Thermal and microchemical characterisation of sol-gel SiO2, TiO2 and xSiO(2)-(1-x)TiO2 ceramic materials

Amorphous SiO2, TiO2 and xSiO(2)-(1-x)TiO2 ceramic materials with selected values of x 0.5, 0.7 and 0.9, have been prepared via sol-gel process using silicon tetraethoxysilane (TEOS) and titanium tetraisopropoxide Ti(OPri)(4). By means of the combined use of differential thermal analysis (DTA), thermogravimetry (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES), the surface microchemical structure and morphology of the sol-gel materials have been studied as a function of thermal treatments carried out in air up to 1200 degreesC. In the range of temperature from 50 to 450 degreesC, DTA-TG results evidence a remarkable mass loss due to the evaporation of organic solvents entrapped in the sol-gel materials and of the remnant organic components of the precursor metal alkoxides. In the range of temperature from 400 to about 1000 degreesC, by means of the combined use of DTA, XRD, XPS and XAES techniques as a function of temperature and of chemical composition, it is possible to evidence the formation of crystalline phases such as quartz, anatase and rutile. Furthermore, lineshape analysis of O 1s XPS peak allows to distinguish between single O-Ti and O-Si bonds and also to disclose the presence of cross linking Si-O-Ti bonds, that act as bridges between SiO2 and TiO2 moieties. As a function of temperature, Si-O-Ti bonds are broken and the formation of new Ti-O and Si-O bonds as in TiO2 and SiO2 takes place as well as a silica segregation phenomenon.