Silicon oxycarbide glasses

The first attempts to introduce carbon into glass date back to 1951. But up until recently, the use of carbon or carbide raw materials, and the oxidat ion, volatilization and decomposition that accompany high temperature melti ng, have limited the synthesis of true silicon oxycarbide glasses. Here, th e term silicon-oxycarbide refers specifically to a carbon-containing silica te glass wherein oxygen and carbon atoms share bonds with silicon in the am orphous, network structure. Thus, there is a distinction between black glas s, which contains only a second-phase dispersion of elemental carbon, and o xycarbide glasses which usually contain both network carbon and elemental c arbon. In addition to exploring the unique properties and applications of t hese glasses, per se, they are also of interest for developing models of th e residual amorphous phases in polymer-derived silicon-carbide and silicon- nitride ceramics. The application of sol/gel techniques to glass synthesis has significantly advanced the development and characterization of silicon oxycarbide glasses . In this approach, alkyl-substituted silicon alkoxides, which are molecula r precursors containing oxygen and carbon functionalities on the silicon, c an be hydrolyzed and condensed without decomposition or loss of the carbon functional group. A low-temperature (< 1000 degrees C) heat-treatment of th e gel creates a glassy silicate material whose molecular structure consists of an oxygen/carbon anionic network. In addition, there is always a blacke ning of the material due to elemental carbon, which forms during pyrolysis and densification of the gel. The nature of the network carbon, and especia lly the distribution and form of the elemental carbon, are fundamental to t he structure and properties of these novel materials. Their chemical and ph ysical characteristics as revealed by NMR, Raman and TEM are discussed in t he overview. In addition, the high temperature stability of these glasses ( up to 1750 degrees C), and the effect of hot-pressing, are described. It will be shown that the silicon oxycarbide network is stable up to 1000-1 200 degrees C. The network carbon is terminated with hydrogen (i.e., CH, =C H2 and -CH3), and with polyaromatic carbon (i.e., nC(6)Hx) wherein most of the elemental carbon resides. These glasses can be described as molecular c omposites of polyaromatic graphene-rings dispersed in a silicon oxycarbide network. After heating to temperatures in excess of 1000-1200 degrees C, th e oxycarbide network decomposes through the loss of hydrogen, and a two- or three-phase glass-ceramic consisting of nanocrystalline graphite, silicon carbide, and amorphous silica or cristobalite, is created. Some of the prop erties and applications of these glasses/glass-ceramics for coatings, compo sites and porous solids are summarized.