How to Get Electricity from Vibrations

Last year, Georgia Institute of Technology researcher 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. "We are creating a portable, adaptable and cost-effective technology for powering small electronic devices," said Wang. "Our nanogenerator allows us to harvest or recycle energy from many sources."

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 to learn how to control the precise length of grown nanowires. Too short, and the wires will not reach the top zigzag electrode when they flex; too long, and they will be permanently shorted out against the upper electrode. "Our last challenge will be finding an inexpensive way to package the nanogenerator," said Wang.

The applications of these findings could mean miniature sensors that could be sprinkled in the environment or implanted in the body.