Van der Waals force assisted heat transfer for vacuum gap spacings

TL;DR

This paper explores how Van der Waals forces enable phonons to transfer heat across vacuum gaps, extending existing models to different materials and comparing results with air conduction for nanostructure applications.

Contribution

It provides a theoretical extension for phonon transmission modeling across vacuum gaps between dissimilar materials, highlighting the role of Van der Waals forces in heat transfer.

Findings

Phonon transmission can occur across vacuum gaps via Van der Waals interactions.

Heat transfer through Van der Waals-assisted phonons can surpass air conduction in small gaps.

Material properties significantly influence the magnitude of phonon-mediated heat transfer.

Abstract

Phonons (collective atomic vibrations in solids) are more effective in transporting heat than photons. This is the reason why the conduction mode of heat transport in nonmetals (mediated by phonons) is dominant compared to the radiation mode of heat transport (mediated by photons). However, since phonons are unable to traverse a vacuum gap (unlike photons) it is commonly believed that two bodies separated by a gap cannot exchange heat via phonons. Recently, a mechanism was proposed by which phonons can transport heat across a vacuum gap - through Van der Waals interaction between two bodies with gap less than wavelength of light. Such heat transfer mechanisms are highly relevant for heating (and cooling) of nanostructures; the heating of the flying heads in magnetic storage disks is a case in point. Here, the theoretical derivation for modeling phonon transmission is revisited and extended to the case of two bodies made of different materials separated by a vacuum gap. Magnitudes of phonon transmission, and hence the heat transfer, for commonly used materials in the micro and nano-electromechanical industry are calculated and compared with the calculation of conduction heat transfer through air for small gaps.