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 °C/min showed an exothermic peak at about 1000 °C associated with mullitization and/or γ-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 γ-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.