Effect of formulations in sol-gel process on monolithicdensity of organic and carbon aerogels from sol-gelpolymerization of phenolic novolak and furfural

摘要

Monolithic density of carbon aerogels can be related to porosity, mechanic and electrical properties of carbon aerogels, and therefore is an important macroscopic property that needs controlling in many applications. In this paper, organic and carbon aerogels are synthesized from sol-gel polymerization of phenolic novolak and furfural, and effect of formulations in sol-gel process on monolithic density of organic and carbon aerogels has been investigated. Volume of gels was recorded in sol-gel, solvent exchange, supercritical drying, and pyrolysis steps by volume measurement. It was found that polymerization of said components led to wet gels with their volume only within 5% less than that of the original solution. After wet gels were exchanged with petroleum ether (30~60°C), slight swelling of gels without exceeding volume of the original solution occurred. The gels were dried in supercritical petroleum ether at 240 °C and 6. 0 Mpa for one hour to obtain organic aerogels. The volume shrinkage from wet gels to organic aerogels increases linearly with increasing the ratios of phenolic novolak to furfural, but is independent of total concentration of reactants. The monolithic density of organic aerogels, p bo, can be fitted by formula as p bo=0.00295+0.018311Ra (R=0.97) , where C is total concentration of reactants and Ra, is the ratios of phenolic novolak to furfural. The organic aerogels were pyrolyzed under protection of nitrogen at 850 °C for three hours to obtain carbon aerogels. The volume shrinkage from organic aerogels to carbon aerogels decreases linearly with increasing the ratios of phenolic novolak to furfural, and is also independent of total concentration of reactants. The monolithic density of carbon aerogels, p bc , can be formulated as p bc/C=0.00484+0.0235Ra (R=0.95) .