Effect of urea on the mullite crystallization

Due to its chemical, physical and mechanical properties, mullite has been used in equipments subjected to extreme mechanical strain and high temperatures. This paper reports the study of the effect of urea on the mullite crystallization, synthesized through the colloidal and polymeric sol–gel processes, using XRD and FT-IR. The colloidal precursors were obtained from silicic acid, water, aluminum nitrate nonahydrate (ANN) and urea. The polymeric precursors were obtained from TEOS, ethanol, ANN and urea. For both methods, urea was used in the molar ratio urea/Al3+ equal to 0/1, 1/1 and 3/1. The urea addition in colloidal gels led to materials with higher concentrations of orthorhombic mullite, besides they crystallized mullite at lower temperatures (positive effect). However, a negative effect was observed when urea was added to polymeric gels. In addition, the amount of crystallized mullite increased with the urea content in the colloidal gels. The colloidal sample with the highest urea content proved to form mullite at a lower temperature. Moreover, this sample did not segregate the α-alumina phase. The positive effect of urea in the colloidal gels is related to its participation in the hydrolysis and condensation steps of aluminum and silicon, avoiding the extensive phase segregation of silica and alumina. The negative effect observed in the polymeric samples may have occurred due to a competition between the silanol, the ANN and urea by water molecules. The only water molecules that are present in the polymeric gel are those that are part of the ANN.