UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

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 given thesis/dissertation in DRUM.

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    Phonon Modeling in Nano- and Micro- scale Crystalline Systems
    (2018) VanGessel, Francis; Chung, Peter; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Submicrometer phonon systems are becoming increasingly relevant in modern day technology. Phonon mechanisms are notably relevant in a number of solid-state devices including lasers, LEDs, transistors, and thermoelectrics. Proliferation of these devices has been driven by advancements in silicon-on-insulator technology. These advancements have allowed for the manufacture of devices with complex nanostructures and dimensions deep in the sub-microscale regime. However, accompanying improvements in the manufacture and design of novel crystalline systems is the requirement for accurate computational approaches for phonon modeling in nanostructured, anisotropic, and complex materials. The phonon Boltzmann transport equation is uniquely well suited to modeling energy transfer at the nano- and micro- meter length scales and is therefore an excellent candidate for this simulation task. However, current Boltzmann modeling approaches utilize a range of assumptions and simplifications that restrict their validity to isotropic, nominally one or two dimensional, or compositionally simple systems. In this dissertation we present an original finite volume-based methodology for the solution of the three dimensional full Brillouin zone phonon Boltzmann transport equation. This methodology allows for separate real and reciprocal space discretization. By taking a sampling of vibrational modes throughout the first Brillouin zone our methodology captures three unique sources of phonon anisotropy. We investigate the effect of phonon anisotropy in a fin field effect transistor, calculating the effect that incorporating various sources of anisotropy has on the resultant temperature fields. In a second study, we consider phonon flow through silicon nanowires with a modified boundary geometry. The three-dimensional flow fields are calculated and thermal transport below the Casimir limit is observed. Reduction in thermal conductivity is a result of maximizing the phonon backscatter that occurs in our phononic system. The backscatter serves to create regions of highly misaligned phonon flux. In addition, our silicon nanowire geometry has properties analogous with a high-pass phonon filter. In the final study we apply our Boltzmann transport methodology to the simulation of phonon transport in the energetic material, RDX. We study phonon transport in the vicinity of a material hotspot, the location at which chemistry initiates in the material. By applying Boltzmann modeling, applied for the first time to this material, we gain valuable insights into the interplay between thermal transport and phonon modes linked with initiation.
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    The importance of female phenotype in determining reproductive potential and recruitment in Atlantic coast striped bass (Morone saxatilis)
    (2012) Peer, Adam Christopher; Miller, Thomas J; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The influence of female phenotype on the reproductive potential of Atlantic coast striped bass is addressed in three key areas of research. The importance of the environment in shaping maternal phenotype was evaluated using a spawning stock time-series to evaluate possible environmental drivers of female migration timing in the Chesapeake Bay. Results showed that local and recent water temperature was the primary factor influencing timing of movement onto spawning grounds, with higher temperatures resulting in early movements. Next, two approaches were used to evaluate the influence of female energetic condition on reproductive potential. First, a field approach was used to test the hypothesis that relative total female condition (hereafter condition) has a positive influence on pre-fertilized indicators of reproductive potential (i.e., probability of spawning, relative fecundity, and relative oocyte volume). Results indicated that condition had a positive influence on residual fecundity, residual oocyte volume and indirectly on the probability of spawning. In the second approach, a laboratory experiment was conducted to test the hypothesis that female condition has a positive effect on offspring size, growth and survival. The null hypothesis that the maternal influences on offspring phenotype did not differ in the Chesapeake Bay and Roanoke River populations also was tested. In contrast to the effects of female condition on pre-fertilized indicators of reproductive potential, condition had no influence on offspring phenotype in either population. Instead, post-spawn gutted weight alone had the greatest influence on offspring phenotype, although to a lesser and potentially insignificant degree in the Roanoke River. Finally, a preliminary field evaluation was conducted in the Patuxent River, MD to determine whether maternal influences can lead to disproportionate numbers of mothers contributing to juvenile recruitment. Specifically, this study evaluated whether the variance in the distribution of half-sibling families was greater than expected by random reproductive success (i.e., Poisson process). If true, it was expected that the effective population size would be orders of magnitude smaller than the census size. Results provide preliminary evidence for higher than expected variance in reproductive success; however, methodological improvements will be necessary to confirm these results in the future
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    Foraging values of Mulinia lateralis and Ischadium recurvum: energetics effects of surf scoters wintering in the Chesapeake Bay.
    (2008-03-26) Berlin, Alicia; Ottinger, Mary Ann; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Surf scoter (Melanitta perspicillata) populations wintering in the Chesapeake Bay primarily prey on two food items, the hooked mussel (Ischadium recurvum) and dwarf surfclam (Mulinia lateralis). The decline of oyster reefs (Crassostrea virginica) has decreased availability of mussels inducing surf scoters to switch to a more opportune food item, the dwarf surfclam. The objectives of this study were: 1) to determine the comparative nutrient composition of these prey items; 2) to evaluate the energy assimilated by surf scoters from these prey items; 3) to determine the functional responses of scoters foraging on four different ecologically relevant densities (30, 100, 1000, and 3000 m-2) of each prey item; and 4) to model the foraging value (costs - benefits) of both prey items for surf scoters. I. recurvum contained higher ash, protein, lipid, and energy per item than M. lateralis. Metabolizable energy from each prey item by surf scoters was 83% for M. lateralis and 87% for I. recurvum. The shell strength of I. recurvum was significantly stronger than M. lateralis. For scoters foraging in a large diving tank 2 m deep, intake (# s-1) for M. lateralis was significantly higher than I. recurvum at high densities, but lower at the low densities. Gross energy intake (kJ s-1) and metabolized energy intake (kJ s-1) were significantly greater for I. recurvum than M. lateralis. Based on nutrient content, metabolizability, behavior and intake rates, and energy expenditure at naturally occurring densities, the foraging value for M. lateralis was significantly lower than I. recurvum. Despite higher ash content and harder shell, which would partly offset the apparent energetic advantages of I. recurvum, greater foraging value of I. recurvum than M. lateralis provides a more beneficial prey item for wintering surf scoters. Therefore, wintering surf scoters must adapt in order to maintain their daily energy requirement. If surf scoters are forced to feed primarily on M. lateralis, the most advantageous and available prey in the Chesapeake Bay, instead of I. recurvum; there may be insufficient energy for them to build fat reserves needed to make migration.