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 p yrolysis 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 mean s of different techniques such as Si-29 magic angle spinning nuclear m agnetic resonance (MAS-NMR) and Raman spectroscopies, X-ray diffractio n (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 conta ins an extra carbon or silicon phase. By increasing the temperature up to 1500 degrees C, the oxycarbide glasses display compositional and w eight stability; however, the amorphous network undergoes structural r earrangements that lead to the precipitation of nano-sized beta-SiC cr ystallites into amorphous silica. Crystallization of metallic silicon is also clearly observed at 1500 degrees C for the samples in which th e presence of Si-Si bonds was postulated at 1000 degrees C.