Impact of thermal coupling effects on the effective thermal conductivity of aerogels

Abstract

Nanoporous aerogels are excellent thermal insulation materials with thermal conductivities down to about 0.012 W m−1 K−1 at ambient conditions. So far, it was assumed that the total thermal conductivity of aerogels can be described by a simple superposition of the different individual heat transport contributions. However, recent investigations reveal that thermal coupling effects can result in a gas pressure dependent contribution that may be up to three times higher than expected from just a gas phase thermal conductivity, which is predicted by the Knudsen equation at given porosity and pore size. In this study, we use data from previous publications covering a gas pressure range from 10−5 to 10 MPa and analyze systematically the impact of pore size as well as solid phase and gas phase thermal conductivity on the coupling effect. The goal is to evaluate the data with respect to practical implications for aerogels in general. This means using the gas pressure dependence of the thermal conductivity of aerogels to determine their average pore size as well as allowing for a targeted optimization of aerogel-based insulations for applications at given gas pressure and temperature.