Analogous to a semiconductor diode, the high-temperature device may well form the basis of a future thermal computer

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 as
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.  

Thermal_Diode_Reverse_Forward

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,” where 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 towards 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 NanoThermoMechanicalRectification (NTMR).