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 ELECTROLYTE AND INTERFACE DESIGNATION FOR HIGH-PERFORMANCE SOLID-STATE LITHIUM METAL BATTERIES(2024) Zhang, Weiran; Wang, Chunsheng; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The demand for advanced battery technology is intensifying as electric energy becomes the foundation of modern technologies, such as smart devices, transportation, and artificial intelligence. Batteries play a crucial role in meeting our increasing energy demands and transitioning towards cleaner and more sustainable energy sources. However, range anxiety and safety concerns still hinder the widespread application of battery technology.Current Li-ion batteries, based on graphite anode, have revolutionized battery technology but are nearing the energy density limits. This necessitates the development of metal batteries, employing lithium metal as anode which eliminates host materials that do not contribute to capacity, thereby offering 10 times higher specific capacity. Recent research on lithium metal batteries has seen a significant surge, with growing knowledge transitioning from Li+ intercalation chemistry (graphite) to Li metal plating/stripping. The electrolyte, which was previously regarded as an inert material and acting as a Li+ ion transportation mediator, has gradually attracted researchers’ attention due to its significant impact on the solid electrolyte interphase (SEI) and the Li metal plating/stripping behaviors. Compared to the traditional liquid electrolytes, solid-state lithium metal batteries (SSLMB) have been regarded as the holy grail, the future of electric vehicles (EVs), due to their high safety and potential for higher energy density. However, there are notable knowledge gaps between liquid electrolytes and solid-state electrolytes (SSEs). The transition from liquid-solid contact to solid-solid contact poses new challenges to the SSLMB. As a result, the development of SSLMB is strongly hindered by interface challenges, including not only the Li/SSE interfaces and SSE/cathode interfaces but also SSE/SSE interfaces. In this dissertation, I detailed our efforts to highlight the role of electrolytes and interfaces and establish our understanding and fundamental criteria for them. Building on this understanding, we propose effective and facile engineering solutions that significantly enhance batterie metrics to meet real-world application demand. Rather than simply introducing new compositions or new designations, we are dedicated to introducing our understanding and mechanism behind it, we hope the scientific understanding, the practical solution, and the applicability to various systems can further guide and inspire the electrolyte and interface designation for next-generation battery technology.Item DFT AND RELATED MODELING OF POST-SILICON VALENCE 4 MATERIALS: SiC AND Ge(2020) Darmody, Christopher; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Though silicon (Si) is in many ways the material of choice for many electronic applications due in part to its mature processing technology, its intrinsic properties are not always suited for every challenge. Specialized high power and high temper- ature devices benefit from using semiconductors with a larger band-gap and higher thermal conductivity such as silicon carbide (SiC). Additionally, the 1.1eV bandgap of Si makes it unable to effectively absorb infrared photons so a material with a smaller bandgap, like germanium (Ge), is more suited to the task. Currently SiC power transistors are commercially available but suffer from poor channel mobility due to interface roughness which limits their performance. To predict the maximum theoretically achievable mobility for different crystallo- graphic interfaces I developed a novel technique for extracting an atomic-roughness scattering rate from an arbitrary atomic surface. The term atomic-roughness here means an interface purely due to the variation of atom species and position without the presence of a crystallographic miscut due to epitaxial growth considerations. I used Density Functional Theory (DFT) to obtain a perturbation potential from which I can calculate a scattering rate. This scattering rate can then be used in a Monte Carlo simulation to predict mobility for a given field configuration. In addition to SiC’s low channel mobility, SiC p-type dopant species also ex- hibit an abnormally large ionization energy compared to its n-type dopants and to the primary dopants in many other semiconductors. This fact can cause is- sues such as unexpectedly high resistance regions at lower operating temperatures - causing the need to dope at significantly higher concentration. To characterize the incomplete ionization fraction p/N A , I first gathered nearly all existing pub- lished data on the ionization energy of aluminum (Al) in 4H-SiC and created an empirical concentration-dependent model of this function. Then I put together a physics-based model of the entire acceptor and valence band system and used my concentration-dependent ionization energy as an input to predict p/N A . I verify my physics-based model result against a separate experimental dataset derived from nearly-exhaustive literature measurements of Hall mobility and resistivity. Finally, I transform fully temperature-dependent result of p/N A from a complex numerical computation to a more easily implementable parameterized function with the use of a genetic algorithm. The remaining part of my work was performed on Germanium which has interesting application in short-wave infrared imaging due its 0.66eV indirect and 0.85 eV direct bandgaps, which corresponds closely to the peak illumination of the “night glow” at 0.75 eV. Optical devices greatly benefit from direct gap band structures to increase photon absorption and emission efficiency. Though Ge is an indirect gap material, it can be alloyed with a direct gap material, namely tin (Sn), to transition it to a direct gap material at a certain molar fraction. Through DFT calculations I investigate the nature of this transition and determine theoretically the minimum molar fraction needed to achieve a direct bandgap.Item HUMAN FACTORS EVALUATION OF OPERATOR INTERFACES FOR TELEOPERATION OF A DEXTEROUS MANIPULATOR(2014) Davis, Kevin Patrick; Akin, David L; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ground