Dehydration and structural development of mullite precursors: an FTIR spectroscopic study

Mullite type, I and III precursors were prepared by a sol-gel process using tetraethoxysilane and aluminum sec.-butoxide as starting materials. The pr ecursors were treated by 15 h heating steps in intervals of 100 degreesC fr om 200 to 1000 degreesC. Type III precursors are characterised by a more di scontinuous decrease of the analytical water content compared to that of ty pe I precursors, The FTIR powder spectra of the preheated precursors show w eak absorption bands at 5160 and 4540 cm(-1) which are due to H2O and OH co mbination modes, thus proving the presence of both, H2O molecules and OH gr oups as structural components of the precursors. (H2O, OH) stretching modes are centered at around 3430 cm(-1) and H2O bending modes at 1635 cm(-1). T he deconvolution of the stretching mode bands reveals non-bridging and brid ging H2O molecules and OH groups. Close similarities in the pattern of the 1400-400 cm(-1) vibrational region between type I and III precursors exist up to preheating temperatures of 800 degreesC. Significant differences are evident at temperatures of 900 degreesC, where the spectrum of the type III precursor still corresponds to that at 800 degreesC, while the type I prec ursor reveals a spectrum with features present in the spectrum of mullite. Bands centered around 1110 and 1010 cm(-1) are assigned to Si-O stretching vibrations of the SiO4 tetrahedral units and are strongly shifted in 900 de greesC treated type I precursors to higher wavenumbers. This band shift is a strong indication for an increasing degree of network condensation and fo r changes in the Si-O distances to tetrahedra dimensions similar to those o f mullite. A significant absorption around 860 cm(-1) is assigned to Al-O s tretching modes of AlO4 tetrahedral units, a band around 570 cm(-1) is assi gned to Al-O stretching vibrations of AlO6 octahedral units. A slightly dec reasing intensity of this band with increasing preheating temperatures, alo ng with a strongly increasing intensity of the 860 cm(-1) band demonstrates a clear preference of Al for a four-fold coordinated structural position i n the precursors preheated at high temperatures. This process is correlated to the dehydration process occurring in the medium- to high-temperature fi eld of network condensation starting at around 400 degreesC. (C) 2001 Elsev ier Science Ltd. All rights reserved.