Power/Performance Bits: April 25

Thermal diode; MoS2 microprocessor; stable lithium metal batteries.

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Thermal diode

Engineers at the University of Nebraska-Lincoln developed a nano-thermal-mechanical device, or thermal diode, which uses heat as an alternative energy source that would allow computing at ultra-high temperatures.

“If you think about it, whatever you do with electricity you should (also) be able to do with heat, because they are similar in many ways,” said Sidy Ndao, assistant professor of mechanical and materials engineering at UNL. “In principle, they are both energy carriers. If you could control heat, you could use it to do computing and avoid the problem of overheating.”

So far, the device works at temperatures that approached 630 degrees Fahrenheit, but the team expects the device could eventually work in heat as extreme as 1,300 degrees Fahrenheit, which could have major implications in many industries.

Sidy Ndao and Mahmoud Elzouka, University of Nebraska-Lincoln College of Engineering, developed this thermal diode that may allow computers to use heat as an alternate energy source. (Source: Karl Vogel / University of Nebraska-Lincoln Engineering)

“We are basically creating a thermal computer,” Ndao said. “It could be used in space exploration, for exploring the core of the earth, for oil drilling, (for) many applications. It could allow us to do calculations and process data in real time in places where we haven’t been able to do so before.”

By taking advantage of an energy source that has long been overlooked, the thermal diode could also help limit the amount of energy that gets wasted.

“It is said now that nearly 60% of the energy produced for consumption in the United States is wasted in heat,” Ndao said. “If you could harness this heat and use it for energy in these devices, you could obviously cut down on waste and the cost of energy.”

Though the researchers have filed for a patent, there is still work to be done to improve the diode and its performance.

The next step is making the device more efficient and making a physical computer that could work in the highest of temperatures, Ndao said.

“If we can achieve high efficiency, show that we can do computations and run a logic system experimentally, then we can have a proof-of-concept,” said Mahmoud Elzouka, a graduate student in mechanical and materials engineering. “(That) is when we can think about the future.”

MoS2 microprocessor

Researchers at the Vienna University of Technology and the Graphene Flagship created fully functional microprocessor logic devices based the three-atom thick 2D material molybdenum disulphide (MoS2).

The microprocessors consist of 115 integrated transistors made from layers of MoS2, and are capable of 1-bit logic operations. The surface area is around 0.6 mm2 and the design is scalable to multi-bit operations.

The ultra-thin MoS2 transistors are inherently flexible and compact, so this result could be directly translated into microprocessors for fully flexible electronic devices. The MoS2 transistors are highly responsive, and could enable low-powered computers to be integrated into everyday objects without adding bulk. “In principle, it’s an advantage to have a thin material for a transistor. The thinner the material, the better the electrostatic control of the transistor channel, and the smaller the power consumption,” said Thomas Mueller, who led the work at TU Vienna.

MoS2 transistors on the microprocessor chip. (Source: Hermann Detz, TU Vienna)

“In general, being a flexible material there are new opportunities for novel applications. One could combine these processor circuits with light emitters that could also be made with MoS2 to make flexible displays and e-paper, or integrate them for logic circuits in smart sensors,” said Mueller.

The devices were tested using simple programs, delivering the correct results with excellent signal quality and low power consumption.

Compared to modern processors, the 115-transistor devices are very simple. “Our goal is to realize significantly larger circuits that can do much more in terms of useful operations. We want to make a full 8-bit design – or even more bits – on a single chip with smaller feature sizes,” said Mueller.

This goal presents a challenge in terms of design and fabrication: “Adding additional bits of course makes everything much more complicated. For example, adding just one bit will roughly double the complexity of the circuit,” said Stefan Wachter, a researcher at TU Vienna.

Dmitry Polyushkin of TU Vienna outlined the team’s next steps: “Our approach is to improve the processing to a point where we can reliably make chips with a few tens of thousands of transistors. For example, growing directly onto the chip would avoid the transfer process, which would give higher yield so that we can go to more complex circuits.”

Stable lithium metal batteries

Researchers at the University of California, Riverside took a new approach to stabilizing the metal anodes present in lithium metal batteries.

While lithium metal batteries are capable of a five to 10 times capacity increase over lithium ion, they suffer from dendrites, microscopic fibers that grow during charge cycles, degrading the performance and posing the risk of a short circuit.

The team discovered that by coating the battery with an organic compound called methyl viologen they were able to stabilize battery performance, eliminate dendrite growth and increase the lifetime of the battery by more than three times compared to the current standard electrolyte used with lithium metal anodes. In addition, methyl viologen is very low in cost and can easily be scaled up.

The methyl viologen molecule used by the researchers can be dissolved in the electrolytes in the charged states. Once the molecules meet the lithium metal, they are immediately reduced to form a stable coating on top of the metal electrode.

Illustrations of the design principles of using methyl viologen to form a stable coating to allow the stable cycling of lithium metal. (Source: UC Riverside)

The stable operation of lithium metal anodes could enable the development of next generation high-capacity batteries, including lithium metal batteries and lithium air batteries.

“This has the potential to change the future,” said Chao Wang, an adjunct assistant professor of chemistry at UC Riverside. “It is low cost, easily manipulated and compatible with the current lithium ion battery industry.”

The researchers, however, cautioned that while the coating improves battery performance, it isn’t a way to prevent batteries from catching fire.