An x-ray and infra-red study of synthetic mullite

Abstract

Attempts have been made to prepare pure mullite over the solid solution range, at different temperatures and under controlled atmospheres. Two methods were used, one by heating mechanically mixed pure alumina and silica at 1500°, the other by preparing co-precipitated gels and heating these at temperatures at or about 1400° and 1500°. The presence of a solid solution was tested by a variation of lattice parameters with composition, and homogeneity of the product was tested by both x-ray and infra-red techniques. The areas of various x-ray peaks and positions of infra-red peaks were investigated to determine whether these characteristic properties of mullite, which are used to determine its rate of formation from kaolinite, vary with the composition of mullite. The data was analysed statistically and the correlation coefficients, R2, were taken to represent the fraction of the variable property of mullite that had been explained in terms of a variation of mullite composition. The first method of preparation was unsuccessful. Mullites obtained by the second method showed a variation of lattice parameters with composition. The variation with composition for a linear correlation showed for a, R2 = 0.843, while for b and c, R2 = 0.086 and 0.065 respectively. For the ratio of the areas under the doublet x-ray peak [(210)(120) planes] and the (110) peak, R2 was 0.874 at 1370° and 0.837 at 1470°, while for the ratio of the areas under the (111) and (110) x-ray peaks, R2 was 0.628 at 1370° and 0.693 at 1470°, for a linear correlation with composition.