Ceramics-Silikáty 45, (2) 48 - 54 (2001)

Crystalline and amorphous orthosilicates of magnesium, zinc, cadmium and copper were prepared by non-traditional sol-gel schemes connected with the use of high-boiling chemically active (HBCA) additives to prevent the polycondensation of silicate anions at early synthesis stages. Metal acetates and nitrates were used as the metal oxide precursors, tetraethoxysilane and γ-aminopropyltriethoxysilane - as the SiO₂ precursors, resorcinol and triethanolamine - as the HBCA additives, water and dimethyl formamide - as the solvents. Gelation and drying of the precursor solutions were carried out at 20 ° or 120 °C, the further thermal treatment of samples were performed in three stages up to 900 °C. Chemical evolution of the systems during the thermal treatment, was controlled using TGA-DTA and XRD techniques, the kinetic "molybdate" method, FTIR and UVVIS spectrophotometry. The use of high-boiling chemically active additives was shown to reduce the content of free metal oxides in the synthesis products, as well as to suppress the formation of high-polymerized silicate species that could complicate the further formation of orthosilicates. The formation of amorphous Cu₂SiO₄ is particularly complicated by thermodynamic instability of the corresponding crystalline silicate and needs addition of quaternary ammonium hydroxide as the highly basic agent.