Power/Performance Bits: Feb. 3

Bulletproof vests for batteries; higher high-voltage cables; optoelectronic amplification.

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Bulletproof vests for batteries

It was almost two years ago that the Boeing Dreamliner was grounded because of fires caused by its lithium-ion batteries. Now researchers at the University of Michigan have used nanofibers extracted from Kevlar, best known as the material in bulletproof vests, to create a new barrier between the electrodes in a lithium-ion battery.

Lithium atoms in batteries can form fern-like structures, called dendrites, which can eventually poke through the membrane separating the electrodes. If they reach the other electrode, the electrons have a path within the battery, shorting out the circuit.

Pores in current membranes are a few hundred nanometers across, while the pores in the new membrane are 15-to-20 nanometers across. They are large enough to let individual lithium ions pass, but small enough to block the 20-to-50-nanometer tips of the dendrites.

The membrane was constructed by layering the fibers on top of each other in thin sheets. This method keeps the chain-like molecules in the plastic stretched out, which is important for good lithium-ion conductivity between the electrodes.

Higher high-voltage cables

You cannot read articles these days without the importance of power being mentioned. This extends from reducing the power consumed by chips, to energy harvesting and the transmission of power over long distances. “Reducing energy losses during electric power transmission is one of the most important factors for the energy systems of the future,” says Chalmers researcher Christian Müller in a recent scientific article.

They have modified the insulation plastic used in high-voltage cables to aid in this.

The researchers at Chalmers have shown that different variants of the C60 carbon ball, a nanomaterial in the fullerene molecular group, provide strong protection against breakdown of the insulation plastic used in high-voltage cables. Today the voltage in the cables must be limited to prevent the insulation layer from getting damaged. The higher the voltage the more electrons can leak out into the insulation material, eventually breaking it down.

Very small amounts of fullerene added to the insulation plastic allow it to withstand a voltage that is 26 per cent higher, without the material breaking down, than the voltage that plastic without the additive can withstand.

Currently, additives are not used in the insulation material, although other researchers have experimented with fullerenes in the electrically conductive parts of high-voltage cables.

An electrical tree, which is a major electrical breakdown mechanism of insulation plastic. Fullerenes prevent electrical trees from forming by capturing electrons that would otherwise destroy chemical bonds in the plastic.

An electrical tree, which is a major electrical breakdown mechanism of insulation plastic. Fullerenes prevent electrical trees from forming by capturing electrons that would otherwise destroy chemical bonds in the plastic. [Image credit: Lina Bertling, Jan-Olof Yxell, Carolina Eek Jaworski, Anette Johansson, Markus Jarvid, Christian Müller]

Large scale testing in complete high-voltage cables for both alternating current and direct current is the researcher’s next step.

Optoelectronic amplification

The researchers in UC San Diego’s Jacobs School of Engineering, led by electrical and computer engineering professor Yuhwa Lo, have discovered a mechanism to amplify signals in optoelectronic systems that is far more efficient than the process long used by the semiconductor industry based on impact ionization. They dubbed it the ‘cycling excitation process,’ or CEP.

The key discovery and innovation for the amplification process was the use of compensating impurities as the intermediate steps for electron-hole pair generation. “It appears that a small modification, such as heavy doping compensation, from a common structure can be used to take advantage of the unusual physical process that results from concerted interactions between electrons in extended and localized (impurity) states and phonons,” said Lo in the announcement.

The team believes that the new signal amplification mechanism could be used in a wide array of devices and semiconductors, creating a new paradigm for the semiconductor industry.