Microstructural evolution of mullite during the sintering of kaolin powder compacts

Kaolin particles are usually flaky in shape. In the present study, kaolin powder compacts were prepared by applying the die-pressing technique. The kaolin flakes tend to lie down on the plane which is perpendicular to the die-pressing direction. The powder compact thus shows anisotropic shrinkage after firing. A series of phase transformations take place as the kaolin is fired at elevated temperature. Mullite is first formed at a temperature as low as 1100°C. If the firing temperature is higher than 1400°C, needle-shaped mullite grains are formed. The size and the aspect ratio of the mullite grains increase with the increase of firing temperature. Furthermore, the long axes of the mullite needles tend to be perpendicular to the die-pressing direction.