Paper published in Journal of Applied Physics: “Minimum thermal conductivity considerations in aerogel thin films”

Our paper, “Minimum thermal conductivity considerations in aerogel thin films,” was recently published in the Journal of Applied Physics (J. Appl. Phys. 111, 113532 (2012)).  In this work, we show that time domain thermoreflectance can be used to measure the thermal conductivity of the solid network of highly porous aerogel films.  Following in depth analysis of the measurement theory of TDTR applied to porous films, we measure the thermal conductivity of thin aerogel films with ~10 and 15% porosity.  We show that at room temperature, the thermal conductivity of aerogel films scales with porosity as predicted by differential effective medium theory.  We develop a modification to the minimum limit to thermal conductivity that accounts for porosity and agrees with our aerogel data, along with other data on fully dense and porous silica structures well.  This was was performed in collaboration with Professors Bryan Kaehr and Jeffrey Brinker at University of New Mexico and Sandia National Laboratories along with Ed Piekos at Sandia National Laboratories.

Abstract

We demonstrate the use time domain thermoreflectance (TDTR) to measure the thermal conductivity of the solid silica network of aerogel thin-films. TDTR presents a unique experimental capability for measuring the thermal conductivity of porous media due to the nanosecond time domain aspect of the measurement. In short, TDTR is capable of explicitly measuring the change in temperature with time of the solid portion of porous media independently from the pores or effective media. This makes TDTR ideal for determining the thermal transport through the solid network of the aerogel film. We measure the thermal conductivity of the solid silica networks of an aerogel film that is 10% solid, and the thermal conductivity of the same type of film that has been calcined to remove the terminating methyl groups. We find that for similar densities, the thermal conductivity through the silica in the aerogel thin films is similar to that of bulk aerogels. We theoretically describe the thermal transport in the aerogel films with a modified minimum limit to thermal conductivity that accounts for porosity through a reduction in phonon velocity. Our porous minimum limit agrees well with a wide range of experimental data in addition to sound agreement with differential effective medium theory. This porous minimum limit therefore demonstrates an approach to predict the thermal conductivity of porous disordered materials with no a priori knowledge of the corresponding bulk phase, unlike differential effective medium theory.

This work was funded by NSF (CBET Award #1134311)