Devices that harvest wasted mechanical energy could make many new advances possible—including clothing that recharges personal electronics with body movements, or implants that tap the motion of blood or organs. But making energy-harvesting devices that are compact, flexible, and, above all, efficient remains a big challenge. Now researchers at Georgia Tech have made the first nanowire-based generators that can harvest sufficient mechanical energy to power small devices, including light-emitting diodes and a liquid-crystal display.
Power flex: This material contains piezoelectric nanowires. When flexed, it produces enough power to drive a liquid-crystal display.
The generators take advantage of materials that exhibit a property called piezoelectricity. When a piezoelectric material is stressed, it can drive an electrical current (applying a current has the reverse effect, making the material flex). Piezoelectrics are already used in microphones, sensors, clocks, and other devices, but efforts to harvest biomechanical energy using them have been stymied by the fact that they are typically rigid. Piezoelectric polymers do exist, but they aren't very efficient.
Zhong Lin Wang, who directs the Center for Nanostructure Characterization at Georgia Tech, has been working on another approach: embedding tiny piezoelectric nanowires in flexible materials. Wang was the first to demonstrate the piezoelectric effect at the nanoscale in 2005; since then he has developed increasingly sophisticated nanowire generators and used them to harvest all sorts of biomechanical energy, including the movement of a running hamster. But until recently, Wang hadn't developed anything capable of harvesting enough power to actually run a device.
In a paper published online last week in the journal Nano Letters, Wang's group describes using a nanogenerator containing more nanowires, over a larger area, to drive a small liquid crystal display. To make the generator, Wang's team dripped a solution containing zinc-oxide nanowires onto a thin metal electrode sitting on a sheet of plastic, creating several layers of the wires. They then covered the material with a polymer and topped it with an electrode. The resulting device is about 1.5 by two centimeters and, when compressed 4 percent every second, it produces about two volts, enough to drive a liquid-crystal display taken from a calculator. "We were generating 50 millivolts in the past, so this is an enhancement of about 20 times," says Wang.
In a paper published in Nano Letters this summer, Wang demonstrated a nanogenerator capable producing 11 milliwatts per cubic centimeter—enough to light up an LED. Wang notes that a pacemaker requires 5 milliwatts to run, an iPod 80 milliwatts. "We're almost there," he says.
The devices made by the Georgia Tech group are "getting into the realm where the power output is reasonable," says Michael McAlpine, professor of mechanical engineering at Princeton University and a 2010 TR35 awardee. "Getting impressive power outputs is a matter of scaling up," he adds.
Both Wang and McAlpine are looking to more efficient materials for making nanogenerators. Both have recently demonstrated making nanowires from PZT, a crystalline material that is standard in commercial piezoelectric devices. PZT, a compound that contains lead, zirconium, and titanium, is the most efficient piezoelectric material known, but making it into nanowires has been tricky because there are no good catalysts for growing PZT nanowires.
Wang and McAlpine have found different solutions to this problem. Wang treats his starting solution at high temperature and pressure, which does away with the need for an efficient catalyst. McAlpine grows a flat film of PZT, and then uses a mask to pattern nanowires through chemical etching. Energy harvesters made from PZT nanowires aren't as efficient as the zinc-oxide ones yet, but McAlpine says this is because he and Wang have only just begun to work with them.
By Katherine Bourzac
From Technology Review