Bridging conduction and radiation : investigating thermal transport in nanoscale gaps

Abstract

Near field radiation transfer between objects separated by small gaps is a widely studied field in heat transfer and has become more important than ever. Many technologies such as heat assisted magnetic recording, aerogels, and composite materials with interfacial transport involve heat transfer between surfaces with separations in the nanometer length scales. At separations of only a few nanometers, the distinction between classical thermal conduction and thermal radiation become blurred. Contact thermal conduction is understood through the means of interfacial transport of phonons, whereas thermal radiation is understood by the exchange of heat through the electromagnetic field. Typically conductance values in the far field radiation regime are on the order of 5 W/m²K, whereas contact conductance is on the order of 108 W/m²K. While near field radiation experiments have reached separations down to on the order of 10 nm and measured 10⁴ W/m²K, there are still 4 orders of magnitude change that occurs over 10 nm of separation. However to this day, there does not exist a single unified formalism that is able to capture the relevant physics at finite gaps all the way down to the contact limit. The success of the continuum electromagnetic theory with a local dielectric constant has allowed accurate modeling of thermal transport for materials separated by tens of nanometers. The validity of this approach breaks down at the contact limit as the theory predicts diverging thermal conductance. The nonlocal dielectric constant formalism has successfully been applied to correct this error and predict transport at nanometer separations for metals and nanoparticles. However, success has been limited for deriving nonlocal dielectric constants for insulators as it is both theoretically and computationally more challenging and requires accurate atomic modeling to retrieve a valid continuum dielectric that reproduces the response of the system. In this work, the continuum approach is avoided and an approach is taken which more closely resembles the conduction picture, by performing atomistic modeling of the thermal transport between two semi-infinite media. The interatomic forces of both short-range chemical bonding forces and long ranged electromagnetic forces are included in an atomistic Green's function formalism in order to accurately calculate thermal transport at finite gaps down to the contact limit. With a single, unified formalism the bridge between conduction and radiation is finally achieved.