Browsing by Subject "Silicon carbide"
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Item Characterization and loss modeling of silicon carbide based power electronic converters(2015-04) Ravi, LakshmiSilicon Carbide (SiC) based power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are great candidates for high-voltage, high-frequency and high-temperature power switching applications because of their favorable material properties when compared with Silicon (Si) power MOSFETs. In this thesis, the design, characterization, and modeling of a power electronic converter based around SiC MOSFETs is investigated. The test converter circuit is designed to be general enough that it can represent a half bridge converter, a DC chopper circuit or an output phase of an inverter for flexibility in testing. A practical characterization procedure is proposed which takes a circuit-level approach, as opposed to a device-level approach, using only the actual power electronic circuit under study and no additional test circuitry. Therefore this study takes into account the inherent parasitic impedances associated with the test circuit and its influence on the SiC devices' high-speed switching behavior. The hardware setup is operated at frequencies up to 200 kHz and efficiencies up to approximately 99% were recorded.Based on the characterization data and analysis, a model is constructed using MATLAB (a mathematical modeling software) for predicting converter and gate driver losses at different load currents, DC bus voltages, and operating temperatures (for both a DC-DC synchronous buck converter and a DC-AC three phase, two-level Voltage Source Inverter). Good agreements are obtained between the model outputs and experimental results. Possible future extensions to the work are discussed.Item Charge Carrier Transport and Strain in Graphene Grown on Nitrogen-Seeded Silicon Carbide(2017-10) Torrey, Ethan R.The interest in graphene as a possible basis for new, faster, smaller and more flexible electronics is tempered by its lack of a band-gap. In recent years, several methods by which a gap might be created have been proposed and explored. The work presented here is a part of that exploration. In this case, the specific gap-inducing mechanism under study is a method of engineered strain. Graphene can be grown on silicon carbide. By pre-treating the silicon carbide in a process that leaves small amounts of nitrogen on its surface, the subsequently grown graphene is made to wrinkle. By controlling the wrinkling, i.e. the strain in the graphene layer, it may be possible to induce a band-gap. Indeed, Angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy results provide experimental support for this theory. At the same time, optical absorption measurements appear to contradict it. The primary focus of this dissertation is strain and transport measurements taken on devices fabricated from this type of graphene, with the expectation that these would aid in resolving the apparent contradiction in previous results.In the course of this work, a new tri-layer method of gate oxide deposition, using reactive electron beam deposition and plasma-assisted atomic layer deposition, was developed. Also, a method of enhanced Raman spectroscopy was developed for graphene-on-silicon-carbide devices. These methods were applied to a set of samples of graphene grown on nitrogen-seeded silicon carbide (NG) with the concentration of nitrogen varying between samples. In this dissertation, several transport characteristics are shown to exhibit a monotonic dependence upon the nitrogen concentration. These include changes in strain, broadening of the longitudinal resistivity peak, an offset between that peak and the zero-crossing of Hall conductivity, and a thermally activated n-doping mechanism, all measured with respect to an applied gate voltage. In addition, more complicated changes in temperature dependence and B-field dependence of the longitudinal resistivity are observed. These results, along with the surprising decrease in resistivity with the addition of nitrogen, are explained in the context of weak localization effects, increased transport by charge puddle-mediated tunneling, and edge states. While the presence of a band-gap could not be demonstrated conclusively in this, the first report of charge transport in this material, the results are in keeping with the presence of a band-gap short-circuited by edge states.Item Stress localization and size dependent toughening effects in SiC composites.(2010-08) Beaber, Aaron RossCoatings with high wear resistance have generated a great deal of interest due to a diverse range of applications, including cutting tools, turbine blades, and biomedical joint replacements. Ceramic nanocomposites offer a potential combination of high strength and toughness that is ideal for such environments. In the current dissertation research, silicon and silicon carbide based films and nanostructures were deposited using a hybrid of chemical vapor deposition and nanoparticle ballistic impaction known as hypersonic plasma particle deposition (HPPD). This included SiC/Ti-based multilayers and Si-SiC core-shell composite nanotowers. Using a combination of nanoindentation and confocal Raman microscopy, the role of plasticity and phase transformations was studied during fracture events at small length scales. In a parallel study, HPPD synthesized Si nanospheres and vapor-liquid-solid (VLS) Si nanotowers were compressed uniaxially inside the TEM. These experiments confirmed inverse length scale dependent relationships for strength and toughness in Si based on dislocation pile-up and crack tip shielding mechanisms, respectively. A transition was also identified in the deformation of Si under anisotropic loading below a critical size and used as the basis for a new toughening mechanism in Si-SiC composites. Overall, these results demonstrate the importance of nanoscale confinement and localized stress in the design of mechanically robust nanocomposites.