Synthesis of bulk silicon oxynitride glass through nitridation of SiO 2 aerogels and determination of T g

The development of transparent glass for use in high‐temperature applications is continuing. In this study, we synthesized bulk silicon oxynitride glasses (a‐Si(O,N) x ) through the nitridation of SiO 2 aerogels containing methyl (CH 3 ‐) groups and evaluated their bulk properties, including their glass transition temperature ( T g ). Tetramethyl orthosilicate and methyltrimethoxysilane were added into the precursor gels, and those gels were subjected to a supercritical CO 2 drying process. The presence of CH 3 ‐group in the gel avoided cracking during ammonolysis at 750°C–1400°C, and the transparency of the gel was remained even after ammonolysis at 1300°C. The ammonolysis successfully introduced nitrogen into the gels even at relatively low temperatures, for example, 750°C, and the highest nitrogen content (11.7 mass%) was achieved in the gel after ammonolysis at 1300°C. As the nitrogen‐related signals in electron spectroscopy indicated presence of nitride ions (N 3− ) after ammonolysis and the infrared absorption signals attributed to Si–N bonds were enhanced with the increase of nitrogen concentration, we successfully obtained oxynitride glasses. Those oxynitride glasses showed increase of T g with their nitrogen concentration.