Crystallization kinetics of orthorhombic mullite from diphasic gels

The kinetics parameters of orthorhombic mullite (o-mullite) crystallization from a diphasic gel were measured using a non-isothermal differential thermal analysis method. The diphasic gel was obtained from aqueous solutions containing silic acid and aluminum nitrate, in amount to keep the silicon and aluminum molar present in mullite (Si:Al equals 1:3). Urea in the molar ratio Al:urea equal to 1:3 was used as drying control chemical agent. The phase changes were verified using dynamic X-ray diffraction. The mullite morphology was analyzed by scanning electron microscopy (SEM). It was observed that mullite might be formed from the amorphous materials or from the spinel phase, since it was not observed the formation of tetragonal mullite and spinel phase disappeared before o-mullite crystallization. The apparent activation energy for the o-mullite crystallization was determined by a non-isothermal method based on Johnson-Mehl-Avrami-Kolmogorov (JMAK) and Arrhenius law for the chemical rate dependence on temperature. The apparent activation energy was equal to (730 +/- 150) kJ mol(-1), the Avrami's exponent was equal to 1.6 +/- 0.1, the frequency factor was (1.8 +/- 0.1) x 10(21) s(-1), and the ratio t(0.75)/t(0.25) was equal to (1.9 +/- 0.1). The SEM analysis showed that mullite growth is plate-like which agreed with JMAK model. (C) 2002 Elsevier Science B.V. All rights reserved.