Chromium containing materials, e.g. stainless steels, are commonly used in high temperature (>500 • C) applications such as solid oxide fuel cell (SOFC) stacks, combustion exhaust systems, and in various chemical process equipment. At these temperatures and in oxidizing atmospheres, chromium oxide (chromia) surface layers form and grow, effectively protecting the underlying alloy. Also known to form under these conditions, however, are volatile chromium species such as CrO 2 (OH) 2 and CrO 3 . Formation of these volatile species may have detrimental effects not only on the source material, but also on surrounding materials in the system which may interact with these species. To better understand how volatile chromium species interact with materials, volatile chromium species were generated from ferritic stainless steel (FSS) T409 at 700 • C and directed into aluminosilicate fibers at 100-230 • C for 150 hours. Post exposure fibers were noted to possess yellow, brown, and/or green stained regions which were isolated and characterized using X-ray photoelectron spectroscopy (XPS). Examination of Cr 2p 3/2 peaks revealed varied Cr(VI) content and Cr(III) multiplet-split components for different discolored regions. Possible explanations for differences in chromium content by discoloration color are discussed. The volatilization of chromium species from chromium containing materials is a well-documented phenomenon which holds consequences for SOFCs, human health, and the environment. Using chromia as an example for a source, in the presence of oxygen and absence of water vapor, the dominant volatilization pathway proceeds according to Reaction 1.If, however, water vapor is present in addition to oxygen, then the dominant volatilization pathway proceeds according to Reaction 2. 1-7The generation of these gas/vapor species is well understood, but the same cannot be said of how they interact with (e.g. condense onto) other materials. This is of great importance for human health and the environment considering that chromium species may take toxic forms (hexavalent) or non-toxic forms (trivalent). This relative dearth of understanding also holds negative implications for SOFCs, and so many investigative efforts have been undertaken to understand how CrO 2 (OH) 2 interacts with ceramic components in SOFCs. Electrochemical reduction of CrO 2 (OH) 2 has often been observed to occur at the triple phase boundary (TPB), where cathode, electrolyte, and gas phase meet. [8][9][10] This electrochemical reaction is not limited to the TPB, however, and can occur away from the TPB given: the formation of a continuous chromia layer for hole transport, a mixed conducting electrolyte, or a mixed conducting cathode. 8 While there is evidence for preferential electrochemical reduction of CrO 2 (OH) 2 , 11,12 open circuit chemical reactions have also been observed with cathodes containing Mn or Sr. 13 It should be noted that Cr transport to lanthanum strontium manganite (LSM) was observed via solid state diffusion, whereas vapor deposition o...
Stainless steels are often used in high temperature (≥500°C) applications such as solid oxide fuel cells (SOFCs), combustion engine exhaust systems, and in power/chemical plant process equipment. At high temperature and in oxidizing conditions, chromium containing oxides, such as chromia, may form protective surface layers on the underlying alloy. Reactive evaporation of chromium, however, may occur from the protective surface layers given these conditions, resulting in the formation of volatile chromium species such as CrO 2 (OH) 2 . These volatile chromium species may then interact with surrounding materials, potentially resulting in hazardous compound formation, or having detrimental effects on system performance, as in the case of SOFCs. To better understand the interaction of volatile chromium condensation/ deposition on material substrates, volatile chromium species were generated from chromia powder at 500°C to 900°C and flowed past coupons of alumina and mica and quartz wool at temperatures ranging from 150°C to 900°C for 24-and 100-hour exposures. The ceramic surfaces were characterized as a function of these exposures using X-ray photoelectron spectroscopy (XPS). 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.
Chlorosilane species are commonly used at high temperatures in the manufacture and refinement of ultra-high purity silicon and silicon materials. The chlorosilane species are often highly corrosive in these processes, necessitating the use of expensive, corrosion resistant alloys for the construction of reactors, pipes, and vessels required to handle and produce them. In this study, iron, the primary alloying component of low cost metals, was exposed to a silicon tetrachloride-hydrogen vapor stream at industrially-relevant times (0-100 hours), temperatures (550-700 • C), and vapor stream compositions. Post exposure analyses including FE-SEM, EDS, XRD, and gravimetric analysis revealed formation and growth of stratified iron silicide surface layers, which vary as a function of time and temperature. The most common stratification after exposure was a thin FeSi layer on the surface followed by a thick stoichiometric Fe 3 Si layer, a silicon activity gradient in an iron lattice, and finally, unreacted iron. Speculated mechanisms to explain these observations were supported by thermodynamic equilibrium simulations of experimental conditions. This study furthers the understanding of metals in chlorosilane environments, which is critically important for manufacturing the high purity silicon required for silicon-based electronic and photovoltaic devices.
Chlorosilane species are commonly used at high temperatures in the manufacture and refinement of ultra-high purity silicon and silicon materials. They are highly corrosive in these processes, necessitating the use of high cost alloys for the construction of reactors, pipes, and vessels required to handle and produce them. In this study, iron, the primary alloying component of low cost metals, was exposed to a variety of silicon tetrachloride-hydrogen-hydrogen chloride vapor streams at industrially-relevant times (0-100 hours), temperatures (500-700°C), and compositions. Post exposure analyses including FE-SEM, EDS, XRD, and gravimetric analysis revealed formation and growth of stratified silicide and chloride surface layers, which vary as a function of time, temperature and gas composition. Additionally, there was evidence for various regimes of diffusion-limited and reaction-limited surface layer growth. Speculated mechanisms to explain these observations were supported by thermodynamic equilibrium simulations of experimental conditions. This study furthers the understanding of metals in chlorosilane environments, which is critically important for manufacturing the high purity silicon required for silicon-based electronic and photovoltaic devices.
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