Mullite ceramics (I)

Mechanochemical activation has been applied to phase formation of mullite from oxide precursors. The activated powders exhibit enhanced reactivity, thus leading to a significant reduction in phase formation temperature of mullite. Owing to its special crystal structure, the mullite formed at low temperatures tend to have anisotropic grain growth. As a result, mullite whiskers are formed in the mixtures with high-energy mechanochemical activation. In comparison, such an anisotropic grain growth is rarely observed when using the conventional solid-state reaction process, where the samples are usually densified before the mullite phase is formed. The densification takes place through the viscous flow sintering with the presence of liquid phase. It is observed that the mullite phase formation, grain growth behavior and microstructure development of the mullite ceramics can be tailored by selecting the milling media (tungsten carbide or stainless steel), as well as by doping with other oxides. This chapter mainly convers the effects of mechanochemical activation and the doping of transitional oxides.