MULLITE AND ITS ISOMORPHIC SUBSTITUTION

Abstract

The phase diagram of SiO₂-Al₂O₃ is considered.  The paper presents an overview of the methods of synthesising mullite, including the calcining of natural aluminosilicates, the Czochralski process (synthesising mullite from melt), the Verneuil process (synthesising mullite from gel), and the solid state method (synthesising mullite from the mixture of stoichiometric composition).  Previous studies have determined that the parameters of the orthorhombic lattice and the habitus of mullite crystals depend directly on the ratio of aluminum and silicon in the initial batch, the content of impurities, the method of heat treatment, and the form of introduction of the initial components into the batch. The size of the mullite crystals is affected by the presence of aluminum fluoride in the batch, which at a certain ratios allows to obtain the final product in the form of whiskers.  The effect of mineralizers on the yield of mullite was considered: alkali metal fluorides reduce the yield, while sulfates and chlorides increase it and also influence the viscosity and structure of the melt.  In order to determine the effect, we studied the influence of various additives on the process, structure, and properties of the synthesis.  Structural features of the mullite crystal lattice affecting isomorphic substitutions were determined. The paper also considers the possibility of obtaining isomorphically substituted mullite varieties, and presents the optical characteristics of isomorphically substituted mullite containing d-elements.