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Using ultrasonic waves to vibrate an array of zinc-oxide nanowires, researchers at Georgia Tech have made a tiny generator that can produce direct current. By taking advantage of the fact that zinc-oxide nanowires are piezoelectric--they can convert mechanical energy into electricity--and by finding a way to collect electricity from multiple nanowires, the researchers have taken a big step toward a practical nanoscale power generator. "We can make each and every wire simultaneously and continuously produce electricity," says Zhong Lin Wang, a professor of materials science at Georgia Tech, who led the work. In a Science paper published this week, Wang and his colleagues demonstrate a prototype device, about two millimeters square, that generates around 0.5 nanoamperes of current for more than an hour. "The technique essentially provides a new method of power generation," says Pulickel Ajayan, a materials engineering professor at Rennselaer Polytechnic Institute. He says that the generator could be coupled with devices that are difficult or inefficient to power using conventional means. One important application is powering implantable biological sensors. According to Thomas Thundat, who researches nanoscale biological sensors at Oak Ridge National Laboratory, current battery technology limits the use of microelectromechanical sensors that measure cancer biomarkers, blood pH, and glucose. These sensors are getting smaller and smaller, but conventional chemical batteries can't keep up. "[The batteries] are huge and they run out of power...most of the time it's the battery that's big compared to the sensing part," Thundat says. "We have always been looking for very small power sources that don't need refilling." The new nanowire generator looks like a promising answer, he says. It could be implanted in the body, and, driven by muscle contractions, blood flow, or external vibrations transmitted through tissue, it could power the sensors. The generator could also drive nanodevices. Wang's research group has previously made nanowire pressure sensors that can detect extremely small piconewton forces as well as nanowire gas sensors. (See "A Nano Pressure Sensor."  Instead of an external battery, these devices could run on wind or water flow using the new generator. A key innovation that has led to the nanowire generator is a new electrode design. The surface of the platinum-coated electrode has a zigzag shape like the teeth of a saw: it has alternating parallel peaks and trenches. This zigzag electrode goes on top of an array of upright zinc-oxide nanowires, and its teeth can push many nanowires at the same time if it moves up and down. To vibrate the electrode, the researchers package the device, put it in water, and expose it to ultrasonic waves. As the zigzag electrode moves up and down, its peaks push and bend the nanowires, which generate electric current that the electrode collects simultaneously. "The wires can be compressed, can be vibrated left or right--it doesn't matter: all the current adds up in the same direction," Wang says. This is the first demonstration of a direct current output from nanowires that are driven by mechanical energy, says Charles Lieber, a chemistry professor at Harvard University. The new development is a "key step towards novel, cost-effective, adaptable, and mobile applications of nanogenerators in nanotechnology," he says. For real-world applications, the current generated by the nanogenerator would need to be higher and more stable. Wang's research group is working on improvements toward that goal. Right now, the nanowires are grown randomly, and the researchers estimate that anywhere between 250 and 1,000 nanowires contribute to the current. This is less than 1 percent of all the wires in the array, Wang says. An important next step is to grow a more regular array of nanowires that are uniform in size and height. Matching the nanowire pattern with the pattern on the electrode would utilize all the nanowires, increasing the current output and making it more stable, he says. The research team also needs to increase the generator's lifetime. It runs for a little more than an hour right now, and Wang says the researchers are not sure why it dies after that time. As a proof of concept, though, Thundat says that this work is a "major advancement in the power-generation area." |
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