A team of researchers at the University of Michigan (USA) has come up with an approach that could lead to a new solid-state cooling technology for microprocessors embedded in the computers of the future. The key? They have run a light-emitting diode (a lifelong LED) with inverted electrodes, managing to cool another device just nanometers away.
In this way, future microprocessors could have a myriad of transistors packed into a small space, eliminating heat quickly, something that current technology does not do fast enough.
“We have demonstrated a second method of using photons to cool the devices,” says Pramod Reddy, professor of mechanical engineering and co-author of the paper published in the journal Nature.
The first method, known as laser cooling, builds on the founding work of Arthur Ashkin, who shared the 2018 Nobel Prize in Physics with Gérard Mourou and Donna Strickland.
In this new work, the researchers instead harnessed the chemical potential of thermal radiation, a concept most commonly used to explain, for example, how a battery works.
The chemical potential in a battery, for example, drives an electric current when placed in a device. Inside the battery, the metal ions want to flow to the other side because they can get rid of some energy (chemical potential energy) and we use that energy as electricity. Electromagnetic radiation, including visible light and infrared thermal radiation, generally do not have this type of potential.
“Generally, for thermal radiation, the intensity only depends on the temperature, but we actually have an additional button to control this radiation, which makes the cooling we investigate possible,” says Linxiao Zhu, a researcher in mechanical engineering.
In theory, reversing the positive and negative electrical connections in an infrared LED will not only prevent it from emitting light, it will suppress the thermal radiation that it should be producing just because it is at room temperature. Thanks to this reverse bias trick , the LED behaves as if it were at a lower temperature.
How to do it
To get enough infrared light to flow from an object to the LED, the two would have to be very close together. Previously, experts had been heating and cooling the nanoscale devices, arranging them so that they were only a few tens of nanometers apart, or less than a thousandth of the width of a hair.
The group tested their theory by building a tiny calorimeter, which is a device that measures changes in energy, and placing it next to a small LED the size of a grain of rice. These two emitted and received thermal photons to each other and to other parts of the environment.
“Any object at room temperature is emitting light. A night vision camera is basically capturing infrared light that comes from a warm body,” the authors comment.
But once the LED was reverse biased, it began to act like a very low temperature object, absorbing photons from the calorimeter. At the same time, the gap prevents heat from returning to the calorimeter through conduction, which produces a cooling effect.
The team demonstrated a cooling of 6 watts per square meter. Theoretically, this effect could produce a cooling equivalent to 1,000 watts per square meter, or about the power of sunlight on the Earth’s surface; which could become key for future smartphones and computers. And it is that with greater computing power in the devices and these being increasingly smaller, the elimination of heat from the microprocessor is beginning to limit the amount of energy that can be compressed in a given space.
With the improvements in efficiency and cooling speeds of this new method, we are presented with an excellent way to quickly move heat away from microprocessors in devices.
Referencia: Near-field photonic cooling through control of the chemical potential of photons, Nature (2019). DOI: 10.1038/s41586-019-0918-8 , https://www.nature.com/articles/s41586-019-0918-8
