Theses and Dissertations at Montana State University (MSU)
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Item Condensation of chromium vapor, generated in high-temperature (>800°C) environments, and interactions with aluminosilicate surfaces(Montana State University - Bozeman, College of Engineering, 2024) van Leeuwen, Travis Kent; Chairperson, Graduate Committee: Paul E. Gannon; This is a manuscript style paper that includes co-authored chapters.This work represents a collection of research and reporting with the goal of improving fundamental understanding of chromium (Cr) vapor reactive condensation, relevant in many high-temperature (>800°C) process environments where Cr-containing alloys are used. While reactive evaporation of Cr from stainless steels used in high-temperature solid oxide electrochemical systems is well-documented, the dynamics of Cr condensation onto surrounding interfaces during complex and dynamic system operation is less understood. Understanding these interactions during operation is critical for improving system performance and safeguarding environmental, health and safety, as some condensed species contain hexavalent chromium (Cr(VI)), a known carcinogen. A series of studies were designed and conducted to investigate the condensation pathways of Cr vapors within representative high-temperature system environments, simulating extreme conditions for Cr evaporation and downstream aluminosilicate fibers used in high-temperature insulation. The first study focuses on the influence of water vapor concentration in the gaseous environment on reactive Cr condensation and speciation onto aluminosilicate fibers. The second study explores the effects of alkaline oxide additives in aluminosilicate fibers on Cr condensation and speciation. The third study investigates the effects of presence of alkaline oxides within the Cr vapor source on reactive evaporation and condensation of Cr vapors onto downstream aluminosilicate fibers. To accomplish the specific objectives of these studies, Cr vapors, produced by high-temperature (>800°C) air exposures of trivalent chromium (Cr(III)) oxide (Cr 2O 3) (chromia) powder with variable moisture content, were condensed onto various ceramic materials (aluminosilicate fibers) downstream at lower temperatures (100-500°C). Total condensed Cr and ratios of oxidation states were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES) and diphenyl carbazide (DPC) colorimetric/direct UV-VIS spectrophotometric analyses, respectively. Results indicate presence of both Cr(III) and Cr(VI) species condensed on all samples investigated. Total Cr and ratio of Cr(VI) to total Cr detected was significantly more on those containing alkaline oxides and at higher atmospheric water vapor concentration, while the presence of alkaline oxides in the Cr vapor source (Cr 2O 3) decreased the evaporation and amount of Cr/Cr(VI) condensed on the samples downstream. Computational thermodynamic equilibrium modelling helps explain experimental results showing the relative stability of alkaline-chromate compounds.Item Temperature dependent second harmonic generation studies of materials used in energy conversion applications(Montana State University - Bozeman, College of Engineering, 2022) McNally, Marshall Traver; Chairperson, Graduate Committee: Robert Walker; This is a manuscript style paper that includes co-authored chapters.Materials in energy conversion devices often undergo a variety of degradation mechanisms. Solid oxide fuel cell cathodes materials, for example, are subject to surface compositional changes due to material segregation. The extreme operating conditions in these energy conversion devices requires the development of an operando technique that is surface and material specific to accurately probe these degradation mechanisms. Second harmonic generation (SHG) is a surface specific technique that probes the electronic structure of a material using the 2nd order polarizability. Using well characterized materials like Au, Si and NiO, we began investigating how high temperatures (260 °C) and atmospheric composition affected the surface electronic structures. To do this, a custom sample chamber dubbed TROPICS was designed and built to achieve temperature, atmospheric compositional and eventually, electrochemical control. We found that gold's SH intensity was enhanced (3.5 times) when O 2 was present in the atmosphere but this enhancement disappeared at high temperatures. Using data from titrating O 2 into a N 2 atmosphere, we concluded that a monolayer of O 2 was forming on the gold surface, providing backbonding opportunities for gold's free electrons into the partially filled O 2 pi* orbitals. Similar behavior was seen in N-type Si which also showed SH enhancement at room temperature. However, P-type and undoped Si showed no such atmospheric dependent behavior. SHG experiments done with NiO showed decoupled behavior in the electronic structure recovery between the bulk and surface. After heating to 260 °C, the