M.Sc. (Physics) / Please refer to full text to view abstract

This master’s thesis deals with design and construction of experimental flyback converter with utilization of novel lossless clamp circuit for switching transistor and with utilization of silicon carbide devices. The issue of flyback converter for higher power and possible control strategies are discussed. The thesis also describes power and control circuits design, design of PCB, construction of the converter and measured waveforms. End of the thesis contains technical documentation of designed board. Designed converter was successfully built and tested.

The favourable physical and mechanical properties of silicon carbide (SiC) nanopowders allow application across many areas, including high-power, high-frequency electronics and high-temperature technologies. Many different synthesis methods for the creation of SiC nanoparticles have been studied, including carbothermic reduction, pulsed laser deposition, sol-gel processes, microwave heating and various plasma techniques. Among the different synthesis methods reported in the literature, very few experiments investigate the microwave-induced plasma synthesis of SiC nanoparticles. The few reported studies show promising results with regard to particle size and production rate. In this work, the synthesis of SiC nanoparticles from methyltrichlorosilane (MTS) is reported using a microwave-induced plasma, operating at atmospheric pressure. The investigation was done experimentally using a 1 500 W power supply, a microwave generator operating at 2.45 GHz, a stub tuner, a waveguide and a sliding short. Quartz tubes were used, in which the plasma was generated and maintained. Hydrogen served as an added reductant for the conversion reaction, and argon served as the MTS carrier gas. The parameters studied were the H2:MTS molar ratio and the total enthalpy, in the ranges 0 to 10 and 70 to 220 MJ/kg respectively. X-ray diffraction studies confirmed the presence of β-SiC and optical emission spectrometry showed the majority of peaks to be that of elementary silicon, carbon and argon, indicative of MTS decomposition in the plasma. Scanning electron microscopy shows average individual particle sizes ranging between 50 and 135 nm, whereas transmission electron microscopy shows average individual particle sizes ranging from 15 to 140 nm. Larger agglomerates are also present, ranging in sizes from 460 to 1 800 nm. Through response surface methodology (RSM), it was shown that the optimum conditions for the production of nanoparticles lie within the range of enthalpy > 180 MJ/kg and H2:MTS ratio of > 5. / Dissertation (MEng)--University of Pretoria, 2015. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

>Magister Scientiae - MSc / Amorphous silicon carbide (a-SiC) is a versatile material due to its interesting mechanical, chemical and optical properties that make it a candidate for application in solar cell technology. As a-SiC stoichiometry can be tuned over a large range, consequently is its bandgap. In this thesis, amorphous silicon carbide thin films for solar cells application have been deposited by means of the electron-beam physical vapour deposition (e-beam PVD) technique and have been isochronally annealed at varying temperatures. The structural and optical properties of the films have been investigated by Fourier transform Infrared and Raman spectroscopies, X-ray diffraction, Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy and UV-VIS-NIR spectroscopy. The effect of annealing is a gradual crystallization of the amorphous network of as-deposited silicon carbide films and consequently the microstructural and optical properties are altered. We showed that the microstructural changes of the as-deposited films depend on the annealing temperature. High temperature enhances the growth of Si and SiC nanocrystals in amorphous SiC matrix. Improved stoichiometry of SiC comes with high band gap of the material up to 2.53 eV which makes the films transparent to the visible radiation and thus they can be applied as window layer in solar cells.

This Ph.D. thesis carries out extensive and in-depth research on the packaging technology of silicon carbide (SiC) power modules, including new packaging structures, multi-physics modeling and optimal design methods for half-bridge power modules, manufacturing processes, and experimental validations. A new packaging scheme, the Silver-Sintered Molybdenum (SSM) packaging, is proposed in this thesis. It contains a molybdenum (Mo) -based insulated-metal-substrate (IMS) structure, nano-silver sintering die-attachments, and planar interconnections. This technology has the potential to increase the operating temperature of SiC power modules to above 200 degrees, and can greatly improve their lifetime. These advantages are verified by active power cycling and passive temperature cycling simulations. Analytical modeling methods for half-bridge power modules with the SSM packaging are also studied. A decoupled Fourier-based thermal model is introduced. This model considers the decoupling effect between different heat source regions and can give a three-dimensional analytical solution for the temperature field of a simplified half-bridge power module structure. In addition, based on the partial inductance model for rectangular busbars, an analytical stray inductance model for half-bridge power modules is also proposed. The accuracy of these two models is estimated by both numerical simulations and experiments. With the proposed analytical models, an optimal design method for half-bridge power modules with the SSM packaging is proposed in this study, which uses the particle swarm optimization algorithm. This method is successfully applied in the design of a prototype power module and is able to minimize the stray inductance and volume while maintaining desired junction temperatures. This thesis also introduces the manufacturing process of the prototype power module. Several new processes are proposed and validated, including a pressure-less nano-silver sintering process to bond SiC dies on Mo substrates, the formation of the Mo-based IMS structure, and the re-metallization of SiC dies. / Thesis / Doctor of Philosophy (PhD)

Femtosecond lasers deliver a high peak concentration of optical power while maintaining low average power. With an accompanying optical setup, this power can be focused and used for high-precision fabrication of metallized polymers via ablation, creating conductive structures on a thin film. These lasers can also be harnessed in tandem with hydrofluoric acid and the two-photon absorption principle to selectively etch silicon carbide, a very durable and machining-resistant semiconductor with desirable properties. This thesis presents improvements made to the Laser-Assisted Chemical Etching (LACE) technique and the ablation system. ��, the two- photon absorption coefficient of silicon carbide, is measured and characterized for each wafer using an optical system. The LACE etch rate of silicon carbide is found to be on average a quarter of a micrometer per 120 seconds. A new destructive imaging technique for characterizing high- aspect-ratio through-wafer non-line-of-sight features in silicon carbide is discussed, with the result of an SEM image of a LACE feature that was impossible to achieve previously. The photolithography process necessary to make use of those through-wafer features is explored. A method of leveraging image processing to maintain the focus of the laser close to the machined sample is explained, resulting in the creation of terahertz metal- mesh filters with a near one-hundred-percent success rate. It is recommended that future work explore the use of an installed confocal microscopy as a method of surface-tracking and result characterization.

