Mullite crystallization mechanism obtained from kinetic parameters determination for seeded and non-seeded gel

Mullite crystallization kinetics was studied using a non-isothermal method. The mullite crystallization parameters obtained from seeded gels were compared with those obtained from non-seeded gels. The phase evolution was studied by X-ray diffraction (XRD) and the thermal events associated to heating treatment by differential thermal analysis (DTA). The crystallization kinetic parameters were obtained using the methodology developed by R.A. Ligero, which is based on Johnson-Mehl-Avrami (JMA) kinetic model. The Avrami exponent (1.9), the apparent activation energy (980 kJ/mol) and the rate constant (3.68 x 1030 s-1) were determined by this model, for both seeded and non-seeded processes, resulting in similar values for each parameter. Based on the apparent activation energy and rate constant values, one should conclude that mullite crystallization is not controlled by nucleation. On the other hand, Avrami exponent ∼2 is an indicative that the process is controlled by the nucleation step. These controversies are discussed in this paper, based on experimental procedure, model limitation and literature information