Pyrolysis induced ordering in the preparation of carbon aerogels from sol-gel polymerization of phenolic resole and methylolated melamine

摘要

Sol-gel polymerization of phenolic resole and methylolated melamine in aqueous solution lead to the formation ofrnorganic gels which, after water in the gels being exchanged with acetone, are dried in supercritical petroleum etherrnto produce organic aerogels. Carbon aerogels are formed by pyrolysis of the organic aerogels under protection ofrnnitrogen at 800℃ for three hours. Small- angle X-ray scattering (SAXS) using synchrotron radiation as X-rayrnsource has been employed to characterize nanostructure of organic and carbon aerogels. It is shown that the SAXSrnintensity profile of some carbon aerogels exhibit peaks in the low value of scattering vector, but no peak isrnobserved for that of organic aerogels. Further study show that existence of the scattering peaks of carbon aerogelsrnis dependent on total concentration of reactants. When the total concentration of reactants is low, no peak is found,rnthe scattering peaks appear as total concentration of reactants is increased above 15g/100ml. Nitrogen adsorptionrncharacterization show that mesopore volume shrinkage from organic aerogels to carbon aerogels increases withrnincreasing total concentration of reactants. Based on the fact that not only first order maximum but also secondrnorder maximum is observed in the SAXS intensity profile of carbon aerogels, we conclude that, accompanied byrnchemical reaction in the pyrolysis step, ordering of gel nanostructure occurs as a result of both linear and nonlinearrnnonequilibrium process.