Study on the Formation mechanism of Tabular Al2O3 and Prismatic Mullite Formation on Alumina Substrate

本研究使用氧化鋁-氧化矽雙相溶膠(diphasic aluminum silicate sol)在多孔及緻密氧化鋁基材上，以低於共晶溫度(1560oC)的熱處理條件(1280oC)形成鍍膜。在多孔氧化鋁基材上，生成片狀(tabular)氧化鋁的互鎖(interlock)結構；而在緻密96%純度氧化鋁基材上，則生成針狀(acicular)莫來石的互鎖結構。 實驗中以矽酸四乙酯(TEOS，tetraethlylorthosilicate)和氫氧化鋁(boehmite)分別當作鋁、矽原子的來源，形成氧化鋁-氧化矽雙相溶膠。在固定鋁矽莫耳比為莫來石當量比3Al2O3•2SiO2的條件下，利用旋轉塗佈法(spin coating)在氧化鋁基材上生成雙相氧化鋁-氧化矽(簡稱DAS)薄膜，經過乾燥和1280oC熱處理得到具有非等向晶粒鍍膜的高溫陶瓷複合材料。所得到的複合材料經由X光繞射儀(XRD)、掃描式電子顯微鏡暨能量分析光譜儀(SEM&EDX)、與分析式穿透電子顯微鏡(AEM)及電子能量損失譜儀(EELS)來進行成分及微結構的觀察。研究中採用不同的純度、鍍膜成分或熱處理條件等製程參數，並製備多種條件的多孔陶瓷TEM樣品，針對非等向晶粒在氧化鋁基材上生成的機制進行探討。 DAS鍍膜在多孔氧化鋁基材上會形成以片狀氧化鋁為主的層狀結構。在薄膜中，片狀晶交錯生成，產生微米級的孔洞，一片狀氧化鋁晶粒中含有數個方向相同的次片狀晶(sub-tabular grain)，片狀晶的最大平面為(000h)AlO。同一個片狀晶中的次片狀晶晶粒生長方向略有不同，但不論成分鑑定或晶相分析皆能証明其為氧化鋁。 在96%緻密氧化鋁基材上的DAS薄膜經燒結後會生成針狀莫來石。經過TEM的晶相分析，針狀莫來石具有的軸向(zone direction)為[001]M，長度為5-20In this study, diphasic aluminum silicate sol (M) was coating on porous or dense alumina substrate to form a covering layer (DAS) and then sintered at 1280oC, which was below eutectic temperature (1560oC) of Al2O3-SiO system. A gradient, porous Al2O3 substrate was prepared with the characteristics of micro-sized tabular grains on surface, fine interlocking of Al2O3 grains in matrix. Acicular mullite grains were formed on dense Al2O3 substrate with interlocking structure. The sample was made by spin-coating alumina substrates with diphasic gel made by pseudo-boehmite (AlOOH) and hydrolyzed TEOS [Si(OC2H5)4]. After drying and sintering at different heat treatments, which at the temperature 1280oC, the crystallography and microstructure of the porous structure was investigated by analytical electron microscopy (AEM), X-ray differactometry (XRD), and scanning electron microscopy (SEM). Detailed micro-beam compositional and structural analyses were conducted in order to verify the glassy phase and abnormal Al2O3/ mullite grains. The abnormal grain growth of Al2O3 grains is possibly due to the transient SiO2-glass phase during sintering above 1250oC. The formation of tabular grains is due to a heterogeneous nucleation from faceted grain of Al2O3/Glass interface, ripening from transient Al2O3 nano-crystals through amorphous SiO2 glass. The reactions are limited to regions where the density is higher, and results in a perpendicular growth of plate-like grains on the substrate. Ostwald ripening in association with the rearrangement of small transient摘要....................................................1 Abstract..............................................III 目錄....................................................V 圖目錄...................................................VIII 表目錄...............................................XIII 第一章 前言.............................................1 第二章 文獻回顧.........................................5 2-1氧化鋁-氧化矽相圖(Al2O3-SiO2 binary phase diagram)...6 2-1-1 氧化鋁、氧化矽、氧化鋁-氧化矽相圖 錯誤! 尚未定義書籤。 2-1-1 介穩定相 (metastable region)......................9 2-1-2相分離區..........................................10 2-1-3 過冷液相.........................................10 2-2 莫來石.............................................12 2-2-1莫來石的形成......................................12 2-2-2固溶反應與鍵能....................................13 2-2-3過飽和及莫來石的生成.....................................................14 2-3-1 晶形定義.........................................19 2-3-2 莫來石的製備與晶形...............................20 2-4 片