The structural studies of aluminosilicate gels and thin films synthesized by the sol-gel method using different Al₂O₃ and SiO₂ precursors

Aluminosilicate materials were obtained by sol-gel method, using different Al₂O₃ and SiO₂ precursors in order to prepare sols based on water and organic solvents. As SiO₂ precursors, Aerosil 200^TM and tetraethoxysilane TEOS: Si(OC₂H₅)₄ were applied, while Disperal^TM and aluminium secondary butoxide ATSB: Al(OC₄H₉)₃ were used for Al₂O₃ ones. Bulk samples were obtained by heating gels at 500 °C, 850 °C and at 1150 °C in air, while thin films were synthesized on carbon, steel and alundum (representing porous ceramics) substrates by the dip coating method. Thin films were annealed in air (steel and alundum) and in argon (carbon) at different temperatures, depending on the substrate type. The samples were synthesized as gels and coatings of the composition corresponding the that of 3Al₂O₃·2SiO₂ mullite because of the specific valuable properties of this material. The structure of the annealed bulk samples and coatings was studied by FT-IR spectroscopy and XRD method (in standard and GID configurations). Additionally, the electron microscopy (SEM) together with EDS microanalysis were applied to describe the morphology and the chemical composition of thin films. The analysis of FT-IR spectra and X-ray diffraction patterns of bulk samples revealed the presence of γ-Al₂O₃ and δ-Al₂O₃ phases, together with the small amount of SiO₂ in the particulate samples. This observation was confirmed by the bands due to vibrations of Al–O bonds occurring in γ-Al₂O₃ and δ-Al₂O₃ structures, in the range of 400 to 900 cm⁻¹. The same phases (γ-Al₂O₃ and δ-Al₂O) were observed in the deposited coatings, but the presence of particulate ones strongly depended on the type of Al₂O₃ and SiO₂ precursor and on the heat treatment temperature. All thin films contained considerable amounts of amorphous phase.