Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item Metals and Metallic Alloys for Energy Harvesting and Storage(2018) Gong, Chen; Leite, Marina S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metals have been widely used for harvesting and storing energy in devices such as superabsorbers and Li-ion batteries. However, incorporating metals into a wider range of energy applications is severely limited by their intrinsic optical and electrochemical properties. Therefore, in this thesis, we provide a new class of metallic materials by forming binary mixtures of Ag, Au, Cu, and Al with novel physical properties for photonics, and a comprehensive understanding of the fundamental electrochemistry in Al and Si anode all-solid-state batteries for energy storage. The first part of my thesis focuses on developing metallic alloys with a tunable optical response. We realize a new family of metallic materials by alloying Ag, Au, and Cu with on-demand dielectric functions, which can be used in superabsorbers and hot carrier devices. We design and fabricate alloyed nanostructures with engineered optical response and spatially resolve the electric field distribution at the nanoscale by utilizing near-field scanning optical microscopy, which can potentially enhance the performance of optoelectronic devices. To understand the physical origin of the optical response of the alloys, we measure the valence band spectra and calculate the band structures of Ag-Au alloys, providing direct evidence that the change in the electronic bands is responsible for its optical property. Further, we obtain a photonic device with superior performance using metallic alloys. Specifically, an Al-Cu/Si bilayer superabsorber is reached in a lithography-free manner with maximum absorption > 99%, which can be used for energy harvesting. The second part of my thesis highlights the importance of understanding the reactions and ion distribution in energy storage devices. We inspect how the Al electrode surface changes upon cycling and directly map the Li distribution in 3-dimensions within all-solid-state batteries by implementing time-of-flight secondary ion mass spectroscopy. This research indicates that undesired chemical reactions, including the formation of an insulating layer on the Al anode surface and the trapping of Li ions at the interfaces, hinder the cycling performance of the devices. Overall, our results will contribute to the design of energy storage devices with enhanced electrochemical performance.Item Miniaturized Power Electronic Interfaces for Ultra-compact Electromechanical Systems(2015) Tang, Yichao; Khaligh, Alireza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Advanced and ultra-compact electromechanical (EM) systems, such as kinetic energy harvesting and microrobotic systems are deemed as enabling solutions to provide efficient energy conversion. One of the most critical challenges in such systems is to develop tiny power electronic interfaces (PEIs) capable of addressing power conditioning between EM devices and energy storage units. This dissertation presents technologies and topological solutions toward fabricating miniaturized PEIs to efficiently regulate erratic power/voltage for kinetic energy harvesting and drive high-voltage actuators for microrobotic systems. High-frequency resonant-switching topologies are introduced as power stages of PEIs that allow small footprint of the circuit without suffering from switching losses. Two types of bridgeless resonant ac-dc converters are first introduced and developed to efficiently convert arbitrary input voltages into a regulated dc output voltage. The proposed topologies provide direct ac-dc power conversion with less number of components, in comparison to other resonant topologies. A 5-mm×6-mm, 100-mg, 2-MHz and 650-mW prototype is fabricated for validation of capability of converting very-low ac voltages into a relatively higher voltage. A resonant gate drive circuit is designed and utilized to further reduce gating losses under high-frequency switching and light-load condition. The closed-loop efficiency reaches higher than 70% across wide range of input voltages and output powers. In a multi-channel energy harvesting system, a multi-input bridgeless resonant ac-dc converter is developed to achieve ac-dc conversion, step up voltage and match optimal impedance. Alternating voltage of each energy harvesting channel is stepped up through the switching LC network and then rectified by a freewheeling diode. The optimal electrical impedance can be adjusted through resonance impedance matching and pulse-frequency-modulation (PFM) control. In addition, a six-input standalone prototype is fabricated to address power conditioning for a six-channel wind panel. Furthermore, the concepts of miniaturization are incorporated in the context of microrobots. In a mobile microrobotic system, conventional bulky power supplies and electronics used to drive electroactive polymer (EAP) actuators are not practical as on-board energy sources for microrobots. A bidirectional single-stage resonant dc-dc step-up converter is introduced and developed to efficiently drive high-voltage EAP actuators. The converter utilizes resonant capacitors and a coupled-inductor as a soft-switched LC network to step up low input voltages. The circuit is capable of generating explicit high-voltage actuation signals, with capability of recovering unused energy from EAP actuators. A 4-mm × 8-mm, 100-mg and 600-mW prototype has been designed and fabricated to drive an in-plane gap-closing electrostatic inchworm motor. Experimental validations have been carried out to verify the circuit’s ability to step up voltage from 2 V to 100 V and generate two 1-kHz, 100-V