| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 100 === Many studies have discussed the thermal decomposition of kyanite; such interest is due both to its importance in the geosciences and in regard to the ceramic process. However, there are no detailed crystallographic studies on the decomposition of kyanite and, therefore, no decisive clues regarding the decomposition reaction mechanism. In this study, a detailed analysis of the microstructure evolution during the decomposition reaction, and two kinetics model: Master Kinetics Curve and Avrami Equation Method have been employed to determine the underlying mechanism of the thermal decomposition of kyanite.
This research consists of three parts: The first is an examination of the crystal phases and chemical composition of kyanite powders and single crystals by X-ray diffractometer (XRD) and Electron Probe Microanalyzer (EPMA). In the second, a series of thermal decomposition experiments of the powders and single crystals by High-T furnace and dilatometer at isothermal conditions is conducted. In the third part, the morphology of kyanite powders and single crystals is observed by scanning electron microscope (SEM). The decomposed experimental data which was generated by High-T tube-furnace and dilatometer was also analyzed by Master Kinetics Curve and Avrami Equation. The results of microstructure evolution, combined with the Master kinetics Curve and Avrami Equation analysis may help us to obtain a more thorough understanding of the decomposition process.
In this work, the SEM images of kyanite powders and single crystals showed that thin needles of mullite and SiO2 liquid phase developed along cleavage planes (100), and the mullite crystallite revealed a distinct tendency to grow along preferred orientation. Moreover, the cristobalite crystallite embedded in a SiO2 liquid phase was observed when the single crystals were heated above 1300oC, indicating that the reaction process may differ between the kyanite powders and single crystals. The experimental results revealed that the Master Kinetics Curve and Avrami Equation can analyze and describe the reaction process of the experimental data of kyanite powders, indicating that the thermal decomposition of kyanite powder is controlled by a single mechanism. However, the results of Master Kinetics Curve indicated that the decomposition of a single crystal is controlled by a multi-mechanism. The Avrami Equation also suggested that the reaction process of a single crystal is diffusion-controlled at reaction temperature between 1250oC and 1300oC; the reaction mechanism was changed to interface-controlled within the temperature range of 1300oC to 1325oC.
In conclusion, the results of MKC and Avrami analysis suggest that at temperatures below 1300oC, the reaction mechanism probably differs from that of temperatures exceeding 1300oC, at which point the cristobalite is presented. Therefore, the reaction mechanism of thermal decomposition of kyanite seems strongly correlated to the presence of cristobalite crystallites.
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