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How Thermal Transistors Could Control MEMs devices

The world’s fastest thermal transistor could lead to thermal logic gates for controlling microscopic machines, say physicists

In recent years, engineers have begun to design and test thermal transistors with some success. Their goal is to exercise the same control over heat that they already have over electric current–the ability to switch it on and off, to modulate it and even to amplify it. 

That would be hugely useful for managing heat dissipation but also for creating thermal logic gates that can process information in the form of heat.

The thermal transistors built so far all work by modulating the flow of phonons, or thermal vibrations, from one material to another. For this to work, materials must be in physical contact with one another.

But there is another way for heat to flow–by radiative transfer. In this case, heat flows with the passage of thermal photons from one material to another. In this case, the materials do not need to be in physical contact.

Today, Philippe Ben-Abdallah at the Université Paris-Sud in France and Svend-Age Biehs at Carl von Ossietzky Universität in Germany, unveil the first thermal transistor to operate on thermal photons. The big advantage of this device is that it works at much higher speed than phonon transistors, potentially at light speed.

The design is simple. The transistor consists of three parts, which Ben-Abdallah and Biehs call the source, drain and gate, in analogy to a conventional transistor. The source and drain are made of silica and held at different temperatures to create a temperature gradient.

The source, which is hotter than the drain, emits thermal photons which transfer heat to the drain.

However, these materials are separated by a thin layer of vanadium oxide, which acts as the gate. Each of these three layers are also separated from each other by a gap of around 50 nanometres to ensure that the heat transfer is radiative only.

Vanadium oxide has the interesting property that it switches from being a photon conductor to an insulator as it cools down. (The critical temperature at which this happens is called the Mott transition temperature.)

The trick behind Ben-Abdallah and Biehs’ transistor is to keep the layer of vanadium oxide close to its transition temperature. When the material is acting as an insulator, the thermal photons cannot pass and the transistor behaves like a switch that is off.

But by raising the temperature of the vanadium oxide above its transition point, the transistor is switched ‘on’ and begins to conduct thermal photons. So a small change in the temperature of the gate leads to a dramatic change in the flow of heat through the device. Voila–a thermal transistor!

That’s a clever idea that Ben-Abdallah and Biehs have developed into a working device that they test in their paper. They show how to use this transistor to modulate heat flow, to switch the flow on and off and even to amplify the flow, just like an ordinary electronic transistor.

This kind of work could have important applications. Ben-Abdallah and Biehs talk about thermal logic gates and thermal memories for the storage and processing of thermal information. And they say these devices should have some attractive properties. “The present concept authorizes much higher operational speeds (speed of light) and should be very competitive compared to the previous ones,” they say.

But there are other uses too, such as in microelectromechanical machines where heat can be used to move microscopic devices such as cantilevers. The ability to switch this motion on and off using  thermal transistors is an idea with great potential.

Just when we’ll see these kinds of transistors in action isn’t clear. But the technology is available now, so sooner rather than later is a strong possibility.

Ref: Near-field Thermal Transistor

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