Power/Performance Bits: Feb. 4

Infrared nanoantenna; watching dendrites; breaking down PCBs.


Infrared nanoantenna
Researchers at the University of Würzburg built a nanoantenna capable of generating directed infrared light. The Yagi-Uda antenna is the smallest of its type yet created.

“Basically, it works in the same way as its big brothers for radio waves ,” said René Kullock, a member of the nano-optics team at Würzburg. An AC voltage is applied that causes electrons in the metal to vibrate and the antennas radiate electromagnetic waves as a result. “In the case of a Yagi-Uda antenna, however, this does not occur evenly in all directions but through the selective superposition of the radiated waves using special elements, the so-called reflectors and directors,” said Kullock. “This results in constructive interference in one direction and destructive interference in all other directions.” Accordingly, such an antenna would only be able to receive light coming from the same direction when operated as a receiver.

The team had previously demonstrated that the principle of an electrically driven light antenna works. Producing one was more difficult, however. “We bombarded gold with gallium ions which enabled us to cut out the antenna shape with all reflectors and directors as well as the necessary connecting wires from high-purity gold crystals with great precision,” explains Bert Hecht, professor and Chair of Experimental Physics 5 at Würzburg.

Let there be light – and it was directional: The world’s first electrically powered Yagi-Uda antenna was built at the University of Würzburg’s Department of Physics. (Picture: Department of Physics) (Image: Physikalisches Institut)

In a next step, the physicists positioned a gold nano particle in the active element so that it touches one wire of the active element while keeping a distance of only one nanometer to the other wire. “This gap is so narrow that electrons can cross it when voltage is applied using a process known as quantum tunnelling,” explains Kullock. This charge motion generates vibrations with optical frequencies in the antenna which are emitted in a specific direction thanks to the special arrangement of the reflectors and directors.

The nanoantenna radiates light in a particular direction, although it is very small. Like radio wave antennas, the directional accuracy of light emission of the new optical antenna is determined by the number of antenna elements. “This has allowed us to build the world’s smallest electrically powered light source to date which is capable of emitting light in a specific direction,” Hecht said.

The team still has work to do on the counterpart that receives light signals, as well as boost the antenna’s efficiency and stability.

Watching dendrites
Researchers at Yanshan University, Pennsylvania State University, and Georgia Institute of Technology propose a way to observe dendrites, a major cause of lithium-ion battery failure.

Dendrites, which resemble thin metal whiskers or needles, can grow on battery electrodes during repeated charge-discharge cycles. If they grow large enough, they can puncture the volatile electrolyte and reach the other electrode, potentially causing reduced battery life, a short circuit, or battery fires. Understanding how they form could contribute to designing safer batteries.

“It is difficult to detect the nucleation of such a whisker and observe its growth because it is tiny,” said Sulin Zhang, professor of mechanical engineering, Penn State. “The extremely high reactivity of lithium also makes it very difficult to experimentally examine its existence and measure its properties.”

The team was able to successfully grow lithium whiskers inside an environmental transmission electron microscope (ETEM) using a carbon dioxide atmosphere. The reaction of carbon dioxide with lithium forms an oxide layer that helps stabilize the whiskers.

They used an atomic force microscope (AFM) tip as a counter electrode and the integrated ETEM-AFM technique allows simultaneous imaging of the whisker growth and measurement of the growth stress. If the growth stress is too high, it would penetrate and fracture the solid electrolyte and allow the whiskers to continue growing and eventually short-circuit the cell.

“Now that we know the limit of the growth stress, we can engineer the solid electrolytes accordingly to prevent it,” Zhang said. Lithium metal-based all-solid-state batteries are desirable because of greater safety and higher energy density.

Next, the team plans to use their technique to look at the dendrite as it forms against a more realistic solid-state electrolyte under TEM to see exactly what happens.

Breaking down PCBs
Researchers at Sun Yat-sen University propose a way to break down potentially harmful components of printed circuit boards, making them safer to dispose. Using a ball-milling method, the team focused on brominated flame retardants, which are added to PCBs to keep them from catching fire but can leech out and contaminate soil and groundwater when disposed of in landfills.

Metallic components, which comprise about 30% of a PCB, can be recovered from crushed circuit boards by magnetic and high-voltage electrostatic separations, leaving behind nonmetallic particles including resins, reinforcing materials, brominated flame retardants and other additives. Scientists have linked compounds in brominated flame retardants to endocrine disorders and fetal tissue damage.

The researchers crushed PCBs and removed the metallic components by magnetic and high-voltage electrostatic separations, as is typically done. Then, they put the nonmetallic particles into a ball mill – a rotating machine that uses small agate balls to grind up materials. They also added iron powder, which prior studies had shown was helpful for removing halogens, such as bromine, from organic compounds. After ball-milling, the bromine content on the surface of the particles had decreased by 50%, and phenolic resin compounds had decomposed. The researchers determined that during the ball-milling process, iron transferred electrons to flame retardant compounds, causing carbon-bromine bonds to stretch and break.

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