Applications of this could be better solar cells, materials that behave like one-way valves for heat flow and heat-based computing platforms.
The heat flow rate between two objects has a limit that depends on details like the size of the objects, the surfaces that are facing one another, their temperatures, and the distance between them. Heat travels between objects as electromagnetic waves, such as infrared radiation and visible light.
Previously, Reddy and Meyhofer had led a study showing that heat can travel 10,000 times faster than expected between objects separated by nanoscale gaps.
They spent many months in the Lurie Nanofabrication Facility making matched pairs of semiconductor rectangular plates but about a thousand times smaller in length and width. The thickness of the rectangular plates was anywhere between 10,000 nanometers (0.01 millimeters) to 270 nanometers. He suspended these on very narrow beams about a hundred times thinner than human hair.
Heat would ordinarily be expected radiate from each of the six sides in proportion to the surface area. Extremely thin structures about half the wavelength of green light—those edges released and absorbed much more heat than anticipated.
After many hours running the model on a supercomputer, Zhu’s results confirmed that the 100-fold enhancement in heat flow occurs because of the way that waves move in the very thin plates. Since the waves run parallel to the plate’s longer dimensions, the heat shoots out the edges. In the identical plate absorbing the energy, the same concept was at work.
Researchers believe they can control 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). 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, for instance.
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.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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