Strongly coupled near-field radiative and conductive heat transfer between planar objects

TL;DR

This paper investigates how conductive and radiative heat transfer interact in planar systems, revealing that radiation-induced temperature gradients significantly influence heat transfer behavior at nanometer gaps, especially in materials like silica, sapphire, and metal oxides.

Contribution

It introduces a rigorous and analytical framework to model coupled conduction and radiation in planar geometries, highlighting the impact of temperature gradients on near-field radiative heat transfer.

Findings

Temperature gradients affect RHT depending on material and geometry.

RHT effects are prominent at tens of nanometers gap size.

RHT saturation occurs due to conduction limits at very small gaps.

Abstract

We study the interplay of conductive and radiative heat transfer (RHT) in planar geometries and predict that temperature gradients induced by radiation can play a significant role on the behavior of RHT with respect to gap sizes, depending largely on geometric and material parameters and not so crucially on operating temperatures. Our findings exploit rigorous calculations based on a closed-form expression for the heat flux between two plates separated by vacuum gaps $d$ and subject to arbitrary temperature profiles, along with an approximate but accurate analytical treatment of coupled conduction--radiation in this geometry. We find that these effects can be prominent in typical materials (e.g. silica and sapphire) at separations of tens of nanometers, and can play an even larger role in metal oxides, which exhibit moderate conductivities and enhanced radiative properties. Broadly speaking, these predictions suggest that the impact of RHT on thermal conduction, and vice versa, could manifest itself as a limit on the possible magnitude of RHT at the nanoscale, which asymptotes to a constant (the conductive transfer rate when the gap is closed) instead of diverging at short separations.