Opportunities for harvesting energy from the environment, for instance from vibrations, has prompted more than a dozen startups to create microsized power generators. Now even smaller, nanoscale generators have been demonstrated to produce electrical power from vibrations.

A professor with Georgia Tech's School of Materials Science and Engineering Zhong Lin Wang demonstrated that zinc oxide nanowires produce current when flexed with an atomic-force microscope. Now Wang has harnessed that nanogenerator effect in an array of nanowires that could produce as much as 4 watts/cubic centimeter.

The technique fixes one end of the piezoelectric nanowires against a gallium arsenide, sapphire or flexible polymer substrate, which serves as one electrode, then grows the wires vertically upward, spaced about a half-micron apart in an aligned array. A zigzag-surfaced upper silicon electrode, made more conductive by a platinum coating, permits the nanowires to protrude into the small gaps between its slanted surfaces.

When the piezoelectric wires are flexed by vibrations—or, as in the Georgia Tech test, by ultrasonic waves—they bend, creating more charge on one side than on the other. When a nanowire bends far enough to touch the slanted surface of the top electrode, its charge is transferred, resulting in an average current proportional to the vibration.

Thus far, the nanogenerator has only produced nanoamperes of power—too little for powering macroscopic devices. But Wang is confident his next prototype will produce microamperes, capable of powering wireless sensor network nodes.

"Our theoretically estimated power density is 1 to 4 W/cubic centimeter," said Wang. "Our next goal is to grow bigger nanowire arrays to raise the nanogenerators' current to microamperes, and to build a three-dimensional structure to raise the voltage to 0.5 V so that it can be used for powering devices."

To meet that goal, the researchers will have go learn how to control the precise length of grown nanowires.

The nanowire project is funded by the Defense Advanced Research Projects Agency, the National Science Foundation and the Emory-Georgia Tech Center of Cancer Nanotechnology Excellence.

By Vasil Sidorov on November 11, 2009 from Georgia Tech

sidorovvasil@gmail.com