Synthesis of Stoichiometric Cordierite Ceramics from Chemically Modified Kaolin by a Controlled Acid Leaching

Abstract

Cordierite ceramic still attracts great interest due to its interesting properties for applications in the field of modern engineering. It is one of the few ceramics that undergoes extremely low thermal expansion, less than 0.001% in the wide temperature range 25 ‒ 800°C. This work aims to synthesize stoichiometric cordierite ceramics according the following molecular formula: Mg2Al4Si5O18, from a single kaolin (Al2Si2O5(OH)4), that is chemically modified by acid leaching. The chemical modification of the kaolin is carried out in a hydrochloric acid solution (6M), at a temperature of 70°C, under magnetic stirring for 2 hours. These optimized acid leaching conditions increase the Si/Al ratio of kaolin, and thus allow a stoichiometric mixture of cordierite, by adding the precipitated magnesium hydroxide. Thermal transformations of the starting mixture in the temperature range 1100 ‒ 1430°C reveals the appearance of several phases, such as sapphirine and enstatite, before the cordierite becomes predominant at 1250°C. These intermediate phases have higher densities than that of cordierite, and thereby their transformations to cordierite phase are accompanied by swelling, which usually causes cracking in the ceramic. Therefore, the densification of structural ceramics is carried out from stoichiometric cordierite powders calcined at 1250°C by solid phase sintering and from the starting mixture by reaction sintering for comparison of the ceramic properties. The results show that in the first case, the maximum densification reaches 92% at 1325°C and then decreases to 78% at 1430°C, due to the formation of a glass phase, and in the second case, the densification reaches 90% at 1325°C, and at 1350°C its becomes very close to the total densification (~95%). The thermal expansion coefficient a (from RT to 700 °C) decreases with increasing sintering temperature;  = 3.1x10-6 °C-1 at 1275°C, and becomes <1.7x10-6 °C-1 at 1400°C.