When Xudong Wang finished his PhD in materials science at Georgia Tech at the end of 2005, he knew he had a good thing going. He opted to stay put in the lab of Zhong Lin Wang (no relation), sure that he and his lab mates were close to creating a new nanotech-based generator–an invention they felt could change the future of nanotechnology.
His risk paid off earlier this year when Science published a paper he coauthored, describing a novel device that converts ultrasonic waves–high-frequency mechanical vibrations–into electricity. The tiny device turns out a steady 0.5 nanoamperes of current that engineers may one day be able use to power implantable biosensors, remote environmental monitors, and more. “It’s a very cool concept,” says Peidong Yang, a nanowire researcher at the University of California, Berkeley. “Vibrational energy is everywhere.” If Wang’s devices can harness it cheaply, “the impact could be big,” Yang says.
The generator is the culmination of several remarkable advances made by Wang since he arrived in Z. L. Wang’s lab from China in 2002. Others had made nanowires of zinc oxide (ZnO), a versatile optical, semiconductor, and piezoelectric material, but the production process typically left them tangled like spaghetti. Many prospective uses of nanowires, however, require that they form an orderly array. By 2004, Xudong had found a way to use gold to catalyze the emergence of an organized forest of wires from a vapor of zinc oxide dust.
While Xudong was finishing up his PhD, Z. L. Wang and Jinhui Song, another graduate student in the lab, showed that they could generate a tiny electric current by bending individual ZnO nanowires with the tip of an atomic force microscope. Still, to make practical energy harvesters, the researchers needed a way to collect energy from thousands of nanowires flexing simultaneously.
They began with one of Xudong Wang’s miniature ZnO forests, grown atop an electrode made from gallium nitride, sapphire, or a conducting polymer. Xudong capped this with a second electrode made of platinum-coated silicon and studded with parallel rows of tiny peaks and trenches, like lines of saw teeth. He then used ultrasound waves to vibrate the electrodes. The motion squeezed the two electrodes together, causing the nanowires between them to flex and generate a current; the current flowed through the platinum coating and into an external circuit.
Conceiving the generator was a group effort, but Z. L. Wang gives Xudong credit for pulling off the demonstration. “Anything you can think up, he can make it work,” Z. L. says.
The two-square-millimeter devices turn out just a trickle of power, but in the months since its Science paper appeared, the team has already boosted the devices’ output current 30-fold. And there’s plenty of room for improvement: ensuring that all the nanowires are actively generating current, for example, could boost the power output to as much as four watts per cubic centimeter. “If we do that, we can power portable electronics such as cell phones,” Xudong says. The group is also trying to make versions of the device that generate current in response to lower-frequency sound waves and to mechanical vibrations. That could allow nanotechnologists to harvest energy anywhere, from the interior of a pulsing blood vessel to the chassis of a car rattling down the highway.
Credit: Animation Technology Review, Source: www.jwestdesign.com
–Robert F. Service