GROUP-II TRIS(GLYCOLATO)SILICATES AS PRECURSORS TO SILICATE-GLASSES AND CERAMICS

Group II tris(glycolato)silicates, MSi(OCH2CH2O)(3) (where M = Ba, Ca, Mg), can be synthesized directly by reaction of silica with ethylene glycol and alkaline-earth (group II) oxides at 200 degrees C, These hexa-alkoxy silicates serve as precursors to silicate glass and ceramic powders, They are readily modified by exchange with longer-chain diols into processable polymer precursors, These theologically useful precursors may provide access to silicate or aluminosilicate powders, thin films, fibers, and coatings, Thus, we have examined the utility of hexacoordinate glycolatosilicates as model precursors, Pyrolysis of the compounds, MSi(OCH2CH2O)(3), in air transforms them to their anticipated ceramic products, MO . SiO2. The phase transformations and chemical changes that occur during pyrolysis were characterized using X-ray powder diffractometry (XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), thermal gravimetric analysis (TGA), differential thermal analysis (DTA), and scanning electron microscopy (SEM). The hexacoordinate glycolatosilicates oxidatively decompose at approximate to 300 degrees C to form amorphous materials, Moderate to significant quantities of the group II carbonates, MCO(3) (15-50 wt%), form coincidentally as the amorphous intermediates trap CO2 generated by ligand oxidation, At approximate to 900 degrees C, the amorphous materials crystallize into the expected, phase-pure, MO . SiO2.