藍晶石熱分解反應微結構變化與動力學探討

Abstract

藍晶石為高壓變質帶典型指標礦物之一，透過研究其相變與熱分解反應動力學，能重建變質帶的演化情形與古溫壓環境，在礦物學、岩石學與反應動力學領域中是重要的研究題材。本研究藉由觀察藍晶石於熱分解反應過程中的微結構變化，以及利用主導動力學曲線(Master Kinetics Curve, MKC)與Avrami方程式等兩種動力學模型，分析熱分解實驗數據，藉由所得結果瞭解藍晶石之熱分解行為，並嘗試釐清其反應機制。 研究方法可分為三部分：首先以X光粉末繞射儀(XRD)與電子微探儀(EPMA)對藍晶石粉末、單晶樣本進行定性與定量分析；第二部分使用管型高溫爐以及熱膨脹儀進行藍晶石熱分解反應，同時擷取文獻中的藍晶石熱分解反應數據和本研究結果進行比較。第三部分則透過掃描式電子顯微鏡(SEM)觀察藍晶石樣本於反應前後所發生之微結構變化，並利用主導動力學曲線與Avrami方程式分析所得到的實驗數據，建立反應預測模型確認動力學模型的適用性。最後則綜合微結構及動力學模型分析結果，探討藍晶石熱分解反應可能的反應機制。 藍晶石在高溫下將分解為富鋁紅柱石與二氧化矽玻璃相或方矽石。觀察藍晶石粉末與單晶的微結構變化，可發現兩者之熱分解反應皆發生於(100)面上，但藍晶石粉末熱分解後僅形成富鋁紅柱石單一晶相，二氧化矽則以非晶質相出現；藍晶石單晶則除了富鋁紅柱石外，亦可觀察到二氧化矽非晶質相與方矽石。在動力學模型分析方面，本研究確認主導動力學曲線與Avrami方程式皆適用於描述藍晶石的熱分解反應，且分析結果顯示藍晶石粉末之熱分解反應可能由單一機制所控制；藍晶石單晶之熱分解反應則具有兩種反應機制。綜合微結構觀察結果與動力學模型之分析結果，本研究推論藍晶石熱分解反應隨生成物變化而有所不同：若熱分解反應產生富鋁紅柱石與二氧化矽非晶質相，此時反應機制為擴散控制；而產物轉變為富鋁紅柱石與方矽石相時，則由介面機制控制其熱分解反應。

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.