No description available.

Discontinuously reinforced MMCs with optimized microstructure are sought after for exceptional high strain rate behavior. The microstructure evolution of a stir-cast A359 aluminum composite reinforced with 30 vol.% SiCp after isothermal anneal, successive hot-rolling, and high strain rate deformation has been investigated. Quantitative microstructural analysis was carried out for the as-cast, annealed (470°C, 538°C and 570°C) and successively hot rolled specimens (64, 75, 88, and 96% rolling reductions). Selected composites were also examined after high strain rate deformation. X-ray diffraction, optical microscopy, scanning electron microscopy and transmission electron microscopy were employed for microstructural characterization. The strength and ductility of the A359 Al alloys, and the composite, were greatly influenced by the brittle eutectic silicon phase and its morphology. Lamellar eutectic silicon spheroidized with isothermal anneal and successive hot rolling with a corresponding decrease in hardness. The hot rolling process also considerably decreased the SiC particle size (approximately 20% after 96% reduction) by breaking-up the hard SiC particles. However, this break-up of particles increased the homogeneity of SiCp size distribution. Successive hot rolling also healed voids due to solidification shrinkage, incomplete infiltration of molten Al and defects originating from fractured particles. Four selected specimens of composites were examined after high strain rate deformation. Fractography and metallographic analysis for the craters, voids, and relevant regions affected by the high velocity impact were carried out. The deposition of impact residuals was frequently iv observed on the exposed fracture surfaces. These residuals were typically observed as “moltenand-solidified” as a consequence of excessive heat generated during and after the damage. Particularly in regions of entry and exit of impact, intermixing of residuals and composite constituents were observed, demonstrating that the Al matrix of the composite also had melted. In all samples examined, cracks were observed to propagate through the eutectic Si network while a small number of broken reinforcement particles were observed. A slight variation in failure mechanisms was observed (e.g., radial, fragmentation, petalling) corresponding to the variation in ductility against high strain rate deformation. In selected specimens, parallel sub-cracks at the exit were observed at 45° and 30°. These sub-cracks were again filled with intermixed constituents from projectile residuals and composites. This observation suggests that the melting of composite constituents that leads to intermixing occured after the crack propagation and other damage.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24630号 / 工博第5136号 / 新制||工||1981(附属図書館) / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 水落 憲和, 教授 関 修平, 教授 梶 弘典 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

<p dir="ltr">Silicon carbide (SiC) is a ceramic with strength retention at elevated temperatures, oxidation resistance and a high thermal conductivity. These properties make SiC a desirable ceramic for compact, high temperature (> 1000 °C), heat exchangers with improved thermal performance. However, fabricating a SiC heat exchanger is difficult due to the low self-diffusion and high melting temperature of SiC. The aim of this dissertation is to show the viability of using co-extrusion and slip casting as low-cost, scalable processes for creating a compact, high temperature, SiC heat exchanger.</p><p dir="ltr">Co-extrusion is an advantageous fabrication technique as it is capable of producing samples with micron-sized features in two dimensions. To fabricate the heat exchanger body via co-extrusion, a SiC-filled polymer blend and a carbon black (CB)-filled polymer blend (sacrificial) were developed. A 54 vol% SiC-filled polymer blend with the addition of 12 wt% alumina and yttria, sintering aids, in a 2:1 ratio, respectively produced samples with the highest relative densities of 94% while maintaining an extrudable rheology. The SiC-filled polymer blend was co-extruded at 80 °C with a 45 vol% CB-filled polymer blend to produce unit cells that were open and continuous after binder burnout and sintering. The unit cells had an average relative density of 90% with an average strength of 165 MPa.</p><p dir="ltr">The unit cell strengths were lower than expected due to the formation of defects that occurred after removal of the polymers. These defects were categorized into macrodelaminations, defects that occur between two laminated unit cells, and microdelaminations, defects that occur within a single unit cell. The mechanisms causing these defects was studied by investigating the lamination and polymer removal processes. Results showed that poor lamination between extrudates mitigated the macrodelaminations and an oxygen-rich debinding atmosphere caused the formation of microdelaminations. Defect-free unit cells were produced though a partial extrusion step and binder removal in a nitrogen atmosphere.</p><p dir="ltr">An aqueous SiC suspension for slip casting was optimized by investigating the rheological properties, zeta potential, and slip casting behavior. It was determined that a suspension with 40 vol% solids, 1.2 wt% dispersant (polyethyleneimine), and a pH of 7.5 resulted in uniform slip cast parts. This optimized suspension was used to fabricate dense, crack-free SiC headers with an average relative density of 96% and an average strength of 266 MPa.</p><p dir="ltr">This dissertation gives insight into important fabrication parameters that must be considered when fabricating high temperature, SiC heat exchanger components. Additionally, this dissertation showcases the capability of using co-extrusion and slip casting as potential pathways for fabricating a high temperature, SiC heat exchanger.</p>