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 Simulating the Fuel Mass Loss Rate in Fire Dynamics Simulator (FDS) Using a New Furniture Calorimeter(2010) McKeever, Meghan Allison; Trouve, Arnaud; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fire Dynamics Simulator (FDS) is widely used in the fire community to simulate and understand in detail enclosure fire dynamics. Fire models require accurate descriptions of the fuel sources to simulate the fire behavior. One approach in FDS is to describe the fuel mass loss rate from furniture calorimeter tests. Unfortunately furniture calorimeter tests do not account for enclosure effects on the fuel sources (i.e. the thermal feedback of the smoke layer and the air vitiation). This work explores a simple pyrolysis model that uses furniture calorimeter data and applies a correction to the data to represent enclosure effects. The study includes: (1) the development of a database which compiles furniture calorimeter data, (2) the development of a modified version of FDS that incorporates a simple pyrolysis model proposed by Professor Quintiere and (3) a performance evaluation of the model by detailed comparisons between FDS results and experimental data from two studies performed at the University of Canterbury.Item MANUFACTURING TECHNIQUES FOR TITANIUM ALUMINIDE BASED ALLOYS AND METAL MATRIX COMPOSITES(2010) Kothari, Kunal B; Wereley, Norman M; Radhakrishnan, Ramachandran; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dual phase titanium aluminides composed vastly of gamma phase (TiAl) with moderate amounts of alpha2 phase (Ti3Al) have been considered for several high temperature aerospace and automobile applications. High specific strength coupled with exceptional high temperature performance in the areas of creep and oxidation resistance makes titanium aluminides "materials of choice" for next generation propulsion systems. Titanium aluminides are primarily being considered as potential replacements for Ni-based superalloys in gas turbine engine components with the aim of developing more efficient and leaner engines with high thrust-to-weight ratio. As titanium aluminides lack room temperature ductility, traditional manufacturing techniques such as casting, forging and rolling are more expensive to perform. To overcome this, research over the past decade has examined powder metallurgy techniques such as hot-isostatic pressing, sintering and hot-pressing to produce titanium aluminides parts. Enhancements in these powder metallurgy techniques has produced near-net shape parts of titanium aluminides possessing a homogeneous and refined microstructure and thereby exhibiting better mechanical performance. This study presents a novel powder metallurgy approach to consolidate titanium aluminide powders. Traditional powder consolidation processes require exposure to high temperatures over a lengthy duration. This exposure leads to grain growth in the consolidated part which adversely affects its mechanical properties. A rapid consolidation process called Plasma Pressure Compaction (P2C) has been introduced and utilized to consolidate titanium aluminide powders to produce titanium aluminide parts with minimal grain growth. The research also explores the role of small alloying additions of Nb and Cr to enhance ductility of the consolidated parts. The grain size of the consolidated parts is further reduced in the sub-micrometer range by milling the as-received powders. Finally, a metal matrix composite with TiAl matrix reinforced with TiB was developed by first blending the matrix and the reinforcement powders and then consolidating the powder blend.Item Experimental Characterization of Vascular Tissue Viscoelasticity with Emphasis on Elastin's Role(2010) Shahmirzadi, Danial; Hsieh, Adam; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Elucidating how cardiovascular biomechanics is regulated during health and disease is critical for developing diagnostic and therapeutic methods. The extracellular matrix of cardiovascular tissue is composed of multiple fibrillar networks embedded in an amorphous ground substance and has been found to reveal time-dependent mechanical behavior. Given the multiscale nature of tissue biomechanics, an accurate description of cardiovascular biomechanics can be obtained only when microstructural morphology is characterized and put together in correlation with tissue-scale mechanics. This study constitutes the initial steps toward a full description of cardiovascular tissue biomechanics by examining two fundamental questions: How does the elastin microstructure change with tissue-level deformations? And how does the extracellular matrix composition affect tissue biomechanics? The outcome of this dissertation is believed to contribute to the field of cardiovascular tissue biomechanics by addressing some of the fundamental existing questions therein. Assessing alterations in