Phase-transformation kinetics in triphasic cordierite gel

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 E_a = 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 E_a = 583±77 kJ/mol.