Doctor of Philosophy, The Ohio State University, 2008, Food Agricultural and Biological Engineering

In microbial fuel cells (MFCs), bacteria generate electricity by mediating the oxidation of organic compounds and transferring the resulting electrons to an anode electrode. The objectives of this study were to: 1) test the possibility of generating electricity in an MFC with rumen microorganisms as biocatalysts and cellulose as the electron donor, 2) analyze the composition of bacterial communities enriched in cellulose-fed MFCs, 3) determine the effect of various external resistances on power output and coulombic efficiency of cellulose-fed MFCs, 4) evaluate bacterial diversity and cellulose metabolism under different circuit loads, 5) assess the influence of methane formation on the performance of cellulose-fed MFCs under long-term operation, and 6) characterize the diversity of methanogens in cellulose-fed MFCs. The results demonstrate that electricity can be generated from cellulose by exploiting rumen microorganisms as biocatalysts. Cloning and analysis of 16S rRNA gene sequences indicated that the most predominant bacteria in the anode-attached consortia were related to Clostridium spp., while Comamonas spp. abounded in the suspended consortia. Results suggest that oxidation of metabolites with the anode as an electron sink was a rate limiting step in the conversion of cellulose to electricity in MFCs. This study also shows that the size of external resistance significantly affects the bacterial diversity and power output of MFCs. A maximum power density of 66 mW/m2 was achieved by the 20-ohm MFCs, while MFCs with 249, 480 and1000 ohms external resistances produced 57.5, 53 and 47 mW/m2, respectively. Thus the external resistance may be a useful tool to control microbial communities and consequently enhance performance of MFCs. Furthermore, this study demonstrates that methanogenesis competes with electricity generation at the early stages of MFC operation but operating conditions suppress methanogenic activity over time. The suppression of methanoge (open full item for complete abstract)

Committee: Ann Christy PhD (Advisor); Burk Dehority PhD (Committee Member); Olli Tuovinen PhD (Committee Member); Alfred Soboyejo PhD (Committee Member); Zhongtang Yu PhD (Committee Member) Subjects: Agricultural Engineering; Chemical Engineering; Energy; Environmental Engineering; Environmental Science; Microbiology

Doctor of Philosophy, The Ohio State University, 2003, Welding Engineering

This dissertation study has developed a fundamental understanding of the contact resistance behavior using the virtual contact volume concept and the equivalent contact resistivity definition. In this research, an integrated experimental-numerical approach was used to demonstrate the proposed equivalent contact resistivity versus temperature relationship. Such relationship was further used to study the expulsion behavior of the resistance spot welding aluminum alloy. A concept of using the constriction ring was demonstrated to be effective in preventing weld expulsion in resistance spot aluminum welds. The a-spot model simulated a single ball contact to define the equivalent contact resistivity. For a specific loading range, the generic equivalent contact resistivity versus temperature relationship, consisting of a proportional term and a exponential term, was derived. The proportionality term represents the increasing contact resistivity with temperature reflecting the Wiedemann-Franz-Lorentz behavior. The exponential term represents the softening effect of the contact surface as temperature increases. With the generic equivalent contact resistivity versus temperature relationship established, an a-spot welding which incorporated the contact pairs with the resistivity relationship leaving three factors as parametric variables and an experimental a-spot welding model were conducted. Both the numerical and experimental analyses show the same relationship that is defined by the theoretical hypothesis of a-spot model. The parametric study was conducted to quantify the parametric factors based on comparisons on nugget size and electrical potential drops across the electrodes. For an ideal combination of parameters at the Cu/Al interface and the Al/Al faying surface, both the comparisons of weld nugget size and potential drops are satisfactory. The mechanical analysis proposed a concept that expulsion would occur when the crack tip is within the solidus nugget zone at an (open full item for complete abstract)

Committee: Chon Tsai (Advisor) Subjects: Engineering, Mechanical