microstructural morphology requires quantified measures which can be challenging given the complex, local and interconnected conformations of tissue structural components embedded in the extracellular matrix. In this study, new image-based methods for quantification of tissue microstructure were developed and examined on aortic tissue under different deformation states. Although in their infancy stages of development, the methods yielded encouraging results consistent with existing perceptions of tissue deformation. Changes in microstructure were investigated by examining histological images of deformed and undeformed tissues. The observations shed light on roles of elastin network in regulating tissue deformation. The viscoelastic behavior of specimens was studied using native, collagen-denatured, and elastin-isolated aortic tissues. The stress-relaxation responses of specimens provide insight into the significance of extracellular matrix composition on tissue biomechanics and how the tissue hydration affects the relaxation behavior. The responses were approximated by traditional spring-dashpot models and the results were interpreted in regards to microstructural composition.Item Integrated Measurement Technique To Measure Curing Process-dependent Mechanical And Thermal Properties Of Polymeric Materials Using Fiber Bragg Grating Sensors(2009) Wang, Yong; Han, Bongtae; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An innovative technique based on a fiber Bragg grating (FBG) sensor is proposed to measure the critical mechanical and thermal properties of polymeric materials. The properties include (1) chemical shrinkage evolution during curing, (2) modulus evolution during curing, (3) glass transition temperature (4) coefficient of thermal expansion (CTE), and (5) visco-elastic properties. Optimum specimen configurations are proposed from the theoretical analysis. Then an efficient numerical procedure is established to determine the material properties from the measured Bragg wavelength (BW) shift. The technique is implemented with various polymeric materials. The measured quantities are verified through a self-consistency test as well as the existing testing methods such as a warpage measurement of a bi-material strip, and a TMA measurement. The evolution properties obtained at a curing temperature are extended further by combining them with the conventional isothermal DSC experiments. Based on the existing theories, the evolution properties can be predicted at any temperatures. The proposed technique greatly enhances the capability to characterize the mechanical properties and behavior of polymeric materials. Since the specimen preparation is very straightforward, the proposed method can be routinely practiced and the measurement can be completely automated. It will provide a much-needed tool for rapid but accurate assessment of polymer properties, which, in turn, will enhance the accuracy of predictive modeling for design optimization of a microelectronics product at the conceptual stage of product development.Item EVALUATION OF THE THERMAL PERFORMANCE OF FIRE FIGHTER PROTECTIVE CLOTHING WITH THE ADDITION OF PHASE CHANGE MATERIAL(2010) McCarthy, Lee K.; di Marzo, Marino; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fire fighters rely on fire fighter protective clothing (FFPC) to provide adequate protection in the various hazardous environments they may encounter during operations. FFPC has seen significant advancement in technology over the past few decades. The addition of phase change material (PCM) to FFPC is a new technology with potential to enhance the thermal protection provided by the FFPC. To explore this technology, data from bench-scale experiments involving FFPC with PCMs are compared with a theoretical finite difference heat transfer model. The results demonstrate an effective method to mathematically model the heat transfer and provide insight into the effectiveness of improving the thermal protection of FFPC. The experiments confirm that the latent heat absorbed during the phase change reduces temperatures that might be experienced at the fire fighter's skin surface, advancing the high temperature performance of FFPC.Item EVOLUTION OF THE MICROSTRUCTURE AND VISCOPLASTIC BEHAVIOR OF MICROSCALE SAC305 SOLDER JOINTS AS A FUNCTION OF MECHANICAL FATIGUE DAMAGE(2009) Cuddalorepatta, Gayatri; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The effect of mechanical cycling fatigue damage and isothermal aging histories on the evolution of the constitutive and fatigue responses, and microstructure of microscale SAC305 solder joints is investigated. In particular, the study examines if joint dependent behavior should be expected from as-fabricated and cycled microscale SAC305 joints that exhibit an initial non-homogenous coarse-grained Sn microstructure. In addition, the ability of traditionally used macroscale constitutive models based on