driving voltages at 2-nF capacitive loads.Item ENERGY HARVESTING MICROGENERATORS FOR BODY SENSOR NETWORKS(2014) Dadfarnia, Mehdi; Baras, John S; Systems Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Body sensor networks have the potential to become an asset for personalizing healthcare delivery to patients in need. A key limitation for a successful implementation of body sensor networks comes from the lack of a continuous, reliable power source for the body-mounted sensors. The aim of this thesis is to model and optimize a micro-energy harvesting generator that prolongs the operational lifetime of body sensors and make them more appealing, especially for personalized healthcare purposes. It explores a model that is suitable for harvesting mechanical power generated from human body motions. Adaptive optimization algorithms are used to maximize the amount of power harvested from this model. Practicality considerations discuss the feasibility of optimization and overall effectiveness of implementing the energy harvester model with respect to body sensor power requirements and its operational lifetime.Item Characterization and applications of FeGa/PZT multiferroic cantilevers(2014) Wang, Yi; Takeuchi, Ichiro; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Multiferroic materials and structures, which possess two or more ferroic properties, have been widely investigated because of their ability to transfer one different form of signals. The magnetoelectric (ME) effect, which results in induced voltage under applied magnetic field, makes multiferroic materials promising in applications for new types of transducers, sensors, and information storage devices. The laminated bulk composite multiferric devices had attracted a lot of attention because of their high ME coefficients, which define the strength of ME coupling. We fabricated mechanically-resonant ME devices by depositing magnetostrictive FeGa and piezoelectric PZT thin films on Si cantilevers. Various sized cantilevers were found to exhibit different behaviors. With a 1 Oe AC magnetic driving field HAC, the small cantilever (0.95 mm × 0.2 mm × 5 μm) shows a high ME coefficient (33 V/(cm×Oe)) with a bias DC magnetic field of 66.1 Oe at the resonant frequency fr of 3833 Hz in vacuum. We found that the fr of the small cantilever continuously shifts with the bias magnetic field. A magnetic cantilever theory was used to explain this shift. In addition, we are able to demonstrate application of magnetic cantilevers in AC magnetic energy harvesters with an efficiency of 0.7 mW/cm3. By driving the cantilever into the nonlinear regime with an AC magnetic field larger than 3 Oe or AC electric field larger than 5 mV, we are able to demonstrate its application in a robust multi-mode memory device based on bistable solutions of the Duffing oscillation. We can use the driving frequency, the driving amplitude, DC magnetic field, or DC electric field as the input, and use bistable vibration amplitudes of the device as the output. We also show that parametric amplification can be used to substantially increase the ME coefficient by adding a pump voltage on the PZT layer. The parametric gain is sensitive to both the phase of pumping signal and the phase of the driving signal. The gain diverges as the pump voltage approaches the threshold. With parametric amplification, the ME coefficient can be boosted to a value as large as 2×106 V/(cm×Oe) from 33 V/(cm×Oe).Item Thermoacoustic-Piezoelectric Systems with Dynamic Magnifiers(2013) Nouh, Mostafa Akram; Baz, Amr; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Thermoacoustic energy conversion is an emergent technology with considerable potential for research, development, and innovation. In thermoacoustic resonators, self-excited acoustic oscillations are induced in a working gas by means of a temperature gradient across a porous body and vice versa with no need of moving parts. In the first part of this dissertation, thermoacoustic resonators are integrated with piezoelectric membranes to create a new class of energy harvesters. The incident acoustic waves impinge on a piezo-diaphragm located at one end of the thermoacoustic-piezoelectric (TAP) resonator to generate an electrical power output. The TAP design is enhanced by appending the resonator with an elastic structure aimed at enhancing the strain experienced by the piezo-element to magnify the electric energy produced for the same input acoustic power. An analytical approach to model the thermal, acoustical, mechanical and electrical domains of the developed harvester is introduced and optimized. The performance of the harvesters is compared with experimental data obtained from an in-house built prototype with similar dimensions. In an attempt to further understand the dynamics and transient behavior of the excited waves in the presence of piezoelectric coupling, a novel approach to compute and accurately predict critical temperature gradients that onset the acoustic waves is discussed. The developed model encompasses tools from electric circuit analogy of the lumped acoustical and mechanical components to unify the modeling domain. In the second part of the dissertation, piezo-driven thermoacoustic refrigerators (PDTARs) are presented. The PDTARs rely on the inverse thermoacoustic effect for their operation. A high amplitude pressure wave in a working medium is used to create a temperature gradient across the ends of a porous body located in an acoustic resonator. Finally, PDTARs with dynamic magnifiers are introduced. The developed design is shown, theoretically and experimentally, as capable of potentially enhancing the cooling effect of PDTARs by increasing the temperature gradient created across the porous body.