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
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Item Nanostructured Reactive Metals, Alloys, and Composites: Aerosol- and Laser-Assisted Synthesis, Assembly, and Characterization for Tunable Energy Release(2022) Ghildiyal, Pankaj; Zachariah, Michael R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nanostructured heterogeneous energetic materials are a class of high-energy materials that utilize intimately mixed fuel and oxidizer particles to rapidly release a large amount of stored chemical energy in the form of heat, light, and intense pressures. Developing robust and scalable strategies to modify the structural features of these materials to tailor their energy release behavior is paramount to their success in large-scale propellant applications, which demand a consistent and predictable delivery of the stored material energy. This dissertation explores a multi-scale structure modification (nano-micro-macro) approach to achieve tunability in the functional energetic properties of reactive metal-based nanoscale fuels. Specifically, I have developed scalable aerosol- and laser-assisted techniques for the synthesis and assembly of nanostructured reactive metals, alloys, and their composites. This dissertation also identifies key fabrication, design, and assembly parameters that enable the tuning of material structural features such as particle size, composition, aggregate morphology, microstructure, and porosity. Additionally, the role of these structural modifications on their functional properties such as energy density, oxidation behavior, reaction pathways, ignition, and energy release characteristics has been extensively studied. Therefore, through these investigations, the dissertation establishes the critical process design-structure-property-function relationships in metal-based fuel systems. To achieve structural and reaction control on the nanoparticle scale, three strategies are explored. First, a vapor-phase route to surface-pure, core−shell nanoscale magnesium particles (Mg NPs) is employed, whereby controlled evaporation and growth are used to tune nanoparticle sizes and their size-dependent oxidation and energy release behavior are evaluated. Through direct observations from extensive in situ characterizations, I demonstrate that the remarkably high reactivity of Mg NPs (up to 10-fold higher than Al NPs) is a direct consequence of enhanced vaporization and Mg release from their high-energy surfaces that result in the accelerated energy release kinetics from their composites. Secondly, the synergistic role of Mg NP additives in inducing heterogeneous etching reactions on the surface of boron nanoparticles is studied. Specifically, I show that Mg NPs rapidly release vapor-phase Mg (~100 µs), which reacts exothermically (∆H_r= -420 kJ mol-1) with the molten B2O3 layer and assists in its removal during the reaction, causing ~6-fold reactivity enhancement and ~60% reduction in the burn times of boron. A third approach utilizes an in-flight surface modification of Mg NPs with a reactive element (Si) to form core-shell Mg-Si nanoparticles. Through mechanistic investigations of these systems, I find that the Si-coated Mg NPs themselves undergo an intraparticle condensed-phase alloying reaction between the Mg core and Si shell at relatively low-temperatures (400-500°C), resulting in highly accelerated reaction rates (~3-9-fold shorter reaction timescales) and lower ignition temperatures (~210°C lowering) than unfunctionalized Mg particles. Next, two aerosol-phase assembly techniques are explored to control the micron-scale structural and aggregation features of metal nanoparticle assemblies. First, an electrospray approach is used to incorporate plasma-synthesized ultrasmall Si particles to fill in the void structure of Al-based microparticles to augment their volumetric energy density and reactivity. This approach results in ~21% enhancement in energy density due to partial filling of structural voids and ~2-3-fold enhancement of reaction rates due to enhanced transport in ultrafine silicon particles. Another vapor-phase assembly approach employing external magnetic fields during synthesis is explored in directing the in-flight assembly of ferromagnetic metal nanoparticles into distinct aggregate morphologies with altered fractal dimensions. For control over the macroscale features of nanostructured composites, three robust and scalable techniques are employed. The first method utilizes spray drying as a highly scalable approach (production rates up to ~275 g h-1) to assemble metal and oxidizer nanoparticles into microparticle composites with ~2-7-fold higher reactivities than their physically mixed counterparts as a result of rapid gas generation and reduced nanoparticle sintering. I further demonstrate that these nanostructured microparticles can be further processed and additively manufactured into macroscale, hierarchical films (macro-micro-nano) without compromising