Theses and Dissertations at Montana State University (MSU)
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Item The effects of atomic oxygen on silicon-carbon systems in extreme environments(Montana State University - Bozeman, College of Letters & Science, 2021) Chen, David Zuyu; Chairperson, Graduate Committee: Timothy Minton; Chenbiao Xu, Vanessa J. Murray and Timothy K. Minton were co-authors of the article, 'Oxidation of silicon carbide through the passive-to-active transition' submitted to the journal 'The journal of chemical physics' which is contained within this dissertation.; Chenbiao Xu and Timothy K. Minton were co-authors of the article, 'Effect of atomic oxygen on CV-1144-0 and RTV-560 silicones' submitted to the journal 'Acta astronautica' which is contained within this dissertation.; Chenbiao Xu and Timothy K. Minton were co-authors of the article, 'Effect of silicone coating on atomic oxygen reactivity with fiberform and phenolic impregnated carbon ablator' submitted to the journal 'Journal of spacecraft and rockets' which is contained within this dissertation.Vehicles traveling at hypersonic speeds require thermal protection systems (TPSs) that can withstand the extreme temperatures and reactive atomic oxygen species present in these environments. Ultra-high temperature ceramics are candidate TPSs, and many of them contain silicon carbide, allowing them to resist chemical attack by forming a protective oxide-containing layer, called passive oxidation. At very high temperatures, however, the layer will decompose, subjecting the material to ablation from reaction with O-atoms, called active oxidation, through a process called the passive-to-active oxidation transition (PAT). We have conducted molecular beam-surface scattering experiments to investigate the interactions of O-atoms with SiC at high temperatures, which revealed that with a lower fluence of O-atoms above the PAT, the SiC surface undergoes graphitization, while a sufficiently higher fluence of O-atoms promotes active oxidation. Analysis of the oxide layer decomposition revealed a decomposition process that initiated at the oxide-SiC interface. These insights will be useful for the development of more accurate predictive models, but they also aided understanding of the ablation of silicone-coated heat shields for atmospheric entry applications. For these applications, phenolic impregnated carbon ablator (PICA), a material composed of a carbon fiber network (FiberForm) and a resole phenolic resin stable against high heat convection and conduction, is used. Silicone is sprayed onto PICA to reduce dust, but the silicone can also form an oxide layer, which, like on SiC, will resist O-atom attack until it decomposes at very high temperatures, exposing the underlying material to reactive O-atoms. We conducted additional experiments in which a beam of atomic oxygen was directed at silicone-coated and uncoated samples of PICA as well as FiberForm, which revealed high nonreactive O-atom product scattering when the oxide layer was present, while with the decomposition of the oxide, product scattering resembled O-atom scattering from the underlying substrate. Additional studies probed the oxidation layer that is formed on pure silicone during O-atom bombardment, which revealed a three orders of magnitude reduction in erosion yield compared to that of Kapton H, a polyimide. This new data on PICA and FiberForm has been provided to NASA Ames for their development of an ablation model.Item Operando optical studies of next generation anode materials in high temperature solid oxide fuel cells(Montana State University - Bozeman, College of Letters & Science, 2020) Welander, Martha Maria; Chairperson, Graduate Committee: Robert Walker; Marley S. Zachariasen, Clay D. Hunt, Stephen W. Sofie and Robert A. Walker were co-authors of the article, 'Operando studies of redox resiliance in alt enhanced NIO-YSZ SOFC anodes' in the journal 'Journal of the electrochemical society' which is contained within this dissertation.; Marley S. Zachariasen, Stephen W. Sofie and Robert A. Walker were co-authors of the article, 'Enhancing Ni-YSZ anode resilience to environmental redox stress with aluminum titanate secondary phases' in the journal 'ACS applied energy materials' which is contained within this dissertation.; Marley S. Zachariasen, Stephen W. Sofie and Robert A. Walker were co-authors of the article, 'Mitigating carbon formation with Al 2TiO 5 enhanced solid oxide fuel cell anodes' in the journal 'The journal of physical chemistry C' which is contained within this dissertation.; Daniel B. Drasbaek, Marie L. Traulsem Bhaskar R. Sudireddy, Peter Holtappels and Robert A. Walker were co-authors of the article, 'What does carbon tolerant really mean? Operando vibrational studies of carbon accumulation on novel solid ocide fuel cell anodes prepared by infiltration' submitted to the journal 'RSC physical chemistry chemical physics' which is contained within this dissertation.; Disseration contains an article of which Martha Maria Welander is not the main author.Solid oxide fuel cells (SOFCs) are high temperature energy conversion devices capable of efficient and sustainable energy production. Because of the need to electrochemically reduce molecular oxygen and the relatively high activation energy required for oxide ions to diffuse through the dense, solid-state electrolyte, SOFCs typically operate at temperatures > or = 500 °C. High operating temperatures endow SOFCs with many advantages, including fuel flexibility and high conversion efficiencies, distinguishing them from other types of fuel cells. However, high temperatures also present challenges related to the stability of the electrode materials, accelerating cell degradation and limiting the development and integration of SOFCs