Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
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.
Browse
10 results
Search Results
Item Analytical Microscopy Applications to Wide-Bandgap Semiconductors and Nanocarbon-Metal Composite Materials(2020) Klingshirn, Christopher J.; Salamanca-Riba, Lourdes G.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding the atomic structure of materials lies at the heart of materials science. Electron microscopy offers myriad techniques to both probe processing-structure-property relationships in materials, and to manipulate those relationships directly. In this thesis, analytical transmission electron microscopy (TEM) was used to investigate two distinct material systems with applications to energy-efficient technologies: wide-bandgap semiconductors and nanocarbon-metal composites. In the first project, TEM and electron energy loss spectroscopy were used to investigate the structure, composition and bonding of metal-oxide-semiconductor devices based on silicon carbide (SiC) and gallium oxide (Ga2O3). The performance of SiC falls short of ideal due to electrically active interfacial defect states. This work confirms that boron doping at the SiC/SiO2 interface is feasible and improves the device channel mobility likely through a stress-relaxation mechanism. Separately, no adverse structural effects were found after antimony ion implantation into the SiC substrate, which independently raises mobility via a counter-doping mechanism. Few atomic-scale studies on Ga2O3 have been reported to date; this thesis aims to bridge the knowledge gap by investigating gate oxide materials and process conditions from a structural perspective. Elevated annealing temperatures reduced interface quality for both SiO2 and Al2O3 gate oxides. Separately, amorphous Al2O3 layers were crystallized under moderate electron irradiation in TEM. One-fourth the dose was required for crystallization with 100-keV electrons compared to 200 keV, indicating an ionization-induced atomic rearrangement mechanism. This unexpected phenomenon will have implications for devices operating in extreme environments. The second project investigated structure-property relationships in novel nano-carbon metal-matrix composites called covetics, which exploit the superior mechanical and electrical properties of carbon nanostructures such as graphene. Aluminum covetics were characterized using TEM and various spectroscopy techniques; complementary quantum-mechanical and effective-medium models were used to predict the performance of covetics with a range of structures. The models suggest that an electrical conductivity enhancement of ≈10% is feasible with a 5 vol.% carbon loading, but oxides and poor Al/C contact often diminish the performance of real covetics.Item New Methodology for Predicting Ultimate Capacity of One-Sided Composite Patch Repaired Aluminum Plate(2019) Hart, Daniel C; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Composite patch repairs are an alternative to traditional weld repair methods to address cracking in aluminum plates. Analytical and numerical design methods use linear elastic fracture mechanics (LEFM) and do not account for elastic-plastic crack tip behavior demonstrated in static tests of one-sided patch repaired ductile panels. This research used digital image correlation (DIC) and three-dimensional finite element analysis (FEA) with first order elements to study crack tip effects due to the one-sided composite patch applied to center crack tension (CCT) specimens loaded monotonically to failure. The measurable effects on crack tip behavior due to the composite patch were ultimate tensile load increase of more than 100% and a total achieved crack opening displacement (COD) increase of 20% over the unpatched behavior. Crack tip fracture behavior was found to be an intrinsic property of the aluminum and directly related to the COD independent of the one-sided composite patch. Increased capacity was related to accumulation of large-strain free surface area and through thickness volume ahead of the crack tip. Test data and numerical predictions correlated with measured load, strain, displacement fields, and J-integral behavior. Correlation of displacement fields with HRR and K fields established a state of small scale yielding prior to failure. Data and predictions indicated critical COD occurs when unpatched and patched large strain area is equivalent, which occurs before crack tip behavior transitions from small scale to large scale yielding and crack growth. Identifying a critical COD for both small and large scale one-sided patch repaired cracked ductile panels results in a predicted failure closer to the ultimate tensile load and 80% greater than predicted with LEFM methods. Observations and predictions demonstrated in this research resulted in three scientific contributions: (1) development of criteria to determine crack growth in cracked ductile panels repaired with a one-sided composite patch using a critical COD, (2) development of a three-dimensional FEA to study development of the plastic zone and evolution of the large-strain region ahead of the crack tip, and (3) development of a numerical methodology to predict ultimate tensile load capacity of cracked ductile panels repaired with a one-sided composite patch.Item SYNTHESIS OF MAIN GROUP ELEMENT CLUSTERS OF GROUPS 13 AND 15 FROM DISPROPORTIONATION PATHWAYS AND ZINTL POLYANIONS(2019) Stevens, Lauren Marie; Eichhorn, Bryan W; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis details the synthesis and characterization of main group element clusters of Groups 13 (aluminum) and 15 (phosphorus, arsenic). Aluminum clusters were