Phase Transformation of Diphasic Aluminosilicate Gels

Aluminosilicate gels with compositions Al 2 O 2 /SiO 2 and 2 were prepared by gelling a mixture of colloidal pseudo‐boehmite and a silica sol prepared from acid‐hydrolyzed Si(OC 2 H 5 ) 4 . Upon heating the pseudo‐boehmite transforms to γ‐Al 2 O 3 around 400°C, then to δ‐Al 2 O 3 at 1050°C, and at 1200°C reacts with amorphous SiO 2 to form mullite. Some twinned θ‐Al 2 O 3 forms before mullite. Nonstoichiometric specimens have a similar transformation sequence, but form mullite grains with inclusions of either Al 2 O 3 or cristobalite, often associated with dislocation networks or micropores. Mullite grains are formed by nucleation and growth and have equiaxed shape.