Phase Transformation of Diphasic Aluminosilicate Gels

Aluminosilicate gels with compositions Al2O2/SiO2 and 2 were prepared by gelling a mixture of colloidal pseudo-boehmite and a silica sol prepared from acid-hydrolyzed Si(OC2H5)4. Upon heating the pseudo-boehmite transforms to γ-Al2O3 around 400°C, then to δ-Al2O3 at 1050°C, and at 1200°C reacts with amorphous SiO2 to form mullite. Some twinned θ-Al2O3 forms before mullite. Nonstoichiometric specimens have a similar transformation sequence, but form mullite grains with inclusions of either Al2O3 or cristobalite, often associated with dislocation networks or micropores. Mullite grains are formed by nucleation and growth and have equiaxed shape.