Structural characterization and high-temperature behavior of silicon oxycarbide glasses prepared from sol-gel precursors containing Si-H bonds
Group ? tris(glycolato)silicates, MSi(OCH//2CH//2O)//3 (where M=Ba, Ca, Mg), can be syntehsized directly by reaction of silica with ethylene glycol and alkaline-earth (group ?) oxides at 200?. 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. Thes rheologcially useful precusors maty provdie access to silicate or aluminosilicate powders, thin films, fibers, and coatings. Thus, we have examiend the utility of hexacoordinate glycoatosicliates as model precursors. Pyrolysis of thecompounds, MSi(OCH/2-CH//2O)//3, in air transforms them to theior anticipated ceramic products, MO·SiO//2. The phase transformaitons and chemical changes that occur during pyrolysis were characterized using X-ray powder diffractometry 9XRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), thermal gravimetric analysis (TGA), diferential thermal analysis (DTA), and scanning electron microscopy (SEM). The hexacoordinate glycolatosilicates oxidativekly decompose at ?300? toform amorphous materials. Moderate to significnat quantities of teh gorup ? carbonates, MCO//3 (15-50 wt%), form coincidentally as the amorphous intermediates trap CO//2 generated by ligand oxidation. At ?900?, the amorphous materials crystallize intot he expected, phase-pure, MO·SiO//2.
Pallavi Kansal?Richard M. Laine