Formation of black glasses and silicon carbide from binary carbonaceous/silica hydrogels

Binary hybrid gels consisting of a carbonaceous hydrogel and a silica gel were prepared as preceramic materials. The pyrolytic conversion into carbon-containing glasses under inert conditions were followed by TG analysis, FTIR spectroscopy and nitrogen adsorption. On pyrolysis up to 700 degrees C, the functional groups of the carbonaceous gel constituent were destroyed and the porosity of the binary gels is nearly completely reduced. Black glasses consisting of amorphous silica and carbon are formed between 700 and 1400 degrees C. TG analysis showed that the dense glasses with a carbon content <10 wt.% are stable up to 1000 degrees C in air. Heat treatment at 1300-1400 degrees C renders the glasses porous and instable against oxidation by air. On heating above 1400 degrees C, the SiO2 crystallizes and converts into SiC by a carbothermal reduction. At 1600 degrees C, the glass with a molar ratio C/Si=3 has transformed into SiC crystallites with grain sizes of 1 mu m and less.