SURFACE-CHEMISTRY AND STRUCTURE OF SILICON OXYCARBIDE GELS AND GLASSES

In this study, the surface chemistry and structure of methyl-substitut ed silica gels and porous oxycarbide glasses were investigated. FTIR w as used to measure the relative concentration of Si-CH3 and Si-OH as a function of the degree of methyl-substitution and the pyrolysis tempe rature. The gels and glasses were further heated, dehydrated or hydrat ed, in situ, within the FTIR spectrometer. In the temperature range of 800-850 degrees C, high surface area oxycarbide glasses were created with no detectable surface hydroxyl groups. Oxycarbide glasses synthes ized in argon at 700 degrees C displayed a weak band for surface hydro xyl groups and reversible physisorption of water, while those synthesi zed at 850/900 degrees C showed a complete absence of surface hydroxyl groups and the formation of vicinal silanols upon chemisorption of wa ter. Isolated silanols were observed upon heat treatment in vacuum. Fo rmation of aromatic carbon species was found to correlate with the dec omposition of the methyl groups. The oxycarbide surface is quite stabl e to densification (presumably due to elemental carbon on the pore sur faces). In the absence of oxygen, porous silicon oxycarbide glass powd ers maintain surface areas >200 m(2)/g at 1200 degrees C. However, oxi dizing species in the atmosphere deplete the aromatic carbon species, and the glasses lose surface area.