Nanodevice Powered by Motion

Every move you make, every step you take, you can generate electricity. By cramming 20,000 nanowires into three square centimeters, scientists from Georgia Tech have created the world's first device powered solely by piezoelectric materials.

A piezoelectric material is something that, when pushed or pulled, generates a mild electrical charge. Within three to five years piezoeleectric nanowires, woven into a cotton shirt or housed in a shoe heel, could charge a cell phone or laptop battery after even a short walk.

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"This is a key step to designing technology that will be useful in the near future," said Z.L. Wang, a professor at Georgia Tech and co-author of two new papers in Nature Nanotechnology and Advanced Materials.

Quartz and cane sugar crystals are common piezoelectric materials; when pressure is applied, a very small electrical current is produced. Over the last five to six years, however, scientists have worked with cheap zinc oxide and powerful lead zirconate titanate or PZT.

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While the power generated from these materials has steadily risen into the millivolt range, it hasn't yet produced enough power to actually power a device. Now, according to the two new papers published by Wang's group at Georgia Tech, piezoelectrics can generate voltages up to 1.26 volts, and soon will produce voltages much higher than that.

Wang's group used plentiful and easy-to-manipulate zinc oxide nanowires to create their nanogenerator. An individual zinc oxide nanowire is so tiny that it's invisible to the human eye, measuring anywhere between 50 and 200 nanometers across and about five microns in length.

Twenty thousand nanowires, placed side by side and end to end, covers three square centimeters, with two thin electrodes hanging off either end.

This unique arrangement maximizes the electricity the piezoelectric nanowires can create. The wires work with each other, amplifying the electrical charge to record levels as the single layer is pushed back and forth with only the most slight and gentle of nudges.

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Pushing the arranged nanowires harder or faster would bump the power output up to 30 times without damaging the device. If more powerful, and more expensive, gallium nitride replaced the cheap zinc oxide nanowires the power output could increase another 10 times.

That's more than enough energy to power most consumer devices, if the piezoelectric material were in motion constantly.

If a device weren't in constant motion however, no energy would flow, and any electrical device connected to the nanowires would shut off. For laptops and cell phones, which have batteries build into them, this doesn't matter; the electricity from the nanowires will charge the battery.

An extra few minutes of talk time would be great for cell phone owners, but Wang envisions this nanowires powering a range of electrical devices.

Other tiny piezoelectric-powered devices could sense fires and gather weather data in areas outside the reach of traditional power grids. To power such small sensors Wang will create tiny batteries or supercapacitors to store the electricity generated by his advanced piezoelectric nanowires.

Other scientists are enthusiastic about the new nanowires.

"I think the major accomplishment is that typical piezoelectric nanowires can produce about 30 milli volts," said Liwei Lin, a professor at the University of California, Berkeley, who also does piezoelectric research. "This time [Wang] actually got a huge output."

Lin's work is in creating single piezoelectric nanowires much longer than Wang's, long enough to be woven into clothing. It will likely be three to five years before either Wang's or Lin's work will be found in a commercial product, but the Lin notes that piezoelectrics has made tremendous progress during the last few years, much of it led by Wang. The next few years will be even more exciting.

"My prediction is that in the next few years you will see commercial products," said Lin.