The commercialization of wide band-gap devices such as silicon carbide and gallium nitride transistors has made it possible for power electronics applications to achieve unprecedented performance in terms of efficiency and power density. However, the device characteristics which make this performance possible also create secondary consequences in these high-performance applications. One such consequence which is particularly difficult to manage in the context of power electronics applications is the occurrence of self-sustained oscillation. This problem has been recognized in the power electronics literature, but heretofore has not received an extensive analytical treatment. This dissertation provides a comprehensive analytical treatment of the self-sustained oscillation phenomenon, logically separated into two components: an initial forced cycle and the subsequent oscillatory behavior. A large-signal model has been developed in order to predict the occurrence of the initial forced cycle based on a set of estimated initial conditions derived from a user-specified operating point. The establishment of the initial forced cycle as predicted by the large-signal model creates the bias conditions necessary for the analytical treatment of the subsequent oscillatory behavior. For this purpose, a small-signal model is presented which describes this phenomenon on the basis of recognizing the wide band-gap device and a minimal set of parasitic components associated with the gate and drain circuits as an unintended negative conductance oscillator. In the context of established oscillator design theory it has been shown both analytically and with simulation that negative differential conductance exhibited by the parasitic model explains the conditions under which self-sustained oscillation is likely to occur. Both the large-signal and small-signal models are shown to demonstrate good agreement with empirical results from pulsed switching experiments obtained over a wide range of operating conditions. In addition, a catalog of known solutions to the problem of self-sustained oscillation is presented, along with a discussion of a method by which the current work can be used by application designers to preclude the occurrence of this phenomenon in practical systems by design.

In high frequency design, harmonic suppression is a persistent struggle. Non-linear devices such as switches and amplifiers produce unwanted harmonics which may interfere with other frequency bands. Filtering is a widely accepted solution, however there are various shortcomings involved. Suppressing multiple harmonics, if desired, with traditional lumped element and distributed component band-stop filters requires using multiple filters. These topologies are not easily made tunable either. A new filter topology is investigated called Electromagnetic Band-Gap (EBG) structures. EBG structures have recently gained the interest of microwave designers due to their periodic nature which prohibits the propagation of certain frequency bands. EBG structures exhibit characteristics similar to that of a band-stop filter, but in periodically repeating intervals making it ideal for harmonic suppression. The band-gap frequency of an EBG structure may be varied by altering the periodicity of the structure. However, EBG materials are generally static in structure making tuning a challenge. In this thesis, a novel solution for tuning the band-gap properties of an EBG structure is investigated. Designs aimed to improve upon existing solutions are reached. These designs involve acoustic and mechanical tuning methods. Performance is simulated using Agilent’s Advanced Design System (ADS) and a device is constructed and evaluated. Comparing all measured test cases to simulation, band-gap center frequency error is on average 4.44% and absolute band-gap rejection error is 1.358 dB.

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>Magister Scientiae - MSc / Tellurium is a member of the chalcogen group in the periodic table and is known to be a better semiconductor material when compared to sulfur and selenium. By introducing tellurium into the kesterite structure there would be an improvement in the semiconducting property of the kesterite material. This research focused on incorporating tellurium into kesterite structure in order to reduce its band-gap thereby improving its light absorption and ultimately lead to a more efficient photovoltaic effect. For a typical synthesis, kesterite nanoparticles were synthesized by anion hot injection process which involved the injection of the anion precursor comprising of sulfur, selenium and tellurium in diethylene glycol into a solution containing the cation precursor which are copper (II) chloride, Zinc chloride and tin (II) chloride which are dissolved in diethylene glycol. The synthesized nanoparticles were copper zinc tin sulfide (CZTS), copper zinc tin sulfide selenide telluride (CZTSSeTe) and copper zinc tin sulfide telluride (CZTSTe). Morphological characterization of the synthesized nanoparticles was carried out by high-resolution scanning electron microscopy (HRSEM) and high-resolution transmission electron microscopy (HRTEM) to obtain the shape of the surface and internal structure of the nanoparticles respectively. The micrograph obtained from HRSEM shows that all synthesized nanoparticles had a flower-like surface appearance which is a common morphology obtained for non-vacuum synthesized kesterite nanoparticles. The micrograph obtained from TEM showed that all nanoparticles were agglomerated and had a black surface covering which attributable to the solvent used during synthesis, washing and centrifugation. The internal structure of the synthesized nanoparticles was obtained through small angle x-ray scattering (SAXS) plot of the shapes. The shape obtained for the nanoparticles were core shell hollow sphere for CZTS, core shell dumb-bell for CZTSSeTe and solid sphere for CZTSTe.

