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Measuring Near-Field Radiative Heat Transfer

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A Novel Nanoscale Platform

How heat is transferred via radiation from a hot to a cold surface in the near field, meaning at sub-wavelength distances predicted by Wien's law, differs significantly from radiative transfer in the macroscopic world.

Scientists have predicted that, at room temperature and for objects less than 10 micrometers, near-field heat transfer becomes dependent on the distance between the emitter and receiver. And it is enhanced by orders of magnitude, both for metals and dielectric surfaces, when the gap is reduced to tens of nanometers and below.

This holds the promise of dramatic improvements and new capabilities for micro- and nanoscale devices to efficiently generate energy or to enhance data storage in heat-assisted, magnetic recording, to name two applications.

To begin to leverage near-field radiative transport for such technical advances, it is critical to quantitatively confirm these predictions and to show that similar transport enhancements can be obtained when thin coatings of materials are used. Such coatings are typical in the fabrication of modern micro- and nanoscale devices.

But until now, no investigator had been able to achieve such measurements of near-field radiative heat transfer, owing to a range of extremely technical challenges.

In an article published in March 2015 in Nature Nanotechnology, a team of researchers from the ME department and collaborators from Spain, led by professors Edgar Meyhofer and Pramod Reddy, describe their experiments. They are the first to demonstrate that near field heat transfer across nanoscale gaps is enhanced at the sub-wavelength scale with the use of thin films.

For the experimental measurements, Meyhofer and Reddy conceived of a calorimetry platform to measure heat transfer between a tiny silica sphere -- 50 microns in diameter, or about half the width of a human hair -- that served as an emitter and a similar-sized silicon nitride plate that functioned as a receiver.

The silicon nitride plate was coated with gold or dielectric, insulating silica films of various thicknesses, ranging from 50 nanometers to three micrometers (thinner than the thermal wavelength). The high-resolution heat transfer measurements were made possible because the receiver was suspended by tiny beams (and therefore thermally well isolated) and equipped with an extremely sensitive thermometer, among many other enabling features.

"Scientists have been making theoretical predictions about near-field heat transfer for a long time, but it's only recently that the technology to measure such nanoscale phenomena come into its own to provide us with experimental capabilities and data," said Meyhofer.

"We knew that radiative heat transfer is strikingly different at the nanoscale. One of the most interesting theoretical predictions to us was that heat transfer could be enhanced with thin films. But no one had demonstrated it, so we began to think about how we could test it," he added. 

The platform and technique Meyhofer and Reddy developed overcome several challenges to measuring near-field heat transfer at the nanoscale, including the effects of temperature changes, forces and mechanical motion.

Their system is able to modulate temperature, enabling resolution of heat flow down to about 100 picowatts (one picowatt equals one-trillionth of a watt) for the experimental conditions used in the study. It can measure as little as one picowatt of power, something Meyhofer and Reddy plan to leverage for obtaining biological measurements in the future. 

Findings showed that near-field radiative heat transfer across nanoscale gaps is indeed dramatically enhanced when compared to the far-field situation. Surprisingly, the high heat fluxes observed with bulk silica can also be obtained with thin films, as long as the gap size between the emitter and receiver is reduced to the thickness of the thin film.

Likewise, the researchers also found that the enhancement is less when the gap between microsphere and plate is larger than the thickness of the film.

"It was important to us to study how this heat transfer would work with dieletric materials in a coating thickness that is relevant from a fabrication standpoint," Meyhofer explained.

"Do those thicknesses still support the high energy fluxes one needs for new nanoscale devices? Magically it works, although at first it was not intuitive to us. In fact, the computational work reported in our paper now provides detailed insights into the gap-dependent, radiant heat transfer of the various films," Meyhofer added.

The research effort took five years and is far from over. Still, the experimental platform developed by Meyhofer and Reddy as well as their findings will enable them to conduct more detailed studies of nanoscale near-field heat transfer phenomena, particularly in biological systems, that have not yet been investigated experimentally.

"Our research is motivated by practically-minded ideas," Meyhofer said. "For example, if one has the appropriate materials and/or engineers suitable nano-structured surfaces, one could enable larger heat fluxes from emitters to receivers in desirable spectral windows. That will allow us to more precisely tune and better control all kinds of current and new technologies and devices."