Breaking the Law at the Nanoscale
When objects get very close, Planck’s law is violated.
Any time you see a piece of metal glowing red-hot, or turning yellow or white as it gets hotter, you’re watching Planck’s law in action. The century-old principle, which describes how energy is radiated from an idealized nonreflective black object, applies to everything from a cast-iron frying pan to the surface of a star. But it turns out to have a loophole.
Planck’s law says that thermal emission of radiation at different wavelengths follows a precise pattern that varies according to the temperature of the object. When the German physicist Max Planck proposed the law, in 1900, he suspected that it wouldn’t apply when two objects were very close together. But it took until this year to prove his hunch, because keeping objects that close without letting them touch is a major challenge. Now MIT researchers have shown that heat transfer between objects a few nanometers apart can be three orders of magnitude greater than the law would predict.
Professor of power engineering Gang Chen and his team, graduate student Sheng Shen and Columbia University professor Arvind Narayanaswamy, PhD ‘07, described how they did it in a paper last summer in the journal Nano Letters. “If we use two parallel surfaces, it is very hard to push to nanometer scale without some parts touching each other,” Chen explains. Instead, they used a small, round glass bead next to a flat surface. The objects came closest to touching at just one point, making the separation much easier to maintain. The researchers were able to test separations as small as 10 nanometers.
The findings could lead to new kinds of photovoltaic devices for harnessing photons emitted by a heat source, making it possible to harvest energy from heat that would otherwise be wasted. They could also be useful in magnetic data-recording systems such as computer hard disks, where the space between the recording head and the disk surface is typically in the range of five to six nanometers. The head tends to heat up, and researchers have been looking for ways to manage the heat or even exploit it. For example, some recording materials need to be heated, usually with a laser beam, before their surfaces can be magnetized by the head. If researchers understand how heat transfer works at these distances, they might be able to design a way for the head to provide its own heating.
Further work is still needed to explore what happens at even smaller distances, Chen says, because the researchers don’t know exactly how much heat can be dissipated in closely spaced systems.
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