teleoperation of a satellite servicing spacecraft is a challenging task for a human operator, especially when there is significant communications delay between the control station and spacecraft. On-orbit operations are further complicated by a communications time delay between the ground and spacecraft. Operator performance can be improved with the use of a graphical simulation of the robot. By displaying the robot's commanded position, graphical simulation can also mitigate some effects of time delay. This work implemented a visualization tool and commanded display to assist operation of a remote dexterous manipulator. A Fitts' Law experiment was designed to determine the effectiveness of the commanded display in reducing the impact of time delay. The experiment was conducted with a six degree of freedom manipulator over a range of time delays, from 0.0 to 6.0 seconds. The experimental results were analyzed to assess the reduction of task completion time and operator workload.Item Solvation, Structure and Organization at Liquid Surfaces(2009) Brindza, Michael Ross; Walker, Robert A; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents the results of nonlinear spectroscopic studies whose goal is to understand how the asymmetric nature of interfaces and intermolecular interactions give rise to interfacial solvation properties and solvent structure. The first part of this thesis uses resonance enhanced second harmonic generation to examine the polarity and hydrogen bonding opportunities at interfaces formed between hydrophilic silica and both weakly and strongly associating organic liquids. Measuring interfacial electronic spectra of probe molecules that exhibit solvatochromic sensitivity to polarity and hydrogen bonding, we saw that small changes in solvent structure affect interfacial polarity, and strongly associating alcohols solvents create a region of heterogeneous polarity at the interface. Silica appears to donate hydrogen bonds to adsorbates no matter what solvent (protic or aprotic) was chosen. The second part of this dissertation uses another nonlinear spectroscopic technique, vibrational sum frequency generation, to determine the structure and orientation of solvent molecules adsorbed to silica/vapor, silica/liquid, and neat liquid/vapor interfaces. By comparing spectral features appearing under different experimental polarization conditions, we have determined average solvent orientations and degree of organization. Our initial studies of alkanes adsorbed to the silica/vapor interface show that despite strong substrate-adsorbate interactions, molecules at the interface show some degree of long range order and organization. In order to examine how the strength of intermolecular forces between adsorbates and either the substrate or neighboring molecules affect interfacial organization, we measured vibrational spectra of octanol isomers as well as different functional group containing n-alkyl molecules at silica/vapor and silica/liquid interfaces. The octanol studies show that strongly associating molecules form ordered monolayers at the silica/vapor interface, but that strength of lateral interactions is important for preserving that order when the liquid is brought into contact. Branched isomers appeared very disordered at solid/liquid interfaces. Further examining this change in order between solvents at silica/vapor and silica/liquid interfaces using equal length but different functional group containing solvents, we see that the energetics of adsorption and solvation are likely to be responsible for the degree of order both at the solid/vapor surface (adsorption) and solid/liquid interface (both adsorption and solvation).Item Formation and Piezoelectricity of Self-Assembled PbTiO3-CoFe2O4 Nanostructural Films(2008-06-13) tan, zhuopeng; Roytburd, Alexander L; Levin, Igor; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Main tasks of our research include: (1) exploring optimum growth conditions for PLD deposition of self-assembled nanophase PbTiO3-CoFe2O4 films with different compositions and orientations; (2) analyzing morphologies and nanostructures of the two-phase films to clarify relative effects of elastic energy and interface energy on the self-assembled film formation; (3) investigating stress state and relaxation of stresses arising as a result of a paraelectric-ferroelectric transformation in PbTiO3; (4) exploring ferroelectric state in the confined PbTiO3 nanophase in the films with {110} and {111} orientations. Principal results of the research are: (1). Optimum PLD growth conditions to obtain high quality films with distinct separation of epitaxial PbTiO3 and CoFe2O4 nanophases are found after systematic studies. (2). Nano-facets along {111} plane between PbTiO3 and CoFe2O4 phases are found to be generic in addition to orientation dependent macroscopic interfaces. We have concluded that accounting of interface and surface energies is important for description of nano-faceting of interfaces and the near substrate zone of the films while the two-phase morphologies are determined by the elastic interactions; (3). The investigation of the stress state of the {001} film arising due to paraelectric-ferroelectric transition of PbTiO3 have discovered the polydomain nanostructure of the ferroelectric phase with ~50-60% c-domains. Piezoresponse of PbTiO3 should be reduced dramatically by combined effects of dissolution of Fe in PbTiO3, a domains and constraints. The relative large dzz from previous research must contain large extrinsic contribution due to movement of nano-domain walls. (4). Switching spectroscopy piezoresponse force microscopy (SS-PFM) is used to characterize local piezo- ferroelectric property of confined ferroelectrics in {110} and {111} films with composition of 1/3PbTiO3-2/3CoFe2O4. It is proved that PbTiO3 nano-inclusions exhibit ferroelectricity in both films. 180o domain switching is observed under measurement condition (<10V) for the {110} films but not for the {111} film. Quantitatively, both films yield a piezoresponse of about 15% compared to bulk single crystal PbTiO3. It is a reasonable value of intrinsic piezoeffect taking into account mechanical and electrical constraints (depolarizing field) as well as the effect of Fe dissolution and possible in-plane domains