Phase segregation of non-stoichiometric aluminosilicate gels characterized by 27Al and 29Si MAS-NMR

Polymeric aluminosilicate gels with Al 2O 3/SiO 2 molar ratios of 1.3/2, 2.5/2, 3/2 and 4.7/2 were prepared by gelling a mixture of tetraethoxysilane and ethyl acetoacetate aluminium diisopropoxide. Mullite, without any other crystalline phase, directly crystallizes from the gel matrix at about 1000 °C for all investigated samples. The Al 2O 3 or SiO 2 crystalline phase can only be detected at relatively high temperatures accompanying the modification of mullite in the lattice structure. 27Al and 29Si MAS-NMR studies indicate that segregation of Al and Si atoms occurs in all samples below 900 °C in the amorphous state, regardless of the gel composition. However, the species of segregated units are very different and strongly dependent on the composition of the starting gels. This segregation may not cause the exothermic effect in differential thermal analysis examination, but it appears to be responsible for the composition change behavior of crystalline mullite with different heat treatment.