synthesized from metastable Al(I)X (X = Cl, Br) solutions, which have proven adept at fostering the growth of metalloid clusters of the form AlnRm (where n > m). Group 15 – transition metal coordination complexes and ligand-free binary anions are synthesized through the use of Zintl ion precursors, originating from phases K3Pn7 (Pn = P, As, Sb). The novel cluster [(Bu2O)3Li][Li4Al5Ph12] has been synthesized and characterized, reported here in seven different crystallographic modifications. In addition to being the first low-valent phenyl aluminum cluster, this anion exhibits an unusual metastability in both the solid-state and solution, as indicated through ESI-MS, LDI-MS, and solid-state NMR analyses. The Zintl-derived coordination complexes [(η4-As7)Co(η3-As3)]3-, [(η4-As7)Rh(COD)]2-, and [(η4-As7)Ir(COD)]2- are reported as the first As / Group 9 clusters, and are isoelectronic to known coordination species of Zintl anions and transition metal carbonyl fragments. Additionally, the product [(η4-As7)Co(η3-As3)]3-is the first known carbon-free binary anion of As / Co. These complexes have been characterized via LDI-MS, NMR, single crystal XRD, and quantum chemical calculations. The doubly substituted coordination complex [(en)(CO)3Mo(η4-P7)Mo(CO)3]3- completes a series of previously reported Group 6 polyphosphide complexes, and is compared to its W congener, [(en)(CO)3W(η4-P7)W(CO)3]3-. Unlike coordination complexes, which retain the nuclearity of the seven-atom precursors [Pn7]3-, binary intermetalloids [Mo2P16]4- and [Rh3As16]3- show extensive reorganization of the original polypnictide cages. These anions feature cyclo-[P10]2- and cyclo-[As5]1- subunits, which are the first to be isolated and described in products of Zintl anions. Additionally, these are the first binary systems to be reported for Mo/P and Rh/As, and could potentially be used for the formation of binary phases (i.e. RhAs2) upon oxidationItem Evaluation of Information Entropy from Acoustic Emission Waveforms as a Fatigue Damage Metric for Al7075-T6(2016) Sauerbrunn, Christine Marie; Modarres, Mohammad; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Information entropy measured from acoustic emission (AE) waveforms is shown to be an indicator of fatigue damage in a high-strength aluminum alloy. Several tension-tension fatigue experiments were performed with dogbone samples of aluminum alloy, Al7075-T6, a commonly used material in aerospace structures. Unlike previous studies in which fatigue damage is simply measured based on visible crack growth, this work investigated fatigue damage prior to crack initiation through the use of instantaneous elastic modulus degradation. Three methods of measuring the AE information entropy, regarded as a direct measure of microstructural disorder, are proposed and compared with traditional damage-related AE features. Results show that one of the three entropy measurement methods appears to better assess damage than the traditional AE features, while the other two entropies have unique trends that can differentiate between small and large cracks.Item The Effect of Package Geometry on Moisture Driven Degradation of Polymer Aluminum Capacitors(2016) Bevensee, Helmut Manfred; Azarian, Michael H.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polymer aluminum electrolytic capacitors were introduced to provide an alternative to liquid electrolytic capacitors. Polymer electrolytic capacitor electric parameters of capacitance and ESR are less temperature dependent than those of liquid aluminum electrolytic capacitors. Furthermore, the electrical conductivity of the polymer used in these capacitors (poly-3,4ethylenedioxithiophene) is orders of magnitude higher than the electrolytes used in liquid aluminum electrolytic capacitors, resulting in capacitors with much lower equivalent series resistance which are suitable for use in high ripple-current applications. The presence of the moisture-sensitive polymer PEDOT introduces concerns on the reliability of polymer aluminum capacitors in high humidity conditions. Highly accelerated stress testing (or HAST) (110ºC, 85% relative humidity) of polymer aluminum capacitors in which the parts were subjected to unbiased HAST conditions for 700 hours was done to understand the design factors that contribute to the susceptibility to degradation of a polymer aluminum electrolytic capacitor exposed to HAST conditions. A large scale study involving capacitors of different electrical ratings (2.5V – 16V, 100µF – 470 µF), mounting types (surface-mount and through-hole) and manufacturers (6 different manufacturers) was done to determine a relationship between package geometry and reliability in high temperature-humidity conditions. A Geometry-Based HAST test in which the part selection limited variations between capacitor samples to geometric differences only was done to analyze the effect of package geometry on humidity-driven degradation more closely. Raman spectroscopy, x-ray imaging, environmental scanning electron microscopy, and destructive analysis of the capacitors after HAST exposure was done to determine the failure mechanisms of polymer aluminum capacitors under high temperature-humidity conditions.Item Synthesis and Characterization of Products Produced from Aluminum Monohalide Precursors(2015) DeCarlo, Samantha; Eichhorn, Bryan W; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, the synthesis, characterization and applications of aluminum compounds and cluster from aluminum monohalide solutions, AlX (where X = Cl or Br) are described. Chemistry of AlX solutions is not well understood, but AlX has proven adept at producing aluminum metalloid clusters (AlnLm where n>m). A brief