continuum mechanics to represent the viscoplastic constitutive behavior of the non-homogenous as-fabricated microscale SAC305 specimens is explored. Insights into the effect of key microstructural features and dominant creep mechanisms influencing the measured viscoplastic behavior of SAC305 are provided using a multi-scale mechanistic modeling framework. Modified lap-shear microscale SAC305 specimens are characterized using the thermomechanical microscale test setup (TMM). Microscale SAC305 solder specimens show significant piece-to-piece variability in the viscoplastic constitutive properties under identical loading histories in the as-fabricated state. The mechanical response is strongly influenced by the grain microstructure across the entire joint, which is non-repeatable and comprises of very few highly anisotropic Sn grains. The statistical non-homogeneity in the microstructure and the associated variability in the mechanical properties in the microscale SAC305 test specimen are far more significant than in similar Sn37Pb specimens, and are consistent with those reported for functional microelectronics solder interconnects. In spite of the scatter, as-fabricated SAC305 specimens exhibit superior creep-resistance (and lower stress relaxation) than Sn37Pb. Macroscale creep model constants represent the non-homogeneous behavior of microscale joints in an average sense. Macroscale modeling results show that the range of scatter measured from macroscale creep model constants is within the range of scatter obtained from the stress relaxation predictions. Stress relaxation predictions are strongly sensitive to the inclusion or exclusion of primary creep models. The proposed multiscale framework effectively captures the dominant creep deformation mechanisms and the influence of key microstructural features on the measured secondary creep response of microscale as-fabricated SAC305 solder specimens. The multiscale model predictions for the effect of alloy composition on SAC solders provide good agreement with test measurements. The multiscale model can be extended to understand the effects of other parameters such as aging and manufacturing profiles, thereby aiding in the effective design and optimization of the viscoplastic behavior of SAC alloys. Accumulated fatigue damage and isothermal aging are found to degrade the constitutive and mechanical fatigue properties of the solder. The scatter gradually decreases with an increasing state of solder damage. Compared to the elastic-plastic and creep measurements, the variability in the fatigue life of these non-homogenous solder joints under mechanical fatigue tests is negligible. Recrystallization is evident under creep and mechanical fatigue loads. Gradual homogenization of the Sn grain microstructure with damage is a possible reason for the observed evolution of scatter in the isothermal mechanical fatigue curves. The yield stress measurements suggest that SAC305 obeys a hardening rule different from that of isotropic or kinematic hardening. The measured degradation in elastic, plastic and yield properties is captured reasonably well with a continuum damage mechanics model from the literature.Item Microstructural Evolution in Friction Stir Welding of Ti-5111(2010) Wolk, Jennifer Nguyen; Salamanca-Riba, Lourdes; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Titanium and titanium alloys have shown excellent mechanical, physical, and corrosion properties. To address the needs of future naval combatants, this research examines an alternative joining technology, friction stir welding (FSW). Friction stir welding uses a non-consumable tool to generate frictional heat to plastically deform and mix metal to form a consolidated joint. This work focuses on FSW of Ti-5111 (Ti-5Al-1Sn-1Zr-1V-0.8Mo), a near alpha alloy. This study aims to gain a fundamental understanding of the relationship between processing parameters, microstructure, and mechanical properties of experimental 12.7mm and 6.35mm Ti-5111 friction stir welds. The resulting weld microstructure shows significant grain refinement within the weld compared to the base metal. The weld microstructures show a fully lamellar colony structure with peak welding temperatures exceeding beta transformation temperature. The friction stir weld shows material texture strengthening of the BCC F fiber component before transformation to D2 shear texture in the stir zone. Transmission electron microscopy results of the base metal and the stir zone show a lath colony-type structure with low dislocation density and no lath grain substructure. In situ TEM heating experiments of Ti-5111 friction stir welded material show transformation to the high temperature beta phase at significantly lower temperatures compared to the base metal. Thermal and deformation mechanisms within Ti-5111 were examined through the use of thermomechanical simulation. Isothermal constant strain rate tests