into large scale power production strategies. These mechanisms are the result of fundamental changes in material properties that remain poorly described and difficult to predict. Studies presented in this work utilized operando Raman spectroscopy and electrochemical measurements to directly correlate material changes with changes in cell performance under various operating conditions. Research focused on developing and characterizing new electro-catalytic materials having improved conversion efficiencies and mechanical resilience to thermal and chemical stress. Because current state of the art SOFC Ni-YSZ cermet anodes are sensitive to oxidation, the first two studies investigated the effects of adding small amounts of Al 2TiO 5 to Ni-YSZ anodes and the impact of resulting secondary (2°) phases that formed on SOFC tolerance to electrochemical and environmental reduction and oxidation (redox) cycling. Results show that Al 2TiO 5 helps improve tolerance to both types of redox cycling by maintaining electrode-electrolyte connectivity and minimizing catalyst coarsening. The third study illustrates how this same dopant improved anode carbon tolerance when operating with hydrocarbon fuels. Because excessive carbon accumulation on SOFC anodes can lead to rapid cell failure, ways to improve carbon tolerance was further explored in the last two studies. These studies investigate the effect of decoupling the electro-catalytic and the electronically conductive phases of the anode under pure methane and biogas-surrogate environments. Collectively, the studies described in this dissertation provide insight into the materials-specific mechanisms responsible for limiting degradation of doped and functionally decoupled anodes to help guide the design of new SOFC electrode materials.Item Pyrolysis of thermal protection system materials: molar yields of volatile products derived from in situ mass spectrometric measurements(Montana State University - Bozeman, College of Letters & Science, 2018) Bessire, Brody Kelly; Chairperson, Graduate Committee: Timothy Minton; Sridhar A. Lahankar and Timothy K. Minton were co-authors of the article, 'Pyrolysis of phenolic impregnated carbon ablartor (PICA)' in the journal 'ACS applied materials and interfaces' which is contained within this thesis.; Timothy K. Minton was a co-author of the article, 'Decomposition of phenolic impregnated carbon ablator (PICA) as a function of temperature and heating rate' in the journal 'ACS applied materials and interfaces' which is contained within this thesis.; Timothy K. Minton was a co-author of the article, 'Pyrolysis of epoxy-novolac materials as a function of time and temperature' submitted to the journal 'The journal of analytical and applied pyrolysis' which is contained within this thesis.Mass spectrometric techniques have been developed to measure the molar yields of pyrolysis products from ablative resins and composite materials at heating rates that are relevant to flight conditions. Thermal decomposition mechanisms of phenolic and an epoxy-novolac resin systems are reviewed. New insights into the thermal decomposition mechanisms of PICA (Phenolic Impregnated Carbon Ablator) and epoxy-novolac D.E.N. 438 (Dow Epoxy-Novolac) are proposed and are based on the measurements of molar yields from these materials. Molar yield data have been provided in the appendices of this thesis for use in material response models. The thermal decomposition of phenolic impregnated carbon ablator (PICA) has been investigated with the objective of measuring molar yields of pyrolysis products at heating rates that are relevant to MSL flight conditions. The relative molar yields of 14 pyrolysis gases were obtained in conjunction with mass loss measurements. These measurements allowed for the calculation of absolute molar yields and mass yields as a function of temperature and heating rate, as well as the simulation of TGA curves. Pyrolysis product yields change as a function of heating rate, and this behavior has been attributed to two mechanisms that compete during the initial stages of thermal decomposition. The results of this study are now available for use in material response models. The thermal decomposition of an epoxy-novolac resin system has also been investigated. Samples of D.E.N. 438 were cured using NMA (methyl-5-norbornene-2,3-dicarboxylic anhydride) as a crosslinking agent and BDMA (N-benzyldimethylamine) as a catalyst. A radiative heating method was developed to minimize experimental uncertainties that may emerge from thermal gradients that are established across the samples as they experience high rates of heating. The molar yields of the six dominant pyrolysis products were measured at a heating rate of 8°C s -1. The molar yields of pyrolysis products provide new insight, and a new thermal decomposition mechanism is proposed.Item In operando spectroscopic studies of high temperature electrocatalysts used for energy conversion(Montana State University - Bozeman, College of Letters & Science, 2016) McIntyre, Melissa Dawn; Chairperson, Graduate Committee: Robert Walker; John D. Kirtley, David M. Halat, Kyle W. Reeping and Robert A. Walker were co-authors of the article, 'In situ spectroscopic studies of carbon formation in SOFCS operating with syn-gas' in the journal 'Electrochemical Society transactions' which is contained within this thesis.; Daniel M. Neuburger and Robert A. Walker were co-authors of the article, 'In operando raman spectroscopy studies of temperature dependent carbon accumulation on SOFCS operating with syn-gas' submitted to the journal 'The journal of the Electrochemical Society' which is contained within this thesis.; John D. Kirtley, Anand Singh, Shamiul Islam, Josephine M. Hill and Robert A. Walker were co-authors of the article, 'Comparing in situ carbon tolerances of Sn-infiltrated and Bao-infiltrated Ni-YSZ cermet anodes in solid oxide fuel cells exposed to methane' in the journal 'The journal of physical chemistry C' which is contained within this thesis.; David R. Driscoll, Martha M. Welander, Josh B. Sinrud, Stephen W.Sofie, Robert A. Walker were co-authors of the article, 'In situ formation of multifunctional ceramics : mixed ion-electron conducting properties of zirconium titanium oxides' submitted to the journal 'The journal of materials chemistry A' which is contained within this thesis.; Thesis contains an article of which Melissa Dawn McIntyre is not the main author.Solid-state electrochemical cells are efficient energy conversion devices that can be used for clean energy production or for removing air pollutants from exhaust gas emitted by combustion processes. For example, solid oxide fuel cells generate electricity with low emissions from a variety of fuel sources; solid oxide electrolysis cells produce zero-emission H2 fuel; and solid-state DeNO x cells remove NO x gases from diesel exhaust. In order to maintain high conversion efficiencies, these systems typically operate at temperatures > or = 500°C. The high operating temperatures, however, accelerate chemical and mechanical cell degradation. To improve device durability, a mechanistic understanding of the surface chemistry occurring at the cell electrodes (anode and cathode) is critical in terms of refining cell design, material selection and operation protocols. The studies presented herein utilized in operando Raman spectroscopy coupled with electrochemical measurements to directly correlate molecular/material changes with device performance in solid oxide cells under various operating conditions. Because excessive carbon accumulation with carbon-based fuels destroys anodes, the first three studies investigated strategies for mitigating carbon accumulation on Ni cermet anodes. Results from the first two studies showed that low amounts of solid carbon stabilized the electrical output and improved performance of solid oxide fuel cells operating with syn-gas (H 2/CO fuel mixture). The third study revealed that infiltrating anodes with Sn or BaO suppressed carbon accumulation with CH 4 fuel and that H 2O was the most effective reforming agent facilitating carbon removal. The last two studies explored how secondary phases formed in traditional solid oxide cell materials doped with metal oxides improve electrochemical performance. Results from the fourth study suggest that the mixed ion-electron conducting Zr 5Ti 7O 24 secondary phase can expand the electrochemically active region and increase electrochemical activity in cermet electrodes. The final study of lanthanum strontium manganite cathodes infiltrated with BaO revealed the reversible decomposition/formation of a Ba 3Mn 2O 8 secondary phase under applied potentials and proposed mechanisms for the enhanced electrocatalytic oxygen reduction associated with this compound under polarizing conditions. Collectively, these studies demonstrate that mechanistic information obtained from molecular/material specific techniques coupled with electrochemical measurements can be used to help optimize materials and operating conditions in solid-state electrochemical cells.Item Spatial distribution of carbon accumulation on solid oxide fuel cells operating under methane(Montana State University - Bozeman, College of Letters & Science, 2015) Neuburger, Daniel Miller; Chairperson, Graduate Committee: Robert WalkerWith the looming threat of climate change, new technologies that can reduce the use of fossil fuels are desirable. One such technology is the solid oxide fuel cell (SOFC). SOFCs convert chemical energy directly into electrical energy, so they can be much more efficient than traditional sources of electrical power generation. SOFCs utilize an oxide-ion conducting electrolyte to generate electricity by reducing molecular oxygen at the cathode and oxidizing fuel at the anode. High temperatures (<650 °C) are required to catalyze oxide conduction through the electrolyte. High temperatures enable SOFCs to use a variety of fuels, but these same conditions also accelerate unfavorable reactions that can cause anode degradation. Research described in this dissertation examined the effects of carbon accumulation in SOFC anodes on overall device performance, and the spatial variation in anode degradation. Fuel was introduced to the fuel cell using a single fuel inlet. This method of fuel introduction created spatial variance in factors such as fuel concentration, and this variance was expected to affect carbon accumulation on the anode. Carbon accumulation is a major contributor to anode degradation through a variety of mechanisms including mass transport obstruction, anode delamination from the electrolyte, and metal dusting, a process describing anode disintegration induced by carbon growth disrupting the anode's electronically conducting network. The amount of carbon accumulation on the surface of the anode was analyzed in operando using Raman spectroscopy. Spectroscopic results showed that more carbon accumulated on the anode further from the fuel inlet than closer to it. In situ electrochemical measurements coupled with post mortem visual and FEM analysis suggested that severe anode degradation occurred early in an experiment, affecting spectroscopic results. We infer that the highest amount of carbon accumulation occurs close to the fuel inlet causing fast and severe anode degradation, decreasing the amount of carbon that can accumulate in further trials when the majority of the spectroscopic data was collected. These results suggest that spectroscopic results should be analyzed with experimental factors such as anode location and trial sequence specifically in mind.