'Nanocable' could be big boon for energy storage

June 7, 2012
This is an artist's impression of Rice University's new coaxial nanocable, which is about a thousand times smaller than a human hair. Credit: Zheng Liu/Rice University

Thanks to a little serendipity, researchers at Rice University have created a tiny coaxial cable that is about a thousand times smaller than a human hair and has higher capacitance than previously reported microcapacitors.

The nanocable, which is described this week in Nature Communications, was produced with techniques pioneered in the nascent graphene research field and could be used to build next-generation energy-storage systems. It could also find use in wiring up components of lab-on-a-chip processors, but its discovery is owed partly to chance.

"We didn't expect to create this when we started," said study co-author Jun Lou, associate professor of mechanical engineering and materials science at Rice. "At the outset, we were just curious to see what would happen electrically and mechanically if we took small known as interconnects and covered them with a thin layer of carbon."

The tiny is remarkably similar in makeup to the ones that carry cable into millions of homes and offices. The heart of the cable is a solid copper wire that is surrounded by a thin sheath of insulating copper oxide. A third layer, another conductor, surrounds that. In the case of TV cables, the third layer is copper again, but in the nanocable it is a thin layer of carbon measuring just a few atoms thick. The coax nanocable is about 100 , or 100 billionths of a meter, wide.

While the coaxial cable is a mainstay of broadband telecommunications, the three-layer, metal-insulator-metal structure can also be used to build energy-storage devices called capacitors. Unlike batteries, which rely on chemical reactions to both store and supply electricity, capacitors use . A capacitor contains two , one negative and the other positive, that are separated by thin layer of insulation. Separating the oppositely charged conductors creates an electrical potential, and that potential increases as the separated charges increase and as the distance between them – occupied by the insulating layer -- decreases. The proportion between the charge density and the separating distance is known as capacitance, and it's the standard measure of efficiency of a capacitor.

The study reports that the capacitance of the nanocable is at least 10 times greater than what would be predicted with classical electrostatics.

"The increase is most likely due to quantum effects that arise because of the small size of the cable," said study co-author Pulickel Ajayan, Rice's Benjamin M. and Mary Greenwood Anderson Professor of Mechanical Engineering and Materials Science.

The team of materials scientists that created Rice's coaxial nanocable included (from left) Pulickel Ajayan, Jun Lou, Zheng Liu and Robert Vajtai. CREDIT: Jeff Fitlow/Rice University

Lou's and Ajayan's laboratories each specialize in fabricating and studying nanoscale materials and nanodevices that exhibit these types of intriguing quantum effects, but Ajayan and Lou said there was an element of chance to the nanocable discovery.

When the project began 18 months ago, Rice postdoctoral researcher Zheng Liu, the lead co-author of the study, intended to make pure copper wires covered with carbon. The techniques for making the wires, which are just a few nanometers wide, are well-established because the wires are often used as "interconnects" in state-of-the-art electronics. Liu used a technique known as chemical vapor deposition (CVD) to cover the wires with a thin coating of carbon. The CVD technique is also used to grow sheets of single-atom-thick carbon called graphene on films of copper.

"When people make graphene, they usually want to study the graphene and they aren't very interested in the copper," Lou said. "It's just used a platform for making the graphene."

When Liu ran some electronic tests on his first few samples, the results were far from what he expected.

"We eventually found that a of -- which is served as a dielectric layer -- was forming between the copper and the carbon," said Liu.

Upon examining other studies more closely, the team found that a few other scientists had made mention of oxidation occurring on the substrates during production.

"It's fairly well-documented, but we couldn't find anyone who'd done a detailed examination of the electronic properties of such complex interfaces," Ajayan said.

The capacitance of the new nanocable is up to 143 microfarads per centimeter squared, better than the best previous results from microcapacitors.

Lou said it may be possible to build a large-scale energy-storage device by arranging millions of the tiny nanocables side by side in large arrays.

"The nanoscale cable might also be used as a transmission line for radio frequency signals at the nanoscale," Liu said. "This could be useful as a fundamental building block in micro- and nano-sized electromechanical systems like lab-on-a-chip devices."

Explore further: Researchers make graphene hybrid

More information: A copy of the Nature Communications paper is available at: dx.doi.org/10.1038/ncomms1833

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3 / 5 (1) Jun 07, 2012

How thick is the layer of copper wires that provided 143uF/cm2?

What is the leakage? I would imagine that there is quite a bit of electron tunneling through that Cu2O layer and the voltage that the capacitors work at is quite low. Making the energy storage not quite a great as stated.
not rated yet Jun 07, 2012
Neat. Lots of capacitance. What's the Q? I presume these are going to leak to some extent, so what becomes of the energy? Are these microscopic coaxial stubs going to move?
not rated yet Jun 07, 2012
Great research. Large arrays of these would have the bulk benifit of allowing slow discharge as subsets of the array are discharged over time.
1 / 5 (1) Jun 07, 2012
It would be better if the copper dioxide could be replaced with a wicked-awesome dielectric like hafnium silicate, zirconium silicate, hafnium dioxide or zirconium dioxide
not rated yet Jun 08, 2012
Except for a short comment at the end, this article is all about capacitors (of a particular shape), not coax cables. FWIW, I prefer coaxial cables with low capacitance. But then I don't use them to store energy.
not rated yet Jun 08, 2012
Stray capacitance effects are very undesirable, and a real problem at those scales, so they cannot be used as transmission cables. Their only practical application is as large scale capacitor arrays that store a massive charge for instantaneous energy bursts, such as might be used in laser guns. They could be designed as single-use disposable discharge cartridges, since their huge energy spikes would destroy them.

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