overview of the renaissance of low-valent aluminum chemistry and select low-valent Al products is presented as background. The neutral mononuclear aluminum tris-bpy complexes [Al(Mebpy)3] and [Al(tBubpy)3] have been synthesized, isolated, and structurally characterized via X-ray single crystal diffraction. These complexes are the first structurally characterized homoleptic tris-bpy complexes and were studied via ESI-MS, d.c. magnetic susceptibility, electrochemical analyses. Electrochemistry demonstrates that six oxidation states are accessible from both neutral complexes: [Al(Rbpy)]n (n = -3 to 3, R = Me or tBu). The [Al(Mebpy)3] complex demonstrates unexpected magnetic ordering at 19 K which is not observed in [Al(tBubpy)3] nor in transition metal centered tris-bpy congeners. Synthesis, isolation, and characterization of the low-valent aluminum cluster [LiOEt2]2[HAl3(PPh2)6] via NMR and ESI-MS studies are also described. These experiments proved the presence of an H atom, and developed a complete and comprehensive picture of the structure, magnetism, and spectroscopy of this compound. Solution studies of reactions of AlBr with tBu-thiolate via ESI-MS show the formation and identification of [Al17Br(StBu)10S3]1-, [Al10(StBu)4S5]1-, [Al13(StBu)4BrS]1-, and [Al5(StBu)7Br]1-¬ in solution. The preparation and characterization of the aluminum (III) thiolate complex, Na[Al(SPh)4], is also described. These studies demonstrate the importance of reaction conditions in the formation of aluminum clusters in solution, and the viability of thiolate ligands to isolate low-valent aluminum products. Al nanoparticles (NP) can be produced from AlX solutions and have been successfully supported on both graphene and graphene oxide. The reduction of AlX solutions are quick, facile, and performed at low temperatures (-78°C). In the presence of graphene, faceted and well-dispersed graphene supported Al-NPs can be obtained. The [AlBrNEt3]4 cluster is isolated from AlBr⋅NEt3 solution and is soluble in toluene and diethyl ether. The burning rate of the hydrocarbon fuel doped with the tetramer is studied. There is an increase in burning rate attributed to the presence of [AlBrNEt3]4.Item Transient Fire Load on Aluminum Ferries(2014) Hall, Brian; Gollner, Michael J; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Transient fire load aboard aluminum ferries is studied to determine the contribution of baggage on increasing compartment overhead temperatures. Single-point and average temperature maximums are compared to critical values. A survey of passenger ferry vessels determined the baggage type, carriage rate, and baggage weights to quantify the fire load aboard the vessels. Ferry vessels were examined for potential problem locations. Fire Dynamics Simulator (FDS) by the National institute of Standards and Technology (NIST) was used to model an aluminum ferry compartment. Multiple scenarios are simulated with spread based on a critical heat flux ignition data. The survey determined that the majority of fully loaded aluminum ferries attain higher fuel loads than allowed by Coast Guard requirements. Simulations revealed that the current level of baggage compromises the structural integrity of the aluminum on an average ferry. It is recommended that regulatory changes be made to ensure protection of life and property.Item IGNITION, COMBUSTION AND TUNING OF NANOCOMPOSITE THERMITES(2010) Sullivan, Kyle Thomas; Zachariah, Michael R; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nanocomposite thermites, or Metastable Intermolecular Composites (MICs), are energetic systems involving the reaction between nanoparticles of a metal fuel and another metal or metal oxide. When nanoparticles are used, the interfacial contact area and homogeneity of mixing are greatly improved, dramatically decreasing the characteristic mass diffusion length between the fuel and the oxidizer. Nano-sized aluminum is commonly used as a fuel, due to a combination of its abundance, good reactivity, and its ability to produce environmentally benign reaction products. A variety of oxidizers have been studied depending on the particular application. Nanocomposite thermites are currently being investigated for uses in propellants, pyrotechnics, and explosives, as well as some more exotic applications such as micro-propulsion and joining applications. Despite the research efforts and potential applications, the reaction mechanism remains poorly understood. As the particle size transitions into the nanometer regime, properties such as the melting temperature, surface energy, drag force, along with the characteristic time scales of thermo-chemical processes can change. In an exothermically reacting system, all of these considerations must be taken into account simultaneously, a rather daunting task. However, if we design parametric experiments to look at relative trends, we can develop scaling laws and determine which parameters are perhaps the most important in the reaction mechanism. This work largely involves combusting thermite materials in a pressure cell, and also uses new techniques such as inducing a reaction inside an electron microscope with a specially designed heating holder. The results suggest that the pressurization and optical emission can arise from fundamentally different phenomena. A reactive sintering mechanism occurs which rapidly decomposes the oxidizer and pressurizes the system. This is followed by the remainder of the fuel burning in a gaseous, pressurized environment, where the burning rate is controlled by the fuel. Also in this work, we combust new fuels and oxidizers such as nano-sized boron, AgIO3, and Ag2O. Boron can be used as an additive to increase