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|Type:||Artigo de periódico|
|Title:||Kinetics of cordierite crystallization from diphasic gels|
|Abstract:||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.|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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