Orientador: Maria Helena Robert / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-23T10:48:15Z (GMT). No. of bitstreams: 1 Ferreira_LygiaMariaPolicarpio_M.pdf: 14529538 bytes, checksum: f34c04de7772ea86ea3d164ba7a34c41 (MD5) Previous issue date: 2013 / Resumo: Este trabalho explora a aplicação da tecnologia de metais semi-sólidos para a fabricação de compósitos de matriz metálica (CMM), e ainda a possibilidade de reciclagem de cavacos de usinagem para a produção destes materiais, buscando o desenvolvimento de uma engenharia de baixo custo. São empregados como matriz a liga AA7075, dada a grande geração de cavacos de usinagem na indústria aeronáutica, e SiC ou NiAl2O4 particulados como reforços. O processo é baseado na tixoconformação de misturas de cavacos e partículas de reforço; é esperada a penetração destas últimas em contornos de glóbulos, contendo líquido, no interior do cavaco no estado semi-sólido. São avaliados parâmetros de processo e sua influência na qualidade do produto, em particular na distribuição de reforço na matriz e interação matriz/reforço. Compósitos tixoconformados contendo 10, 20 e 30% em peso de SiC são avaliados mecanicamente através de ensaios de microindentação instrumentada e desgaste micro-abrasivo. Os resultados mostraram, de modo geral, a viabilidade do processo proposto para a fabricação de diferentes tipos de compósitos, o qual envolve procedimento simples e de reduzido custo, além de mostrar a possibilidade de produção de materiais com boas propriedades mecânicas a partir da reciclagem de cavacos, particularmente importante em uma indústria que envolve elevada demanda de energia, como a do Al. Os resultados indicaram que a qualidade geral do produto, em termos de distribuição do reforço e interação reforço/matriz dependem da adequada seleção dos parâmetros de processo: temperatura, tempo de aquecimento, pressão de tixoconformação. Entre as várias condições estudadas, as composições nas quais foi utilizado NiAl2O4 como reforço apresentaram melhores características microestruturais, com melhor interface entre matriz e reforço e baixa porosidade. Boa dispersão das partículas de reforço e baixa porosidade também foram observados para compósitos reforçados com SiC nos quais foram adicionadas partículas finas de silício e alumina / Abstract: This work explores the application of semi-solid technology to produce metal matrix composites, and also the possibility of using machining chips as raw material. The main aim is to develop a process inserted in a low cost engineering concept. To achieve this general objective, the alloy AA7075 is used as matrix, once a significant amount of rejected chips of this high resistance, low weight alloy is generated in the aeronautical industry. As reinforcing material, SiC or NiAl2O4 particles are used. The proposed process is based on the thixoforming of pre-compacted mixtures of chips and reinforcing particles; it is expected the penetration of reinforcing particles within the semi-solid, thixotropic material. It is analyzed the influence of processing parameters in the final quality of products, particularly in the reinforcement dispersion in the matrix and matrix/reinforcement interface. Thixoformed composites containing 10, 20 and 30% weight SiC are produced and evaluated concerning mechanical properties through indentation tests and micro wear. Results showed the general viability of producing composites by the proposed technique, based on a simple and low cost procedure. It was also shown the possibility of producing materials with good mechanical properties from recycled chips, which is particularly important in the high energy demanding Aluminium industry. Results showed the importance of choosing appropriate processing parameters (temperature, heating rate / soaking time and thixoforming pressure), to achieve desired product quality. Among the various conditions studied, the compositions in which NiAl2O4 was used as reinforcement showed better microstructural characteristics with better interface between matrix and reinforcement, and lower porosity. Good dispersion of the reinforcement particles and low porosity were also observed for SiC reinforced composites in which fine particles of silicon and alumina were added / Mestrado / Materiais e Processos de Fabricação / Mestra em Engenharia Mecânica

M.Sc. (Physics) / Amorphous 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 latter problem were investigated. The photo-degradation was investigated by illuminating films of a-Si:H with simulated sunlight for different periods of time and then thermally annealing them. The change in the optical properties were investigated with the aid of optical transmission spectroscopy. The films were also characterized by Fourier Transform Infra-Red (FTIR) spectroscopy. The change in the electrical properties of the intrinsic films was determined as function of temperature and total photon flux. No change in the optical properties could be detected. The illumination had-no effect on the FTIR measurements. It seems as if the hydrogen is not involved in the microscopic processes leading to the Staebler-Wronski Effect (SWE). The effect of the photo-degradation manifests itself in a drop in the the dark conductivity and photoconductivity over the total temperature range that was investigated. The observed phenomena are 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. The measurements of photoconductivity as function of photon energy show that these defect levels increase the absorption 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. It was found that the light introduced defects deep in the band gap. The concentration of the defects increases with illumination, but saturates after about 24 hours of illumination. The defects could be annealed almost completely. The microscopic processes causing the photo-degradation of α Si:H solar cells were investigated by comparing the different theoretical models explaining the SWE with the results obtained during this project.

