The aim of this thesis is to investigate the effects of interstellar processing using presolar samples. Dust in the interstellar medium is predicted to have experienced grain-grain and grain-gas collisions, cosmic-ray bombardment, or the formation of ices on their surfaces. Each process is likely to have altered presolar grains. The grains are extracted from meteorites and can be analyzed in the laboratory to try and understand these processes. The main analytical tool used in this research was a new time-of-flight secondary ion mass spectrometry instrument equipped with a Au-cluster primary ion source. Analysis of presolar grains required that a rigorous experimental procedure was developed. A depth-profiling technique for the analysis of micron-sized samples was produced and the limitations of the technique considered. Secondary ion mass spectrometry suffers from matrix effects, so homogeneous silicate glass standards were analyzed. The use of Au-cluster primary ions was shown to enhance practical secondary ion yields relative to those with Au+, consistent with increased sputter rates. Relative sensitivity factors for major and trace elements in the standards were obtained using both normal and delayed secondary ion extraction techniques. Depth-profiles of Li, B, Mg, Al, K, Ca, Ti, V, Cr and Fe were obtained from eleven presolar SiC grains. In some SiC grains, the abundances of several elements were up to orders-of-magnitude higher in the outer ~200nm relative to the grain cores. This was attributed to the implantation of interstellar matter, accelerated to velocities of ~1000kms-1 by supernovae shockwaves. Other SiC grains contained homogeneously distributed trace elements, or evidence of elemental zoning, which could be explained by condensation processes around the grains' parent stars. These grains must have experienced minimal processing in the interstellar medium. It is suggested that the two populations represent SiC grains whose residence times in the interstellar medium significantly differed, consistent with previous findings of noble gas and Li isotopic studies. A further study investigated carbonaceous grains isolated from the Murchison meteorite using a size and density procedure adapted for presolar graphite. No graphite grains were found and possible reasons for this are discussed. The structural and isotopic natures of thirty-three carbonaceous grains were determined by correlated, multi-instrument analyses. The grains contained solar C, N and O isotopic compositions. Deuterium was enriched in the grains with δD values up to +333 ± 110‰. These enrichments suggest exchange of H with cold interstellar gas in the outer part of the early solar nebula or interstellar medium. Raman spectroscopic and transmission electron microscopic analysis showed the grains to be composed of carbon more structurally disordered and amorphous than most carbonaceous phases observed in extra-terrestrial samples. It is argued that amorphization of the grains occurred through solar wind ion irradiation in the proto-solar nebula. This model is supported by previous studies of terrestrial soot and carbon-rich ices irradiated by H⁺ and He⁺ ion doses of ~10¹⁵ - 10¹⁶ ions cm⁻². Implantation and mixing of H⁺ ions is likely to have diluted the grains' original H isotopic composition.

In a sub-wavelength scale, even approaching to the atomic scale, nanoscale physics shows various novel phenomena. Since it has been named, nanoscience and nanotechnology has been employed to explore and exploit this small scale world. For example, with various functionalized features, nanowire (NW) has been making its leading position in the researches of physics, chemistry, biology, and engineering as a miniaturized building block. Its individual characteristic shows superior and unique features compared with its bulk counterpart. As one part of these research efforts and progresses, and with a part of the fulfillment of degree study, novel methodologies and device structures in nanoscale were devised and developed to show the abilities of high performing thermoelectrical, biological, and optical applications. A single β-SiC NW was characterized for its thermoelectric properties (thermal conductivity, Seebeck coefficient, and figure of merit) to compare with its bulk counterpart. The combined structure of Ag NW and ND was made to exhibit its ability of clear imaging of a fluorescent cell. And a plasmonic nanosture of silver (Ag) nanodot array and a β-SiC NW was fabricated to show a high efficient light harvesting device that allows us to make a better efficient solar cell. Novel nanomanipulation techniques were developed and employed in order to fabricate all of these measurement platforms. Additionally, one of these methodological approaches was used to successfully isolate a few layer graphene.

