Power/Performance Bits: May 16

Chaos-based IC; electrolytes for sodium, magnesium batteries; graphene speaker.


Chaos-based IC

Researchers at North Carolina State University and the College of Wooster developed a three transistor nonlinear, chaos-based integrated circuit combining digital and analog components, which they hope can improve computational power by enabling processing of a larger number of inputs.

In chaos-based, nonlinear circuits, one circuit can perform multiple computations instead of the current “one circuit, one task” design. However, the number of inputs that can be processed in chaos-based computing is limited by ambient noise, which decreases accuracy.

“Noise has always been a big problem in almost all engineering applications including computing devices and communications,” says Vivek Kohar, postdoctoral research scholar at NC State. “Our system is nonlinear and so noise can be even more problematic.”

Schematic diagram of chaos computing in hybrid digital-analog systems. (Source: Vivek Kohar, Behnam Kia, John F. Lindner, and William L. Ditto / Physical Review Applied)

To address the problem, the researchers created a hybrid system which uses a digital block of AND gates and an analog nonlinear circuit to distribute the computation between the digital and analog circuits. The result is an exponential reduction in computational time, which means that the output can be measured while the noise-based deviations are still small. In short, the computations are performed so quickly that noise doesn’t have time to affect their accuracy.

To further improve the accuracy, the proposed solution couples multiple systems. This coupling provides a safety net that reduces the effect of noise-based deviations at the final stage.

“The systems are tuned in such a way that at the time of measurement, our system is at the maxima or minima – the points where the effects of noise are low in general and much lower if the systems are coupled,” said Kohar.

Electrolytes for sodium, magnesium batteries

As part of the Novel Ionic Conductors project, scientists at Empa and the University of Geneva, supported by the Swiss National Science Foundation, developed prototype solid electrolytes for solid-state batteries based on sodium and magnesium.

In working towards a sodium or magnesium battery, the electrolytes’ crystalline structure had to be completely redesigned to facilitate the movement of each element’s ions.

One of the team’s solid electrolytes facilitates good mobility of sodium ions at 20 degrees C (68 F). Ions require a source of heat in order to move, and inducing a reaction at room temperature poses a technical challenge. The electrolyte is also non-flammable and is chemically stable up to 300 degrees C (572 F), which addresses the various safety concerns associated with lithium-ion batteries. A team at the University of Geneva has been working in parallel to develop cheaper technology for the production of this new solid electrolyte.

Unlike lithium, there are huge reserves of sodium: it’s one of the two components of table salt. “Availability is our key argument,” said Léo Duchêne of Empa. “However, it stores less energy than the equivalent mass of lithium and thus could prove to be a good solution if the size of the battery isn’t a factor for its application.”

Little research had been done into the team’s other project, a solid magnesium-based electrolyte. While magnesium is more difficult to set in motion, it is available in abundance, it’s light, and there’s no risk of it exploding. More importantly, a magnesium ion has two positive charges, whereas lithium only has one. Essentially, this means that it stores almost twice as much energy in the same volume.

Some experimental electrolytes have already been used to stimulate magnesium ions to move, but at temperatures in excess of 400 degrees C (752 F). The electrolytes have already recorded similar conductivities at 70 degrees C (158 F). “This is pioneering research and a proof of concept,” said Elsa Roedern of Empa, who led the experiments. “We are still a long way from having a complete and functional prototype, but we have taken the first important step towards achieving our goal.”

Graphene speaker

Researchers from the University of Exeter have devised a method to use graphene to generate complex and controllable sound signals. In essence, it combines speaker, amplifier and graphic equalizer into a chip the size of a thumbnail.

Unlike a traditional speaker, the technique uses no moving parts. A layer of graphene is rapidly heated and cooled by an alternating electric current, and transfer of this thermal variation to the air causes it to expand and contract, thereby generating sound waves.

Dr. David Horsell, a Senior Lecturer in the Quantum Systems and Nanomaterials Group at Exeter, explained, “Thermoacoustics (conversion of heat into sound) has been overlooked because it is regarded as such an inefficient process that it has no practical applications. We looked instead at the way the sound is actually produced and found that by controlling the electrical current through the graphene we could not only produce sound but could change its volume and specify how each frequency component is amplified. Such amplification and control opens up a range of real-world applications we had not envisaged.”

The graphene speaker chip. (Source: University of Exeter)

Ultrasound imaging is one promising application for the device, says the team. The known high strength and flexibility of graphene would allow intimate surface contact leading to much better imaging. The device’s low cost could also lend itself to intelligent bandages.

Another possibility the researchers raise is combined audio-visual technologies, given graphene’s transparency.

Horsell added, “The frequency mixing is key to new applications. The sound generating mechanism allows us to take two or more different sound sources and multiply them together. This leads to the efficient generation of ultrasound (and infrasound). However, the most exciting thing is that it does this trick of multiplication in a remarkably simple and controllable way. This could have a real impact in the telecommunications industry, which needs to combine signals this way but currently uses rather complex and, therefore, costly methods to do so.”

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