The kinetic and mechanistic understanding of cooperatively catalyzed aldol and nitroaldol condensations is probed using a series of mesoporous silicas functionalized with aminosilanes to provide bifunctional acid−base character. Mechanistically, a Hammett analysis is performed to determine the effects of electrondonating and electron-withdrawing groups of para-substituted benzaldehyde derivatives on the catalytic activity of each condensation reaction. This information is also used to discuss the validity of previously proposed catalytic mechanisms and to propose a revised mechanism with plausible reaction intermediates. For both reactions, electron-withdrawing groups increase the observed rates of reaction, though resonance effects play an important, yet subtle, role in the nitroaldol condensation, in which a pmethoxy electron-donating group is also able to stabilize the proposed carbocation intermediate. Additionally, activation energies and pre-exponential factors are calculated via the Arrhenius analysis of two catalysts with similar amine loadings: one catalyst had silanols available for cooperative interactions (acid−base catalysis), while the other was treated with a silanol-capping reagent to prevent such cooperativity (base-only catalysis). The values obtained for activation energies and pre-exponential factors in each reaction are discussed in the context of the proposed mechanisms and the importance of cooperative interactions in each reaction. The catalytic activity decreases for all reactions when the silanols are capped with trimethylsilyl groups, and higher temperatures are required to make accurate rate measurements, emphasizing the vital role the weakly acidic silanols play in the catalytic cycles. The results indicate that loss of acid sites is more detrimental to the catalytic activity of the aldol condensation than the nitroaldol condensation, as evidenced by the significant decrease in the pre-exponential factor for the aldol condensation when silanols are unavailable for cooperative interactions. Cooperative catalysis is evidenced by significant changes in the preexponential factor, rather than the activation energy for the aldol condensation.

The mechanical response of ceramic matrix composites depends critically on the slip along the matrix-fiber interface, which is usually achieved with thin coatings on the fibers. Environmental attack of such coatings (enabled by ingress of reactants through matrix cracks) often leads to significant degradation, through the removal of the coating via volatilization and oxidation of exposed SiC surfaces.The extent of the volatilization region extending from the matrix crack plane (i.e., recession length) is strongly coupled to the formation of oxide, which ultimately fills open gaps and arrests further reactions. This paper presents models to quantify these effects over a broad range of environmental conditions, coating thickness, and matrix crack opening. Analytical solutions are presented for the time to close recession gaps via oxidation, and the associated terminal recession lengths obtained near free surfaces. A broad parameter study illustrates that recession behaviors are controlled by a competition between volatilization and oxidation rates. As such, the extent of recession is highly sensitive to water vapor and temperature, providing an explanation for disparate observations of recession under seemingly similar conditions. The extent of recession in the interior of composites is also illustrated, using a straightforward reaction-transport model.Recession lengths decay rapidly away from the free surface, with the extent of recession penetration scaling with maximum recession at the free surface.

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Oxidation of SiC plays a critical role in the durability of ceramic matrix composites, as large changes in molar volume generate significant stresses that can drive cracking in adjacent features. The analysis of such phenomena requires coupling between descriptions of diffusion through the oxide, growth of the oxide domain (i.e., evolution of the oxide/SiC interface), and creep relaxation in the oxide at elevated temperatures. This paper presents a two‐dimensional finite element framework that uses a single finite element mesh to predict these behaviors; large changes in the oxide geometry are simulated by tracking the oxidation front as a discrete interface and periodically re‐meshing. The numerical performance of the framework is illustrated using an analytical description of oxidation of a flat surface and subsequent stress evolution. The framework is then used to analyze internal oxidation within a cylindrical cavity for a wide range of temperatures and water vapor concentrations pertinent to gas turbines. The results are used to discuss the roles of oxide growth rate, creep, and geometry with respect to the tensile stresses that develop in the adjacent SiC, and their implications for oxidation‐driven damage at high temperatures.