The diffusion of iodine implanted 6H-SiC has been investigated using Rutherford backscattering Spectrometry (RBS). SiC is used as the main barrier in the modern high temperature gas cooled reactors. An understanding of the transport behaviour of iodine in 6H-SiC will shed some light into SiC’s effectiveness in the retention of fission products. Room temperature iodine implantations were performed to a fluence of 1 x 1016 iodine ions per cm2. Iodine depth profiles were determined by Rutherford backscattering spectroscopy. Previous studies indicate that iodine diffusion cannot be detected for temperatures below 1000oC. Isochronal annealing experiments for 5 hours cycles were then performed starting from 1000oC. 5 hour annealing cycles at 1200oC and 1300oC were also performed. These relatively short annealing cycles were used to study the evolution of the iodine depth profiles and to determine the temperature where the first noticeable broadening of the iodine implanted profile is observed. Broadening of the iodine profile was initially observed at temperatures around 1200oC. Isothermal annealing cycles were then performed at this temperature of 1200oC where the broadening was first observed. The annealing experiments were performed for a total annealing time of 60 hours. Isothermal annealing experiments were also performed at a lower temperature of 1100oC also for a total annealing time of 60 hours. The experimental data was analyzed by fitting the iodine depth profiles to a Gaussian function using the GENPLOT program. Diffusion coefficients were obtained by comparing the full widths at half maximum, (FWHM) of the iodine profiles before and after annealing. Results for 1100oC show that the diffusion coefficient is below 10-21 m2 s-1 . Solving the diffusion equation numerically for this diffusion coefficient shows that the iodine does not traverse more than 2 ìm through SiC after annealing for one year. Intact SiC coatings which have a thickness of 35 ìm in the fuel particles will therefore be able to prevent iodine release from the particle at this temperature. Results for 1200oC show that the diffusion coefficient is in the order of 10-20 m2 s-1. The results also reveal that iodine loss through the front surface is relatively low with only 15% of iodine loss observed after annealing for a total time of 60 hours. Further indications of SiC ability to retain iodine are observed when an iodine peak is still present after annealing for 30 hours at a temperature of 1500oC. Radiation damaged produced during ion implantation was investigated by Rutherford backscattering in conjunction with channeling. The results of the RBS/channeling reveal that the room temperature implantations produce an amorphous layer from the surface up to a depth of 260 nm. Some epitaxial regrowth is observed starting at approximately 1000oC. / Dissertation (MSc)--University of Pretoria, 2011. / Physics / unrestricted

The need for high-density power electronics converters becomes more critical by the day as energy consumption continues to grow across the world. Specifically, the need for medium-voltage (MV) high-density converters in power distribution systems, electric ships, and airplanes become more critical as weight and space becomes more a premium. The limited space and footprint require new packaging technologies and methods to develop an integrated power converter. The advancement of wide-bandgap (WBG) devices like silicon carbide (SiC) allows converters to have higher power and faster switching... To benefit from these devices, the packaging of the converter needs to be carefully considered. This thesis presents the design of a 250 kW integrated power electronics building block (iPEBB) for future electric system applications. This work explores the common substrate concept that would serve as the electrical, thermal, and mechanical foundation for the converter. State-of-the-art organic direct-bonded copper (ODBC) is explored to serve as the material foundation for the common substrate. Multi-domain simulations are used to design the integrated SiC bridges to achieve a power loop inductance of 3.5 nH, a maximum temperature of 175 °C, and a weight of 16 kg. ODBC and silicon nitride switching cells are packaged and analyzed in order to see the benefits on a multi-layer design as well as determining electrical and thermal trade-offs. The insights gained from hardware testing will help in the redesign and refinement of the iPEBB. / M.S. / This thesis presents the design of an integrated power electronics building block (iPEBB) for high-density systems. The PEBB concept allows for modular converters that can perform various power conversions. The design begins with exploring state-of-the-art substrates that will serve as the foundation for the iPEBB. Due to the integrated design, the substrate plays a vital role in the thermal, electrical, and mechanical performance, and contributes to the weight and reliability of the iPEBB. State-of-the-art organic direct-bonded copper (ODBC) substrates and multi-layer silicon nitride substrates are explored in this work. The ODBC is used to develop a common substrate for the converter, which allows for a high level of integration between different SiC half-bridges. Switching-cell prototypes based on the ODBC and multi-layer silicon nitride are fabricated to provide insight into the electrical and thermal performance of different substrates. This information will aid in the further redesign and refinement of the iPEBB concept.

