Formation of unique nanocrystalline Cu-In-Se bulk pn homojunctions for opto-electronic devices

Semiconductor pn junctions, integrated in optoelectronic devices require high quality crystals, made by expensive, technically difficult processes. Bulk heterojunction (BHJ) structures offer practical alternatives to circumvent the cost, flexibility and scale-up challenges of crystalline planar pn junctions. Fabrication methods for the current organic or inorganic BHJ structures invariably create interface mismatch and low doping issues. To overcome such issues, we devised an innovative approach, founded on novel inorganic material system that ensued from single-step electrodeposited copper-indium-selenide compounds. Surface analytical microscopies and spectroscopies reveal unusual phenomena, electro-optical properties and quantum effects. They support the formation of highly-ordered, sharp, abrupt 3-dimensional nanoscale pn BHJs that facilitate efficient charge carrier separation and transport, and essentially perform the same functions as crystalline planar pn junctions. This approach offers a low-cost processing platform to create nanocrystalline films, with the attributes necessary for efficient BHJ operation. It allows roll-to-roll processing of flexible devices in simple thin-film form factor.Partial funding for this work is provided by customers of Xcel Energy through a grant from the Renewable Development Fund. The authors gratefully acknowledge sample preparation, analytical contributions and useful discussions with Sharmila Menezes and Yan Li (InterPhases Solar); Senli Guo (Brucker Nano); Terrence McGuckin (Ephemeron Labs); and Nassim Rahimi (HORIBA Scientific). A. Samantilleke acknowledges Prof. L. M. Peter (Bath University, UK) for introducing EER technique

Folded RF-excited CO₂ waveguide lasers

This thesis describes theoretical and experimental work on RF excited CO₂ waveguide lasers and amplifiers.The mode coupling losses at a bend in a folded waveguide have been evaluated as a function of the selectable parameters to determine the low-loss folding geometries. A direct comparison is made between three types of optical arrangement used for folding in a compact, sealed-off, Z-fold CO₂ waveguide laser excited by a transverse RF discharge. In particular, the measured laser output power as a function of discharge conditions and mirror alignment for plane and curved mirror, and partial waveguide folded resonators are compared.The Z-fold laser output power is predicted by incorporating the known and estimated laser parameters into a Rigrod-type analysis. A simultaneous solution of the Rigrod equations predicting the laser powers for different intra-cavity gain lengths is used with the experimental data to derive the discharge and resonator parameters. Experimental results are in good agreement with the theoretical predictions, and suggest that a M% power loss per fold has been achieved with partial waveguide folding. Also, the preliminary theoretical results of a multi-mode resonator model predicting the laser output power as a function of the angular mis-alignment of one of the Z-fold laser folding mirrors are in qualitative agreement with the experimental determinations.Experiments related to laser efficiency and frequency stability are discussed briefly. These include an investigation into an automatic impedance matching scheme for dynamic optimisation of the power transfer efficiency between RF generator and the laser head; the Opto-Hertzian effect (RF equivalent to the opto-Galvanic effect) for laser frequency stabilisation; a novel parallel-resonant distributed inductance excitation technique using a multi-start solenoid; and finally, identification of hooting laser resonator modes responsible for impeding heterodyne measurements Mien a clean RF spectrum is required.In addition, theoretical and experimental studies of laser amplification are presented. The suitability of folded waveguide and non-waveguide structures for power amplification or pre-amplification is assessed to determine their applicability to coherent LiDAR. Optical amplification of wideband transmitter and/or receiver signals is considered a favourable way of improving the discrimination of range and velocity determinations.Finally, as a result of this work, up to 53.4 Watts output power in a high quality fundamental Gaussian beam has been obtained from a compact, sealed-off, Z-fold CO₂ waveguide laser with a 115 cm discharge length, which implies a specific power performance of 0.46 W/cm. Efficiencies (laser output power/RF input power) of up to 9.2% have also been observed

