Phase-transformation kinetics in triphasic cordierite gel
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A triphasic cordierite-type gel was prepared from silica sol, boehmite sol, and an aqueous solution of Mg(NO3)(2). 6H(2)O. 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 get 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 alpha -cordierite, At higher temperatures, alpha -cordierite transformed into modulated beta -cordierite. Kinetic parameters of alpha -cordierite formation and alpha -cordierite to modulated beta -cordierite transformation were determined by differential scanning calorimetry under nonisothermal conditions. Formation of alpha -cordierite was found to be a diffusion-controlled process. with an overall activation energy Of E-a = 1232 +/- 66 kJ/mol. During t...he alpha -- GT beta -cordierite transformation, a modulated phase was formed by surface transformation of alpha -cordierite, The overall activation energy for the formation of the modulated structure is E-a = 583 +/- 77 kJ/mol.