Shape-Independent Limits to Near-Field Radiative Heat Transfer

TL;DR

This paper establishes fundamental, shape-independent limits on near-field radiative heat transfer between bodies, revealing potential for significant enhancement over existing structures and approaching conductive heat transfer levels.

Contribution

It generalizes the black body concept to near-field energy transfer, deriving bounds based on material susceptibility and near-field photon transfer, and compares these limits with common structures.

Findings

Dipole and dipole-plate structures approach the limits but do not fully reach them.

Particle arrays can potentially achieve the bounds, indicating orders-of-magnitude improvements.

Radiative heat transfer could match or exceed conductive heat transfer at room temperature.

Abstract

We derive shape-independent limits to the spectral radiative heat-transfer rate between two closely spaced bodies, generalizing the concept of a black body to the case of near-field energy transfer. Through conservation of energy and reciprocity, we show that each body of susceptibility $\chi$ can emit and absorb radiation at enhanced rates bounded by $|\chi|^2 / \textrm{Im} \chi$, optimally mediated by near-field photon transfer proportional to $1/d^2$ across a separation distance $d$. Dipole--dipole and dipole--plate structures approach restricted versions of the limit, but common large-area structures do not exhibit the material enhancement factor and thus fall short of the general limit. By contrast, we find that particle arrays interacting in an idealized Born approximation (i.e., neglecting multiple scattering) exhibit both enhancement factors, suggesting the possibility of orders-of-magnitude improvement beyond previous designs and the potential for radiative heat transfer to be comparable to conductive heat transfer through air at room temperature, and significantly greater at higher temperatures.