By Josh Perry, Editor
[email protected]

Researchers from the University of Michigan (Ann Arbor, Mich.) and the College of William and Mary (Williamsburg, Va.) have found a surprising heat transfer twist, determining that 100 times more heat can flow between nanoscale objects (even at larger than nanoscale distances) than was predicted by standard radiation theory.

An electron microscope image of the experimental set-up with two plates, each 0.06 x 0.08 mm. At their thinnest, with a thickness of just 0.00027 mm, the heat flow between them was 100 times higher than expected. (Dakotah Thompson, Michigan Engineering)

According to the report from Michigan, the theory of radiation was proposed by Max Planck in 1900 and has been consistent for the past century, until researchers saw a much higher than predicted amount of heat flow between objects that should have been insulated from each other.

Researchers initially questioned the calculations, but they were found to be correct, so a series of experiments were established to test why this was happening.

Matching pairs of semiconductor plates were designed with a thickness of 10,000 nm to 270 nm. The plates were suspended on narrow beams that were 100 times thinner than a human hair.

“In an object the size and shape of a credit card, heat would ordinarily radiate from each of the six sides in proportion to the surface area,” the article explained. “But the team found that when the structures were extremely thin—at the thinnest, about half the wavelength of green light—those edges released and absorbed much more heat than anticipated.”

A mathematical model was created based on the experimental data. The simulations confirmed that 100 times as much heat transfer was occurring. It was determined that, because the heat waves were moving parallel to the plate’s longer dimensions, “heat shoots out the edges.” A similar principle was at work in the plate absorbing the heat.

“Examples [of applications] proposed by the team include controlling the flow of heat in a way similar to how electronics manage electrons, making heat transistors for next-generation computers and diodes (like one-way valves),” the article added. “For example, future building materials could let heat out during cool summer nights but keep it in during the winter. Solar cells could harness the portion of the sun’s spectrum that isn’t converted to electricity for other purposes. A roof installation could send this lost energy to heat water.”

The research was recently published in Nature. The abstract read:

“Radiative heat transfer (RHT) has a central role in entropy generation and energy transfer at length scales ranging from nanometers to light years. The blackbody limit, as established in Max Planck’s theory of RHT, provides a convenient metric for quantifying rates of RHT because it represents the maximum possible rate of RHT between macroscopic objects in the far field—that is, at separations greater than Wien’s wavelength.

“Recent experimental work has verified the feasibility of overcoming the blackbody limit in the near field, but heat-transfer rates exceeding the blackbody limit have not previously been demonstrated in the far field. Here we use custom-fabricated calorimetric nanostructures with embedded thermometers to show that RHT between planar membranes with sub-wavelength dimensions can exceed the blackbody limit in the far field by more than two orders of magnitude. The heat-transfer rates that we observe are in good agreement with calculations based on fluctuational electrodynamics.

“These findings may be directly relevant to various fields, such as energy conversion, atmospheric sciences and astrophysics, in which RHT is important.”