show evidence of dynamic recrystallization and deformation above beta transus when compared with the FSW thermal profile without deformation. Subtransus deformation shows kinking and bending of the existing colony structure without recrystallization. Applying the friction stir thermal profile to constant strain rate deformation successfully reproduced the friction stir microstructure at a peak temperature of 1000ºC and a strain rate of 10/s. These results provide unique insight into the strain, strain rates, and temperatures regime within the process. Finally, the experimental thermal and deformation fields were compared using ISAIAH, a Eulerian based three-dimensional model of friction stir welding. These results are preliminary but show promise for the ability of the model to compute thermal fields for material flow, model damage prediction, and decouple texture evolution for specific thermomechanical histories in the friction stir process.Item Atomic Layer Deposition Conformality and Process Optimization: Transitioning from 2-Dimensional Planar Systems to 3-Dimensional Nanostructures(2010) Robertson Cleveland, Erin Darcy; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Conformal coatings are becoming increasingly important as technology heads towards the nanoscale. The exceptional thickness control (atomic scale) and conformality (uniformity over nanoscale 3D features) of atomic layer deposition (ALD) has made it the process of choice for numerous applications found in microelectronics and nanotechnology with a wide variety of ALD processes and resulting materials. While its benefits derive from self-limited saturating surface reactions of alternating gas precursors, process optimization for ALD conformality is often difficult as process parameters, such as dosage, purge, temperature and pressure are often interdependent with one another, especially within the confines of an ultra-high aspect ratio nanopore. Therefore, processes must be optimized to achieve self-limiting saturated surfaces and avoid parasitic CVD-like reactions in order to maintain thickness control and achieve uniformity and conformality at the atomic level while preserving the desired materials' properties (electrical, optical, compositional, etc.). This work investigates novel approaches to optimize ALD conformality when transitioning from a 2D planar system to a 3D ultra-high aspect ratio nanopore in the context of a cross-flow wafer-scale reactor used to highlight deviations from ideal ALD behavior. Porous anodic alumina (PAA) is used as a versatile platform to analyze TiO2 ALD profiles via ex-situ SEM, EDS and TEM. Results of TiO2 ALD illustrate enhanced growth rates that can occur when the precursors titanium tetraisopropoxide and ozone were used at minimal saturation doses for ALD and for considerably higher doses. The results also demonstrate that ALD process recipes that achieve excellent across-wafer uniformity across full 100 mm wafers do not produce conformal films in ultra-high aspect ratio nanopores. The results further demonstrate that conformality is determined by precursor dose, surface residence time, and purge time, creating large depletion gradients down the length of the nanopore. Also, deposition of ALD films over sharp surface features are very uniform, and verified by profile evolution modeling. This behavior, in contrast to that in high aspect ratio structures, suggests strongly that detailed dynamics, local flow conditions (e.g. viscous vs molecular), surface residence time, and ALD surface reaction kinetics play a complex role in determining ALD profiles for high aspect ratio features.Item RATIONAL DESIGN OF NON-DAMAGING CAPACITIVELY COUPLED PLASMA ETCHING AND PHOTORESIST STRIPPING PROCESSES FOR ULTRALOW K DIELECTRIC MATERIALS(2010) Kuo, Ming-Shu; Oehrlein, Gottlieb S.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Resistance-capacitance delay, crosstalk, and power dissipation associated with the increasing capacitance of interconnect structures limits the performance of high-speed microelectronics and leads to the demand for porous ultralow dielectric constant (ULK) material introduction. Process integration of ULK dielectrics requires plasma etching of dielectric material, stripping of the post-etching photoresist (PR) mask, and surface cleaning of plasma-etching-related residues, without damaging the dielectric. Dual frequency capacitively coupled plasma (CCP) reactor are becoming the standard for etching of ULK materials. In this work, we evaluated ULK-compatible PR stripping using both remote plasma and in situ ashing processes coordinated with CCP fluorocarbon (FC)-based ULK etching. Remote H2 plasma enabled a high PR ashing rate while introducing little ULK damage at an elevated substrate temperature (275 °C), and was the best for our remote plasma ashing processes. In situ ashing, with the advantage of no need for an additional dedicated reactor, is