Coupling of gaseous and solid thermal conduction in porous solids

Abstract

Porous material systems are commonly used for thermal insulation due to their low total effective thermal conductivities. However, the prediction of this physical property is difficult for some porous materials, since there is no straightforward relation to describe the impact of coupling between solid and gaseous thermal conduction. Thus, the aim of this work was to get a better general understanding of the coupling effect. For this purpose, different carbon and organic (resorcinol-formaldehyde) aerogels were used as model system with high and low intrinsic backbone conductivity and were investigated by means of gas-pressure dependent hot-wire measurements in different gas atmospheres (Ar, N2, He and CO2) up to 10MPa. The thermal conductivity data received for all samples analyzed show a significant amount of thermal coupling between gas and solid phase: The experimental gas-pressure dependent thermal conductivities are up to a factor of 3.3 higher than the expected pure gaseous thermal conductivities.A simple theoretical approach (“scaling model”) is introduced providing a good description of the experimental gas-pressure dependent thermal conductivity data. Thus, this model is useful for easily estimating the mean pore sizes of porous samples. Moreover, applying the scaling model to the helium data reveals that the accommodation coefficient of helium must be close to one for all aerogel samples investigated.Furthermore, the coupling term for a simple layer system is analytically described by a series connection of the thermal resistances of the two phases, in order to analyze the impact of different parameters on the coupling term: coupling increases with the ratio of solid phase thickness to mean pore size and with the ratio of solid to gaseous thermal conductivity. Overall, the theoretical results are consistent with the experimental results.