Thesis (M.S.)--University of Missouri--Rolla, 2007. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed November 14, 2007) Includes bibliographical references.

It is essential to make performance measurements of photovoltaics modules in order to quantify the power they will produce under operational conditions. Performance measurements are fundamental throughout the photovoltaic industry, from product development to quality control in manufacturing and installation in the field. Rapid and economic evaluation of photovoltaic performance requires measurements using pulsed illumination solar simulators. However some devices have characteristics which can cause difficulties making these measurements. The aim of this thesis is to overcome these measurement problems focusing particularly on two of the most prevalent and pressing of these problematic characteristics: high capacitance and metastability. A new method for measuring high capacitance modules in a pulsed simulator, based on tailor made voltage ramps, was developed. The voltage ramp is tailor made such that the measurement time is minimised while maintaining high accuracy (0.5 %), allowing the measurement of high capacitance modules in a single 10ms illumination pulse. The necessary inputs for this method are the capacitance and dark current as a function of voltage for each module. In order to make these measurements, at the high forward bias voltages required, a new system was developed. The tailored voltage ramp can be created individually for each module, since the process is rapid an automatic. This makes the method applicable to a production line or to test house measurements. In addition to their use as inputs for the voltage ramp design, the capacitance and dark current also contain other valuable information, including effective minority carrier lifetime. In several thin film technologies, such as CIGS, the efficiency is not a fixed value, rather the module is metastable and the efficiency changes depending on the previous exposure /preconditioning of the device. Preconditioning is normally applied to these devices before measurement in order to put them in a specific state that is repeatable and representative of outdoor operation. Improved preconditioning practices are vital for performance measurements in CIGS modules. Therefore the preconditioning behaviour of a variety of CIGS modules from different manufacturers was investigated. The effect of preconditioning varied for different modules, commonly the fill factor improved substantially, but often changes in open circuit voltage were also seen and in some cases also substantial changes in short circuit current. The rates of preconditioning and relaxation were found to follow stretched exponential behaviour, such that the changes occur linearly on a logarithmic timescale over several orders of magnitude in time. The total time for performance stabilisation was found to vary significantly between different types of module. Because of this stretched exponential behaviour, even though the module took days to fully relax to the dark state, there was significant relaxation within the tens of minutes that it would normally take a module to cool down after light soaking before it could be measured. The major implication of observed kinetics is that in order to achieve repeatable measurement the timing in each element of a preconditioning routine should be controlled such that the fractional error in the duration of each step is small. During the investigation an unexpectedly short timescale preconditioning effect was observed, which occurs on a millisecond timescale and relaxes in seconds. It was shown that the measurement artefacts introduced using this method can be eliminated by using electrical forward bias until immediately before the measurement. Another measurement system was developed to track the dark current and C-V characteristic of the modules during electrical bias preconditioning and subsequent relaxation. These measurements demonstrate that more than one process involved during preconditioning in CIGS. Changes occur both in the doping in the bulk of the absorber and also in charge accumulation occurring near to the absorber / buffer interface. The theoretical models for preconditioning in CIGS were reviewed and compared to the experimental results. A rate model was developed based on the theory of the metastable VSe-VCu defect. This model was shown to correspond well to the rates of preconditioning and relaxation in CIGS. The non-exponential behaviour was shown to be compatible with a distribution of activation energies for the transition between different defect states. The difference in the time taken for modules to stabilise is explained by differences in doping density and the density of VSe-VCu defects. The work presented facilitates more accurate, economical performance measurements for high capacitance devices and CIGS devices, thereby contributing to the large scale implementation of photovoltaics as power source.

The purpose of this project is to verify if the so called capacitance method is a sensitive enough method to measure micrometer distance changes in an electrode system. This is done by heating up an electrode system to 60°C and make capacitance measurements during the procedure. With the inverse relationship between the capacitance and the distance between the surfaces the distance is attained. The incitement to find a sensitive measurement method is that in a particle accelerator the temperature is down to 4K and the components of an electrode system will contract which can lead to a breakdown of the system. The change in temperature gives a theoretical change in "gap distance" that is close to the measured change in gap distance, therefore the method is a reliable enough method to measure micrometer distance changes. A possible reason for the small deviation is that the measured temperature is the temperature of the surface of the electrode system. It would be preferable to measure the total temperature of the electrodes, not just the surfaces because it is the whole electrode that expands and contracts with the temperature. This could be done with an IR-camera.

