Synthesis of mullite powders through a suspension combustion process

This work reports on the synthesis of mullite powders by a suspension combustion process. Aluminum nitrate, as a source of Al, and a colloidal silica suspension, as a source of Si, are used as reagents, and urea serves as the fuel. This colloidal suspension allows a complete mixing of the reactants, thus promoting a core-shell reaction to produce the mullite. The powders obtained need to be treated at a high temperature to form the mullite. However, when ammonium nitrate is also added as a combustion aid, the mullite phase is directly obtained without any further treatment. A chemical reaction involving all these compounds is proposed, and the thermodynamic parameters are calculated in order to predict the temperature reached during the combustion process. The addition of ammonium nitrate reduces the temperature of the reaction by ca. 150 degrees C. Once the conditions for the combustion reaction are optimized, the Al/Si ratio is modified in an attempt to obtain monophasic mullite.