Preparation and characterization of C/SiO2/SiC aerogels based on novolac/silica hybrid hyperporous materials

• Sol–gel polymerization in vapor of solvent saturated atmosphere is developed.
• Highly porous carbon-silica-SiC composite aerogels are synthesized by pyrolizing novalac-silica composite aerogels.
• The incorporation of silica resulted in higher porosity and lower density in carbon and polymer aerogel composites.
• Presence of silica decreased the shrinkage during drying stage and increased shrinkage during the ultimate pyrolysing step.
• By adding silica the oxidation temperature increased and the rate of oxidation decreased in carbon composite aerogels.

Novolac/silica hybrid gels (N/SiO2) are synthesized using the sol–gel polymerization method under a solvent-vapor saturated atmosphere, followed by an ambient-pressure drying technique. Moreover, prepared N/SiO2 aerogels are converted to carbon/silica (C/SiO2) and carbon/silica/silicon carbide (C/SiO2/SiC) nanocomposite aerogels through pyrolysis and carbothermal reduction at 800 and 1500 °C, respectively. X-ray diffraction studies reveal the formation of graphite, cristobalite and β-SiC portions in the structure of prepared C/SiO2/SiC aerogels. Mercury porosimetry and scanning electron microscopy results demonstrate that the structural characteristics of fabricated nanocomposite aerogels retained after heat treatment steps. Thermogravimetric analysis and compressive strength measurements are used to investigate thermal and mechanical properties of prepared aerogels, respectively. The maximum oxidation rate temperature increased from 588 °C for carbon aerogel to about 750 °C for C/SiO2/SiC aerogels. The thermal conductivity coefficient of C/SiO2 and C/SiO2/SiC aerogels at room temperature are about 0.058 and 0.121 (W m− 1 K− 1), respectively. Compressive strength of C/SiO2/SiC aerogels is in the range of 1.8–2.3 MPa. This is very impressive as the compressive strength of novolac/silica aerogels is in the range of 0.2–1.4 MPa.