their structural integrity. The third technique I have developed for macroscale structure modulation is by employing spatially and temporally resolved CO2 laser pulses to fabricate and write a high concentration of unaggregated, sub-10 nm metal nanoparticles directly in polymer films. Using this approach, I demonstrate that laser parameters – pulse duration, laser energy flux, and pulsed thermal loads – can be used for direct, in-situ modulation of particle size distributions of metal nanoclusters in polymer matrices. Rapid heating timescales employed in this approach allow for the scalable manufacturing and structural control of metal nanoclusters with production rates up to 1 g min-1. In conjunction with each other, all three techniques enable high-yield manufacturing of metal-based composites with a broad, nano- to macro-scale structural control. Finally, the structure and reaction modulation strategies are suggested for other fuel systems such as nanoscale reactive alloys (Al-Mg) to achieve controllable energy release behavior through further modifications of fuel composition and morphological features. The techniques developed in this dissertation will allow the strategic design of metal-based nanostructured energetic composites with tailored energy release rates and controllable structural features over a wide range of length scales.Item Microstructural Evolution and the Resultant Mechanical Behavior of Duplex Stainless Steels(2018) Schwarm, Samuel Christian; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As the current generation of commercial light water nuclear reactors approach initial design life specifications (40-50 years), the plausibility of extending the operational life of duplex stainless steel piping to 80 years has become an important research focus. Successful evaluation of this potential requires an improved understanding of microstructural evolution and corresponding changes in mechanical behavior that occur during continuous operation at temperatures up to 320 °C, which notably results in aging embrittlement in these systems. This investigation characterizes the effects of thermal aging on the mechanical properties of cast CF–3 and CF–8 stainless steels at operational (280 °C, 320 °C) and accelerated temperatures (360 °C, 400 °C) by a variety of test methods. Bulk mechanical tests have been performed to measure changes in properties such as tensile strength, impact energy, and ductility during aging embrittlement. The results show an increase in strength and decrease in ductility and impact energy after aging to 17,200 h. The phase structure is investigated by electron microscopy and correlated to the mechanical properties and aging conditions in order to form a comprehensive understanding of the progression of embrittlement and elucidate trends. Smaller length scale tests, such as instrumented nanoindentation, reveal the effects of aging on local properties of the constituent ferrite and austenite phases. The resulting data are utilized to evaluate the influence of local microstructural changes, such as spinodal decomposition, on thermal aging embrittlement of the steels. Finite element method (FEM) models have been developed based on the real microstructure and local properties of the steels in order to analyze the micromechanical relationships between phases at different stages in the aging process. This research combines mechanical, microstructural, and computational characterization methods to build a comprehensive evaluation of the effects of thermal aging on structure-property relationships of these important structural stainless steels.Item DEVELOPMENT OF ANALYTICAL METHODS FOR CHARACTERIZATION OF NANOPARTICLES FOR BIO-MEDICAL AND ENVIRONMENTAL APPLICATION BY ION MOBILITY-ICP-MS(2017) Tan, Jiaojie; Zachariah, Michael R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The development of nanotechnology necessitates appropriate tools for nanoparticle characterization to assure product quality, evaluate safety and facilitate manufacturing. The properties of interest particularly relevant to nanomedicine and environmental ecotoxicology include size, shape, aggregation, concentration, dissolution, surface chemistry, and composition etc. Engineered nanoparticles in a complex matrix, at realistic concentration are two of the major challenges for analytical scientist. Potential transformation of pristine engineered nanomaterials when put in contact with either biological or environmental media further complicate the analytical task. In this dissertation, I aim to optimize and extend the application of novel hyphenated instruments consisting of differential mobility analysis (DMA) and inductively coupled plasma-mass spectrometry (ICP-MS) for real time size classification and elemental detection in biomedical and environmental fields. I have applied DMA-ICP-MS in quantitatively characterizing anti-tumor drug delivery platform to assist design and performance evaluation. Optimal balance among drug