| Summary: | 碩士 === 國立成功大學 === 資源工程學系 === 102 === Particle size effects of starting materials on mullite formation in the α-Al2O3/cristobalite powder systems was examined. α-Al2O3 and cristobalite powders with D50 values of 200, 300, and 400 nm were mixed in a stoichiometric composition of 3Al2O3∙2SiO2 (71.8 wt% α-Al2O3 and 28.2 wt% SiO2) as starting powder systems. Differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM) techniques were used in this study.
The results showed that mullite formation is related to the amorphization of silica. Cristobalite powder amorphized in advance during the thermal treatment, and then the Si component migrates to contact the α-Al2O3 particle to form mullite. A reduction in the size of α-Al2O3 particles resulted in a decrease in the amorphization temperatuer. Thus, the temperature of initiating mullite formation was lowered. A reduction in cristobalite particle sizes accelerated the amorphization reaction and resulted in higher rates of mullite formation. Thus it lowered the temperatures at which the powder system entirely converted into mullite. The activation energy calculated by isothermal experiments shows the reduction in particle sizes of cristobalite powders experienced more impact on the generation of the activation energy of mullite than that of α-Al2O3. As a result, in the α-Al2O3/cristobalite powder systems, size of cristobalite particles determined the rate of mullite formation.The crystal orientation of the mullite was controlled by the α-Al2O3 matrix, that is, [001] α-Al2O3 → [001] mullite. These results indicate that the amorphization of cristobalite may trigger the reaction of SiO2 with α-Al2O3, initiating the nucleation of mullite. The α-Al2O3 particles act as the hosts for mullite formation and determine the size of the mullite particles.
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