Dynamic gas flow measurements on aerogels

Abstract

Diffusivities, pore volumes and effective pore radii of porous systems can be determined using dynamic gas flow measurements. All pores are detected in scattering investigations, but gas flow measurements are only sensitive to open pores. The latter are the relevant ones in filter or catalyst applications. A base-catalyzed silica aerogel monolith ( ρ = 280 kg/m 3 , specific surface = 430 m 2 /g, size 45×30×30 mm 3 ) was investigated by a dynamic gas flow method. At ambient temperature and different average pressures, P av (10 mbar ≤ P av ≤ 1000 mbar), small pressure pulses were applied to the specimen. The dependence reflected in the diffusion constant, D , can be used to determine an effective capillary radius, R eff . Different gases were used to check the D ( M -relation ( M = molar mass of the gases) expected for the diffusivities, D , at low pressure and to distinguish between free diffusion and adsorption or surface diffusion, respectively. The measured diffusion constant for helium was found to increase from 7.5×10 −6 m 2 /s to 11×10 −6 m 2 /s with increasing average pressure. D -values for trifluoromethane, the heaviest gas used, were about 1/5 that of He. The D ( p av )-dependence was found to be in good agreement with theory. Detailed evaluation of the data shows that, even at ambient temperatures, adsorption processes may be significant. Therefore, an effective capillary radius, R eff , could only be reliably derived from the helium data. R eff is in good agreement with results from X-ray scattering investigations.