loading, stability and release performance was achieved and evaluated by DMA-ICP-MS. I have further developed a novel analytical methodology including DMA and ICP-MS operating in single particle mode (i.e. spICP-MS). I successfully demonstrated and validated the method for accurate and simultaneous size, mass and concentration measurement by NIST reference materials. DMA-spICP-MS was shown with the capability to characterize nanoparticle aggregation state and surface coating. In addition, this technique was shown to be useful for real-world samples with high ionic background due to its ability to remove dissolved ions yielding a cleaner background. Given this validated DMA-spICP-MS method, I applied it to quantifying the geometries of seven gold nanorod samples with different geometries. It was demonstrated that DMA-spICP-MS can achieve fast quantification of both length and diameter with accuracy comparable with TEM analysis. This method provided the capability to separate nanorods from spheres quantifying the geometry for each population. Finally, an interesting open and high-order rosette protein structure was investigated by electrospray-DMA. The staining procedure was optimized and effect of electrospray process on protein particle structure was evaluated. Protein particle after electrospray was largely maintained. Mobility simulation by MOBCAL showed close matches with experimental data and enabled peak assignment to various particle assembly structures.Item MEMS SENSOR PLATFORMS FOR IN SITU CHARACTERIZATION OF LI-ION BATTERY ELECTRODES(2016) Jung, Hyun; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lithium-ion batteries provide high energy density while being compact and light-weight and are the most pervasive energy storage technology powering portable electronic devices such as smartphones, laptops, and tablet PCs. Considerable efforts have been made to develop new electrode materials with ever higher capacity, while being able to maintain long cycle life. A key challenge in those efforts has been characterizing and understanding these materials during battery operation. While it is generally accepted that the repeated strain/stress cycles play a role in long-term battery degradation, the detailed mechanisms creating these mechanical effects and the damage they create still remain unclear. Therefore, development of techniques which are capable of capturing in real time the microstructural changes and the associated stress during operation are crucial for unravelling lithium-ion battery degradation mechanisms and further improving lithium-ion battery performance. This dissertation presents the development of two microelectromechanical systems sensor platforms for in situ characterization of stress and microstructural changes in thin film lithium-ion battery electrodes, which can be leveraged as a characterization platform for advancing battery performance. First, a Fabry-Perot microelectromechanical systems sensor based in situ characterization platform is developed which allows simultaneous measurement of microstructural changes using Raman spectroscopy in parallel with qualitative stress changes via optical interferometry. Evolutions in the microstructure creating a Raman shift from 145 cm−1 to 154 cm−1 and stress in the various crystal phases in the LixV2O5 system are observed, including both reversible and irreversible phase transitions. Also, a unique way of controlling electrochemically-driven stress and stress gradient in lithium-ion battery electrodes is demonstrated using the Fabry-Perot microelectromechanical systems sensor integrated with an optical measurement setup. By stacking alternately stressed layers, the average stress in the stacked electrode is greatly reduced by 75% compared to an unmodified electrode. After 2,000 discharge-charge cycles, the stacked electrodes retain only 83% of their maximum capacity while unmodified electrodes retain 91%, illuminating the importance of the stress gradient within the electrode. Second, a buckled membrane microelectromechanical systems sensor is developed to enable in situ characterization of quantitative stress and microstructure evolutions in a V2O5 lithium-ion battery cathode by integrating atomic force microscopy and Raman spectroscopy. Using dual-mode measurements in the voltage range of the voltage range of 2.8V – 3.5V, both the induced stress (~ 40 MPa) and Raman intensity changes due to lithium cycling are observed. Upon lithium insertion, tensile stress in the V2O5 increases gradually until the α- to ε-phase and ε- to δ-phase transitions occur. The Raman intensity change at 148 cm−1 shows that the level of disorder increases during lithium insertion and progressively recovers the V2O5 lattice during lithium extraction. Results are in good agreement with the expected mechanical behavior and disorder change in V2O5, highlighting the potential of microelectromechanical systems as enabling tools for advanced scientific investigations. The work presented here will be