Power/Performance Bits: June 22

Terahertz silicon multiplexer; conductive n-type ink; simpler battery recycling.


Terahertz silicon multiplexer
Researchers from Osaka University and University of Adelaide designed a silicon multiplexer for terahertz-range communications in the 300-GHz band.

“In order to control the great spectral bandwidth of terahertz waves, a multiplexer, which is used to split and join signals, is critical for dividing the information into manageable chunks that can be more easily processed and so can be transmitted faster from one device to another,” said Withawat Withayachumnankul, an associate professor from the University of Adelaide’s School of Electrical and Electronic Engineering. “Up until now compact and practical multiplexers have not been developed for the terahertz range. The new terahertz multiplexers, which are economical to manufacture, will be extremely useful for ultra-broadband wireless communications.”

The multiplexer covers a spectral bandwidth over 30 times the total spectrum allocated for 4G/LTE and 5G in Japan, making ultra-high-speed digital transmission possible.

“Our four-channel multiplexer can potentially support aggregate data rate of 48 gigabits per second (Gbit/s), equivalent to that of uncompressed 8K ultrahigh definition video being streamed in real time,” said Masayuki Fujita, an associate professor at Osaka University. “To make the entire system portable, we plan to integrate this multiplexer with resonant tunnelling diodes to provide compact, multi-channel terahertz transceivers.”

Photograph of the silicon multiplexer. (Credit: Osaka University / Masayuki Fujita, et al)

The device is also quite compact, said Dr Daniel Headland from Osaka University. “A typical four-channel optical multiplexer might span more than 2000 wavelengths. This would be about two meters in length in the 300-GHz band. Our device is merely 25 wavelengths across, which offers dramatic size reduction by a factor of 6000.”

The researchers said that large-scale commercial production is feasible and could enable 6G as well as applications such as communication between compact aircraft and drones.

Conductive n-type ink
Researchers from Linköping University, University of Washington, and Korea University developed a new type of conductive polymer ink. The n-type polymer ink is stable in air and at high temperatures.

The most common conducting polymer used for ink is PEDOT:PSS. While it has high conductivity and stability, it is a p-type conductor, which limits how it can be used on its own. The new n-type polymer, called BBL:PEI (poly(benzimidazobenzophenanthroline):poly(ethyleneimine)), is seen by the researchers as a complement that add new capabilities to printed electronics.

“This is a major advance that makes the next generation of printed electronic devices possible. The lack of a suitable n-type polymer has been like walking on one leg when designing functional electronic devices. We can now provide the second leg,” said Simone Fabiano, senior lecturer in the Department of Science and Technology at Linköping University.

“Everything possible with PEDOT:PSS is also possible with our new polymer,” added Chi-Yuan Yang, a postdoc at Linköping University. “The combination of PEDOT:PSS and BBL:PEI opens new possibilities for the development of stable and efficient electronic circuits.”

The n-type material is in the form of ink with ethanol as the solvent, which can be deposited by spraying the solution onto a surface. The team said it is more eco-friendly than other n-type organic conductors that use harmful solvents, and that it is ready for use.

“Large-scale production is already feasible, and we are thrilled to have come so far in a relatively short time. We expect BBL:PEI to have the same impact as PEDOT:PSS. At the same time, much remains to be done to adapt the ink to various technologies, and we need to learn more about the material,” said Fabiano.

Simpler battery recycling
Researchers at Aalto University propose a way to recycle lithium-ion batteries containing cobalt without the time- and energy-consuming processes of breaking them down into purified component parts and re-assembling them.

“In our earlier study of how lithium cobalt oxide batteries age, we noticed that one of the main causes of battery deterioration is the depletion of lithium in the electrode material. The structures can nevertheless remain relatively stable, so we wanted to see if they can be reused,” said Tanja Kallio, a professor at Aalto University.

In one common formulation of rechargeable lithium-ion batteries, one electrode is comprised of lithium cobalt oxide, while the other is typically carbon and copper. Charged particles move between the two electrodes.

In the usual battery recycling process, the barriers are either crushed or dissolved. To return the cobalt to a state that can be reused in electrodes requires a lengthy chemical refinement process.

Instead, the new method can replenish the spent lithium in the electrode through an electrolysis process, enabling the cobalt compound to be directly reused.

The team found that the performance of electrodes newly saturated with lithium is almost as good as that of those made of new material. Kallio believes that with further development the method would also work on an industrial scale.

“By reusing the structures of batteries we can avoid a lot of the labor that is common in recycling and potentially save energy at the same time. We believe that the method could help companies that are developing industrial recycling,” Kallio said.

Next, they plan to investigate whether the same method could be used with nickel-based batteries commonly used in electric vehicles.

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