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). 6H(2)O. The silica sol was obtained from water glass by the ion exchange method, while boehmite sol was obtain ed by peptization of freshly prepared Al(OH)(3), Phase transformations occu rring in the get were studied by differential scanning calorimetry, x-ray d iffractometry, and Fourier transform infrared spectrometry. Spinel was obse rved to crystallize from the gel prior to cristobalite; their reaction subs equently yielded alpha -cordierite, At higher temperatures, alpha -cordieri te 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 n onisothermal conditions. Formation of alpha -cordierite was found to be a d iffusion-controlled process. with an overall activation energy Of E-a = 123 2 +/- 66 kJ/mol. During the alpha --> beta -cordierite transformation, a mo dulated phase was formed by surface transformation of alpha -cordierite, Th e overall activation energy for the formation of the modulated structure is E-a = 583 +/- 77 kJ/mol.