Power/Performance Bits: Dec. 13

3D porous microsupercapacitors

A research team from the King Abdullah University of Science and Technology (KAUST) developed an integrated microsupercapacitor targeted at self-powered system applications where the power source may be intermittent, such as sensors for wearables, security, and structural health monitoring.

The key to the microsupercapacitors is vertically-scaled three-dimensional porous current collectors made from nickel foams to improve microsupercapacitor performance. The pores in the foam work to increase the surface area.

“This three-dimensional porous architecture allows excellent electrolyte permeability, good conductivity and faster ion transportation with maximum mass-loading of active material, which increase energy and power density in a given area,” said Husam Alshareef, KAUST Professor of Material Science and Engineering.

Three-dimensional porous electrodes could lead to smaller and efficient integrated power sources. (Source: KAUST/ John Wiley and Sons)

The microsupercapacitors were also asymmetric, using two different electrode materials for the cathode (nickel cobalt sulfide) and anode (carbon nanofiber), which nearly doubled the operating voltage. As a result, while delivering high power density (four milliwatts per square centimeter), the microsupercapacitors had an energy density of 200 microwatt-hours per square centimeter.

This is superior to state-of-the-art microsupercapacitors, which achieve between one and forty microwatt-hours per square centimeter, and is comparable to various types of thin film batteries. The high capacities were maintained even after 10,000 operating cycles.

Energy harvesting film

Michigan State University engineering researchers developed a new method to harvest energy from human motion, using a film-like device that can be folded to create more power. With the low-cost nanogenerator, the scientists successfully operated an LCD touch screen, a bank of 20 LED lights and a flexible keyboard, with a simple touching or pressing motion and without the aid of a battery.

The process starts with a silicone wafer, which is then fabricated with several layers of environmentally friendly substances including silver, polyimide and polypropylene ferroelectret. Ions are added so that each layer in the device contains charged particles. Electrical energy is created when the device is compressed by human motion, or mechanical energy.

The completed device is called a biocompatible ferroelectret nanogenerator, or FENG. The device is as thin as a sheet of paper and can be adapted to many applications and sizes. The device used to power the LED lights was palm-sized, for example, while the device used to power the touch screen was as small as a finger.

This foldable keyboard, created by Michigan State University engineer Nelson Sepulveda and his research team, operates by touch; no battery is needed. (Source: MSU)

Advantages such as being lightweight, flexible, biocompatible, scalable, low-cost and robust could make FENG “a promising and alternative method in the field of mechanical-energy harvesting” for many autonomous electronics such as wireless headsets, cell phones and other touch-screen devices, the study says.

The device also becomes more powerful when folded. “Each time you fold it you are increasing exponentially the amount of voltage you are creating,” said Nelson Sepulveda, associate professor of electrical and computer engineering at MSU. “You can start with a large device, but when you fold it once, and again, and again, it’s now much smaller and has more energy. Now it may be small enough to put in a specially made heel of your shoe so it creates power each time your heel strikes the ground.”

The team is working on technology that would transmit the power generated from the heel strike to wireless devices.

Bumpy heatsinks

In a new theoretical study, Rice University scientists suggest bumpy surfaces with graphene between could help dissipate heat in microelectronics.

Rice computer models replaced the flat interface between gallium nitride semiconductors and diamond heat sinks with a nanostructured pattern and added a layer of graphene as a way to dramatically improve heat transfer.

Gallium nitride has become a strong candidate for use in high-power, high-temperature applications like uninterruptible power supplies, motors, solar converters and hybrid vehicles. Diamond is an excellent heat sink, but its atomic interface with gallium nitride is hard for phonons to traverse.

“Oftentimes, the individual materials in hybrid nano- and microelectronic devices function well but the interface of different materials is the bottleneck for heat diffusion,” said Rouzbeh Shahsavari, materials scientist at Rice.

Simulations show that graphene between patterned gallium nitride and diamond would offer excellent heat transfer in next-generation hybrids of nano- and microelectronics. (Source: Lei Tao/Rice University)

The researchers simulated 48 distinct grid patterns with square or round graphene pillars and tuned them to match phonon vibration frequencies between the materials. Sinking a dense pattern of small squares into the diamond showed a dramatic decrease in thermal boundary resistance of up to 80%. A layer of graphene between the materials further reduced resistance by 33%.

The strategy is applicable to several other hybrid materials, according to Shahsavari.

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