Thermal Conductance of Nanoparticles: A Study of Phonon Transport in Functionalized Nanodiamond Suspensions

Abstract Nanoparticle additives, with their anomalous thermal conductivity, have attracted attention in research and industry as a novel mode of enhancing the heat transfer mediums. Most studies conducted on nanoparticle suspensions in liquids, pastes, or composites at present have relied on constitutive relations using properties of the bulk substance and of the nanoparticle to explain the effective thermal conductivity. In order to utilize nanoparticles in real world engineering applications, chemical functionalization of the surface of the nanoparticle is frequently employed, either to suspend in liquid applications or to stabilize in arrays. In this study, we have sought to explain the underlying mechanisms of thermal conductivity enhancement taking into consideration the nanoscale effects, such as phonon transport in the nanoparticle coupled with vibrational modes of the surface functional molecules, in order to tailor the functional groups not only for suspension stability but also for minimizing Kapitza resistance at the surface of the nanoparticle. Density functional theory simulations in SIESTA and equilibrium transport theory analysis via GOLLUM2 were used in tandem to evaluate the thermal transport at the nanoparticle to surface ligand junction. By treating the nanoparticle surface and the polymer or acid coating as distinct homogeneous substrates, a model for thermal conductivity becomes more tractable.