One of the hidden heroes of the modern world is heat transfer. Sucking the heat out of modern electronics is a technical feat all by itself. And we've tackled even stickier situations, like cooling objects in the vacuum of space.

Heat transfer is governed by laws that have been tested for over a century. These elderly laws are mostly undefeated and undefeatable. But there are a few exceptions. And researchers have now found another exception: beating the limits of radiative heat transfer.

So hot it glows

In electronics, heat can be conducted away to large surfaces, where convection can take care of the rest. In space, there is no convection; everything has to be radiated. The nice feature about radiative heat transfer (often referred to as "blackbody radiation") is that it is radiative. Heating and cooling can take place over long distances—light from the Sun warms the Earth through radiative heat transfer.

A less nice feature of radiative transfer is its efficiency. The temperature difference between the heat source and the heat sink has to be quite large before any reasonable amount of energy is transferred. In most cases, radiative heat transfer is negligible, but as soon as the temperature goes up substantially, it dominates conduction and convection.

Two of the assumptions used to obtain the rules of blackbody radiation provide loopholes. It was assumed that the radiating and absorbing body were far away from each other compared to the wavelength of the light transferring the heat and that the source and sink were also large compared to the wavelength.

Hey, Tiny, are you packing heat?

A more detailed exploration showed that if either of these two conditions were violated, radiative heat transfer could be made more efficient. In particular, the new theory predicted that there might not even be a limit for heat transfer from objects that were physically smaller than the wavelength of light they radiate. That seems pretty cool (or hot, depending on the perspective).

Previous work had shown that if the source and sink were close to each other, radiative heat transfer could significantly exceed the limits of blackbody radiation. However, all experiments on small emitters and receivers had failed to do so. In the latest installment of the story, researchers have finally shown that the blackbody law can be beaten for objects that are far enough apart.

The researchers made a couple of very thin silicon nitride membranes that were next to each other. By facing each other end-on, the thickness of the membrane becomes the critical size of the radiator. The researchers used membranes that varied in thickness from 270nm to 11 micrometers. For comparison, the wavelength at the peak of the blackbody radiation spectrum for the membrane was about 10 micrometers—this corresponds to room temperature.

The measurement technique is really cool, too. Both membranes had a tiny wire deposited on them. In one membrane, the wire had an alternating current applied to periodically heat it. At the other membrane, the researchers measured the resistance of the wire, which is temperature-dependent. They only looked for a signal at half the period of the heating current. In that way, even if there any environmental drifts or other forms of noise, the researchers only picked up the temperature changes due to the test membrane.

The researchers show that the thinnest membrane transfers heat at about 100 times the rate predicted by the blackbody radiation law. The thickest membrane, however, could only transfer heat at the rate given by the law.

Tiny edges remove tiny amounts of heat

That's pretty incredible. It may even make for a good party story. But it's probably not going to do any major cooling just yet. Hidden in the details is an inconvenient fact. The thinnest membranes also had the smallest absolute heat transfer rate. The blackbody radiation law scales with the size of the emitter (and sink), so in absolute terms, the thick membranes still win. They just don’t win by as much as expected.

Don’t take that as a disappointment, though. First, this gives some hope that we can improve heat transfer at larger scales by combining many small emitters. It's also important to remember that extracting heat from very tiny devices is still a very significant problem. This work provides another way to improve thermal characteristics in the very first steps on the journey from tiny device to big heat sink.

Nature, 2018, DOI: 10.1038/s41586-018-0480-9. (About DOIs)