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 he xa-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 precu rsors may provide access to silicate or aluminosilicate powders, thin films, fibers, and coatings, Thus, we have examined the utility of hex acoordinate glycolatosilicates as model precursors, Pyrolysis of the c ompounds, MSi(OCH2CH2O)(3), in air transforms them to their anticipate d ceramic products, MO . SiO2. The phase transformations and chemical changes that occur during pyrolysis were characterized using X-ray pow der diffractometry (XRD), diffuse reflectance infrared Fourier transfo rm spectroscopy (DRIFTS), thermal gravimetric analysis (TGA), differen tial thermal analysis (DTA), and scanning electron microscopy (SEM). T he hexacoordinate glycolatosilicates oxidatively decompose at approxim ate to 300 degrees C to form amorphous materials, Moderate to signific ant quantities of the group II carbonates, MCO(3) (15-50 wt%), form co incidentally as the amorphous intermediates trap CO2 generated by liga nd oxidation, At approximate to 900 degrees C, the amorphous materials crystallize into the expected, phase-pure, MO . SiO2.