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Graphene, a two dimensional form of carbon, could revolutionize a number of products,  including  smart textiles, conductive inks, energy storage platforms, and even water purification techniques.

A recent experiment indicated the material has one more potential future application— in the audio space.

A research team from the University of Exeter created a novel method that uses graphene to produce complex and controllable sound signals.

Traditional speakers mechanically vibrate to produce sound using a coil or membrane to shift the air back and forth.

By contrast, this alternative requires no moving parts. An alternating electrical current is used to heat and cool sheet of graphene.This mechanism initiates the transfer of thermal variation to the air causing the membrane to expand and contract.

Essentially, this process enabled sound generation and could allow sound frequencies to be mixed together and amplified. 

R&D Magazine interviewed Dr. David Horsell, Ph.D., the lead researcher and senior lecturer at the University of Exeter’s Quantum Systems and Nanomaterials Group,  to learn more about his experiment and gain insight into how this material could change audio and telecommunications.

R&D Magazine: What was the inspiration for this experiment?

David Horsell: For the last few years, we have been looking at how heat and electricity flow through graphene and how heat is lost from it. This was very much from a fundamental physics perspective rather than an applied physics/engineering one. Graphene is a rather odd material in that its thermal conductivity is one of the highest known for any material, but its thermal capacity (how much heat it can store) is very small, by virtue of the fact that it is just a single layer of atoms.

Thermoacoustics, or the conversion of heat into sound, has been understood as a physical concept since the 1940s. In recent years, such sound has been generated by periodically heating a thin conducting material with an electrical current. We wanted to investigate the origins of this transduction to see how the energy converted into sound adds to the overall picture of heat loss in graphene.

R&D: Why is heat a better way to generate soundwaves compared to vibrations?

Horsell: It is not inherently better. However, there are several applications that would benefit from the fact that sound is produced by a motionless surface. For instance, there is a drive now towards flexible technologies for all kinds of applications. Graphene is both flexible and optically transparent so to create a flexible display where the graphene electrode both transmits images and sound, would be quite a technological step forward.

R&D: Describe the engineering process. How did your team ensure the graphene component was able to consistently produce these frequencies?

Horsell: "The graphene we used in this particular study was bought commercially from a couple of different suppliers. It is grown in a furnace under controlled conditions-- a process known as chemical vapour deposition. The devices we produced from this material are not particularly complex-- basically, two metal electrodes with graphene between them-- and the sound output depends only on simple device parameters such as electrical resistance. As a result, data are easily reproduced across many devices. We show in our work that changes to the geometry and materials of the device are easily accounted for. The main thing we had to be careful of is the interface between the electrodes and the graphene. The resistance of this interface needs to be negligible with respect to that of the graphene so that the heat (and, therefore, the resulting sound) is not generated at this interface instead of in the graphene. To maximise the sound power from our devices, we had to optimise the geometry and substrate material the graphene was attached to. We found that using fine, comb-like interdigitated electrodes and substrates with poor thermal properties allowed sounds to be produced at appreciable levels."

R&D: How is this technique different than a traditional speaker system?

Horsell: In the way that it generates the sound waves. Traditional speakers, including moving coil, membrane and piezo-based ones, involve some sort of mechanical movement to create pressure variations in the air (which are sound waves); thermoacoustic speakers have no mechanical movement and instead create these pressure variations by periodically heating the air, causing it to expand and contract.

R&D: What potential field applications do you envision graphene having in the telecommunications and the audio industry?

Horsell: One of the main things we demonstrated was that the thermoacoustic devices are perfect frequency mixers. In other words, if you drive two signals at two different frequencies through the device, you not only get sound resulting from the two frequencies, but also sound at the sum and difference of these frequencies, so-called heterodynes. Heterodyning is a frequently used technique in the telecommunications industry and our thermoacoustic way of realising it appears to be both significantly simpler and potentially more controllable. The other benefit of the mixing is that we can finely tune the volume of the sound output from the device over a wide frequency range, well into the ultrasonic region of the spectrum. One result of this is that you can easily create a calibrated 'white' sound source for metrology and use in the audio industry.

R&D: Where do you see the most challenges/opportunities when it comes to R&D for this material and why?

Horsell: “From announcements in recent years, it appears that the first commercial applications of the material are in coatings and composites rather than electronic devices. The reason is that it is a relatively simple and commercially scalable process to create a powder-like graphene material where each graphene crystallite is much less than a micron wide. Such materials can be mixed with or coated on others to benefit from some of the graphene properties such as strength. It is proving much harder to find a reliable, scalable method to produce graphene for electronic devices. Large area graphene production is possible via the chemical vapour deposition method (and others) but the resulting product is not the perfect single atom thick layer of carbon atoms arranged in a hexagonal array that is often portrayed in the media; instead, these methods produce a single atom thick layer that is formed of small crystallites in a mosaic-like arrangement. As such, electronic devices depend somewhat on the nature of this polycrystalline material. If all you need is a conducting transparent film, then this does not really matter and several companies have demonstrated large TFT displays with this type of graphene acting as the top electrode. However, for other applications such as sensing or logic gates then inconsistency between devices matters a lot. Still, there has been much progress on this front in recent years so it should not be long before these methods are perfected.

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