Gas permeability of carbon aerogels
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Carbon aerogels are synthesized via the aqueous polycondensation of resorcinol with formaldehyde, followed by supercritical drying and subsequent pyrolysis at 1050 °C. As a result of their interconnected porosity, ultrafine cell/pore size, and high surface area, carbon aerogels have many potential applications such as supercapacitors, battery electrodes, catalyst supports, and gas filters. The performance of carbon aerogels in the latter two applications depends on the permeability or gas flow conductance in these materials. By measuring the pressure differential across a thin specimen and the nitrogen gas flow rate in the viscous regime, the permeability of carbon aerogels was calculated from equations based upon Darcy's law. Our measurements show that carbon aerogels have permeabilities on the order of 10"12 to 10~10 cm2 over the density range from 0.05-0.44 g/cm 3 . Like many other aerogel properties, the permeability of carbon aerogels follows a power law relationship with density, reflecting differences in the average mesopore size. Comparing the results from this study with the permeability of silica aerogels reported by other workers, we found that the permeability of aerogels is governed by a simple universal flow equation. This paper discusses the relationship among permeability, pore size, and density in carbon aerogels.
I. INTRODUCTION Aerogels are a special class of foams with the following characteristics: a tortuous open-cell structure, an ultrafine cell/pore size ( 1, the flow is dominated by molecular diffusion; if k = 1, transport is controlled by viscous and molecular flow. In this study, we used nitrogen gas at room temperature and atmospheric pressure. Given A = 66 nm and d ~ 20 nm from gas adsorption data, k is calculated to be 3.3. Based on k, nitrogen flow through carbon aerogels would appear to be controlled by both viscous and molecular diffusion. Stumpf et al, however, revealed that nitrogen gas in a base-catalyzed silica aerogel sample with a density of 0.280 g/cc changed from viscous to molecular flow only at mean pressures less than 100 mbar.13 Since the mean pressures used in this study were §>100 mbar, we assumed that transport was predominately controlled by viscous flow. To describe the fluid flow behavior in porous media, one uses Reynold's number, Re:
V
V
Re =
QO valve N 2 out FIG. 2. Schematic diagram of the permeability apparatus.
In an ordinary measurement, bottled dry nitrogen gas was introduced via an inlet needle valve into the sample chamber, through the sample, and then out to the atmosphere through an outlet needle valve. The inlet gas pressure was maintained constant at 1, 2, 4, and 8 psig. By adjusting the inlet and outlet valves, the flow rate and the pressure drop were measured after reaching steady state flow, typically after 30 min to several hours. Only small increments of pressure drop could be applied during adjustment to avoid breaking the samples. To minimize errors, we checked and confirmed that the pressure drop caused by the apparatus was insignificant
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