Philosophiae Doctor - PhD / The continuous increase in energy consumption and decrease in fossil fuels reserves are a primary concern worldwide; especially for South Africa. Therefore, there is an urgent need for alternative energy resources that will be sustainable, and environmentally friendly in order to tackle the ecological degradation generated by the use of fossil fuels. Among many energy ‘niches’, solar energy appears to be one of the most promising and reliable for the African continent because of the constant availability of sun light. Organic conjugated polymers have been identified as suitable materials to ensure proper design and fabrication of flexible, easy to process and cost-effective solar cells. Their tendency to exhibit good semiconducting properties and their capability to absorb photons from the sunlight and convert it into electrical energy are important features that justify their use in organic photovoltaic cells. Many different polymers have been investigated as either electron donating or electron accepting materials. Among them, poly(3-hexylthiophene) is one of the best electron donor materials that have been used in organic photovoltaic cells. It is a good light absorber and its Highest Occupied Molecular Orbital (HOMO) energy level is suitable to allow electron transfer into an appropriate electron acceptor. On the other hand, the molecular ordering found in dendrimers attracted some interest in the field of photovoltaics as this feature can ensure a constant flow of charges. In this work, I hereby report for the first time, the chemical synthesis of a highly crystalline dendritic star copolymer generation 1 poly(propylene thiophenoimine)-co-poly(3-hexylthiophene) (G1PPT-co-P3HT) with high molecular weight and investigate its application as donating material in bulk heterojunction organic photovoltaics.

The main subject of this thesis is the study of electronic and magnetic properties of materials containing 3d transition metal atoms. Our motivation stems mainly from the modern fields of spintronic computing and solar energy conversion. The two primary goals of this work are to determine (i) why certain transition metal impurities in certain semiconductors can induce magnetic properties suitable for spintronic computing applications, and (ii) how transition metal impurities can be used to modify the electronic band gaps of semiconductors and insulators in ways useful for harnessing solar energy and for other applications. To accomplish these goals, we have applied both experimental and theoretical tools. We studied high quality materials prepared by advanced synthesis techniques using x-ray spectroscopy methods at synchrotron light sources. The results of these experiments were interpreted using a variety of theoretical techniques, primarily using computational software developed as part of this thesis and discussed herein. Regarding the study of introducing transition metal impurities into semiconductors to induce magnetic properties, we first developed and demonstrated a method to determine the location of impurity atoms within the host semiconductor lattice. This allowed to us explain the presence and absence of ferromagnetism in samples prepared under only slightly different synthesis conditions, which helped to address some long--standing issues in the spintronics field. We then studied an advanced and promising material -- indium (III) oxide with iron impurities -- to determine how magnetic ordering was maintained up to room temperatures. Our techniques unveiled that a portion of the iron atoms were coupled to oxygen vacancies in the material to create conditions which propelled the observed magnetism. This finding confirmed some earlier theoretical predictions by others in the field. For the study of electronic band gap modifications in semiconductors and insulators via the incorporation of transition metal atoms, we investigated a wide range of materials synthesized using different techniques. Again, we used experimental techniques to determine the location of impurity atoms within the materials, and used this to understand how band gaps were modified upon the introduction of the impurities. For Ti implantation into SiO2, Ni substitution into ZnO, and a new material, MnNCN, we have determined the electronic band gaps and used our techniques to explain how the values for the gaps arise. Finally, an additional outcome of this thesis work is a software program capable of simulating x-ray spectra using various advanced quantum models. We rewrote and built upon powerful existing programs and applied the result to the above studies. Our software was further applied in a collaborative effort with other researchers at the Canadian Light Source to study the differences in two experimental techniques for measuring x-ray absorption: partial and inverse partial fluorescence yields. By using the proper absorption and scattering formalisms to simulate each technique, we were able to explain the differences between the experimental spectra obtained from each. We explain fluorescence yield deviations using an analysis based on the spin configuration of different states, suggesting that the technique can be further extended as a quantitative spin state probe. These results could have significant implications for the field of soft x-ray absorption spectroscopy.

This paper discussed a way of applied stress to control the photonic crystal fiber dispersion curve, so that it can act on the anomalous dispersion or normal dispersion region area. By this way, we can design the pulse compressor and pulse stretcher for higher peak power laser system. Recently, high-power shortpulse laser has become an indispensable tool in many field, using short-pulse laser oscillator, combined with chirped-frequency amplification technology to produce high-power short-pulse laser system can be used for industrial or medical applications. The all-fiber laser system not only provide better pulse quality and also increased pulse laser system on the stability of the environment.

