Power/Performance Bits: Sept. 15

Stretchy metal; carbon nanotube ICs; reducing smartphone battery drain.

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Stretchy metal

Washington State University researchers stretched metal films used in flexible electronics to twice their size without breaking.

The discovery could lead to dramatic improvements and addresses one of the biggest challenges in flexible electronics, an industry still in its infancy with applications such as bendable batteries, robotic skins, wearable monitoring devices and sensors, and connected fabrics.

Researchers have struggled for years with designing and manufacturing the tiny metal connections that go into flexible electronics. The metal has to undergo severe stretching and bending while continuing to conduct electricity. Manufacturers have so far used tiny metal springs that can stretch and still maintain connectivity, but the springs take up space and make it difficult to design complicated, high-density circuitry. Furthermore, electricity has to travel farther in coiled springs, requiring more power and bigger batteries.

“The circuitry ends up requiring a ton of real estate and bulky batteries,” said Rahul Panat, a researcher at WSU.

(Source: Washington State University)

(Source: Washington State University)

The WSU team found that when they made a metal film out of indium, a fairly inexpensive metal compared to gold, and periodically bonded it to a plastic layer commonly used in electronics, they were able to stretch the metal film to twice its original length. When the pieces broke, it was the plastic layer that failed, not the metal.

“A metal film doubling its size and not failing is very unusual,” he said. “We have proposed a model for the stretchy metal but much work is needed to validate it.”

Carbon nanotube ICs

Individual transistors made from carbon nanotubes are faster and more energy efficient than those made from other materials. Going from a single transistor to an integrated circuit full of transistors, however, is a giant leap.

When trying to make the leap from an individual, nanotube-based transistor to wafer-scale integrated circuits, many research teams have met challenges. The process is incredibly expensive, often requiring billion-dollar cleanrooms to keep the components safe from the potentially damaging effects of air, water, and dust. Researchers have also struggled to create a carbon nanotube-based integrated circuit in which the transistors are spatially uniform across the material, which is needed for the overall system to work.

A team at Northwestern University found a key to solving such issues with newly developed encapsulation layers that protect carbon nanotubes from environmental degradation.

“One of the realities of a nanomaterial, such as a carbon nanotube, is that essentially all of its atoms on the surface,” said Mark Hersam, professor of materials science and engineering at Northwestern. “So anything that touches the surface of these materials can influence their properties. If we made a series of transistors and left them out in the air, water and oxygen would stick to the surface of the nanotubes, degrading them over time. We thought that adding a protective encapsulation layer could arrest this degradation process to achieve substantially longer lifetimes.”

Hersam compares the solution to one currently used for organic LEDs, which experienced similar problems after they were first realized. Many people assumed that organic LEDs would have no future because they degraded in air, before an encapsulation layer for the material was developed. The researchers’ encapsulation layer, made from polymers and inorganic oxides, is based on the same idea but tailored for carbon nanotubes.

To demonstrate proof of concept, researchers developed nanotube-based SRAM circuits. To create the encapsulated carbon nanotubes, the team first deposited the carbon nanotubes from a solution previously developed in their lab. Then they coated the tubes with their encapsulation layers.

Using the encapsulated carbon nanotubes, the team designed and fabricated arrays of working SRAM circuits. The team found that the encapsulation layers protected the sensitive device from the environment, as well as improved spatial uniformity among individual transistors across the wafer.

Reducing smartphone battery drain

The first large-scale study of smartphones in everyday use by consumers has revealed that apps drain 28.9 percent of battery power while the screen is off. To address the problem, researchers have created a software tool that reduces the energy drain by about 16 percent.

Researchers at Purdue University, Intel Corp. and startup company Mobile Enerlytics studied the use of 2,000 Samsung Galaxy S3 and S4 phones served by 191 mobile operators in 61 countries.

“This was the first large-scale study of smartphone energy drain ‘in the wild,’ or in everyday use by consumers,” said Y. Charlie Hu, a Purdue professor of electrical and computer engineering.

“We presented the first study a few years back showing wakelock bugs could cause significant energy drain,” Hu, said. “But this is the first study showing that wakelock bugs appear prevalent on real users’ phones.”

“Being able to reduce the total daily energy drain by about 16 percent is rather significant because you can extend the battery charge by one-sixth,” Hu said.

The key insight behind the team’s proposed solution is that background activities of individual apps are not equally important to individual smartphone users. For example, frequent Facebook updates during screen-off may be useful to a user who checks Facebook feeds and reacts to notifications often, but they are much less useful to another user who rarely checks such updates. Their system dynamically identifies app background activities that are not useful to the user experience on a per-app basis and suppresses such background app activities during screen-off to reduce the battery drain.