Kinetics of cordierite crystallization from diphasic gels

Diphasic cordierite gels were prepared from colloidal silica, aluminum and magnesium nitrates and citric acid. The mechanism of xerogel decomposition was studied by infrared spectroscopy (FT-IR) and thermal gravimetric analysis (TGA). The thermal decomposition of the xerogel forms a solid mixture of MgO, Al(2)O(3) and SiO(2) at around 250 degrees C. Cordierite crystallization was studied by X-ray diffraction (XRD) and differential thermal analysis (DTA). Xerogels were initially thermally treated, and this sample crystallized to mu-cordierite at 850 degrees C, at 900 degrees C alpha-cordierite crystallizes and at 1150 degrees C alpha-cordierite is the major phase and mu-cordierite is totally consumed. The apparent activation energy for cordierite crystallization process was determined based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory, Ligero methods and the Arrhenius law for dependence of activation energy with temperature. The apparent activation energy was (466.8 +/- 34.3) kJ/mol, the exponent of Avrami was (1.9 +/- 0.2) and the frequency factor was (1.55 x 10(20)) s(-1). The Avrami value indicates a nucleation controlled process, which can be a consequence of the high xerogel homogeneity, a consequence of the early and simultaneous formation of the MgO, Al(2)O(3) and SiO(2) mixture.47214014