Doctor of Philosophy

dissertationMagnetic resonance-guided focused ultrasound surgery (MRgFUS) is a noninvasive means of causing selective tissue necrosis using high-power ultrasound and MR temperature imaging. Inhomogeneities in the medium of propagation can cause significant distortion of the ultrasound beam, resulting in changes in focal-zone amplitude, location and shape. Current ultrasound beam simulation techniques are either only applicable to homogeneous media or are relatively slow in calculating power deposition patterns in inhomogeneous media. Further, these techniques use table-value estimates of the acoustic parameters for predicting ultrasound beam propagation in inhomogeneous media, resulting in at best an approximate power deposition pattern. This work improves numerical analysis of ultrasound beam propagation by developing techniques for: 1) fast, accurate predictions of ultrasound beam propagation in inhomogeneous media, 2) noninvasive estimation of acoustic parameters (speed of sound and attenuation coefficient) of tissue types present in inhomogeneous media, 3) noninvasive determination of changes in tissue acoustic properties due to treatment. These beam simulation techniques utilizing subject-specific tissue parameters will rapidly predict power deposition patterns in real patient geometries and estimate changes in tissue acoustic parameters during treatment, leading to treatment-responsive patientspecific treatment plans that will improve the safety, efficacy and effectiveness of MRgFUS

Factors Governing the Performance and Stability of Solid Oxide Fuel Cell Cathodes Prepared by Infiltration

Infiltration method, developed at the University of Pennsylvania, is a unique analytical platform for investigating the effect of material properties and electrode microstructure on the performance of solid oxide fuel cell (SOFC) electrodes. During cell fabrication by infiltration, the ion-conducting electrolyte phase is sintered first, followed by the addition of the catalytically active perovskite phase into the pores of the electrolyte. The use of separate sintering steps for the electrolyte and the active phase gives one a high degree of control over the microstructure of both phases, unattainable with traditional fabrication methods. In this thesis, the infiltration approach has been used to conduct a systematic investigation into the factors that govern the performance and stability of solid oxide fuel cell cathodes. As a result, a number of microstructural and material properties, crucial for obtaining high electrode activity, were identified. In particular, the effect of varying the ionic conductivity of the porous electrolyte, the specific surface area of the perovskite as well as the specific surface area of the porous electrolyte, and the effect of solid-state reactions between the two phases were studied and were found to significantly affect performance. The experimental findings agreed well with the predictions of a mathematical model that was developed to describe the electrochemical characteristics of SOFC composite cathodes. Both theoretical and experimental evidence suggests the performance of SOFC cathodes prepared by infiltration is limited by slow oxygen adsorption on the perovskite surface. The chemical composition of the perovskite surface therefore plays an important role in determining the overall performance of the electrode. The last chapter of this thesis introduces a novel method that may allow one to characterize the active sites on the perovskite surface under SOFC cathode operating conditions (600-700°C, ambient air atmosphere, polarization), unattainable with traditional surface characterization techniques

The development of an in-vivo method for assessing the antithrombotic properties of pharmaceutical compounds

The formation of a thrombus stems from the malfunction of a normal physiological function referred to as haemostasis and the activity of blood platelets; such thrombi give rise to debilitating and often fatal strokes. Consequently much effort is associated with the search for pharmacological compounds capable of their prevention or dispersion. · Most of the primary screens associated with such work rely on in-vitro tests and in separating the blood from it's vasculature, the influence and results associated with several naturally occuring moderators may be lost. There therefore exists the incentive to develop more representative in-vivo screening methods. Following an introduction to the underlying physiology and pharmacology and a review of established screening methods, this thesis proceeds to describe the development of a novel technique suitable for such in-vivo studies. It's inception is shown to be a consequence of an amalgamation of ultrasonic methods associated with the clinical detection of occlusions and laser Doppler velocimetry. Both topics are individually surveyed and then brought together through a concept whereby the efficacy of compounds might be evaluated in animal models by measuring the velocity of blood in the fluid jet formed distal to an induced thrombus.The main underlying assumption is that the jet velocity will reflect the degree of encroachment of the thrombus into the vasculature. In accord with the evolved measurement rationale there then follows a description of a specific laser Doppler velocimeter and some associated experiments, designed to qualitatively appraise the validity of the underlying assumptions. The ensuing results in turn give rise to the design of a laser Doppler microscope, an analyser for extracting the required velocity information from the Doppler shift spectrum and an additional series of experiments. Central to this latter stage of validation is the use of a thrombus analogue in a narrow bored glass flow tube. Finally, some preliminary in-vivo experiments and results are presented

Methods for designing treatments to reduce interior noise of predominant sources and paths in a single engine light aircraft