eventually utilized for optimization of thin film battery electrode performance by achieving fundamental understanding of how stress and microstructural changes are correlated, which will also provide valuable insight into a battery performance degradation mechanism.Item Mechanical and electrical properties of metal-carbon connections for battery applications(2014) Bilger, Christopher John; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Material selection and processing techniques were investigated to form carbon-metal bonds. Mechanical and electrical characterization was performed to more fully comprehend the bonding mechanisms and properties. Utilizing carbon fibers as a primary conduction medium, the specimens from the processes investigated were utilized with lithium-ion cells to further characterize the electrical performance. Electroplating nickel onto the ends of the carbon fibers provides a relatively simple processing technique which improves fiber adhesion to nickel tabs by over 4.7 times when compared to conductive silver epoxy and over 5 times greater than a 1 inch immersion of carbon fiber into a SAC305 solder ingot. Additionally, a reduction of electrical resistance by 0.7 times over the solder ingot is achieved with the electroplating technique. The results of the electroplating are achieved by using about 25% less available contact area than the solder ingot and are scalable for usage in electrical circuits.Item Genetic Characterization of Toxic Resistant Mutants in C. elegans(2010) Walston, Jonathan Dean; Hamza, Iqbal; Animal Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Heme is an essential cofactor that plays a key role in diverse biological processes. Free heme, however, is hydrophobic and toxic to cellular macromolecules. C. elegans lack the heme biosynthetic pathway, and therefore contains a highly regulated trafficking network to redistribute heme throughout the worm. A forward genetic screen in C. elegans identified thirteen mutants which grow at toxic concentrations of heme in axenic liquid media. These mutants, termed them for Toxic HEMe resistant, belong to five complementation groups of which IQ7280, IQ7310, IQ8280 and IQ9110 strains were characterized. them mutants exhibit abnormal responses to heme analogs, mating defects, and growth on heme-deficient bacteria. Pyrosequencing analysis mapped IQ7310, IQ8280, and IQ7280 to a common genetic interval on chromosome I and IQ9110 to chromosome V. Solexa deep sequencing identified mutations in novel genes which may play an essential role in organismal heme homeostasis.Item Characterization of Linear Electro-Optic Effect of Poled Organic Thin Films(2008-02-29) Park, Dong Hun; Lee, Chi H; Herman, Warren N; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The goal of this thesis is to re-evaluate both Teng-Man and attenuated total reflection (ATR) methods for measuring the linear electro-optic (EO) coefficients of poled organic thin films based on a multilayer structure containing a transparent conducting oxide (TCO) layer. The linear EO properties are often characterized using the Teng-Man reflection method. However, it has been reported that experimental error can result from ignoring multiple reflections and that an accurate determination of the EO effect could be achieved only by a numerical calculation that applies anisotropic Fresnel equations to the multilayer structure. We present new closed-form expressions for analysis of Teng-Man measurements of the EO coefficients of poled polymer thin films. These expressions account for multiple reflection effects using a rigorous analysis of the multilayered structure for varying angles of incidence. The analysis based on plane waves is applicable to both transparent and absorptive films and takes into account the properties of the TCO electrode layer and buffer layers. Methods for fitting data are presented and the error introduced by ignoring the TCO layer and multiple reflections is discussed. We also discuss the effect of Gaussian beam optics and the suitability of a thick z-cut LiNbO3 crystal as a reference to validate the Teng-Man measurement. Simply taking the metal electrode off the Teng-Man sample makes it feasible to use the ATR method using a metal-coated prism. This technique has the capability of measuring anisotropic indices of refraction along with film thicknesses. In addition, it enables measurement of r13 and r33 separately without an assumption for the ratio of r13 to r33 as required in the Teng-Man method. We have found that the ATR analysis based on a three-layer waveguide structure (air/film/substrate) can produce a large error especially when the film supports a single guided mode and the ATR analysis based on a multilayer structure containing a TCO layer gives you a more reliable estimation. We discuss the error introduced by using the three-layer waveguide structure and compare to using the multilayer structure. Finally, we discuss the characterization of the optical property of