Relationship between pore size and the gas pressure dependence of the gaseous thermal conductivity

In porous materials, such as foams or pressed powder boards, the thermal conductivity via the gas phase represents at ambient temperatures a significant or even dominant contribution to the total thermal conductivity. The variation of the thermal conductivity of an open porous material with gas pressure is a function of the pore size. In the pressure range below 1 bar, measurement of thermal conductivity offers a non-destructive probe of pore sizes larger than 100 nm. If the thermal conductivity setup also allows for measurements at higher pressures, the pore size range can be extended to even smaller values.Experimental data for a variety of different porous materials such as open porous foams, granular porous and nonporous materials and aerogels, porous solids with an adjustable mean pore size in the meso- or macro-pore range, are presented. The pore characteristics determined from thermal conductivity measurements are compared to morphological data derived from nitrogen sorption and small angle X-ray scattering data.The potential and limits of this method in terms of the extraction of an average pore size and a pore size distribution are discussed.