Low Temperature Routes to Cordierite-Like Ceramics using Chemical Processing

Abstract

ABSTRACTThe reaction of SiO2 with basic magnesium compounds, in the presence of excess ethylene glycol, provides access to the hexacoordinate silicate complex, MgSi(OCH2CH2O)3. Alkali metal hydroxides provide access to pentacoordinate silicates of the type M2Si2(OCH2CH2O)5, where M = Li, Na, K, and Cs. Alumina also reacts with alkali metal hydroxides and ethylene glycol to give compounds of the type MAI(OCH2CH2O)2. HCl neutralization of the alkali metal silicates and aluminates provides the neutral metal glycolates of Si and Al. Stoichiometrically correct mixtures of these complexes form homogeneous, cordierite precursor solutions that are potentially useful for forming films, fibers and membranes of cordierite. These polymer-like materials can be heated directly in air to generate first α-cordierite and then µ-cordierite. Hydrolysis and condensation reactions, which occur in sol-gel processing, are not required. The phase transformations and chemical changes that occur during pyrolytic transformation of the preceramic materials to their target ceramic products were characterized. Methods developed during the course of these studies led to the development of heating schedules wherein well crystallized g.-cordierite could be formed at 850 °C.