La presente tesis se centra en el estudio y desarrollo de nuevos materiales porosos basados en carburo de molibdeno y silicio (Mo2C y SiC) para ser utilizado en reacciones catalíticas de interés medioambiental y económico. Concretamente, en la hidrogenación de crotonaldehído, Reverse water gas shift (RWGS) y hidrogenación de nitroarenos. Respecto al carburo de silicio se ha desarrollado un material mesoporoso de alta área y estudiado su limitación como soporte de catalizadores a alta temperatura. Mientras que para el carburo de molibdeno se han desarrollado dos materiales porosos a partir de diferentes precursores y se ha estudiado su actividad catalítica obteniendo resultados prometedores.

Silicon carbide is known for its potential in high power, high radiation and high temperature applications. It is also one of the first materials observed with phenomenon of electroluminescence. Depending on the mechanism of recombination, carriers inside silicon carbide recombine and release photons at different wavelengths. As one of the third-generation semiconductors, many studies focus on the effects of defects on silicon carbide device stability and performance. Especially for defects like stacking faults, which can be generated either during fabrication or induced by current under forward bias, can cause severe device degradation and limits the use of silicon carbide. By testing electroluminescence of silicon carbide, one can analyses the recombination event and identify the defects that trapped carriers, as each recombination mechanism would be shown as a unique emission peak on the sample EL spectra. In addition to the as-grown and recombination-induced defects, the changes of spectrum due to stress and chemical etching indicate the influence of external factors to the defects that are either existed prior to the external forces or that were induced during the testing. Such analysis could be helpful to understand the defect generation mechanism, reduce the density of the defects and to create innovative ideas for future applications. A general introduction to silicon carbide will be given in Chapter 1 with some detailed description of silicon carbide defect generation and characterization mechanisms in Chapter 2. In Chapters 3 and 4, the focus is to analyse the external effects to the spectrum of 4H silicon carbide, like chemical etching and mechanical stress. Before giving the conclusion in Chapter 6, Chapter 5 will be focusing on analysing the effect of external forces on the silicon carbide with stacking faults existed prior to the testing. / Thesis / Master of Applied Science (MASc)

The goal of this research project was to develop a new implant annealing process using silane overpressure to maintain crystal integrity. After ion implantation the surface of the SiC wafer is damaged due to high energy of the implant ions. In addition the doping activation is very low. To overcome these problems a new implant annealing process was developed to rectify the surface damage and increase the dopant activation. SiC implant annealing was performed in the silicon carbide (SiC) chemical vapor deposition (CVD) reactor in the Emerging Materials Research Laboratory (EMRL) at Mississippi State University. A process was developed to eliminate surface step bunching, which is evident in argon annealed crystals. The process gas used in the new technique was silane (3 % SiH4 in 97% UHP Ar). The anneal run time was 30 minutes with argon flow rate at 6 slm and silane flow rate at 6 sccm. SiC material (n and p type epitaxial layers) and devices (JBS Diodes and LDMOSFET?s) were annealed using the silane over pressure developed during this research. The process results were characterized using tools such as optical micrograph, capacitance-voltage (C-V), Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). These characterization tools were mainly used to determine the surface roughness of the SiC crystal and the dopant activation after annealing. As compared to an Ar anneal, the SiC material and devices annealed in the silane ambient had a better surface. An empirical process chemistry model was developed to support the experimental results. The model developed showed that the partial pressure of Si is greater than the vapor pressure of SiC in the substrate. Thus it is believed that the partial pressure of Si suppressed any Si out-diffusion from the SiC substrate, thereby maintaining the crystal surface integrity. The model also provided silane flow rates for higher temperature anneals which may be necessary to fully activate other ion species.