preferable to the remote plasma ashing for industry, and we studied in detail CO2 in situ ashing process. The ULK damage introduced during CO2 in situ ashing increased with atomic oxygen density as a function of chamber pressure. To compare the performance of different ashing processes for PR stripping from ULK material, we introduced an ashing efficiency (AE) parameter which is defined as the thickness of PR removed over the thickness of ULK simultaneously damaged, and can be considered a process figure of merit. A high AE can be obtained under low pressure operation, which suppresses ULK damage with minimal atomic oxygen while combining with a RF bias to enhance the PR ashing rate. The preceding ULK etching process using 10% C4F8/Ar plasma deposits FC coating on ULK feature sidewalls. For H2-based remote plasma at high temperature, most of FC coating was removed rapidly and its impact on ULK ashing damage was minor. For CO2 in situ ashing, FC coating remained on ULK sidewalls and provided effective protection of ULK. FC protection was essential for the success of the CO2 in situ ashing process. A strong decrease of ULK post-ashing damage with increasing FC coverage was found, which may be due to surface protection by FC surface coverage along with pore-sealing by the FC material.Item Fundamental Studies of Tin Whiskering in Microelectronics Finishes(2010) Piñol, Lesly Agnes; Melngailis, John; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fundamental Studies of Tin Whiskering in Microelectronics Finishes Abstract Common electronics materials, such as tin, copper, steel, and brass, are ambient reactive under common use conditions, and as such are prone to corrosion. During the early 1940s, reports of failures due to electrical shorting of components caused by `whisker' (i.e., filamentary surface protrusion) growth on many surface types - including the aforementioned metals - began to emerge. Lead alloying of tin (3-10% by weight, typically in the eutectic proportion) eliminated whiskering risk for decades, until the July 2006 adoption of the Restriction of Hazardous Substances (RoHS) directive was issued by the European Union. This directive, which has since been adopted by California and parts of China, severely restricted the use of lead (<1000 ppm) in all electrical and electronics equipment being placed on the EU market, imposing the need for developing reliable new "lead-free" alternatives to SnPb. In spite of the abundance of modern-day anecdotes chronicling whisker-related failures in satellites, nuclear power stations, missiles, pacemakers, and spacecraft navigation equipment, pure tin finishes are still increasingly being employed today, and the root cause(s) of tin whiskering remains elusive. This work describes a series of structured experiments exploring the fundamental relationships between the incidence of tin whiskering (as dependent variable) and numerous independent variables. These variables included deposition method (electroplating, electroless plating, template-based electrochemical synthesis, and various physical vapor deposition techniques, including resistive evaporation, electron beam evaporation, and sputtering), the inclusion of microparticles and organic contamination, the effects of sample geometry, and nanostructuring. Key findings pertain to correlations between sample geometry and whisker propensity, and also to the stress evolution across a series of 4"-diameter silicon wafers of varying thicknesses with respect to the degree of post-metallization whiskering. Regarding sample geometry, it was found that smaller, thinner substrates displayed a more rapid onset of whiskering immediately following metallization. Changes in wafer-level stress were not found to correlate with whiskering morphology (number, density, length) after 6 weeks of aging. This result points either to the irrelevance of macrostress in the substrate/film composite, or to a difference in whiskering mechanism for rigid substrates (whose stress gradient over time is significant) when compared with thinner, flexible susbtrates (whose stress is less variable with time). Organic contamination was found to have no appreciable effect when explicitly introduced. Furthermore, electron-beam evaporated films whiskered more readily than films deposited via electroplating from baths containing organic "brighteners." Beyond such findings, novel in themselves, our work is also unique in that we emphasize the "clean" deposition of tin (with chromium adhesion layers and copper underlayers) by vacuum-based physical vapor deposition, to circumvent the question of contamination entirely. By employing silicon substrates exclusively, we have distinguished ourselves from other works (which, for example, use copper coupons fabricated from rolled shim stock) because we have better sample-to-sample consistency in terms of material properties, machinability, and orientation.