Theses and Dissertations at Montana State University (MSU)
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Item Reactive vaporization of oxides in solid oxide fuel cell systems :(Montana State University - Bozeman, College of Letters & Science, 2013) Key, Camas Fought; Chairperson, Graduate Committee: Richard J. SmithMetals such as chromium, aluminum and silicon are of extreme technological and industrial importance due to the corrosion resistance they offer in oxidizing environments at high temperature. Much of this robustness is based on the formation of a thin, well-adhered metal-oxide (MO) layer on the surface of the metal. In particularly corrosive environments or at high-enough temperatures and or pressures, the MO will chemically react with constituents in the surrounding gas, removing atoms from the solid. For many systems, material loss and subsequent mechanical failure is the foremost concern. However, in solid oxide fuel cell (SOFC) systems, the presence of gaseous metal species leads to severe degradation in electrochemical performance well before mechanical limits are reached. Reactive vaporization from ferritic stainless steels, chromia, aluminosilicates and a candidate electrode material (Sr 2VMoO 6), was investigated using the transpiration method. Two novel collection methods were employed: condensation of vapors on wafer collectors analyzed with Rutherford backscattering spectrometry (RBS); and, condensation of vapors on quartz wool analyzed via inductively coupled plasma mass spectroscopy (ICP-MS). Identification and quantification of vapor species provided assessment of material performance in SOFC environments. Experiments demonstrated that Cr vapor species from ferritic stainless steels used for SOFC interconnect applications could be reduced by as much as one order of magnitude through the application of barrier coatings. Base alloys were compared and exhibited a variety of Cr vaporization rates despite being similar in composition, thus illustrating the importance of minor elemental constituents in the alloy. Measurements identified Si as the primary volatile element in aluminosilicate materials when Si concentrations in the bulk material were as low as one percent. Aluminosilicate materials demonstrated a burn out phase during the first hundred hours at 800°C in humid oxidizing environments, where large amounts of Si and other elements were vaporizing. Mass transport rate versus flow rate experiments on pure Cr 2O 3 indicated that sample surface area influences the measured Cr vapor pressure. This finding helps explain the range of values reported in literature for the equilibrium rate constant of the most common Cr vaporization reaction in SOFC environments.Item Sr 2-x VMoO 6-Y double perovskites : a new generation of solid oxide fuel cell anodes(Montana State University - Bozeman, College of Letters & Science, 2013) Childs, Nicholas Brule; Chairperson, Graduate Committee: Richard J. Smith; Cameron Law, Richard Smith, Stephen Sofie, Camas Key, Michael Kopcyzk and Michael Lerch were co-authors of the article, 'Electronic current distribution calculation for a NI-YSZ solid oxide fuel cell anode' in the journal 'Fuel cells' which is contained within this thesis.Fuel cells are an attractive power source due to their ability to efficiently convert chemical energy stored in fuel directly into electricity. The ability of Solid Oxide Fuel Cells (SOFCs) to reform hydrocarbons at the anode provides for fuel flexibility, an advantage over other types of fuel cell technologies. The primary goals of this dissertation were to investigate the limitations of the currently used anode cermet material, synthesize a double perovskite material (Sr ₂₋xVMoO ₆₋y) without these limitations and investigate the electrical conduction properties of this mixed ionic and electronic conductor (MEIC) in a SOFC anode environment. The electronic current density limitation of a Ni-YSZ anode was determined through the development of a computer simulation and use of experimental data. The electronic current density distribution for nickel particles in a Ni-YSZ anode was calculated via a Monte-Carlo percolation model. Experiments were performed to determine the failure current densities of thin nickel wires in a SOFC anode environment. The results show a current density limitation of Ni-YSZ anodes that is not expected with MEIC anodes. A MEIC anode material, Sr ₂₋xVMoO ₆₋y, was synthesized and characterized using a variety of techniques. The expected MEIC nature of this perovskite material eliminates a potential anode limitation, while adding other benefits over Ni-YSZ. X-ray diffraction (XRD) was used to verify crystal structure. In contrast to the trace amounts of secondary insulating phases found through XRD, XPS shows a high percentage (85-90%) of these secondary phases at the surface. The electrical conductivity of Sr ₂₋xVMoO ₆₋y was found to exceed that reported for Ni-YSZ anodes in a typical SOFC anode environment. Polycrystalline Sr 1.9VMoO ₆₋y'' samples exhibited higher electrical conductivity than that reported for SrMoO ₃ polycrystalline samples, making it a candidate for being the highest electrical conducting oxide known. These conduction values were only measured after specific thermal treatments in a reducing atmosphere. These treatments reduced secondary surface phases, Sr ₃V ₂O ₈ and SrMoO ₄, into their more conducting counterparts, SrVO ₃ and SrMoO ₃. Vanadium and molybdenum valence state XPS fitting parameters for primary and secondary phases are reported.Item Ceramic processing and electrochemical analysis of proton conductive solid oxide fuel cell(Montana State University - Bozeman, College of Letters & Science, 2010) Tsai, Chih-Long; Chairperson, Graduate Committee: V. Hugo SchmidtBa(Zr 0.8-xCe xY 0.2)O 3-delta (0 < or = x < or = 0.4) (BZCYs) powders were successfully fabricated by both solid state reaction and glycine-nitrate process. Lithium fluoride (LiF) was selected as a liquid phase sintering additive to lower the sintering temperature of BZCYs. Using LiF as an additive, high density BZCYs ceramics can be obtained at sintering temperatures 200~300 °C lower than the usual 1700 °C with much shorter soaking time. Nuclear reaction investigations showed no lithium and a small amount of fluorine reside in the sample which indicates the non-concomitant evaporation of lithium and fluorine during the sintering process. Scanning electron microscopic investigations showed the bimodal structure of BZCY ceramics and grain growth as Ce content increases. In a water saturated hydrogen containing atmosphere, BZCY ceramics have higher conductivity when LiF is used in the sintering process. LiF-added BZCY electrolyte-supported fuel cells with different cathodes were tested at temperatures from 500 ~ 850 °C. Results show that Pt cathode gives much higher power output than ceramic cathodes, indicating much larger polarization from ceramic cathodes than Pt. Ba(Zr 0.6Ce 0.2Y 0.2)O 3-delta anode supported proton conductive solid oxide fuel cells (H-SOFCs) show low power output due to its low proton conductivity. Ba(Ce 0.8Y 0.2)O 3-delta anode supported H-SOFCs show excellent power output. Different H 2 and O 2 partial pressures were used for fuel and oxidative gas, respectively, to obtain information for V(i) modeling. Different thicknesses of supporting anode were used to obtain saturation current densities of H-SOFC. Using the dusty-gas model which includes Stefan-Maxwell equation and Knudsen terms, the calculation gave tortuosity of our supporting anode 1.95 ± 0.1. The gas concentrations across the anode were also calculated by knowing the tortuosity of the supporting anode. An electrochemical model of H-SOFC was developed. The excellent agreement between model and experimental data implies that our model is close to the true physical picture of H-SOFC. The more accurate prediction of our model, based on a physical picture of electrochemical processes, also provides a replacement for using the Butler-Volmer equation in SOFC modeling. In the parametric analysis, our model shows that ohmic polarization and cathodic polarization limit the performance of H-SOFC. Research for improving H-SOFC performance should be focused on reducing electrolyte thickness, increasing proton conductivity of electrolyte and finding a compatible cathode material.