L’objectif de cette thèse est d’étudier le comportement plastique du carbure de silicium dans le domaine fragile. A cette fin, des essais de déformation par micro et nanoindentation ont été réalisés sur des échantillons monocristallins de SiC (4H et 3C). Des couches homoépitaxiées de 4H-SiC de différents dopages électroniques ont été étudiées. Ces couches présentent des caractéristiques mécaniques différentes en fonction du dopage : le dopage de type p durcit le matériau par rapport au dopage de type n ou au matériau intrinsèque. De plus, l'analyse des courbes charge-enfoncement obtenue en nanoindentation montre que la nucléation des dislocations est plus difficile lorsque le matériau est dopé de type p par rapport au matériau dopé n ou intrinsèque. Ceci est confirmé par les microstructures de déformation observées en Microscopie Electronique en Transmission (MET). Les observations par MET montrent que les dislocations introduites à l'ambiante autour des empreintes sont parfaites et glissent dans les plans {0001} dans le 4H-SiC et dans les plans {111} dans le 3C-SiC. Elles sont orientées principalement le long de la direction vis. Les sites de nanoindentation à température ambiante des couches 4H homoepitaxiées ont été particulièrement étudiés. On met en évidence que les sites de nucléation des dislocations sont vraisemblablement situés dans les plans {1100}, les dislocations se développant par la suite dans le plan basal. La nature des cœurs des dislocations parfaites a été déterminée par la technique LACBED. Ces dislocations parfaites ont un cœur silicium en mode shuffle. Un changement de mécanisme de plasticité est observé par MET pour les échantillons indentés 800 / The aim of this thesis is to study the plastic behaviour of silicon carbide in the brittle domain. For this purpose micro and nanoindentation deformation tests were performed on single crystal specimens of SiC (4H and 3C). Homoepitaxial layers of 4H-SiC with different doping have been studied. These layers show different mechanical characteristics as a function of doping: p-type doping hardens the material as compared to n-type doping or intrinsic material. In addition, load-penetration depthcurves show that the nucleation of dislocations is more difficult in p-doped material as compared to intrinsic or n-doped material. This is confirmed by deformation microstructures observations using Transmission Electron Microscope (TEM). TEM observations show that dislocations introduced around the imprints at room temperature are perfect dislocations gliding in the {0001} plane in 4H-SiC and in the {111} plane in 3C-SiC. They are mostly screw oriented. Room temperature nanoidentation imprints of4H homoepitaxied layers have been extensively studied. It is evidenced that dislocation nucleation sites are likely to be located in {1100} planes and that dislocations bow out subsequently in the basal plane. The core nature of perfect dislocations has been determined using the LACBED technique. Those perfect dislocations have a silicon core in the shuffle mode. TEM observations on specimens indented at 800ºC indicate a change in plastic deformation mechanism. At this temperature, partial dislocations with large stacking faults are observed.

In modern high temperature nuclear reactors, silicon carbide (SiC) is used as the main diffusion barrier for the fission products in coated fuel spheres called TRISO particles. In the TRISO particle, pyrolytic carbon and SiC layers retain most of the important fission products like xenon, krypton and cesium effectively at temperatures up to 1000 oC. Previous studies have shown that 400 oC to 600 oC implantation of heavy ions into single crystal 6H-SiC causes the SiC to remain crystalline with many point defects and dislocation loops (damage). The release of Xe at annealing temperatures above 1400 oC is governed by the normal volume diffusion without any hindrance of trapping effects. In this study two phenomena in single crystal 6H-SiC implanted by 360 keV Xenon ions were studied using Rutherford Backscattering Spectroscopy (RBS) and channeling. Radiation damage and its annealing behavior at annealing temperatures ranging from 1000 oC to 1500 oC, and the diffusion of xenon in 6H-SiC at these annealing temperatures were investigated. 360keV xenon ions were implanted into a single crystalline wafer (6H-SiC) at 600 oC with a fluence of 1 × 1016 cm-2. The sample was vacuum annealed in a computer control Webb 77 graphite furnace. Depth profiles were obtained by Rutherford backscattering spectrometry (RBS). The same set-up was used to investigate radiation damage of the 6H-SiC sample by channeling spectroscopy. Isochronal annealing was performed at temperatures ranging from 1000 to 1500 °C in steps of 100 oC for 5 hours. Channeling revealed that the 6H-SiC sample retained most of its crystal structure when xenon was implanted at 600 °C. Annealing of the radiation damage took place when the sample was heat treated at temperatures ranging from 1000 oC to 1500 oC. The damage peak almost disappears at 1500 oC but the virgin spectrum was not achieved. This happened because of dechanneling due to extended defects like dislocations remaining in the implanted region. RBS profiles showed that no diffusion of the Xe occurred when the sample was annealed at temperatures from 1000 oC to 1400 oC. A slight shift of the xenon peak position towards the surface after annealing at 1400 °C was observed for 600 oC implantation. After annealing at 1500o C, a shift toward the surface accompanied by a broadening of the Xe peak indicating that diffusion took place. This diffusion was not accompanied by a loss of xenon from the SiC surface. The shift towards the surface is due to thermal etching of the SiC at 1400-1500 °C. Modern high temperature gas-cooled reactors operate at temperatures above 600 oC in the range of 750 oC to 950 oC. Consequently, our results indicate that the volume diffusion of Xenon in SiC is not significant in SiC coated fuel particles. / Dissertation (MSc)--University of Pretoria, 2014. / gm2014 / Physics / unrestricted

