A solid state Si-29 and C-13 NMR study on the synthesis of thin silicon-oxycarbide glass sheets by a sol-gel route

Si-29 and C-13 MAS NMR spectroscopy, DTA-TG and elemental analysis were use d to study the evolution of metastable oxycarbide phases starting from thin gel sheets derived from methyltriethoxysilane (MTEOS), tetraethoxysilane ( TEOS) and a colloidal silica containing sol, up to 1450 degrees C under a n itrogen atmosphere. The stoichiometric formulae of the glasses obtained by the semi-quantitative analysis of Si-29 MAS NMR spectra were found to be in good agreement with those obtained by chemical analysis. It was found that the addition of colloidal silica sol delays the decomposition of methyl gr oups in MTEOS containing condensates. This delayed decomposition of the met hyl groups was attributed to facilitate the strengthening of the matrix, th us leading to crack free sintering of sheets up to 400 degrees C in air and for T > 900 degrees C under nitrogen. A significant portion (> 40%) of the total carbon present in the glasses was found to exist as free carbon. C-1 3 NMR showed that unsaturated C=C bonds exist in gels heat treated at 1000 degrees C, but they disappear in heat treated gels at 1200 and 1450 degrees C. Both Si-29 and C-13 NMR spectra indicate the formation of a ca. 15 atom % SiC phase in the glass sheets sintered at 1450 degrees C, which were foun d to be amorphous up to 1200 degrees C.