Abstract
Silica‐free phases which have a structure similar to that of mullite can be crystallized from gels in the Na2O‐Al2O3 and (Na,K)2O‐BaO‐Al2O3 systems. A gelation step appears to be necessary, since a solid‐state reaction between Na2CO3 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‐Al2O3 and corundum are formed. The mullite‐type phase appears to crystallize with an increase in temperature at the expense of a γ‐Al2O3 phase, indicating adsorption of Na on the defect spinel structure, which is then rearranged to give the mullite‐type phases.
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