Experimental and Monte Carlo simulation: the role of urea in mullite synthesis

Mullite was synthesized by a sol–gel process employing aqueous solution of silicic acid, aluminum nitrate and urea in high concentration (9 mol/l). t-Mullite crystallizes at temperatures around 1050°C, the fraction of the spinel phase formed was small and the apparent activation energy obtained by fitting the XRD results with the JMAK model was (770±4) kJ mol−1. Monte Carlo simulations were performed to investigate the site-site correlation function for ion (I)–urea (Ou) and ion (I)–water (Ow) interactions, g(rIOu) and g(rIOw,), respectively. The integration of these two correlation functions indicated the presence of four water and two urea molecules in the first Al3+ coordination shell. The 27Al nuclear magnetic resonance (NMR) results also indicate the presence of species like [Al(H2O)4(urea)2]3+.