TCO's using ellipsometeric analysis, which is required for both the rigorous Teng-Man and ATR analysis. Representative experimental results showing that the result from the ATR method based on the multilayer structure shows a good agreement with that from the rigorous Teng-Man analysis are presented. We have measured a very high linear electro-optic coefficient (r33=350 pm/V) from a NLO film (AJ-TTE-II, synthesized by Alex Jen's group at University of Washington) at 1310 nm wavelength, which is ~12 times higher than the best inorganic electro-optic crystal LiNbO3.Item Processing-Structure-Microstructure-Property Relationships in Polymer Nanocomposites(2008-01-31) Kota, Arun Kumar; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The optimal development of polymer nanocomposites using carbon nanotube (CNTs) and carbon nanofiber (CNFs) fillers requires a complete understanding of processing-structure-property relationships. The purpose of this understanding is to determine the optimal approach for processing polymer nanocomposites with engineered microstructures and enhanced material properties. In this research, two processing techniques were investigated: solvent processing and twin screw extrusion. The former is a batch process which employs mixing a polymer solution with a filler suspension using long mixing times and low levels of shear mixing. The latter is a continuous process that mixes polymer melts with solid nanoscale ingredients using high levels of shear mixing for a short mixing time. Previous studies conducted on polymer-CNT/CNF using these processes have focused mainly on processing-microstructure and structure-property relationships using one technique or the other. This research focuses on understanding the processing-property relationships by comparing the structure-property relationships resulting from the two processes. Furthermore, the effect of ingredients and processing parameters within each process on microstructure and structure-property relationships was investigated. The microstructural features, namely, distribution of agglomerates, dispersion, alignment, and aspect ratio of the filler were studied using optical, scanning electron, confocal and transmission electron microscopy, respectively. The composition of the filler was determined using thermogravimetric analysis. The electrical, rheological, thermo-oxidative and mechanical properties of the composites were also investigated. Many significant insights related to processing-structure-property relationships were obtained including: (a) deagglomeration is a critical combination of the magnitude of shear rate and the residence time, (b) the structure-property relationships can be modeled using a new methodology based on the degree of percolation by representing the material as an interpenetrating phase composite, (c) annealing can re-establish interconnectivity and improve electrical properties, (d) the degree of dispersion can be resolved using thermogravimetric analysis, and (e) increasing extrusion speed inhibits thermal decomposition and begins to asymptotically increase strength and stiffness through reduction in aspect ratio and size of agglomerates. Finally, a new combinatorial approach was developed for rapidly determining processing-structure relationships of polymer nanocomposites. This dissertation has broad implications in the processing of high performance and multifunctional polymer nanocomposites, combinatorial materials science, and histopathology.Item WLAN Workload Characterization(2005-08-25) Yeo, Jihwang; Agrawala, Ashok K; Computer Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, we address the problem of workload characterization in a wireless LAN (WLAN). Workload is generated by applications and users trying to carry out some of their functions. We attempt to capture such application- and user-level characteristics from the information gathered at the MAC level. Developing an understandable description of the workload requires making some abstractions at the application- and user-level. Our approach is to consider the workload in terms of ``sessions", where a session is an application- and user-level sequence of exchanges. We attempt to capture the session by considering an inactive duration in the activities between a wireless end-point and the network. We consider workload to consist of a population of sessions for which a probability distribution function can be defined. Considering this distribution function to be a mixture distribution, we attempt to find the components by using non-parametric clustering technique. As the number of types of user level activities is not likely to be very large, we expect that we can associate a distinct activity with each such component. In this work, we identify such components and analyze the traffic and protocol characteristics of each component. Moreover, we empirically show that the identified workload components can effectively represent the actual WLAN workload and its daily variations.