STRUCTURAL CHARACTERIZATION AND HIGH-TEMPERATURE BEHAVIOR OF SILICON OXYCARBIDE CLASSES PREPARED FROM SOL-GEL PRECURSORS CONTAINING SI-H BONDS

Silicon oxycarbide glasses have been synthesized by inert atmosphere pyrolysis at 1000 degrees C of gel precursors obtained by cohydrolysis of triethosysilane, HSi(OEt)(3), and methyldiethoysilane, HMeSi(OEt)(2). The oxycarbide structures have been carefully characterized by means of different techniques such as Si-29 magic angle spinning nuclear magnetic resonance (MAS-NMR) and Raman spectroscopies, X-ray diffraction (XRD), and chemical analysis. Experimental results clearly indicate that, depending on the composition of the starting gels, the resulting oxycarbide glass either is formed by a pure oxycarbide phase or contains an extra carbon or silicon phase. By increasing the temperature up to 1500 degrees C, the oxycarbide glasses display compositional and weight stability; however, the amorphous network undergoes structural rearrangements that lead to the precipitation of nano-sized beta-SiC crystallites into amorphous silica. Crystallization of metallic silicon is also clearly observed at 1500 degrees C for the samples in which the presence of Si-Si bonds was postulated at 1000 degrees C.