Dans ce travail, deux matériaux réfractaires à base de SiC et à liaison nitrurée sont caractérisés au niveau microstructural et thermomécanique. Ces matériaux sont utilisés dans les Unités de Valorisation Energétique d'Ordures Ménagères (U.V.E.O.M.). Dans cette application, les revêtements réfractaires sont soumis à de fortes sollicitations comme des hautes températures, des gradients thermiques, des gradients de composition chimique ainsi que de la corrosion. Les matériaux SiC-Si3N4 et SiC-SiAlON étudiés sont constitués d'agrégats de SiC reliés entre eux par une phase liante. Cette dernière présente un fort taux de porosité ouverte sous la forme d'un réseau complexe. L'analyse microstructurale de la phase liante a été réalisée par microscopie électronique à balayage et le comportement thermomécanique de chaque matériau a été caractérisé en flexion 4-points et par échographie ultrasonore. Lors de l'utilisation dans les U.V.E.O.M., les réfractaires sont exposés à des phénomènes d'oxydation, en particulier en présence d'humidité. Ainsi, le comportement à l'oxydation de ces matériaux a été étudié sur la plage 800°C - 1200°C sous air sec, sous air ambiant et sous air humide. Les évolutions microstructurales liées à l'oxydation montrent l'influence du réseau de porosité sur les régimes d'oxydation qui caractérisent ces matériaux en fonction du niveau de température et des conditions environnementales. Par ailleurs, le comportement thermomécanique a été déterminé après des vieillissements dans chacun de ces régimes. Cette démarche de caractérisation a permis l'établissement de relations entre le comportement thermomécanique et les évolutions microstructurales associées à l'oxydation. Des modèles phénoménologiques d'oxydation sont présentés. Pour finir, des traitements sont proposés afin d'améliorer la durabilité des réfractaires SiC-Si3N4 et SiC-SiAlON dans les U.V.E.O.M. / In this work, the microstructure and the thermomechanical behaviour of two SiC-based refractories are studied.In waste-to-energy plants, these materials are subjected to severe sollicitations such as thermal gradients, chemical composition gradients and corrosion. SiC-Si3N4 and SiC-SiAlON refractories are composed by SiC agregates bonded with silicon nitride based phases. A complex porosity network characterises the bonding phase. Its microstructure is analysed with Scanning Electron Microscopy (SEM). The thermomechanical behaviour of each refractory is characterised with 4-points bending tests and ultrasonic pulse echo measurement of the Young's modulus. During the running of waste-to-energy plants, refractories are exposed to oxidation phenomenons. They are more critical with an increase of a water content in the air. Thus, the oxidation behaviour is studied between 800°C and 1200°C under dry air, ambiant air and humid air. The microstructural evolutions due to oxidation are influenced by the porosity network, which leads to two oxidation stages depending on temperature level and environnemental conditions. Thermomechanical behaviour is also determined for each oxidation stage. This characterisation approach permits to establish relationships between the thermomechanical behaviour and the microstructural evolutions during oxidation. Phenomenological modellings are presented. Finally, thermal treatments are proposed in order to improve the durability of SiC-Si3N4 and SiC-SiAlON refractories in waste-toenergy plants.

Les matériaux composites connaissent un large succès. En effet les Composites à Matrice Céramique (CMC) fonctionnant à haute température ont des performances inégalées en termes de fatigue thermomécanique. La durée de vie des CMC est pourtant limitée en raison de l'apparition précoce de fissures matricielles, ouvrant autant de portes à des environnements agressifs, entraînant un abattement prématuré des propriétés mécaniques. Arriver à retarder la fissuration matricielle devient donc une étape clé pour une future importante utilisation des CMC dans l'aéronautique ou l'aérospatial. Les travaux de cette thèse se sont inscrits dans cette logique, où pour protéger les fibres et l'interphase de l'oxydation et de la corrosion, les propriétés de la matrice céramique ont tenté d'être modifiées par l'incorporation de nanofibres en leur sein et par l'émoussement de leurs macropores résiduels. / Abstract

