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ItemEnhancing air electrode performance of solid oxide cells by surface modification(Georgia Institute of Technology, 2022-04-15) Evans, ConorReversible solid oxide cells based on proton conductors (P-rSOCs) offer an efficient and clean option for energy storage and conversion. However, one issue holding back this renewable technology is the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics that take place at the air electrode. The air electrode in a P-rSOC is also subject to harsh environments (e.g., high concentration of steam) that can cause degradation over time. Catalyst infiltration into the air electrode offers a possible solution to each of these issues. Several catalyst candidates were investigated using the state-of-the-art double perovskite air electrode material, PrBa0.8Ca0.2Co2O5+δ (PBCC), as the air electrode backbone. Symmetrical cells with catalyst coated PBCC electrodes were primarily used to screen catalyst solutions and isolate the air electrode performance. Electrochemical impedance spectroscopy (EIS) was utilized to characterize the electrochemical performance and the long-term stability of catalyst infiltrated symmetrical cells under various testing conditions containing either steam and/or Cr contaminants. Electrochemical performance of single cells with a catalyst coated PBCC electrode was measured in both the fuel cell mode and the electrolysis cell mode. X- ray diffraction (XRD), scanning electron microcopy (SEM), and Raman spectroscopy were used to characterize phase composition, electrode microstructure and morphology, as well as surface chemistry to gain better understanding of the air electrode degradation mechanism during testing. Several catalysts were screened and optimized via symmetrical cell tests, including LaNiO3, La2NiO4, BaCoO3, LaNi0.6Fe0.4O3, La2Ni0.6Fe0.4O4, and PrCoO3. Symmetrical cells infiltrated with a PrCoO3 catalyst demonstrated particularly excellent stability and electrochemical performance (with a polarization resistance as low as 0.147 Ω cm2 and minimal degradation over 500 hours) against various sources of Cr contaminations at steam concentrations as high as 30% at 600 °C. Single cells infiltrated with PrCoO3 exhibit a peak power density of 2.02 W cm-2 at 650°C in the fuel cell mode, a 35.5% increase in performance from the single cells without catalyst modification. When run in electrolysis mode these same infiltrated single cells demonstrate a current density of 3.22 A cm-2 at 650 °C, a 22.4% improvement from the performance of the cells without catalyst modification. The single cell based on a PrCoO3 infiltrated cathode was among the best performing P-rSOCs ever reported in literature
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ItemEffect of hot rolling and intermittent annealing on the texture evolution of monotectoid BCC Zr-Nb(Georgia Institute of Technology, 2017-12-07) Dave, TanviIn the case of illicit trafficking of nuclear materials or interdicting nuclear debris, it is necessary to have the capabilities to intercept, collect, and analyze the nuclear material and determine its origin in a timely fashion. Nuclear forensics aims to determine robust and predictive signatures of nuclear materials in order to determine the origin of the material. Isotopic signatures, microstructure morphology, and chemical composition all contribute to identification of the material and its origin. The data that is collected is then compared to empirical data from various databases or is computationally interpolated from existing models of tested material systems. The incorporation of material science in nuclear forensics allows for the analysis of microstructure-processing relationships of interdicted materials in order to determine the material’s provenance. The processing steps of actinide alloys can introduce discriminating microstructural elements such as phase or morphological changes which can act as signatures that connect the microstructure to its process path. Texture analysis of a material at each stage in its process path may provide distinct signatures which can be input into an inverse processing model and reverse engineer a fabrication process for a given final microstructure. In this work, the first step is taken towards predicting a material’s processing history by understanding the structure-processing relationship of an actinide alloy surrogate material as it undergoes thermo-mechanical processing. The chosen system for this study was the Zr-18wt% Nb which has applications in the nuclear industry and is a suitable surrogate for U-6wt% Nb. The as-received Zr-18wt% Nb alloy was β-quenched to retain the high temperature BCC phase during subsequent processing. The sample was hot rolled to height reductions of 10%, 20%, 30% and 40% and intermittently annealed. The microstructure evolution was captured by characterizing each stage of this process path using optical microscopy, X-ray diffraction (XRD), and electron back scattered diffraction (EBSD). The texture and microtexture of the β-phase Zr-Nb alloy is analyzed by calculating the orientation distribution functions (ODF) and discussed in terms of the rolling and recrystallization textures of BCC metals and alloys. The typical rolling and recrystallization textures are well understood for BCC materials that undergo such processing over one pass however the overarching effect on final texture of a material that is repeatedly rolled and annealed has been less frequently studied. Changes in the initial texture of the Zr-18wt% Nb alloy as it undergoes each hot-rolling and annealing step leads to unconventional patterns of texture evolution as sample reduction is increased. This work aims to report and explain how such patterns may arise and how these patterns differ when comparing texture to microtexture.
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ItemRecovery boiler superheater corrosion - solubility of metal oxides in molten salt(Georgia Institute of Technology, 2013-04-15) Meyer, Joseph FreemanThe recovery boiler in a pulp and paper mill plays a dual role of recovering pulping chemicals and generating steam for either chemical processes or producing electricity. The efficiency of producing steam in the recovery boiler is limited by the first melting temperature of ash deposits that accumulate on the superheater tubes. Above the first melting temperature, the molten salt reacts with the protective oxide film that develops and dissolves it. The most protective oxide is determined by evaluating how little it dissolves and how its solubility changes in the molten salt. Solubility tests were done on several protective oxides in a known salt composition from a recovery boiler that burns hardwood derived fuel. ICP-OES was used to measure concentration of dissolved metal in the exposure tests while EDS and XRD were used to verify chemical compositions in exposure tests. NiO was found to be the least soluble oxide while Cr₂O₃ and Al₂O₃ had similar solubility with Fe₂O₃ being less soluble than Cr₂O₃ but more soluble than NiO. Exposure tests with pure metals and selected alloys indicated that even though Fe₂O₃ has little solubility, it is not a protective oxide and causes severe corrosion in stainless steels. The change in performance of iron based alloys was due to the development of a negative solubility gradient for Fe₂O₃ where Fe₂O₃ precipitated out of solution and created a continuous leaching of oxide. Manganese was found to be beneficial in stainless steels but its role is still unknown. Nickel based alloys were found to be least corroded due to nickel's low solubility and because it did not form a negative solubility gradient.
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ItemStress corrosion cracking of 316L austenitic stainless steel in high temperature ethanol/water environments(Georgia Institute of Technology, 2012-06) Gulbrandsen, StephaniThere has been an increase in the production of bio-fuels. Organosolv delignification, high temperature ethanol/water environments, can be used to separate lignin, cellulose, and hemicelluloses in the bio-mass for bio-fuel production. These environments have been shown to induce stress corrosion cracking (SCC) in 316L stainless steel. Previous research has been done in mixed solvent environments at room temperature to understand SCC for stainless steels, but little is known about the behavior in high temperature environments. Simulated organosolv delignification environments were studied, varying water content, temperature, pHe, and Cl- content to understand how these constituents impact SCC. In order for SCC to occur in 316L, there needs to be between 10 and 90 volume % water and the environment needs to be at a temperature around 200°C. Once these two conditions are met, the environment needs to either have pHe < 4 or have more than 10 ppm Cl-. These threshold conditions are based on the organosolv delignification simulated environments tested. SCC severity was seen to increase as water content, temperature, and Cl- content increased and as pHe decreased. To prevent failure of industrial vessels encountering organosolv delignification environments, care needs to be taken to monitor and adjust the constituents to prevent SCC.