Crystallization kinetics of mullite from polymeric Al2O3-SiO2 xerogels

The kinetics of mullite crystallization from polymeric Al2O3-SiO2 xerogels with chemical compositions of 60 and 80 mol% Al2O3 was investigated under non-isothermal conditions using DTA. The polymeric xerogels were prepared by slow hydrolysis of tetraethoxysilane and aluminum nitrate nonahydrate dissolved in ethanol. The DTA conducted at heating rates of 2, 5, 10 and 20 degreesC/min showed an exothermic peak at about 1000 degreesC associated with mullitization and/or gamma-Al2O3 formation. The activation energy for the nucleation-growth mechanism of mullite, calculated by the Kissinger method, was 1202 +/- 27 kJ/mol in the 60 mol% xerogel and that of mullite and gamma-Al2O3 was 1092 +/- 42 kJ/mol in the 80 mol% xerogels. Since these activation energies are similar to those reported for glasses, diphasic xerogels and hybrid xerogels, irrespective of their mullitization routes and temperatures, all these mullitization processes are considered to occur by a similar diffusion-controlled mechanism. (C) 2003 Elsevier Science B.V. All rights reserved.