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

Abstract

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 °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 °C. TG analysis showed that the dense glasses with a carbon content < 10 wt.% are stable up to 1000 °C in air. Heat treatment at 1300–1400 °C renders the glasses porous and instable against oxidation by air. On heating above 1400 °C, the SiO 2 crystallizes and converts into SiC by a carbothermal reduction. At 1600 °C, the glass with a molar ratio C Si = 3 has transformed into SiC crystallites with grain sizes of 1 μm and less.