Preparation of Dense Cordierite Ceramics from Ultrafine Particles of Magnesium Hydroxide and Kaolin

Abstract

Thermal reactions and sintering characteristics were investigated to obtain dense cordierite ceramics using kaolin minerals (Al2O3⋅2SiO2⋅nH2O), such as kaolinite or halloysite, and chemically synthesized magnesium hydroxide in the form of ultrafine hexagonal tabular particles. Mixtures of magnesium hydroxide and submicron tabular-kaolinite particles in composition of MgO⋅Al2O3⋅2SiO2 resulted in the formation of amorphous state at about 800-900°C after a reaction of thermal decomposition. The amorphous materials abruptly shrank at about 900°C up to a relative density of about 90%, above which crystallization of a μ-cordierite phase restrained further densification. Firing the mixtures to about 1300-1350°C yielded dense α-cordierite ceramics with a relative density higher than 95% and a negligible apparent porosity and showing a linear thermal expansion coefficient of 2.2×10-6/K. On the other hand, mixtures of magnesium hydroxide and submicron needlelike-halloysite particles resulted in insufficient thermal reaction at about 800-900°C because of the less contact area of mixed particles which, affecting sintering and crystallization characteristics, produced several percents of apparent porosity by firing at 1350°C for 1h.