Phase-transformation kinetics in triphasic cordierite gel

Abstract

A triphasic cordierite-type gel was prepared from silica sol, boehmite sol, and an aqueous solution of Mg(NO3)2 · 6H2O. The silica sol was obtained from water glass by the ion exchange method, while boehmite sol was obtained by peptization of freshly prepared Al(OH)3. Phase transformations occurring in the gel were studied by differential scanning calorimetry, x-ray diffractometry, and Fourier transform infrared spectrometry. Spinel was observed to crystallize from the gel prior to cristobalite; their reaction subsequently yielded α-cordierite. At higher temperatures, α-cordierite transformed into modulated β-cordierite. Kinetic parameters of α-cordierite formation and α-cordierite to modulated β-cordierite transformation were determined by differential scanning calorimetry under nonisothermal conditions. Formation of a-cordierite was found to be a diffusion-controlled process, with an overall activation energy of Ea = 1242 ± 66 kJ/mol. During the α → β-cordierite transformation, a modulated phase was formed by surface transformation of α-cordierite. The overall activation energy for the formation of the modulated structure is Ea = 583±77 kJ/mol.