By Gary Elinoff, contributing writer
We’re all familiar with semiconductor diodes, whose function is to allow electricity to easily flow in one direction and impede its progress in the other. A thermal diode performs a similar gating function, only with heat energy instead of electrical energy. Up until now, this function has only been demonstrated in cold temperatures and at room temperatures. A team at the University of Nebraska-Lincoln, headlined by assistant professor Sidy Ndao and graduate student Mahmoud Elzouka, has come up with a device that can operate at temperatures as high as 600 K (620°F).
Ndao sees the possibility of devices such as these forming the basis for thermal computers, employing heat rather than electricity as the digital currency. Modern electronics, unless well-shielded and well-protected, can’t survive in heat anything like 620°F, and Ndao expects future iterations of thermal devices to be able to operate at temperatures as high as 1,300°F.
This raises the possibility in the future of installing computational devices in the harshest industrial environments and even for use in space exploration. Ndao also sees the possibility of using such devices as a way of using the vast amount of wasted heat generated every day to useful purpose.
How is a heat diode constructed?
The thermal diode is composed of a fixed and a moving terminal, and the illustrations below are separate depictions of the device. In both of them, the moving terminal is illustrated on the bottom, and the fixed terminal is shown on the top. In the illustration on the left, the diode is not conducting heat, while on the right, it is.
A thermal diode. Image source: Nature (edited).
Notice how the moving terminal comes closer to the fixed terminal for the forward illustration on the right and is further away on the reverse illustration on the left. This happens because in “reverse,” when the moving terminal is colder than the fixed, it doesn’t expand upward; rather, it keeps its distance. On “forward,” the moving terminal is hotter than the fixed, and the moving terminal expands upward toward the fixed terminal.
How is heat conducted — or not conducted?
Near-Field Thermal Radiation (NTFR) is the process by which heat is transferred via thermal radiation between two surfaces. The gap has to be very small, actually comparable to the radiation’s wavelength. And the intensity of the transfer is exponentially related to the distance between the points.
Thus is the nature of the rectification. As the moving terminal comes closer to the fixed terminal, it is close enough to affect NFTR; when further away, in reverse, it’s too far away and there is no heat transfer.
As Niday and Elaouka describe in their formal paper in the journal, Nature, the nanoscale vacuum gaps between the terminals were a challenge to fabricate. They have named their new technique as NanoThermoMechanical Rectification (NTMR).
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