Characterization of SiO 2, RF and carbon aerogels by dynamic gas expansion

Abstract

Three different types of aerogel, i.e., silica (SiO 2), resorcinol-formaldehyde (RF) and carbon aerogels were investigated with respect to skeletal density, gas transport and adsorption at room temperature using a new dynamic gas expansion (DGE) method. The skeletal densities determined by DGE for the SiO 2, RF and carbon aerogels are (2000–2600 ± 200), (1550 ± 25) and (2190 ± 25) kg/m 3, respectively. These values are about a factor of 1.3 higher than the skeletal densities derived from liquid nitrogen (LN 2) adsorption or scattering data. This effect is probably due to micropore adsorption. For all aerogels investigated, the He and N 2 transport studied in the pressure range below 0.1 MPa was found to be dominated by molecular diffusion. While for virgin and sintered SiO 2 samples the diffusion coefficients, D, scale with density as ρ −0.9 and ρ −1.28, the scaling exponent derived for both RF and carbon aerogels is about −1.4. The monolayer adsorption of the samples deduced from N 2-DGE is characterized by a monolayer capacity, n m, of (1.5 ± 0.5) mmol/g and Langmuir parameters, B, of (3.2 ± 1.0) × 10 −7 and (18 ± 8) × 10 −7 Pa −1 for the RF and the carbon aerogels, respectively. Investigation of one of the RF samples by CO 2-DGE yields n m = 1.6 mmol/g and B = 45 × 10 −7 Pa −1. In sintered SiO 2 aerogels the ratio n m B (S/m) , with S/ m the specific surface area, was found to take a common value for all samples of 5.2 × 10 −10 mmol/(Pa m 2).