Organic thin-film transistors (OTFTs) have been widely studied because of their promising potential for application in low-cost, large-area and flexible electronics. However, several challenges remain on the way towards practical OTFT devices, such as a high operating voltage (> 20 V) induced by the low charge carrier mobility of organic semiconductors and low capacitance of organic gate dielectrics. A low operating voltage is essential for various OTFTs applications, such as portable displays, radio frequency identification tags (RFIDs), smart textiles and sensors. The key to low voltage operation of OTFTs is reduction of the threshold voltage, inverse subthreshold slope which can be fulfilled by using a high-capacitance gate dielectric with superior interface properties. Since field-effect current is proportional to field-induced charge density, using a gate dielectric layer with high dielectric constant (high-k) enhances output current densities at much lower applied voltages. Very thin dielectric layers have reportedly suffered from poor dielectric properties, while very high-k gate dielectrics have led to inferior dielectric-semiconductor interface. As a result, unsatisfactory device performance, such as low charge carrier mobility and high gate leakage current, has been obtained. In addition, solution-processability on a variety of substrates and compatibility with most common semiconducting materials make high-k dielectric materials an unrivalled candidate for low-voltage, low-cost applications. Consequently, the aim of this project was to produce a high-quality, high-capacitance gate dielectric with excellent properties which is consistent with cheap, basic solution-processing manufacturing techniques. With great promise in hybrid materials, a novel, high-k dielectric material based on alternative organic-inorganic nanocomposites that combine very high dielectric constant values intrinsic to ferroelectric ceramic materials (nanoparticles) with mechanical flexibility, low-cost and easy processing of polymers was developed. Both low- and high-k polymer matrices have been used in formulating high-k nanocomposite dielectric suspensions. The uniformity of suspensions has been improved by surface modification of nanoparticles in the case of low-k polymers, while a combination of polymer choice, solvents and nanoparticle-to-polymer ratio led to homogenous suspensions based on high-k polymers. The nanocomposite preparation technique was also unique to this work and gave reproducibly stable nanocomposite suspensions. Finally, ultralow-voltage (~ 1) OTFTs have been successfully demonstrated by integrating nanocomposite bilayer dielectrics using a high-k fluorinated polymer. Bilayer dielectrics were formed by (partially) capping the surface of the nanocomposite films with an ultrathin capping layer. The capping layer was the key to the operation of low-voltage OTFTs as it allowed remarkable and advantageous use of the nanocomposite surface roughness while improving the dielectric-semiconductor surface roughness. Ultimately, such nanocomposite bilayers have a potential to pave the way towards low-cost fabrication and integration of low-voltage components and circuits on flexible substrates.

Carbonaceous materials, especially in micro and nanoscale, are useful in optical, energy storage, electronic, and biomedical devices or technologies. Techniques have been developed for preparation and modification of the carbonaceous materials, while it is still challenging to tailor the properties of carbonaceous materials effectively and economically. Laser is a powerful tool in academic and industrial laboratories, which also plays important roles in the preparation and modification of high-performance carbonaceous nanomaterials. In this study low-cost hydrothermal synthesis, high-temperature annealing, and Laser ablation (LAL) methods are developed to prepare functionalized carbon nanomaterials and modify their electrochemical and optical properties. Sub-micro hollow carbon spheres are synthesized via hydrothermal carbonization and high-temperature activation without any templates. Good capacitive properties are obtained after activation. The electrochemical properties of the activated carbon spheres depend on the media of the activation. The capacitance of the activated carbon spheres significantly increases with the addition of water as an activation agent. Carbon dots (CDs) are synthesized via a facile and economic hydrothermal (HTC) process using both small-molecule sugar (Xylose) and ground soybean residuals as precursors. The photoluminescence (PL) properties of the as-prepared and further-treated CDs are systematically studied. For the xylose-synthesized CDs, the initial green PL emission disappears after high-temperature treatment at 850 ℃ for 2 h. With further LAL treatment in NH4OH solutions, the PL emission is re-acquired, and a blue shift in emission is observed. Thus, the LAL is found to be an effective method to modify the CDs and their PL properties. For the nitrogen-doped soybean waste-derived carbon particles, they show strong blue emissions, which essentially disappear after 850 ℃ annealing for 2 hours in an argon flow. Then, PL appears again after laser ablation in a 10% NH4OH solution. The conversion from the blue emission to no emission and then back to blue emission again implies the effect of the functional groups on the PL properties of the CDs.

