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
2 results
Search Results
Item UNDERSTANDING DIRECT BOROHYDRIDE - HYDROGEN PEROXIDE FUEL CELL PERFORMANCE(2013) Stroman, Richard O'Neil; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Direct borohydride fuel cells (DBFCs) generate electrical power by oxidizing aqueous BH4- at the anode and reducing an oxidizer, like aqueous H2O2 for an all-liquid fuel cell, at the cathode. Interest in DBFCs has grown due to high theoretical energy densities of the reactants, yet DBFC technology faces challenges such as side reactions and other processes that reduce cell efficiency and power generation. Relationships linking performance to cell design and operation will benefit from detailed and calibrated cell design models, and this study presents the development and calibration of a 2D, single-cell DBFC model that includes transport in reactant channels and complex charge transfer reactions at each electrode. Initial modeling was performed assuming ideal reactions without undesirable side reactions. Results were valuable for showing how design parameters impact ideal performance limits and DBFC cell voltage (efficiency). Model results showed that concentration boundary layers in the reactant flow channels limit power density and single-pass reactant utilization. Shallower channels and recirculation improve utilization, but at the expense of lower cell voltage and power per unit membrane area. Reactant coulombic efficiency grows with decreasing inlet reactant concentration, reactant flow rate and cell potential, as the relative reaction rates at each electrode shift to favor charge transfer reactions. To incorporate more realistic reaction mechanisms into the model, experiments in a single cell DBFC were performed to guide reaction mechanism selection by showing which processes were important to capture. Kinetic parameters for both electrochemical and critical heterogeneous reactions at each electrode were subsequently fitted to the measurements. Single-cell experiments showed that undesirable side reactions identified by gas production were reduced with lower reactant concentration and higher supporting electrolyte concentration and these results provided the basis for calibrating multi-step kinetic mechanism. Model results with the resulting calibrated mechanism showed that cell thermodynamic efficiency falls with cell voltage while coulombic utilization rises, yielding a maximum overall efficiency operating point. For this DBFC, maximum overall efficiency coincides with maximum power density, suggesting the existence of preferred operating point for a given geometry and operating conditions.Item The Protective Behaviors of Student Victims: Responses to Direct and Indirect Bullying(2007-04-19) DeVoe, Jill Fleury; Gottfredson, Denise; Criminology and Criminal Justice; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Most research on school bullying has focused on its prevalence, characteristics of bullies and victims, and programmatic responses undertaken by schools to prevent or reduce bullying. Few studies have investigated victims' responses to bullying at school. While the public, national media, and recent studies implicate bullying as a factor in cases of school violence, little research to date examines the self-protective behaviors of bullying victims. This raises the question; do victims of bullying take measures to protect themselves, despite the fact that these measures may endanger other students at school or school climate? Are they more likely to adopt these behaviors when they perceive that their school is not a capable guardian from such victimization? And finally, do their choices of protective behaviors vary by the type of bullying they endure? The purpose of this dissertation is to examine self-protective behaviors exhibited by victims bullying. Findings indicate that student victims of bullying were more likely than non-bullied students to adopt self-protective behaviors that further endanger school safety and school climate. Specifically, controlling for relevant student and school characteristics, bullied students were three times more likely to carry a weapon to school, to engage in fighting behaviors, and to avoid certain places at school, and were six times more likely to be truant from a school activity. No support for an interaction between measures of school guardianship and student protective behaviors was found, meaning that student perceptions of school security or rule enforcement did not play a role in bullied students' decisions to engage in avoidance, truancy, weapon carrying, or fighting. In addition, the adoption of these behaviors did not differ by the type of bullying, direct or indirect, endured by the victim. Theoretical and policy implications are discussed.