SH signal did not return to pre-heating intensities but required ~60 and ~90 minutes in N 2 and air respectively. The difference in recovery time between N 2 and air could be attributed to interactions between the still paramagnetic NiO electrons and the partially filled O 2 pi* orbitals.Item Reactive evaporation of chromium from stainless steel and the reactive condensation of chromium vapor species on ceramic surfaces(Montana State University - Bozeman, College of Engineering, 2018) Tatar, Gregory Steven; Chairperson, Graduate Committee: Paul E. GannonStainless steels are often used in high temperature (greater than or equal to 500°C) applications such as solid oxide fuel cells (SOFCs), combustion engine exhaust systems, and various power/chemical plant process equipment. At high temperatures and in oxidizing conditions, chromium containing oxides, such as chromia (Cr2O 3), form protective surface layers on the underlying stainless steel. Reactive evaporation of these surface layers, however, may form volatile chromium species such as CrO 2 (OH) 2 and CrO 3, compromise the protection of stainless steels, and cause deleterious downstream effects. Such effects include SOFC performance degradation and hazardous materials generation. This study focuses on both the reactive evaporation and reactive condensation processes and their dependencies on materials and environmental conditions. First, the corrosion behaviors of stainless steels were investigated in a variety of exposure conditions and then the nature of chromium vapor condensation was investigated on ceramic surfaces under various conditions. Ferritic stainless steel samples (T409) were examined after 700°C exposures (94 h) to dry or wet air or nitrogen, and with or without contacting aluminosilicate fibers. Surface compositions and structures were characterized using field emission scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction. The fibers had a substantial impact on corrosion behaviors; likely serving as a mass transport barrier for corrosive gas species. Observed corrosion behaviors under these different environments and their potential mechanisms are presented and discussed. Additionally, quantification of chromium content on fibers was performed using inductively coupled plasma mass spectroscopy. Fibers were observed to collect chromium in dry/moist air consistent with the formation of CrO 3 and CrO 2(OH) 2, respectively, and their subsequent reactive condensation. To better understand the reactive condensation of volatile chromium species onto various ceramic surfaces, volatile chromium species were generated from chromium containing sources at 500-900°C and flowed past samples of aluminosilicate fibers, alumina, mica, and quartz wool at temperatures ranging from 100-900°C for 24-150 hours. The ceramic surfaces were characterized using x-ray photoelectron spectroscopy. Analysis of Cr 2p 3/2 peak positions revealed the influence of temperature, material, and exposure time on the oxidation states of surface chromium compounds, and extent of chromium deposition. Potential mechanisms are proposed to help explain the observed trends.Item Oxidation behavior of (Co,Mn) 3O 4 coatings on preoxidized stainless steel(Montana State University - Bozeman, College of Engineering, 2011) Hoyt, Kathryn Olivia; Chairperson, Graduate Committee: Paul E. GannonAs global energy challenges grow, alternative energy technologies like fuel cells are being investigated. Solid Oxide Fuel Cells (SOFCs) provide the advantages of high energy conversion efficiency, low emissions, fuel flexibility and both portable and stationary application. High material cost and need for longer material lifespan still impede the wider use of SOFCs. To produce substantial voltage, planar SOFCs are joined into stacks using interconnects. Interconnects both separate and connect each individual fuel, separating gas flow and conducting current. For SOFCs that operate at less than 800°C, metal alloys are being considered for the interconnect, particularly ferritic stainless steel. Ceramic coatings are being explored to improve the surface conductivity over time and significantly reduce Cr volatility from the steel. In addition, the coating must have a matching coefficient of thermal expansion (CTE) and be compatible with electrode and seal materials. One promising coating is (Co,Mn) 3O 4 spinel, which is deposited using various techniques, resulting in different thicknesses, compositions and microstructures. In this study, stainless steel 441HP samples were subjected to three levels of preoxidation prior to coating with 2 micron CoMn alloy using magnetron sputtering. Samples were subsequently annealed to Co 1.5Mn 1.5O 4 in 800°C air. Oxidation behaviors were evaluated as a function of exposure to laboratory air and dual atmospheres (3% H 2O and H 2 on one side, 3% H 2O and air on the other) and area specific resistance (ASR) measurements in lab air, all at 800°C. In addition, chemical and phase composition, mass gain, and