Silica‐Free Phases with Mullite‐Type Structures

Silica‐free phases which have a structure similar to that of mullite can be crystallized from gels in the Na 2 O‐Al 2 O 3 and (Na,K) 2 O‐BaO‐Al 2 O 3 systems. A gelation step appears to be necessary, since a solid‐state reaction between Na 2 CO 3 and Al(OH) 3 does not give the mullite‐type phase. Crystallization of this phase requires a high alkali content during formation of the gel. A well‐crystallized phase is formed at 950°C and is stable to at least 1000°C; at higher temperatures (i.e. 1200°C), β‐Al 2 O 3 and corundum are formed. The mullite‐type phase appears to crystallize with an increase in temperature at the expense of a γ‐Al 2 O 3 phase, indicating adsorption of Na on the defect spinel structure, which is then rearranged to give the mullite‐type phases.