Power/Performance Bits: June 4

Flexible high-temp dielectric; cathode coating; sensing light and heat.


Flexible high-temp dielectric
Researchers at Rice University, Georgia Institute of Technology, and Cornell University developed a new high-temperature dielectric nanocomposite for flexible electronics, energy storage, and electric devices that combines one-dimensional polymer nanofibers and two-dimensional boron nitride nanosheets.

The polymer nanofibers act as a structural reinforcement, while the 2D boron nitride provides a thermally conductive network that allows it to withstand the heat that breaks down common dielectrics, the polarized insulators in batteries, and other devices that separate positive and negative electrodes.

Finding just the right combination of attributes was important. “Ceramic is a very good dielectric, but it is mechanically brittle,” said M.M. Rahman, a research scientist at Rice. This makes it unsuitable for new types of flexible electronics. “On the other hand, polymer is a good dielectric with good mechanical properties, but its thermal tolerance is very low.”

Boron nitride is an electrical insulator, but happily disperses heat, said Rahman. “When we combined the polymer nanofiber with boron nitride, we got a material that’s mechanically exceptional, and thermally and chemically very stable.”

A lab video shows how quickly heat disperses from a composite of a polymer nanoscale fiber layer and boron nitride nanosheets. When exposed to light, both materials heat up, but the plain polymer nanofiber layer on the left retains the heat far longer than the composite at right. Courtesy of the Ajayan Research Group

In the nanocomposite, a single layer of polyaramid nanofibers binds via van der Waals forces to a sprinkling of boron nitride flakes. The boron nitride flakes make up 10% of the weight of the final product, just dense enough to form a heat-dissipating network while retaining flexibility and robustness. Layering polyaramid and boron nitride can make the material thicker but still flexible.

The material is 12-to-15-microns thick and acts as an effective heat sink up to 250 degrees Celsius (482 degrees Fahrenheit), according to the researchers. Tests showed the polymer nanofibers-boron nitride combination dispersed heat four times better than the polymer alone. The researchers say the material is scalable and should be easily incorporated into manufacturing.

Cathode coating
Scientists from Argonne National Laboratory, Hong Kong University of Science and Technology, Tsinghua University, Drexel University, Peking University, and Hong Kong Polytechnic University created a coating that provides a protective layer for cathodes in lithium-ion batteries.

The battery’s nickel-manganese-cobalt (NMC) cathode material is encapsulated with a sulfur-containing polymer called PEDOT using an oxidative chemical vapor deposition technique. While most coatings just cover the exterior, this one can penetrate the cathode particle’s interior, adding more shielding.

The coating solves several battery issues at once, including keeping the cathode electrically and ionically conductive and making sure that the battery stays safe after many cycles.

Argonne scientists have developed a new coating (shown in blue) for battery cathodes that can improve the electronic and ionic conductivity of a battery while improving its safety and cycling performance. (Image by Argonne National Laboratory)

“This coating is essentially friendly to all of the processes and chemistry that makes the battery work and unfriendly to all of the potential reactions that would cause the battery to degrade or malfunction,” said Argonne chemist Guiliang Xu.

The coating also largely prevents another reaction that causes the battery’s cathode to deactivate: a reaction in which the cathode material converts to another form called spinel. “The combination of almost no spinel formation with its other properties makes this coating a very exciting material,” said Khalil Amine, Argonne distinguished fellow and battery scientist.

Amine notes that the PEDOT material also demonstrated the ability to prevent oxygen release, a major factor for the degradation of NMC cathode materials at high voltage. “This PEDOT coating was also found to be able to suppress oxygen release during charging, which leads to better structural stability and also improves safety.”

The coating could be scaled up for use in nickel-rich NMC-containing batteries, Amine said. With the coating applied, the researchers believe that the NMC-containing batteries could either run at higher voltages, thus increasing their energy output, or have longer lifetimes, or both.

Sensing light and heat
Researchers at Linköping University developed a new sensor that could be part of an electronic skin. The sensor reacts to changes in body temperature as well as sunlight and warm touch using a combination of pyroelectric and thermoelectric effects along with plasmons.

When pyroelectric materials are heated or cooled, a voltage arises. The signal is rapid and strong, but decays quickly. In contrast, voltage arises in thermoelectric materials when the material has one cold and one hot side; this signal happens gradually, and takes time before it can be measured.

“We wanted to enjoy the best of both worlds, so we combined a pyroelectric polymer with a thermoelectric gel developed in a previous project by Dan Zhao, Simone Fabiano and other colleagues at the Laboratory of Organic Electronics. The combination gives a rapid and strong signal that lasts as long as the stimulus is present,” said Magnus Jonsson, leader of the Organic Photonics and Nano-optics group at Linköping.

Additionally, the two materials interact in a way that reinforces the signal. Plasmons also help to boost the sensor, said Jonsson. “Plasmons arise when light interacts with nanoparticles of metals such as gold and silver. The incident light causes the electrons in the particles to oscillate in unison, which forms the plasmon. This phenomenon provides the nanostructures with extraordinary optical properties, such as high scattering and high absorption.”

Heat and light sensing with hybrid nanooptics. (Thor Balkhed / Linköping University)

Previous work showed that a gold electrode perforated with nanoholes absorbs light efficiently with the aid of plasmons. The absorbed light is subsequently converted to heat. With such an electrode on the side that faces the sun, the sensor can also convert visible light rapidly to a stable signal.

The sensor is also pressure sensitive, but only to warm things. “A signal arises when we press the sensor with a finger, but not when we subject it to the same pressure with a piece of plastic. It reacts to the heat of the hand,” said Jonsson.

The team sees uses for the sensor in robotics, touch-reactive prostheses, and health monitoring applications.

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