Mullite phase evolution in clay with hydrated or anhydrous AlF3

Porous mullite ceramics having needle-shaped interlocking microstructure have been developed from China clay with hydrated aluminium fluoride (AlF3 center dot 3H(2)O) or anhydrous AlF3. The mullite phase evolution, microstructure and the reaction mechanism in both the systems have been compared. In the presence of anhydrous AlF3, needle-shaped mullite crystals grow mainly by the decomposition of topaz crystals, which are formed during the intermediate stages of sintering. Whereas in the case of hydrated AlF3 samples, the presence of water vapour and HF generated in the reaction zone help in direct nucleation of mullite crystals without the formation of topaz. However, in the powder mixture containing clay and 40 wt% AlF3 center dot 3H(2)O, the mullite nucleation happens from the topaz decomposition also, resulting in high mullitisation overall. Mullite crystals formed from the 40 wt% AlF3 center dot 3H(2)O and clay have good needle-shaped interlocking microstructure, and can be used as a better alternative for anhydrous AlF3 to reduce the cost.