Abstract In this study, copper oxide thin films prepared by DC reactive magnetron sputtering using a Cu target were studied. By changing the oxygen partial pressure ratios and sputtering power and deposition temperatures during sputtering, we obtained copper oxide thin films with different properties. The structures of copper oxide thin films were characterized by glancing incident angle X-ray diffraction. Clear crystal orientation at (002) plane were observed at 50% and 60% oxygen partial pressure ratio. The preferred orientation at (111) plane were observed with heating substrate to 200¢J. The optical and electrical properties of cupric oxide thin films were measured by UV-VIS spectrophotometer and four-point probe system. The cupric oxide thin films deposited with heating substrate to 200¢J exhibited the resistivity of 0.77£[-cm and optical band gap of 1.57 eV. Keywords¡G cupric oxide, thin film, magnetron sputtering, band gap

Recently, synthesis of Diamond-Like Carbon (DLC) films has received considerable interest. Owing to their similar characteristics of diamonds, such as extreme hardness, chemical stability, and high heat conductivity etc, DLC films are regarded as one of the most promising materials. But the practical applications have been limited due to their high internal stress and insufficient adhesion at the interface between DLC film and substrate. Several methods used to the deposition of Me-DLC films have been proposed. Studies have shown that the internal stress was released and the adhesion also improved by doping metallic element into DLC films. Conventionally, metal incorporation in DLC films were prepared by vapor deposition. The requirement of high vacuum equipment makes the process complicated. Besides, there are many merits in electrodeposition, such as low cost, simplicity of experimental set up, and availability for deposition on complex shapes substrate in large area. In this study, electrodepositing technique was used to synthesize the amorphous Cu-DLC films deposited on ITO substrate, in which the pH value of electrolyte varied, to study the characteristics and the composition of DLC films. According to the I-t curves of deposition, the end of current density was used for the impedance comparison of films. With the addition of Cu, the resistance of the electron transportation in Cu-DLC was reduced, and the awl-shaped surface morphology was observed by AFM measurement, which could enhance the electron field emission properties of thin films. For Raman analysis, the effect of Cu addition would promote the sp2 bonding¡F this result corresponds with the increasing ID/IG value. It indicates that film becomes graphitization due to the addition of Cu and leads the shift of G-peak position toward lower wavenumber. ESCA spectra of C1s and Cu2p indicate no obvious evidence of Cu-C formation. The sp2/(sp2+sp3) ratio increases with the pH value. In addition, we found that Cu-DLC in acidic environmental condition, or doping as [Cu(NH3)n]2+ complex is more conducive to the growth of copper metal in DLC films, and has the lowest optical band gap value deduced by n&k analyzer. Finally, we discussed the thin film growth mechanisms and the characteristic of electron field emission for the applications in the future.

<p>The focus of this dissertation is on the dynamics of electromagnetic systems for energy harvesting and filtering applications. The inclusion of magnets into systems generates nonlinearity due to the nature of electromagnetic interactions. In this work, magnetic nonlinearity manifests in tip interactions for cantilever beams, coupling effects for electromagnetic transduction, and bistable potential wells for a two beam system. These electromagnetic interactions are used to add non-contact coupling effects for the creation of bistable oscillators or arrays of coupled beams for energy filtering.</p><p>Nonlinearity at the tip of cantilever beams acts to change the dynamic and static behavior of the system. In this dissertation, these interactions are analyzed both with and without the nonlinear tip interactions. A linear analysis of the system without the tip interaction first provides insight into the shifting frequencies of the first four natural oscillation modes when considering a rigid body tip mass with rotational inertia and a center of mass that is offset from the tip of the beam. Then, the characterization of the nonlinearities in the beam stiffness and magnetic interaction provide insight into the static and dynamic behavior of the beam. The analytical and numerical investigations, using Rayleigh-Ritz methods and an assumed static deflection, are shown to be consistent with experimental tests. These methods provide a framework for theoretically establishing nonlinear static modes and small-amplitude linear modes that are consistent with physical behavior.</p><p>In electromagnetic coupling, the role of nonlinearity can have a detrimental or beneficial effect on energy harvesting. This work includes an investigation of the response of an energy harvester that uses electromagnetic induction to convert ambient vibration into electrical energy. The system's response behavior with linear coupling or a physically motivated form of nonlinear coupling is compared with single and multi-frequency base excitation. This analysis is performed with combined theoretical and numerical studies.</p><p>The ability of magnets to add nonlinearity to a system allows for the expansion of the phenomenological behavior of said system and potential advantages and disadvantages for energy harvesting. This work studies a two beam system made up of carbon fiber cantilever beams and attached magnetic tip masses with a focus on energy harvesting potential. Numerical and experimental investigations reveal an array of phenomena from static bifurcations, chaotic oscillations, and sub-harmonic orbits. These features are used to highlight the harvesting prospects for a similarly coupled system.</p><p>Beyond nonlinearity, the non-contacting coupling effects of magnets allow for the hypothetical creation of energy filtering systems. In this work, the band structure of a two dimensional lattice of oscillating beams with magnetic tip masses is explored. The focus of the wave propagation analysis is primarily on regions in the band structure where propagation does not occur for the infinite construction of the system. These band gaps are created in this system of 2 x 2 repeating unit cells by periodically varying the mass properties and, for certain configurations, the frequency band gaps manifest in different size and band location. Uncertainty in these regions is analyzed using potential variations associated with specific physical parameters in order to elucidate their influence on the band gap regions. Boundary effects and damping are also investigated for a finite-dimensional array, revealing an erosion of band gaps that could limit the expected functionality.</p> / Dissertation