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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    Ceramic Materials Development for Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC)
    (2016) Pan, Ke-Ji; Wachsman, Eric D; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Solid oxide fuel cell (SOFC) is an electrochemical device that converts chemical energy into electric power with high efficiency. Traditional SOFC has its disadvantages, such as redox cycling instability and carbon deposition while using hydrocarbon fuels. It is because traditional SOFC uses Ni-cermet as anode. In order to solve these problems, ceramic anode is a good candidate to replace Ni. However, the conductivity of most ceramic anode materials are much lower than Ni metal, and it introduces high ohmic resistance. How to increase the conductivity is a hot topic in this research field. Based on our proposed mechanism, several types of ceramic materials have been developed. Vanadium doped perovskite, Sr1-x/2VxTi1-xO3 (SVT) and Sr0.2Na0.8Nb1-xVxO3 (SNNV), achieved the conductivity as high as 300 S*cm-1 in hydrogen, without any high temperature reduction. GDC electrolyte supported cell was fabricated with Sr0.2Na0.8Nb0.9V0.1O3 and the performance was measured in hydrogen and methane respectively. Due to vanadium’s intrinsic problems, the anode supported cell is not easy. Fe doped double perovskite Sr2CoMoO6 (SFCM) was also developed. By carefully doping Fe, the conductivity was improved over one magnitude, without any vigorous reducing conditions. SFCM anode supported cell was successfully fabricated with GDC as the electrolyte. By impregnating Ni-GDC nano particles into the anode, the cell can be operated at lower temperatures while having higher performance than the traditional Ni-cermet cells. Meanwhile, this SFCM anode supported SOFC has long term stability in the reformate containing methane. During the anode development, cathode improvement caused by a thin Co-GDC layer was observed. By adding this Co-GDC layer between the electrolyte and the cathode, the interfacial resistance decreases due to fast oxygen ion transport. This mechanism was confirmed via isotope exchange. This Co-GDC layer works with multiple kinds of cathodes and the modified cell’s performance is 3 times as the traditional Ni-GDC cell. With this new method, lowering the SOFC operation temperature is feasible.
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    The Effects of Environmental Stresses on the Reliability of Flexible and Standard Termination Multilayer Ceramic Capacitors
    (2009) Brock, Garry Robert; Pecht, Michael G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Flexible termination capacitors were designed to reduce stresses transmitted to the ceramic dielectric of a capacitor and thereby prevent flex cracking. Two studies were conducted to examine the reliability of flexible termination multilayer ceramic capacitors (MLCCs) subjected to environmental stresses. The first study used temperature-humidity-bias to compare the effects of termination type (standard vs. flexible), presence of a conformal coating (acrylic coating vs. no coating), and voltage bias level. In situ monitoring demonstrated similar failure statistics between the flexible and standard termination capacitors, presence of conformal coating, and voltage bias level. Upon removal from THB conditions recovery occurred only in the standard termination MLCCs. Flexible termination capacitors at the rated voltage bias were found to have more permanent failures after exposure to THB testing as compared to standard termination capacitors. Failure analysis indicated that silver and palladium migration between electrodes was the failure mechanism in the biased flexible termination capacitors. In the second study flexible and standard termination MLCCs experienced a storage test in which they were exposed to elevated temperature and humidity conditions. It was found that the standard termination MLCCs had a lower reliability with the majority of the MLCCs failing compared to the flexible termination MLCCs where only one MLCC failed. Nearly all failures were for insulation resistance with few capacitors failing for other parameters. Subsequent bake-out of the MLCCs showed some recovery, however more failures were still occurring in the standard termination MLCCs compared to the flexible termination MLCCs. X-ray photoelectron spectroscopy and cross-sectioning were used to examine the failure mechanisms of the capacitors. A bulk migration of silver into the dielectric was determined to be one of the failure mechanisms in the capacitors.