| This thesis describes the characterization and design of the chemical structure, porosity, and surface chemistry of silicon oxycarbides.; The first successful synthetic approach to reduce the free carbon content of oxycarbide glasses is described. The organic groups were used to bridge the network by initiating low-temperature crosslinking reactions with reactive Si-H bonds introduced in the precursors. The reduced Si-C bond cleavage in the more crosslinked network reduces the free carbon content of the glasses. The structure-development and SiC-crystallization in these glasses at 900{dollar}spcirc{dollar}C-1500{dollar}spcirc{dollar}C is also described.; The effect of organic-modification on surface area and porosity was investigated. Thermally stable porous gels and glasses were synthesized through controlled carbon-incorporation, and optimized processing conditions. Carbon's role in the enhanced thermal stability was established through comparisons with pure silica and with oxycarbide glasses where all the carbon groups were removed through low-temperature plasma-oxidation treatments. Carbon's effect is discussed in terms of its effects on viscous sintering kinetics, viscous flow, and most importantly, the surface chemistry.; The surface chemistry of porous gels/glasses was investigated using in situ FTIR spectroscopy. Dramatic reductions in surface Si-OH concentration occur upon bulk organic modification. Preferential presence of a majority of the methyl groups on the oxycarbide surface is indicated. Between 80{dollar}spcirc{dollar}C-700{dollar}spcirc{dollar}C, surface Si-OH groups condense to form strained siloxane sites, while the Si-CH{dollar}sb3{dollar} groups are largely unaffected. The 700{dollar}spcirc{dollar}C-oxycarbide surface contains methyl groups, siloxane sites and a low concentration of Si-OH groups. Decomposition of the methyl groups correlated with the formation of Si-CH{dollar}sb2{dollar}-Si linkages, Si-H bonds, allene sites, and aromatic carbon occurs from 700{dollar}spcirc{dollar}C-900{dollar}spcirc{dollar}C. High surface area hydroxyl-free silicon oxycarbide glasses are obtained after pyrolysis at 800-900{dollar}spcirc{dollar}C under argon.; The methyl-dominated 700{dollar}spcirc{dollar}C-oxycarbide surfaces display reversible physisorption of water, while the hydroxyl-free 850/900{dollar}spcirc{dollar}C-oxycarbide surfaces are hydroxylated upon exposure to water and form surface Si-OH groups. This is attributed mainly to the increased accessibility of water to the strained siloxane sites after decomposition of the surface methyl groups. The thermal evolution of oxycarbide surface chemical structure is described. A better understanding of carbon's role in the enhanced thermal stability of the surface area is obtained. |