adhesion were investigated using a complimentary suite of analytical techniques. Preoxidation was found to inhibit Fe transport from the stainless steel into the coating and exhibited a substantially thinner surface oxide layer after oxidation. Preoxidized samples also maintained slightly lower ASR values after 1650 hours in 800°C air compared to non-preoxidized samples. Oxidation behaviors, their possible mechanisms, and implications for SOFC interconnects will be presented and discussed.Item Study of praseodymium strontium manganite for the potential use as a solid oxide fuel cell cathode(Montana State University - Bozeman, College of Engineering, 2005) Pfluge, Matthew Edward; Chairperson, Graduate Committee: Max DeibertExtensive research has been performed on solid oxide fuel cell cathodes. These cathodes must be stable in the oxidation environment and have sufficient electrical conductivity and catalytic activity for the oxidant gas reaction at the appropriate operating temperature. Also, the cathode must be chemically and thermally compatible with the other cell components at room temperature, operating temperatures, and higher fabrication temperatures. Praseodymium strontium manganite (PSM) has shown promising electrical properties with respect to ideal properties of cathodes in solid oxide fuel cells. Various dopant levels of strontium in the perovskite structure were investigated, which include Pr 1-xSr xMnO 3-delta where x = 0.10, 0.20, 0.30 and (Pr 1- xSr x)0.98MnO 3-delta where x = 0.20 and 0.30. This cathodic material has shown electrical conductivity over twice as high as a traditionally used cathode, La 0.8Sr 0.2MnO 3. Through this investigation, the electrical and ionic conductivities of this ceramic series were measured from 200°C to 950°C. Another important electrical measurement investigated was the Seebeck coefficient within the same temperature range. This coefficient is a measurement of the change in voltage across a temperature gradient and thus can be referred to as its thermal power. Conductor types have been interpolated from the measurements. This measurement provides an improved understanding of the high electrical properties displayed within the material. Cathodic overpotential was also measured using half cell reactions performed in the temperature range of 650°C to 850°C under both air and pure oxygen. This measurement was used to calculate the current exchange density of the cathode and the area specific resistance. Overall, as the strontium concentration increased, the electrical activity of the ceramic subsequently increased. Furthermore, in relation to the traditional cathode material, La 0.8Sr 0.2MnO 3, the substitution of lanthanum with praseodymium has produced more effective cathodic performance.Item Study of solid oxide fuel cell interconnects, protective coatings and advanced physical vapor deposition techniques(Montana State University - Bozeman, College of Engineering, 2007) Gannon, Paul Edward; Chairperson, Graduate Committee: Max DeibertHigh energy conversion efficiency, decreased environmentally-sensitive emissions and fuel flexibility have attracted increasing attention toward solid oxide fuel cell (SOFC) systems for stationary, transportation and portable power generation. Critical durability and cost issues, however, continue to impede wide-spread deployment. Many intermediate temperature (600-800°C) planar SOFC systems employ metallic alloy interconnect components, which physically connect individual fuel cells into electric series, facilitate gas distribution to appropriate SOFC electrode chambers (fuel/anode and oxidant[air]/cathode) and provide SOFC stack mechanical support. These demanding multifunctional requirements challenge commercially-available and inexpensive metallic alloys due to corrosion and related effects. Many ongoing investigations are aimed at enabling inexpensive metallic alloys (via bulk and/or surface modifications) as SOFC interconnects (SOFC(IC)s). In this study, two advanced physical vapor deposition (PVD) techniques: large area filtered vacuum arc deposition (LAFAD), and filtered arc plasma-assisted electron beam PVD (FA-EBPVD) were used to deposit a wide-variety of protective nanocomposite (amorphous/nanocrystalline) ceramic thin-film (<5micron) coatings on commercial and specialty stainless steels with different surface finishes. Both bare and coated steel specimens were subjected to SOFC(IC)-relevant exposures and evaluated using complimentary surface analysis techniques. Significant improvements were observed under simulated SOFC(IC) exposures with many coated specimens at ~800°C relative to uncoated specimens: stable surface morphology; low area specific resistance (ASR <100mOmega x cm 2 >1,000 hours); and, dramatically reduced Cr volatility (>30-fold). Analyses and discussions of SOFC(IC) corrosion, advanced PVD processes and protective coating behavior are intended to advance understanding and accelerate the development of durable and commercially-viable SOFC systems.