Doctor of Philosophy / Department of Chemical Engineering / James H. Edgar / The effects of process conditions on the material’s properties were investigated for the sublimation growth of aluminum nitride and the epitaxial growth of silicon carbide. Since the mid 1990’s, these semiconductors have made new types of high power electronics and short wavelength optoelectronics that were never before feasible. The sublimation growth of AlN crystals on SiC seeds was carried out to produce high quality AlN bulk crystals. Si-face, 3.5 º off-axis 6H-SiC (0001) and 8 º off-axis 4H-SiC (0001) wafers were used as the substrates. An investigation of the initial growth demonstrated 1800 – 1850ºC was the optimum temperature for AlN growth. By optimizing the temperature gradient, large area AlN layer was deposited. Consecutive growths and continuous growth were performed to enlarge the crystal thickness. Single-crystalline AlN layers, each with a thickness of 2 mm and a diameter of 20 mm, were produced. X-ray diffraction confirmed the grown AlN had good crystal quality. Approximately 3 – 6 at% of Si and 5 – 8 at% of C were detected in the crystals by x-ray photoelectron spectroscopy, which came from the decomposition of SiC seeds and the degradation of the graphite components in the furnace. Molten KOH/NaOH etching revealed the dislocation density decreased from 108 cm-2 to 106 cm-2 as the AlN layer thickness increased from 30 μm to 2 mm. Epitaxial growth of SiC was carried out in a chemical vapor deposition system. High-quality 6H-SiC and 4H-SiC homoepitaxial films were produced at growth rates up to 80 μm/hr by using a novel single precursor, methyltrichlorosilane (MTS). Inclusions of 3C-SiC were circumvented by employing 8º mis-orientated substrates. Adjusting the H2/Ar flow ratio in the carrier gas effectively changed the C/Si ratio in the gas phase due to the reaction between H2 and the graphite heater; thereby, influencing surface roughness and dislocation density. Low H2/Ar ratios of 0.1 and 0.125 produced smooth surfaces without step-bunching. Higher H2/Ar ratios of 0.2 and 0.33 enhanced the conversion of basal plane dislocations into threading edge dislocations, and reduced the density of basal plane dislocations to approximately 600 cm-2.

Master of Science / Department of Chemical Engineering / James H. Edgar / 4H-silicon carbide (4H-SiC) is a wide band gap semiconductor with outstanding capabilities for high temperature, high power, and high frequency electronic device applications. Advances in its processing technology have resulted in large micropipe-free single crystals and high speed epitaxial growth on off-axis silicon face substrates. Extraordinarily high growth rates of high quality epitaxial films (>100 [Mu]m per hour) have been achieved, but only on off-axis substrates (misoriented 4° to 8° from the (0001) crystallographic plane). There is a strong incentive to procure an on-axis growth procedure, due to the excessive waste of high quality single crystal associated with wafering off-axis substrates. The purpose of this research was to develop a reliable process for homoepitaxial growth of 4H-SiC on on-axis 4H-SiC. Typically the use of on-axis SiC for epitaxial growth is undesired due to the increased probability of 3C-SiC inclusions and polycrystalline growth. However, it is believed that the presence of chlorine during reaction may reduce the presence of 3C-SiC and improve the quality of the epitaxial film. Therefore homoepitaxial SiC was deposited using methyltrichlorosilane (MTS) and ethane sources with carrier gases consisting of argon-hydrogen mixtures. Ethane was used to increase the C/Si ratio, to aid in the prevention of 3C-SiC, and to help eliminate silicon droplets deposited during epitaxial growth. Deposition occurred in a homemade, quartz, cold wall chemical vapor deposition reactor. Epitaxial films on on-axis 4H-SiC were deposited without the presence of 3C-SiC inclusions or polycrystalline SiC, as observed by defect selective etching, scanning electron microscopy and optical microscopy. Large defect free areas, [similar to]5 mm[superscript]2, with epitaxial film thicknesses of [similar to]6 [Mu]m were grown on on-axis 4H-SiC. Epitaxial films had approximately an 80%, [similar to]20 cm[superscript]-2, decrease in defect density as compared to the substrates. The growth rate was independent of face polarity and orientation of the substrate. The optimal temperature for hydrogen etching, to promote the smoothest epitaxial films for on-axis substrates (both C- and Si-polarities), is [similar to]1550 °C for 10 minutes in the presence of 2 slm hydrogen. The optimum C/Si ratio for epitaxial growth on on-axis 4H-SiC is 1; excess carbon resulted in the codeposition of graphite and cone-shaped silicon carbide defects.