Na produção de cerâmicas de SiC, a utilização de aditivos que promovam a formação de uma fase líquida, diminui a temperatura de sinterização das cerâmicas produzidas. Neste trabalho foi escolhida a sinterização via fase líquida das cerâmicas de SiC com novos aditivos, o sistema Al2O3/TiO2 e Y2O3/TiO2, com pouca informação na literatura. Foi avaliado a molhabilidade dos aditivos nas cerâmicas de SiC, medindo os ângulos de contato e pontos de fusão, obtendo-se a melhor composição de cada aditivo para sinterizar o SiC. Para a produção das cerâmicas de SiC, barras foram sinterizadas nas temperaturas de 1850 e 1950°C, com taxas de aquecimento de 5 e 20°C/min e tempos de 30 e 60 minutos. O estudo da molhabilidade mostrou que as composições eutética e 5% abaixo da eutética, para os sistemas Al2O3/TiO2 e Y2O3/TiO2, respectivamente, possuem menores temperaturas de fusão e ângulos de contatos, evitando assim reações de decomposições da titânia, e, portanto, essas composições foram utilizadas para sinterização do SiC. As cerâmicas de SiC+Al2O3/TiO2, apresentaram porosidades entre 32-42%, exceto a tratada à 1950°C, 20°C/min por 30 minutos, com porosidadede 7%, estas com KIc de 3,4 MPa.m1/2, MOR de 122 MPa e E de 328 GPa. As cerâmicas de SiC+Y2O3/TiO2, apresentaram porosidades entre 33-43%. Na condição de 1850°C, 5°C/min por 30 minutos, foi obtida a maior porosidade de 43%, e apresentou valores de KIc 0,85, MOR de 86 MPa, e E de 107 GPa, valores menores se comparados com as cerâmicas de SiC+Al2O3/TiO2, devido à maior probabilidade de falha pela alta concentração de poros nas amostras. Houve um aumento do tamanho dos grãos à 1950°C e foram identificadas placas hexagonais características da fase ?-SiC. Os resultados mostraram que um maior tempo de residência no forno das amostras, com influência das reações de decomposição da titânia, influenciaram a densificação das cerâmicas de SiC. Com o aditivo Al2O3/TiO2 foi possível obter, em uma condição, cerâmicas densas. Para SiC+Y2O3/TiO2 foram obtidas cerâmicas porosas. / In the production of SiC- based ceramics, the use of additives that promote the formation of a liquid phase decreases the sintering temperature of the produced ceramics. In this work, liquid phase sintering of SiC ceramics was evaluated by introducing new additives, Al2O3/TiO2 and Y2O3/TiO2. It was evaluated the wettability of the additives in the SiC ceramics, measuring the contact angles and melting points, obtaining the best composition of each additive to sintering the SiC- based ceramics. To produce the SiC ceramics, the bars were sintered at the temperatures of 1850 and 1950°C, with heating rates of 5 and 20°C/min and times of 30 and 60 minutes. The study of wettability showed that the eutectic and 5% below eutectic compositions for the Al2O3/TiO2 and Y2O3/TiO2 systems, respectively, have lower contact angles and melting temperatures, avoiding decomposition reactions, and therefore, were used to sintering the SiC ceramics. The SiC+Al2O3/TiO2 ceramics showed porosities between 32-42%, except for those treated at 1950°C, 20°C/min for 30 minutes, with porosity at 7%, with KIC of 3,4 MPa.m1/2, MOR of 122 MPa and E of 328 GPa. The SiC+ Y2O3/TiO2 ceramics presented porosities between 33-43%. A higher porosity of 43% was obtained at 1850°C, 5°C/min for 30 minutes, and presented values of KIC 0,85, MOR of 86 MPa, and E of 107 GPa, lower values when compared with the SiC+Al2O3/TiO2 ceramics, due to the higher probability of failure due to the high pore concentration in the samples. There was an increase in grain size at 1950°C and hexagonal plates, characteristic of the ?-SiC phase, were identified. The results showed that a longer residence time of the samples in the furnace, influenced by the decomposition reactions of TiO2, influenced its densification. With the additive Al2O3/TiO2 it was possible to obtain, in one condition, dense ceramics. Porous ceramics were obtained for SiC+ Y2O3/TiO2.