the energy density in thermites. The silver-based oxidizers are currently being investigated in nanocomposite thermites for their ability to generate a product which can effectively destroy harmful biological spores, such as Anthrax.Item MOLECULAR DYNAMICS STUDIES OF METALLIC NANOPARTICLES(2009) Henz, Brian John; Zachariah, Michael R; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metal nanoparticles have many desirable electrical, magnetic, optical, chemi-cal, and physical properties. In order to utilize these properties effectively it is neces-sary to be able to accurately predict their size-dependent properties. One common method used to predict these properties is with numerical simulation. The numerical simulation technique used throughout this effort is the molecular dynamics (MD) si-mulation method. Using MD simulations I have investigated various metallic nano-particle systems including gold nanoparticles coated with an organic self-assembled monolayer (SAM), the self-propagating high-temperature synthesis (SHS) reaction of nickel and aluminum nanoparticles, and the mechano-chemical behavior of oxide coated aluminum nanoparticles. The model definition, boundary conditions, and re-sults of these simulations are presented in the following dissertation. In the first material system investigated MD simulations are used to probe the structure and stability of alkanethiolate self-assembled monolayers (SAMs) on gold nanoparticles. Numerous results and observations from this parametric study are pre-sented here. By analyzing the mechanical and chemical properties of gold nanopar-ticles at temperatures below the melting point of gold, with different SAM chain lengths and surface coverage properties, we have determined that the material system is metastable. The model and computational results that provide support for this hy-pothesis are presented. The second material system investigated, namely sintering of aluminum and nickel, is explored in chapter 4. In this chapter MD simulations are used to simulate the kinetic reaction of Ni and Al particles at the nanometer scale. The affect of par-ticle size on reaction time and temperature for separate nanoparticles has been consi-dered as a model system for a powder metallurgy process. Coated nanoparticles in the form of Ni-coated Al nanoparticles and Al-coated Ni nanoparticles are also analyzed as a model for nanoparticles of one material embedded within a matrix of the second. Simulation results show that the sintering time for separate and coated nanoparticles is dependent upon the number of atoms or volume of the sintering nanoparticles and their surface area. We have also found that nanoparticle size and surface energy is an important factor in determining the adiabatic reaction temperature for both systems, coated and separate, at nanoparticle sizes of less than 10nm in diameter. The final material system investigated in chapters 5 and 6 is the oxide coated aluminum nanoparticle. This material system is simulated using the reactive force field (ReaxFF) potential which is capable of considering the charge transfer that occurs during oxidation. The oxidation process of oxide coated aluminum nanoparticles has been observed to occur at a lower temperature and a faster rate than micron sized nanoparticles, suggesting a different oxidation mechanism. From this effort we have discovered that the oxidation process for nanometer sized oxide coated aluminum particles is the result of an enhanced transport due to a built-in electric field induced by the oxide shell. In contrast to the currently assumed pressure driven diffusion process the results presented here demonstrate that the high temperature oxidation process is driven by the electric field present in the oxide layer. This electric field ac-counts for over 90% of the mass flux of aluminum ions through the oxide shell. The computed electric fields show good agreement with published theoretical and experi-mental results. The final chapter includes some important conclusions from this work and highlights some future work in these areas. Future work that is outlined includes ef-forts that are currently underway to analyze the interactions of multiple alkanethiolate coated gold nanoparticles in vacuum and in solvent. Other future efforts are farther out over the horizon and include using advanced computing techniques such as gen-eral purpose graphical processing units (GPGPU) to expand simulation sizes and physical details over what it is currently possible to simulate.Item Modeling of a High Energy Density Propulsion System Based on the Combustion of Aluminum and Steam(2007-12-13) Eagle, W. Ethan; Cadou, Christopher P; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents a thermodynamic analysis of a novel Rankine cycle aluminum/steam combustion power system being developed for use in Unmanned Underwater Vehicles (UUVs). The analysis is performed using a system modeling tool developed by the NASA Glenn Research Center called Numerical Propulsion System Solver (NPSS). Thermodynamic models of the individual components are created and linked together in NPSS, which then solves the system by enforcing mass and energy conservation. Design and off-design conditions are simulated and predicted performance is compared with predictions made by two other research groups. The simulations predict that this power system could provide at least five-fold increases in range and endurance for the US Navy's 'Sea Horse' UUV. A rudimentary sensitivity analysis is used to identify the factors which most strongly influence the performance of the design. Lastly, recommendations for future work and possible model improvements are discussed.