Reduced graphene oxide as outstanding candidate electrode material for supercapacitor has been investigated. This thesis includes two topics. One is that three kinds of reduced graphene oxides were prepared by hydrothermal reduction under different pH conditions. The pH values were found to have great influence on the reduction of graphene oxides. Acidic and neutral media yielded reduced graphene oxides with more oxygen-functional groups, lower specific surface areas but broader pore size distributions than those in basic medium. Variations induced by the pH changes resulted in great differences in the supercapacitor performance. The graphene produced in the basic solution presented mainly electric double layer behavior with specific capacitance of 185 F/g, while the other two showed additional pseudocapacitance behavior with specific capacitance of 225 F/g (acidic) and 230 F/g (neutral), all at a constant current density of 1A/g. The other one is that different reduced graphene oxides were prepared via solution based hydrazine reduction, low temperature thermal reduction, and hydrothermal reduction. The as- prepared samples were then investigated by UV-vis spectroscopy, X-ray diffraction, Raman spectroscopy, and Scanning electron microscope. The supercapacitor performances were also studied and the hydrothermally reduced graphene oxide exhibited the highest specific capacitance.

Wireless power transfer is becoming relevant today because of its effectiveness and convenience. It has been employed into consumer electronics such as cellular charging and electric vehicle charging. In general, inductive wireless power transfer (IPT) is mostly used for WPT. IPT requires coils and power transfer enhancing material such as ferrite to transfer power. However, Capacitive wireless Power Transfer (CPT) appears as an alternative because it requires cost effective and light metal plate couplers. Among CPT couplers, Vertical (stacked) Four-Plate Coupler (V4PC) structure offers the advantage of higher input and output self-capacitances, rotational misalignment. Safety is one of the most important aspect of wireless power transfer. This thesis proposes a solution to minimize the leakage electric field of Vertical 4-Plate Couplers (V4PCs). It does so by finding the optimum value of circuit parameters. The effectiveness of the proposed solution is shown by experimental results.

Energy usage within the world is increasing at a drastic rate. Buildings currently consume a major amount of the total energy used within the United States, and most of this energy usage supports heating and cooling. This demand shows that new passive energy management systems are needed. The use of Increased Thermal Capacitance (ITC) is proposed as a new passive energy management system. To increase thermal capacitance, a piping system is either added into a building’s walls or ceiling. In this paper, a building with ITC added is compared to a similar building without ITC using the simulation program TRNSYS. Along with a comparison between the walls and ceiling, several parameters are analyzed for their effect on the performance of the ITC. ITC was found to be effective especially when located in the ceiling, with the location, specific heat and tank size being the most important factors.

This thesis characterizes the transient thermal response of a 12/16 External Rotor Switched Reluctance Machine (ERSRM) for an E-bike application. A method for calculating coil capacitance based on machine design parameters was introduced and implemented into a standard commercial Lumped Parameter Thermal Network (LPTN). A sizing criterion was proposed for the cuboid number in a physically accurate LPTN coil model design. This sizing criterion considers the change in model size with motor speed or forced convection. The LPTN with a more accurate calculation of capacitance within the coil and a known number of cuboids in the coil was validated with experimental results. An analytical proof was provided that a small number of capacitances is not sufficient to model a typical power-dense coil design. The validated model was used to study the impact of a more accurate capacitance calculation method on motor temperature. Both overload and rated operation were investigated. During overload conditions, it was found that the standard capacitance calculation from commercial software massively underestimated the heating rate and peak temperature of the coil hot spot, even with the same number of cuboids. The capacitance of the rest of the motor was able to be varied and investigated for its effects on cooldown dynamics. It was found that for short-time transients the coil could be assumed to act adiabatically in this operating range. Operating points across the operating envelope for the motor under study were mapped to determine the region where the adiabatic assumption could be made. It was shown that a transition occurred where the adiabatic assumption ceases to be valid. / Thesis / Doctor of Philosophy (PhD) / This thesis deals with the thermal modelling of electric machines for traction applications using lumped parameter thermal modelling. A novel approach is presented for calculating and distributing thermal capacitance in motor coils. A 12/16 External Rotor Switched Reluctance Motor is characterized based on its transient thermal response and the novel methods proposed are validated. The sizing of a coil-based thermal model is discussed and a criterion for physical validity proposed. The validated model is used in a sensitivity analysis of coil and motor capacitances. For severe overload conditions and short periods, a result is obtained showing the coil can be modelled as adiabatic. Finally, a rated load condition is tested, and a transition is suggested between overload conditions and non-overload conditions.

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