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.
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Item Response inhibition and the cortico-striatal circuit(2015) Bryden, Daniel William; Roesch, Matthew R; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ability to flexibly control or inhibit unwanted actions is critical for everyday behavior. Lack of this capacity is characteristic of numerous psychiatric diseases including attention deficit hyperactivity disorder (ADHD). My project is designed to study the neural underpinnings of response inhibition and to what extent these mechanisms are disrupted in animals with impaired impulse control. I therefore recorded single neurons from dorsal striatum, orbitofrontal cortex, and medial prefrontal cortex from rats performing a novel rodent variant of the classic "stop signal" task used in clinical settings. This task asks motivated rats to repeatedly produce simple actions to obtain rewards while needing to semi-occasionally inhibit an already initiated response. To take this a step further, I compared normal rats to rats prenatally exposed to nicotine in order to better understand the mechanism underlying inhibitory control. Rats exposed to nicotine before birth show abnormal attention, poor inhibitory control, and brain deficits consistent with impairments seen in humans prenatally exposed to nicotine and those with ADHD. I found that dorsal striatum neurons tend to encode the direction of a response and the motor refinement necessary to guide behaviors within the task rather than playing a causal role in response inhibition. However the orbitofrontal cortex, a direct afferent of dorsal striatum, possesses the capacity to inform the striatum of the correct action during response inhibition within the critical time window required to flexibly alter an initiated movement. On the other hand, medial prefrontal cortex functions as a conflict “monitor” to broadly increase preparedness for flexible response inhibition by aggregating current and past conflict history. Lastly, rat pups exposed to nicotine during gestation exhibit faster movement speeds and reduced capacity for inhibitory behavior. Physiologically, prenatal nicotine exposure manifests in a hypoactive prefrontal cortex, diminished encoding of task parameters, and reduced capacity to maintain conflict information.Item PERFORMANCE OF Ni/CEO2/YSZ SOFC ANODES WITH CARBONACEOUS FUELS(2009) Patel, Siddharth; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study explores the impact of ceria incorporation into Ni/YSZ cermet anode support layers on the performance of button-cell solid oxide fuel cells operating with syngas and n-butane/steam fuel feeds. Ceria is incorporated into the porous anode support by co-firing ceria powders with NiO, YSZ, and graphite pore formers. Comparison of the performance with and without the co-fired ceria indicated improvements for operation with doped ceria for both syngas (by almost 20% higher power density) and direct n-butane/steam feeds (by over 25% higher power density). For initial cell performance, ceria addition to the support layer offered improved performance at high current densities with syngas suggesting that ceria enhanced water-gas-shift reactions, thereby increasing H2 availability for more effective electrochemical oxidation in the functional layer. For longer-term testing with direct-butane feeds, ceria doped cells not only showed better performance, but also indicated suppression of carbon deposition thus improving long term operability. Ex situ characterization of the ceria-doped anodes using SEM and Raman spectroscopy indicated that ceria addition helped the anodes maintain their structural integrity. To better understand the experiments, a previous through-the-MEA 1-D model has been updated and used with C-H-O microkinetics for Ni/YSZ anodes to characterize the experimentally observed cell performance. The model was enhanced by developing a leakage mechanism to account for leakage through the electrolyte and by incorporating non-isothermal effects to account for temperature gradients due to endothermic internal reforming and exothermic oxidation within the anode layers. Studies with internal methane reforming in a Ni/YSZ anode showed that the non-isothermal effects in 1-D button cell experiments are very small. This through-the-MEA model was used to fit experimental data and provided a basis for assessing key micro-structural parameters for the Ni/YSZ cells tested in this study. The model fits with syngas at various compositions provided a basis for assessing the most sensitive micro-structural parameters on the fuel cell performance such as anode support layer porosity and tortuosity.