Porosity in Carbons and Graphites

Summary In this Chapter various types of porosity in carbons and graphites are described together with their effects on some important properties, e.g. density and strength. Various methods for characterising micropores, mesopores and macropores are reviewed critically. Surface areas can be estimated from the Brunauer-Emmett-Teller (BET) equation applied to gas adsorption, or from the methods of Debye, Porod and Guinier applied to small angle X-ray scattering, SAXS. Gas adsorption and SAXS can also be used to estimate the fractal dimensions of pore surfaces. The BET equation is severely limited when applied to micropores, because micropore sizes are commensurate with the dimensions of adsorbate molecules. Alternatively, micropore volumes can be obtained from the Dubinin-Radushkevich equation applied to gas adsorption. Mesopore volumes and sizes can be obtained from capillary condensation theory applied to adsorption hysteresis, although the method is subject to limitations, particularly as a result of neglecting of pore interconnections. Mercury porosimetry is used to obtain macropore volumes and sizes, but also neglects pore interconnections. Alternative estimates of macropore sizes are available from measurements of gaseous permeation and diffusion and mercury permeametry. Image analysis can provide measurements such as macropore shape factors and spatial distributions, which cannot be obtained by other methods, but a limitation is that the measurements are obtained from a two-dimensional image of a three dimensional object.