Effect of water vapor on the thermal resistance between amorphous silica nanoparticles

Nanoparticle-based materials are of interest because of their unique thermal properties. Possessing the lowest thermal conductivities of any solid materials known, they have been widely used as insulating materials. However, the presence of water vapor has been shown to have a large influence on those properties. In this work, we investigate the effect of water vapor on the heat transfer between nanoparticles using non-equilibrium molecular dynamics simulations. We calculate the absolute thermal resistance and Kapitza resistance between adjacent amorphous spherical silica nanoparticles, when water molecules are allowed to diffuse as vapor into the interstitial pores between particles. The thermal resistance between nanoparticles is shown to decrease rapidly when water vapor is introduced into the pores between particles. The largest decrease in interparticle resistance occurs as a result of the silanization of the silica particle surfaces. A secondary decrease is attributable to the liquid bridge that forms as water molecules condense around the contact point between nanoparticles. Most of the decrease in resistance between nanoparticles occurs when water vapor is first introduced at relative humidities (rh) of less than 1%. As the relative humidity increases above 1%, the interparticle thermal resistance decreases more slowly, approaching a constant value near 50% rh. Numerical results are compared to experimental measurements of heat transfer across packed beds of 20nm silica nanoparticles exposed to water vapor. The simulation results are shown to be consistent with the experimental measurements for relative humidities below 15% rh, while underpredicting the experimental measurements above 15% rh. Published by AIP Publishing.