Radiative heat transfer in the extreme near field

Condensed Matter Physics Centre at the Universidad Autónoma de Madrid (IFIMAC-UAM)
  • Researchers dispelled doubts regarding a theory key in nanotechnology research and develop.
  • Heat transfer between surfaces of the tested materials exhibit dramatic enhancements when their separation is reduced to only a few nanometers
  • Established a firm basis for the future design of novel technologies making use of nanoscale radiative heat transfer.

 

About radiative heat transfer:

Radiative heat transfer entails, as the wording indicates, the transfer of heat via radiation. Radiative heat transfer between objects at different temperatures is of fundamental importance in applications such as energy conversion, thermal management, lithography, data storage, and thermal microscopy.

In these contexts, the thermodynamic concept of black body takes practical importance. The black body represents an idealized object with the characteristic of absorbing all incident electromagnetic radiation (which includes visible light) completely. The black body also emits radiation in a way that is directly dependent on its temperature. Relatedly, the law of Stefan-Boltzmann for black bodies describes this radiation emission mathematically, setting a theoretical limit on the radiation that can be emitted.

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Steel foundry. Metal at high temperature emits radiation of a wavelength dependent on its temperature.

In the past, it was predicted that when the separation between two objects is small enough, the radiative heat transfer could be greatly enhanced above the theoretical limit set by the Stefan-Boltzmann law. If the distance is smaller than the “thermal wavelength” (in the order of 10 µm at room temperature), such heat transfer would be greatly increased. This enhancing effect is possible due to the contribution of waves that concentrate around the surface of the radiation-emitting bodies. That is, around the “near field” region of the electromagnetic field around the object. Such waves, also called evanescent waves, decay strongly with increasing distance.

 

Heat transfer predictions translate into reality:

In recent years, different experimental studies have confirmed the enhanced heat transfer predictions. Despite that, experiments exploring the radiative thermal transport in nanometric gaps have seriously questioned the understanding of how thermal radiation works at the nanoscale level.

As a result, doubts arose about the validity of the theory of fluctuational electrodynamics, the standard theory for the description of the phenomenon of near-field radiative heat transfer (NFRHT). NFRHT is a form of heat transfer that takes place at a very close distance to the emiting body: a distance smaller than λ/2π, being λ the wavelength of the radiation. NFRHT has enormous repercussions in Nano devices in which heat-transfer plays a significant role.

The doubts were resolved in a work published in Nature by a collaboration between the groups of Pramod Reddy and Edgar Meyhofer (University of Michigan), the IFIMAC researchers Víctor Fernández-Hurtado, Johannes Feist, Francisco J. García-Vidal and Juan Carlos Cuevas, and Homer Reid (Massachusetts Institute of Technology).

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Calculated distribution of radiative heat transfer between a silica AFM tip and silica substrate separated by a few nanometers.

The authors used a novel type of scanning thermal probe. The instrument was equipped with embedded thermocouples, that are devices used as high-precision temperature sensors. Measurements of NFRHT between different materials with separations as small as 2 nm were undertaken. It was shown how heat transfer between silica-silica (SiO2), silicon nitride-silicon nitride (Si3N4) and gold-gold (Au) surfaces exhibit dramatic enhancements of such heat transfer when the gap is reduced down to only a few nanometers.

 

Aftermath of the experiments:

After the experiments, state-of-the-art simulations using the framework of fluctuational electrodynamics were able to reproduce all the experimental observations flawlessly. By that reason, the results as a whole unambiguously demonstrated that the theory of fluctuational electrodynamics based on Maxwell’s equations remains valid. It provides an accurate description of the NFRHT between both metals and dielectrics all the way down to nanometer-size gaps.

This work by IFIMAC clarifies the fundamental mechanisms that govern the radiative heat transfer at the nanoscale and establishes a firm basis for the future design of novel technologies that make use of nanoscale radiative heat transfer.

 

Image credits:

Steel works picture is in the public domain and was downloaded from Pxhere.

Atomic Force Microscopy (AFM) picture was kindly provided by IFIMAC.

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