Characterisation of stoichiometric sol–gel mullite by fourier transform infrared spectroscopy

Fourier Transform Infrared Spectroscopic investigation has been carried out to study the structural changes and coordination of Al and Si in the mullite system as a function of temperature (60–1400°C). The FTIR spectral patterns of the gel and the gel calcined at 200°C show bands corresponding to Al–O–Al and Si–O–Si vibrations. At higher temperatures hydroxyl groups and organics present have been removed. The absence of fine structure in the region 400–900 cm−1 in the temperature range 400–1000°C is interpreted as due to the existence of nanostructured alumina, silica and alumina–silica agglomeration in the amorphous state. Analysis of the spectra indicates the presence of both octahedral and tetrahedral coordination at 1000°C, the temperature around which spinel is formed. The shifting of the asymmetric stretching frequencies of the Si–O–Si and Al–O–Si networks to higher wave numbers indicate the formation of mullite at 1200°C. Crystalline nature of the product is indicated by the sharpening of spectral peaks. The spectra also indicates the completion of mullite formation at 1250°C.