Point defects and diffusion in alumina

The sintering of ceramics, like their creep and their reactivity, depends primarily on the diffusion processes, these processes themselves depending on the stracture of the compounds, the defects that they contain and the microstmcture. 

Argillaceous minerals of the sillimanite group were used as early as in the first era of ceramics, but it is mullite 3Al203.Si02 that is the only stable ciystallized compound in the binary phase diagram of the Al203-Si02. However, mullite is very 

Mullite crystallizes in the orthorhombic system, space group Pbam. Its complex structure accommodates stoichiometiy variations (from 3Al203.Si02 to 3Al203.i02 thanks to the presence of oxygen vacancies [EPI91]. 

The manufacture of sintered mullite is not easy. On the one hand, mullite powders are not easily available. Furthermore, the diffusion phenomena are slow in this compoimd, a characteristic which justifies the good meehanieal hot properties -because less diffusion means less creep - but also more difBcult sintering. The 

Mullites with high mechanical performances have hardly made any progress over the last ten years [SOM 90]. The efforts that their development would require have perhaps been overshadowed by the domination of alumina. 

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