High thermal conductivity in polaritonic SiO2 nanoparticle beds

Recent theoretical works predict the ability of surface phonon polaritons to increase thermal conduction along nanoparticle chains, but measurement of this effect has proven elusive. Here we demonstrate a new approach to observe thermal conduction by surface phonon polaritons - packed beds of SiO2 nanoparticles. When we modify the interstitial material with adsorbed water or a coating of ethylene glycol, we experimentally resolve thermal conductivities as high as 1.5 and 18 times the phonon value, respectively. Although existing models do not fully explain our results, we develop a new scaling relation that suggests thermal conduction is primarily dependent on the 3D density of states. An examination of classical heat transport mechanisms shows that these are unlikely sources for the observed thermal conductivities, suggesting that surface phonon polaritons play an observable role in SiO2 nanoparticle bed heat transfer properties.