中文摘要

　　富鋁紅柱石之機械性質優異且化學穩定性佳，在傳統與尖端陶瓷中皆是歷久彌新的研究主題。探索高嶺土–氧化鋁陶瓷中一次及二次富鋁紅柱石之形成與成長，將有助於富鋁紅柱石陶瓷之功能設計與製備。
　　本研究以高嶺土–氧化鋁粉末為原料，利用X光繞射、熱分析、高解析分析電子顯微鏡、熱膨脹儀與水銀測孔儀等，分析其相變化、一次及二次富鋁紅柱石之形成、晶體結構與化學組成及試樣之燒結與孔隙特性。
　　研究結果顯示，燒結60 wt. %高嶺土– 40 wt. %氧化鋁陶瓷時，「一次富鋁紅柱石」在1273至1573 K階段由高嶺土中相變化形成，「二次富鋁紅柱石」在1573至1873 K階段以「溶解–析出」機制由玻璃相中析出。
　　一次富鋁紅柱石之形成活化能為1182.3 kJ/mol且為固定晶核數之「體成核機制」。試樣經1573 K燒結180分鐘後，再於1673 ~ 1873 K經不同時間燒結之試樣可明確得二次富鋁紅柱石含量，其形成活化能為454.6 kJ/mol。兩者皆為平板狀晶粒析出且隨溫度升高而長為伸長型板狀，隨晶粒寬度成長各達70及40 nm時，晶粒中Al含量增高但斜方晶格常數降低而趨於3/2富鋁紅柱石。
　　高嶺土–氧化鋁陶瓷燒結過程之收縮現象可區分為四個階段，一為高嶺土轉變為偏高嶺土之813 ~ 1223 K，二為形成一次富鋁紅柱石與玻璃相之1223 ~ 1273 K，三為燒結緻密化過程在1273 ~ 1473 K之間，最後為1473 ~ 1723 K時伸長型板狀富鋁紅柱石形成。在1373 ~ 1873 K燒結1小時後之燒結體孔隙為「單峰分佈模式」，孔隙體積隨溫度之升高而減小，但平均孔徑由0.6增為1.5 mm。伸長型板狀富鋁紅柱石形成之骨架結構，不只阻礙緻密化進行，更使平均孔徑與開孔孔隙率增大，亦因伸長型板狀晶粒在黏滯流動的玻璃相中成長時，促使燒結體外觀尺寸增大。

Abstract

　　Mullite (3Al2O3×2SiO2), a well-known component in conventional ceramics, has been considered to have potential in some advanced applications as a structural ceramic material due to it’s excellent mechanical strength and chemical stability. Seemingly, a thorough understanding of the mullite formation may provide some unique features of processing advanced ceramics with respect to its structural as well as functional characteristics.
　　In this study, the phase transformation of kaolin-Al2O3 ceramics and the formation, crystal structure and chemical composition of the “primary” and “secondary” mullites have been identified using the XRD, HR-AEM. Sintering behavior and pore properties have been investigated using TMA and mercury porosimeter.
　　For the phase transformation of 60 wt. % kaolin- 40 wt. % Al2O3 ceramics, the primary mullite is transformed from kaolin in 1273 to 1573 K and the secondary mullite is precipitated from the glassy phase by the “solution- precipitation mechanism” in 1573 to 1873 K.
　　The formation activation energy of the primary mullite is 1182.3 kJ/mol and reveals the “bulk nucleation mechanism” with constant nuclei. The activation energy of secondary mullite formation is 454.6 kJ/mol as quantitative analysis as sintered at 1573 K for 180 min and subsequently heating at 1673 to 1873 K for various times. Both of primary and secondary mullites, the square plate-like crystal grow to elongated plate-like one with increasing the sintering temperature. The Al content in the mullite crystal increases but the lattice parameters of the orthorhombic structure decrease, tending to be a 3/2 mullite, as the primary and secondary mullite grains grow to 70 and 40 nm in width, respectively.
　　In the sintering behavior and pore structure development of kaolin-Al2O3 ceramics, linear shrinkage of the heated sample occurs at four stages during sintering. The sintered sample shrinks during the dehydration and transformation of kaolin as heated at 813 ~ 1223 K and 1223 ~ 1273 K, respectively. On other hand, the sintered sample shrinks during sintering process as heated at 1273 ~ 1473 K. Then, the sample shrinks during the elongated plate-like mullite formation as heated at 1473 ~ 1723 K. The sintered sample shows a monodispersed pore size distribution and the pore volume decrease but the average pore diameter increase from 0.6 to 1.5 mm with increasing the sintering temperature from 1373 to 1873 K for 1 h. By increasing the sintering temperature or duration, the in-situ skeleton mullite crystals and glassy phase promotes the expansion of the heated body, therefore increases the pore diameter and improves the open porosity, thus limits the overall densification.

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