Urea effect on the mechanism of mullite crystallization

Mullite is an excellent structural material due to its physical and mechanical properties. In this study, mullite was obtained by the sol–gel process, using silicic acid, aluminum nitrate, and urea. The urea effect was studied by evaluating samples obtained from urea/Al 3+ ratio equal to 0, 1, and 3. The kinetic study was conducted using the isoconversional, non-isothermal, Flynn–Wall–Ozawa method. The sample prepared without urea, which is the least homogeneous one, formed spinel and α-alumina at 1150 °C, and Al-poor mullite together with α-alumina, at 1200 °C. The Al-poor mullite crystallization process from this sample showed the lowest yield. The sample prepared with urea/Al 3+ ratio equal to 1, which has an intermediate behavior, formed spinel at 1100 °C, Al-poor mullite at 1150 °C, and α-alumina together with Al-poor mullite at 1250 °C. However, the sample prepared with urea/Al 3+ ratio equal to 3, the most homogeneous, formed spinel and Al-rich mullite at 1100 °C. This sample formed Al-poor mullite at 1200 °C with the highest yield. Moreover, the sample synthesized without urea showed a higher porosity and a greater amount of hexacoordinated aluminum at 350 °C. All samples showed the same kinetic model, Šesták and Berggren (SB) for Al-poor mullite crystallization. The samples synthesized with urea crystallized mullite through the same kinetic parameters and constant values of the activation energy, but the sample prepared without urea followed different kinetic parameters and values of activation energy which changed over the course of the crystallization.