Mullite formation in kaolin-Al2O3 ceramics

• Main Authors: Yung-Feng Chen, 陳永鋒
• Other Authors: Min-Hsiung Hon
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/48522232085947535154

博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 92 === 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.