First principles calculations of ceramics surfaces and interfaces : examples from beta-silicon nitride and alpha-alumina

Access restricted to the OSU CommunityThe goal of this thesis was to use first principles calculations to provide a fundamental understanding at the atomistic level of the mechanisms (e.g. structural relaxations of ceramic surfaces/interfaces, charge transfer reactions, adsorption and dissociation phenomena, localized debonding) behind macroscopic behavior in ceramics (e.g. fracture toughness, corrosion, catalysis). This thesis includes the results from three independent Density Functional Theory (DFT) studies of beta-Si₃N₄ and alpha-AI₂O₃. Due to the computational complexity of first principles calculations, the models in this thesis do not consider temperature or pressure effects and are limited to describing the behavior of systems containing less than 200 atoms. In future studies, these calculations can be used to train a reactive molecular dynamics force field (REAXFF) so that larger scale phenomena including temperature effects can be explicitly simulated. In the first study, the effect of over 30 dopants on the stability of the interface between beta-Si₃N₄ grains and the intergranular glassy SiON film (IGF) was investigated. The dopants chosen not only represented commonly known glass modifiers and sintering aides but also enabled us to search for dependencies based on atomic size and electronic orbital configuration. To ensure that the approximations used in our model captured the key physical phenomena occurring on the beta-Si₃N₄ (100) surface and at the Si₃N₄/ IGF interface, we compared to experimental data (i.e. High Angle Annual Dark Field- Scanning Transmission Electron Microscopy atomic positions and fracture toughness values (Mikijelj B. , 2009)). We identified a computational metric (the interfacial stability factor S) which correlates with experimentally measured fracture toughness values. The interfacial stability factor S is defined as the binding energy of the doped system minus the binding energy of the undoped system, where the binding energy is the total energy of the system minus the sum of the energies of the constituent atoms. In the second study, we performed constrained geometry barrier calculations of the interaction of CO with the (001) beta-Si₃N₄ surface to answer the following questions: 1) Does the CO combustion product interact with the Si₃N₄ surface and if so, what is the mechanism? 2) Once adsorbed, can CO further dissociate into isolated surface active C and O species? 3) Is it more energetically favorable for C to diffuse into the bulk beta-Si₃N₄ or along its surface? and 4) What is the barrier to C diffusing into an amorphous SiO₂ intergranular film? Our calculations indicated that CO spontaneously adsorbs to the (001) beta-Si₃N₄ surface. However, at ambient temperatures, further dissociation into isolated surface adsorbed C and O species was not thermodynamically or kinetically feasible. The barrier to C diffusing interstitially lA and 5A into the bulk crystalline lattice is 2.12 and 4.42 eV respectively for a defect free, clean surface. However, the barrier for C surface diffusion is much smaller, 0.87 eV. Therefore, we concluded that surface is rich in C which can diffuse to the Si₃N₄/SiO₂ interface and contribute to chemical erosion near the grain boundary interface. In the final study, we created a DFT model to investigate the 'inverse spillover effect' that occurs during hydrogen combustion on catalytically active Pt clusters supported by alpha-AI₂O₃. Our results indicated that the dissociation of O₂ was not thermodynamically favored on the alpha-Al₂O₃ surface. However, both H₂ and H₂O dissociated, forming hydroxyls with oxygen atoms in the second atomic layer. Once dissociated, the oxygen species could diffuse locally but encountered a large barrier to long-range surface diffusion in the absence of defects or other species. In contrast, the barrier to the long-range surface diffusion of hydrogen was modest under ideal conditions. We also identified several adsorption and dissociation products for Pt, Pt-O[superscript ads], Pt₃, O, H, O₂, H₂, and H₂O on the a-Al₂O₃ (0001) surface and described how these structures changed the surface reconstruction. Specifically, we concluded that the adsorption of molecular H₂O, atomic Pt, and Pt trimers changed the termination for the a- AI₂O₃ (0001) surface from aluminum to oxygen terminated in the vicinity of the adsorption products. This should have a dramatic affect on catalytic activity and surface diffusion. We confirmed this for O surface diffusion near surface Al where the presence of atomic Pt decreased the diffusion barrier from 1.17 to 0.22 eV