Sol-gel synthesis and microstructural characterization of silicon oxycarbide glass sheets with high fracture strength and high modulus

A synthesis route to silicon oxycarbide glass sheets (thickness 40 to 1000 mu m and area up to 20 x 35 cm(2)) has been developed for the first time st arting from a methyl modified sol containing nano-particulate SiO2 and havi ng a solid content of similar to 70 wt%. The gel sheets obtained by casting and drying of this sol were sintered in N-2 atmosphere at temperatures bet ween 900-1650 degrees C. Only by the incorporation of colloidal silica (0.1 0-0.35 mole per mole alkoxide) to the sol could crack-free, large area glas s sheets be obtained. Fracture strength (three point bending) was found to attain a maximum (200-300 MPa) for the sheets sintered at 1000 degrees C. Y oung's modulus attained a peak value between 120 and 130 GPa for the sheets sintered at 1200 degrees C. HR-TEM studies showed an amorphous and homogen eous matrix up to a sintering temperature of 1200 degrees C, whereas at 145 0 degrees C and 1650 degrees C, crystallites of SiC and lamellar graphite w ere formed. It is concluded that addition of colloidal silica to the sol do es not lead to inhomogeneities after sintering and therefore does not decre ase the strength and elastic modulus.