Molecular dynamics simulation study of thermal resistance between amorphous silica nanoparticles under vacuum and in the presence of water vapor
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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 in a variety of macroscale and microscale applications. The present work focuses on two main aspects of thermal transport between amorphous silica nanoparticles. The first area of focus was to investigate the effect of interfacial force strength on thermal transport between amorphous silica nanoparticles under vacuum. Using non-equilibrium molecular dynamics (NEMD) simulations, we calculate the total thermal resistance and thermal boundary resistance between adjacent silica nanoparticles. Numerical results are compared to interparticle resistances determined from experimental measurements of heat transfer across packed silica nanoparticle beds. The thermal resistance between nanoparticles is shown to increase rapidly as the particle contact radius decreases. More significantly, the interparticle resistance depends strongly on the forces between particles, in particular, the presence or absence of chemical bonds between nanoparticles. In addition, the effect of interfacial force strength on thermal resistance increases as the nanoparticle diameter decreases. The simulations results are shown to be in good agreement with experimental results for 20 nm silica nanoparticles. The second area of focus was to investigate the effect of water vapor on thermal transport between amorphous silica nanoparticles. Using NEMD simulations, we calculate the total thermal resistance and thermal boundary resistance between adjacent 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. Most of the 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. Numerical results are compared to experimental measurements of heat transfer across packed beds of 20 nm 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.