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 m and area up to 20 × 35 cm2) has been developed for the first time starting from a methyl modified sol containing nano-particulate SiO2 and having a solid content of 70 wt%. The gel sheets obtained by casting and drying of this sol were sintered in N2 atmosphere at temperatures between 900–1650°C. Only by the incorporation of colloidal silica (0.10–0.35 mole per mole alkoxide) to the sol could crack-free, large area glass sheets be obtained. Fracture strength (three point bending) was found to attain a maximum (200–300 MPa) for the sheets sintered at 1000°C. Young's modulus attained a peak value between 120 and 130 GPa for the sheets sintered at 1200°C. HR-TEM studies showed an amorphous and homogeneous matrix up to a sintering temperature of 1200°C, whereas at 1450°C and 1650°C, crystallites of SiC and lamellar graphite were formed. It is concluded that addition of colloidal silica to the sol does not lead to inhomogeneities after sintering and therefore does not decrease the strength and elastic modulus.