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Solid-state reaction and sol-gel, processing techniques were used extensively to form
chemically-modified mullite solid solutions in an effort to lower their thermal expansion
coefficients. TiO2, B203, CrPO4, P205, Ga203, Cr203, and WO3 and the half-breed
Si02 compounds AlPas, BPO4, GaPO4, BAsO4, AIAsO4, GaAsO4, and Ge02 were chosen as the modifiers.
The results indicate that, apart from Ti02, none of the substitutions made in mullite
significantly change the thermal expansion properties. The solubility of 3 wt% TiO2 in
mullite reduces the coefficient of thermal expansion by about 10%; That corresponds
to a reduction in A1203/Si02 molar ratio ( < 1.5) compared to stoichiometric mullite
(3A1203â ¢2Si02). The formation of Ti02-modified mullite depends on processing condition and heat treatment.
The possible mechanism of lowering the CTE of mullite by the addition of TiO2 is discussed in terms of the bond strength. The axial expansion of a Ga203-modified
mullite was measured' up to 1200Â°C to show that the expansion is increased along the c-axis compared with that of the stoichiometric mullite.