Determination of the Permeability of Carbon Aerogels by Gas Flow Measurements
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DETERMINATION OF THE PERMEABILITY OF CARBON AEROGELS BY GAS FLOW MEASUREMENTS * F-M.KONG, S.S. HULSEY, C.T. ALVISO, AND R.W. PEKALA Chemistry and Materials Science Department, Lawrence Livermore National Laboratory, Livermore, CA.
ABSTRACT Carbon aerogels are synthesized via the polycondensation of resorcinol and formaldehyde, followed by supercritical drying and pyrolysis at 1050 0 C in nitrogen. Because of their interconnected porosity, ultrafine cell structure and high surface area, carbon aerogels have many potential applications, such as in 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, we calculated the permeability of carbon aerogels from equations based upon Darcy's law. Our measurements show that carbon aerogels have apparent permeabilities on the order of 10-12 to 10-10 cm 2 for densities ranging from 0.44 to 0.05 g/cm 3 . Like their mechanical properties, the permeability of carbon aerogels follows a power law relationship with density and average pore size. Such findings help us to estimate the average pore sizes of carbon aerogels once their densities are known. This paper reveals the relationships among permeability, pore size and density in carbon aerogels. 1.
INTRODUCTION
Carbon aerogels, or specifically, carbonized resorcinol-formaldehyde aerogels (CRF), are one of the unique open-cell foams which we have developed at Lawrence Livermore National Laboratory. In a similar fashion to silica aerogels, the resorcinol formaldehyde aerogels (RF) are synthesized via a sol-gel polymerization, followed by supercritical extraction. The carbon aerogels are produced by heating the RF aerogels at 10500C in nitrogen. A detailed description of their synthesis and chemistry has been reported previously [1]. All the aerogels share some common features: a microstructure composed of interconnected particles or fibrous chains with diameters ranging from 3-20 nm, high surface areas (350-1000 m2/g), ultrafine pore sizes (100 mbar, we could assume from Fricke's analysis that transport in carbon aerogels was predominately controlled by viscous flow. To define the flow velocity regime, one uses Reynold's number, Re: Re = dpf v/r1
(2)
where pf is fluid density, and il is fluid viscosity. If Re
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