Crystallization behavior of alkoxy-derived cordierite gels
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Alkoxy-derived cordierite gels were synthesized from tetraethylorthosilicate (TEOS), aluminum isopropoxide (Al(OPri)(3)), and magnesium ethoxide (Mg(OEt)(2)). TEOS was partially hydrolyzed at molar ratios H2O/TEOS=1.2, in the presence of hydrochloric acid as a catalyst, HCl/TEOS=0.1. Aluminum and magnesium alkoxides were added successively or as a double alkoxide. Phase transformations occurring in the gel were studied by differential thermal analysis, x-ray diffractometry, and Fourier-transform infrared spectroscopy. In all cases, mu-cordierite crystallized at similar temperatures (950-1000degreesC) with small amounts of spinel, which confirms dominant influence of the optimal conditions for partial hydrolysis of TEOS on the gels homogeneity. The transformation of mu- into alpha-cordierite began at about 1100degreesC. Broadening of diffraction peaks and appearance of new bands in the FT IR spectra confirmed the transformation of alpha- into modulated beta-cordierite at temperatures ab...ove 1300degreesC. Differential thermal analysis under nonisothermal conditions also proved homogeneous nucleation with constant rate and three-dimensional crystallite growth during mu-cordierite crystallization. The overall activation energy of the crystallization of mu-cordierite is 580+/-81 kJ/mol.