Structural evolution in carbon aerogels as a function of precursor material and pyrolysis temperature

Several organic reactions that proceed through a sol-gel transition have been identified at LLNL. The most-studied reaction involves the aqueous polycondensation of resorcinol (1,3-dihydroxybenzene) with formaldehyde. Recently, we have shown that phenol can be added to this polymerization as a comonomer. The resultant crosslinked gels are supercritically dried from carbon dioxide (T-c = 31 degrees C, P-c = 7.4 MPa) to give resorcinol-phenol-formaldehyde (RPF) aerogels. Because RPF aerogels are composed of a highly crosslinked aromatic polymer, they can be pyrolyzed in an inert atmosphere to form vitreous carbon monoliths (CRPF). The resultant aerogels are black in color and no longer transparent, yet they retain the high porosity (40-98%), ultrafine cell/pore size (< 50 nm), high surface area (600-800 m(2)/g), and interconnected particle (similar to 10 nm) morphology of their organic precursors. In this study, we examine the acoustic and mechanical properties of these materials as a function of precursor material and pyrolysis temperature. It is shown that the elastic moduli of RPF and CRPF is higher than that of pure RF/CRF aerogels at a given density. Upon pyrolysis RPF aerogels tend to shrink to a larger extent.