Nas condições utilizadas, a temperatura máxima de aplicação do SiC sinterizado via fase líquida deve ser limitada a 750°C, permitindo seu uso como trocadores de calor, rolamentos, mancais de bombas submersas, turbinas a gás e sensor de motores automotivos e aeronáuticos. / The behavior of ceramic materials towards thermal shock resistance is a topic of great interest, due to applications in which the reliability against sudden temperature variations is required. In this thesis, it was studied how the variation in the proportion of Y2O3:Al2O3 additives and different parameters on the processing of liquid phase sintered silicon carbide may influence thermal shock resistance of this material. Samples were prepared with molar composition 90%SiC+10%Y2O3:Al2O3, by varying oxides molar proportion between 2:1 and 1:4, with and without previous reaction of the additives. Mixtures were compacted and sintered in a resistive graphite furnace at 1750, 1850 and 1950°C, and by hot pressing at 1750 and 1850°C, and evaluated for densification. After analysis of the first results, pressureless sintering and the mixtures with proportions of 1:3 and 1:4 of previously reacted Y2O3:Al2O3 were selected for the study of thermal shock resistance. Thermal cycles were performed by heating at temperatures of 600, 750 and 900°C and sudden cooling in water at room temperature. The evaluation of samples regarding thermal shock resistance was conducted by determination of elasticity modulus, porosity, flexural strength and microstructural analysis of the cracks. The samples sintered at 1950°C temperature are those that exhibit the best performance in relation to thermal shock resistance, while the variation in the proportions Y2O3:Al2O3 from 1:3 to 1:4 do not significantly change this property. Under the conditions used, the maximum temperature for liquid phase sintered SiC application must be limited to 750°C, which allows its use as a component of heat exchanges, bearings, pump bearings, gas turbines and sensors of automotive and aeronautical engines.

Silicon carbide is a useful monolithic and matrix ceramic due to its excellent mechanical properties and corrosion/oxidation resistance at high temperature. This makes it an attractive material for use in advanced applications, such as aircraft engines and high-speed flight. In this study, additively manufactured monolithic SiC and Cf/SiC CMCs, processed via direct ink writing (DIW) of a 53 vol% colloidal suspension, achieved >96% theoretical density through pressureless sintering. When present, fibers are aligned in the direction of the print path. Five different print paths were studied, including a 0o path, 90o path, 0/90o path, 0/15/30/45/60/75/90o path, and 0/30/60/90/60/30/0o path. Four-point bend testing was performed to determine flexural strength and Weibull analysis was performed. Strengths were highest for the 0o print path. The characteristic strength, σo, of this print path was 375 MPa with a Weibull modulus of 7.4 for monolithic SiC and a σo of 361 MPa with a Weibull modulus of 10.7 for Cf/SiC. Weibull modulus was greater for Cf/SiC samples compared to identically printed monolithic SiC samples. SEM and optical microscopy were used to analyze printed parts which showed a high degree of fiber alignment in the direction of the print. Fiber pullout was observed on the fracture surface, as well as intragranular fracture.

Philosophiae Doctor - PhD / The framework agreement in the year 2000 by the international community to launch Generation IV program with 10 nations, to develop safe and reliable nuclear reactors gave rise to the increased interest in the studies of SiC and the effect of different irradiations on solids. Silicon carbide is a preferred candidate used in harsh environments due to its excellent properties such as high chemical stability and strong mechanical strength. The PBMR technology promises to be the safest of all nuclear technology that have been developed before. SiC has been considered one candidate material being used in the fabrication of pebble bed fuel cell. Its outstanding physical and chemical properties even at high temperatures render it a material of choice for the future nuclear industry as whole and PBMR in particular. Due to the hostile environment created during the normal reactor operation, some of these excellent properties are compromised. In order to use this material in such conditions, it should have at least a near perfect crystal lattice to prevent defects that could compromise its strength and performance. A proper knowledge of the behavior of radiation-induced defects in SiC is vital. During irradiation, a disordered crystal lattice occurs, resulting in the production of defects in the lattice. These defects lead to the degradation of these excellent properties of a particular material. This thesis investigates the effects of various radiation effects to 6H-SiC. We have investigated the effects of radiation induced damages to SiC, with a description of the beds and the importance of the stability of the SiC-C interface upon the effects of radiations (y-rays, hot neutrons). The irradiated samples of 6H-SiC have been studied with various spectroscopic and structural characterization methods. The surface sensitive techniques such as Raman spectroscopy, UV-Vis, Photoluminescence and Atomic Force Microscopy will be employed in several complimentary ways to probe the effect of irradiation on SiC. The obtained results are discussed in details.

