On the solenoidal heat-flux in quasi-ballistic thermal conduction

TL;DR

This paper derives a coordinate-invariant form of the phonon heat-flux equation, revealing a solenoidal component that is significant in quasi-ballistic thermal transport, especially in silicon membranes at room temperature.

Contribution

It introduces a novel decomposition of heat-flux into solenoidal and irrotational parts using a second-order spherical harmonic expansion, advancing understanding of phonon transport.

Findings

Identification of the solenoidal heat-flux component in phonon transport.

Explicit demonstration of solenoidal heat-flux in a right-circular cylinder.

Relevance to phonon resonators and quasi-ballistic thermal conduction in silicon.

Abstract

The Boltzmann transport equation for phonons is recast directly in terms of the heat-flux by means of iteration followed by truncation at the second order in the spherical harmonic expansion of the distribution function. This procedure displays the heat-flux in an explicitly coordinate-invariant form, and leads to a natural decomposition into two components, namely the solenoidal component in addition to the usual irrotational component. The solenoidal heat-flux is explicitly shown to arise by applying the heat-flux equation to a right-circular cylinder. These findings are important in the context of phonon resonators that utilize the strong quasi-ballistic thermal transport reported recently in silicon membranes at room temperature.