The sources and paths by which noise enters the cabin of a small single engine aircraft were determined through a combination of flight and laboratory tests. The primary sources of noise were found to be airborne noise from the propeller and engine casing, airborne noise from the engine exhaust, structureborne noise from the engine/propeller combination and noise associated with air flow over the fuselage. For the propeller, the primary airborne paths were through the firewall, windshield and roof. For the engine, the most important airborne path was through the firewall. Exhaust noise was found to enter the cabin primarily through the panels in the vicinity of the exhaust outlet although exhaust noise entering the cabin through the firewall is a distinct possibility. A number of noise control techniques were tried, including firewall stiffening to reduce engine and propeller airborne noise, to stage isolators and engine mounting spider stiffening to reduce structure-borne noise, and wheel well covers to reduce air flow noise

Comparison of Conventional and Bayesian Analysis for the Ultrasonic Characterization of Cancellous Bone

This dissertation investigates the physics underlying the propagation of ultrasonic waves in cancellous bone. Although quantitative ultrasound has the potential to evaluate bone quality even better than the current gold standard X-ray based modality, its clinical utility has been hampered by the incomplete understanding of the mechanisms governing the interaction between ultrasound and bone. Therefore, studies that extend the understanding of the fundamental physics of the relationship between ultrasound and trabecular bone tissue may result in improved clinical capabilities. Ultrasonic measurements were carried out on excised human calcaneal specimens in order to study the effects of overlapping fast and slow compressional mode waves on the ultrasonic parameters of attenuation and velocity. Conventional analysis methods were applied to received sample signals that appeared to contain only a single wave mode. The same signals were also analyzed using a Bayesian parameter estimation technique that showed that the signals, which appeared to be only a single wave, could be separated into fast and slow wave components. Results demonstrated that analyzing the data under the assumption that only a single wave mode is present, instead of two interfering waves, yielded a phase velocity that lay between the fast and slow wave velocities and a broadband ultrasound attenuation that was much larger than the ultrasound attenuations of the individual fast and slow waves. The fast and slow wave ultrasonic parameters were found to correlate with microstructural parameters, including porosity, determined by microCT measurements. Simulations of fast and slow wave propagation in cancellous bone were carried out to demonstrate the plausibility of a proposed explanation for an anticipated sample-thickness dependence of the apparent attenuation in bovine bone. The results showed that an apparent sample-thickness dependence could arise if the fast and slow waves are not separated sufficiently and if frequency-domain analysis is not performed on broadband data. The sample-thickness dependence of the ultrasonic parameters was explored further using experimental data acquired on an equine cancellous bone specimen that was systematically shortened. The thickness of the sample varied the degree to which the fast and slow waves overlapped, permitting the use of conventional analysis methods for sufficiently long sample lengths. Bayesian parameter estimation was performed successfully on data from all sample lengths. The ultrasonic parameters obtained by both conventional and Bayesian analysis methods were found unexpectedly to display small, systematic variations with sample thickness. A very thorough and systematic series of studies were carried out on one-mode Lexan phantoms to investigate the potential cause of the observed sample-thickness dependence. These studies ruled out a series of potential contributors to the sample-thickness dependence, but yielded no clear cause. Although the clinical implications of the small but systematic sample-thickness dependence may be negligible, these studies may provide additional insights into the propagation of ultrasonic waves in cancellous bone and how to maximize the quality of information obtained

A study of the prediction of cruise noise and laminar flow control noise criteria for subsonic air transports

General procedures for the prediction of component noise levels incident upon airframe surfaces during cruise are developed. Contributing noise sources are those associated with the propulsion system, the airframe and the laminar flow control (LFC) system. Transformation procedures from the best prediction base of each noise source to the transonic cruise condition are established. Two approaches to LFC/acoustic criteria are developed. The first is a semi-empirical extension of the X-21 LFC/acoustic criteria to include sensitivity to the spectrum and directionality of the sound field. In the second, the more fundamental problem of how sound excites boundary layer disturbances is analyzed by deriving and solving an inhomogeneous Orr-Sommerfeld equation in which the source terms are proportional to the production and dissipation of sound induced fluctuating vorticity. Numerical solutions are obtained and compared with corresponding measurements. Recommendations are made to improve and validate both the cruise noise prediction methods and the LFC/acoustic criteria

The effect of rotor and casing geometry on the performance of cross-flow fans.

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