The current temperature operating range of semiconducting materials is given to be between -10°C and 105°C. Current plans are to increase this range due to an industry demand for semiconducting materials able to operate outside these bounds. Wide bandgap semiconductor materials have been researched for some time in an effort to manufacture commercially viable controlled semiconductor switches. Samples of such a controlled switch have been obtained and promises to be close to commercial rollout. This controlled switch takes on the form of a Junction Field Effect Transistor (JFET), which is a depletion mode component. Depletion mode switches are normally-on, and have thus far not been used in the power electronics field. Due to difficult switching and availibility of component which are switched more easily, these components were quicklier and easierly adopted for use. The Silicon Carbide JFET promises to circumvent these cons with the ability to block higher voltages, at higher temperatures and at higher speeds. Lower on resistance and smaller size of substrates are other benefits that the use of Silicon Carbide semiconducting material promises. A variety of properties of the Silicon Carbide JFET are investigated. Some of these properties include the component on-resistance, gate charge, breakthrough voltage of the gate and leakage current of the channel. Switching performance is also investigated as well as the reverse recovery of the body diode. All properties were measured as a function of temperature in an effort to describe the difference in performance at high temperatures. / Prof. Ivan Hofsajer

Philosophiae Doctor - PhD / In this thesis, intrinsic hydrogenated nanocrystalline silicon thin films for solar cells application have been deposited by means of the hot – wire chemical vapour deposition (HWCVD) technique and have been characterised for their performance. It is noticed that hydrogenated nanocrystalline silicon is similar in some aspects (mainly optical) to its counterpart amorphous silicon actually used as the intrinsic layer in the photovoltaic industry. Substantial differences between the two materials have been found however in their respective structural and electronic properties. We show that hydrogenated nanocrystalline silicon retains good absorption coefficients known for amorphous silicon in the visible region. The order improvement and a reduced content of the bonded hydrogen in the films are linked to their good stability. We argue that provided a moderate hydrogen dilution ratio in the monosilane gas and efficient process pressure in the deposition chamber, intrinsic hydrogenated nanocrystalline silicon with photosensitivity better than 102 and most importantly resistant to the Staebler Wronski effect (SWE) can be produced. This work explores the optical, structural and electronic properties of this promising material whose study – samples have been exclusively produced in the HWCVD reactors based in the Solar Cells laboratory of the Physics department at the University of the Western Cape. / South Africa

In the TRISO (tristructural isotropic) coated fuel particle used in the High Temperature Reactor, the most important layer is a silicon carbide layer which acts as a pressure vessel. In this study, we have focused our study on the investigation of the Young’s modulus, hardness, residual stress, and fracture toughness of the SiC layer. Moreover, microstructures and impurities in silicon carbide were characterised and then related to both Young’s modulus and hardness of the SiC layer. Both nanoindentation and micro-indentation were used to determine Young’s modulus and hardness of the SiC. Raman spectroscopy, X-ray diffraction, and scanning electron microscopy techniques were used to examine impurities, phases and microstructure of silicon carbide layers, respectively. Young’s modulus was measured at different positions of a polished surface of the SiC with different CVD growth and crystal orientations. With help from the finite element modelling, it has been found that Young’s modulus of the SiC is dependent on the grain orientation of the SiC. Mechanical properties of silicon carbide are affected by the presence of excess silicon, excess carbon, stacking faults, texture, grain size, property of grain boundary. The effect of these factors on Young’s modulus and hardness, are investigated with the orthogonal analysis. The analysis concludes that the most important factor on Young’s modulus is texture while the most significant factor on hardness is grain boundary. Grain size is secondarily important factor to affect hardness. Stacking faults and impurities almost have no influence on Young’s modulus and hardness. The residual stress in the silicon carbide layer was measured based on the peak shift in Raman spectra of the SiC and is in a range of 150-300 MPa. Fracture resistance in the radial direction of the SiC layer is larger than those in the circumferential direction. The difference is controlled by the layer-like structure of the SiC coating.

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