GAS-PERMEABILITY OF CARBON AEROGELS

Carbon aerogels are synthesized via the aqueous polycondensation of re sorcinol with formaldehyde, followed by supercritical drying and subse quent pyrolysis at 1050-degrees-C. As a result of their interconnected porosity, ultrafine cell/pore size, and high surface area, carbon aer ogels have many potential applications such as supercapacitors, batter y electrodes, catalyst supports, and gas filters. The performance of c arbon aerogels in the latter two applications depends on the permeabil ity or gas flow conductance in these materials. By measuring the press ure differential across a thin specimen and the nitrogen gas flow rate in the viscous regime, the permeability of carbon aerogels was calcul ated 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/cm3. Like many other aer ogel properties, the permeability of carbon aerogels follows a power l aw relationship with density, reflecting differences in the average me sopore size. Comparing the results from this study with the permeabili ty of silica aerogels reported by other workers, we found that the per meability of aerogels is governed by a simple universal flow equation. This paper discusses the relationship among permeability, pore size, and density in carbon aerogels.