The solid-state interaction of palladium (Pd) with single crystal silicon carbide (6H- SiC) before and after annealing has been investigated using Rutherford backscattering spectrometry (RBS) in conjunction with RUMP simulation package, time-of-flight elastic recoil detector analysis (ToF-ERDA), glancing incident X-ray diffraction (GIXRD) and scanning electron microscopy (SEM). A thin layer of Pd (500 A) was deposited onto a clean 6H-SiC substrate at room temper- ature. The prepared difusion couples were then annealed in vacuum at different annealing temperatures for a maximum period of 1 h. The annealing temperature ranged from 2000C to 8000C. The composition of the as-deposited and the annealed samples was measured by using a He+ beam with an energy of 1.6 MeV. The ToF-ERDA measurements were per- formed on the as-deposited sample by using a high energy copper beam (about 30 MeV) for elemental depth distribution. The GIXRD measurements performed on the samples were able to identify the phases that form before and after annealing. The SEM micrograph obtained during this study gave some insight on the surface morphology of the samples before and after annealing. Our results obtained during this study showed that Pd reacts with SiC after annealing at 4000C resulting in the formation of metal-rich silicides and some unreacted Pd. Annealing at higher temperatures (5000C and 6000C) produced metal-rich silicides, which continued to grow until all the Pd has been consumed. Annealing at even higher temperatures (7000C and 8000C), the metal-rich silicides disappear and the silicon rich silicides start appearing. These appear by simply consuming the metal-rich silicides, resulting in the formation of two or more phases. The behaviour of the interaction between Pd with 6H-SiC is different than the Pd-Si system. The reaction temperature of the Pd/SiC are much more higher than those of the Pd/Si system. That is, Pd reacts with Si at temperatures as low as 2500C, while it starts to react with SiC at an annealing temperature of 4000C. In addition to this silicides such as Pd9Si2, Pd4Si form at the initial reaction temperature followed by the formation of the Pd2Si phase at the temperatures above 6000C for the Pd/SiC system. Meanwhile in the Pd/Si system the Pd2Si phase remains stable even after annealing at 8000C. No carbon compounds were observed in the temperature range used in this study and the formation of silicides were found to be accompanied by the formation of free carbon which remained immobile in the system. / Dissertation (MSc)--University of Pretoria, 2012. / Physics / unrestricted

In this thesis, growth of ZrB2 thin films by direct current magnetron sputtering is investigatedusing a high vacuum industrial scale deposition system and an ultra-high vacuum laboratory scalesystem. The films were grown from ZrB2 compound targets at temperatures ranging from ambient (without external heating) to 900 °C and with substrate biases from -20 to -120 V. Short deposition times of typically 100 or 300 s and high growth rates of 80-180 nm/min were emphasized to yield films with thicknesses of 300-400 nm. The films were characterized by thinfilm X-ray diffraction with the techniques θ/2θ and ω scans, pole figure measurements andreciprocal space mapping, scanning and transmission electron microscopy, elastic recoil detection analysis and four point probe measurements. The substrates applied were Si(100), Si(111),4H-SiC(0001) and GaN(0001) epilayers grown on 4H-SiC. The Si(111), 4H-SiC(0001) substrates and GaN(0001) epilayers were chosen given their small lattice mismatches to ZrB2 making them suitable for epitaxial growth.The films deposited in the industrial system were found to be close to stoichiometric with a low degree of contaminants, with O being the most abundant at a level of &lt; 1 at.%. Furthermore, the structure of the films is temperature dependent as films deposited in this system without external heating are fiber textured with a 0001-orientation while the films deposited at 550 °C exhibitrandom orientation. In contrast, epitaxial growth was demonstrated in the laboratory scale system on etched 4H-SiC(0001) and Si(111) deposited at 900 °C following outgassing of the substrates at 300 °C and in-situ heat treatment at the applied growth temperature to remove the native oxides. However, films grown on GaN(0001) were found to be 0001 textured at the applied deposition conditions, which make further studies necessary to enable epitaxial growth on this substrate material. Four point probe measurements on the films deposited in the industrial system show typical resistivity values ranging from ˜95 to 200 μΩcm with a trend to lower values for the films deposited at higher temperatures and at higher substrate bias voltages.

The work function and electronic structure of epitaxial graphene as well as of quasi-freestanding graphene multilayer samples were studied by Kelvin probe and angle resolved photoelectron spectroscopy. The work function converges towards the value of graphite as the number of layers is increased. Thereby, n-type doped epitaxial graphene layers have a work function lower than graphite and p-type doped quasi-freestanding graphene layers exhibit a work function higher than graphite. We explain the behaviour by the flling of the pi-bands due to substrate interactions.