| Summary: | 碩士 === 國立臺灣大學 === 土木工程學研究所 === 101 === Mullite is the major phase of conventional and advanced silicate-based ceramics, coatings, fibers and ceramic matrix composites. It has low thermal expansion, low thermal conductivity, good chemical stability and high creep resistance in high temperature. These features have made mullite materials one of the best candidates for structural and high-temperature applications.
Mullite is known to exhibit an anomalous heat capacity and thermal expansion and isotropic character of diffusion. In this study, the first-principles density functional theory (DFT) simulation and nudge elastic band calculations were applied to elucidate the mechanisms responsible for these counter-intuitive phenomena at high temperature. We design two migration paths. The first one is oxygen hopping to vacancy along the c-axis. The other one is vacancy diffuses on the a-b plane. The two paths are perpendicular to each other.
From the calculations, we found that oxygen starts hopping intensely after temperature exceeds 1200K. This findings support the “dynamical site-exchange” mechanism proposed previously and is deemed as the main reason for anomaly of heat capacity in the temperature after 1200 degrees (K). Comparing the result of diffusivity of two different paths to experimental data, we asserted that the result of vacancy diffusion on the a-b plane is the plausible path. The proposed in-plane diffusion path supports the isotropic character of diffusion coefficient measurements and clarifies the role of structural vacancies for diffusion in mullite.
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