2000 Microstructural evolution of mullite during the sintering of kaolin powder compacts.pdfVIP

Microstructural evolution of mullite during the sintering of kaolin powder compacts C.Y. Chen, G.S. Lan, W.H. Tuan * Institute of Materials Science and Engineering, National Taiwan University, Taipei, 106 Taiwan, ROC Received 20 September 1999; received in revised form 12 November 1999; accepted 19 November 1999 Abstract Kaolin particles are usually ˉaky in shape. In the present study, kaolin powder compacts were prepared by applying the die- pressing technique. The kaolin ˉakes tend to lie down on the plane which is perpendicular to the die-pressing direction. The powder compact thus shows anisotropic shrinkage after ?ring. A series of phase transformations take place as the kaolin is ?red at elevated temperature. Mullite is ?rst formed at a temperature as low as 1100C. If the ?ring temperature is higher than 1400C, needle- shaped mullite grains are formed. The size and the aspect ratio of the mullite grains increase with the increase of ?ring temperature. Furthermore, the long axes of the mullite needles tend to be perpendicular to the die-pressing direction. # 2000 Elsevier Science Ltd and Techna S.r.l. All rights reserved. Keywords: A. Sintering; B. Microstructure-?nal; D. Mullite 1. Introduction Kaolin, a relatively pure clay, has been widely used in ceramic industries for centuries [1]. Kaolin is, therefore, one of the most important raw materials for ceramic industries. Though kaolin has been used for many years, to explore the complexities involved in its phase trans- formation and microstructural evolution at elevated temperature is still a challenging task [2]. The main product phase after ?ring kaolin at high temperature is mullite. Mullite is, thus, an important constituent in refractories, whitewares and structural clay products for kaolin is frequently used as the raw material. Mullite itself is very stable at high temperature. Fur- thermore, its thermal expansion coe?cient and dielec- tric loss are low; mullite is, therefore, widely used as thermal