Skin formation on RF aerogel sheets

Aerogel sheets a few hundred mum thick were made from fiber-reinforced resorcinol-formaldehyde (RF) gels with a high molar resorcinol/catalyst (R/C) ratio and a target density of 300 kg/m(3). The obtained gels were ambient pressure dried without noticeable shrinkage. The bulk of these samples show a coarse structure with micron-sized particles and pores while on the sheets' surface a nanostructured skin with a thickness of a few microns formed. The surface skill was peeled off the aerogel sheet. In addition, a non-reinforced aerogel film with the same thickness as the peeled-off skin was made. Both were investigated via small angle X-ray scattering (SAXS) and atomic force microscopy (AI;M). It was found, that the surface skin on a Novoloid-fiber-reinforced RF aerogel possesses a higher density and smaller entities than the non-reinforced RF aerogel film. This is explained by a local superposition of the catalytic influence of the fibers and the sol-mold interface. If pyrolyzed, the skin covered aerogel sheets are most suitable e.g., as gas diffusion electrodes in PEM fuel cells. (C) 2001 Elsevier Science B.V. All rights reserved.