May 24, 2006 12:00 PM PDT

Sensors: Living off scraps of energy

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The sensors devised by Georgia Tech's Wang derive their energy from zinc oxide nanowires. When the wires are bent by a probe, a negative charge is created on the side of the wire that gets stretched (the outside surface) while a positive charge builds on the compressed inside surface of the wire due to what is known as the piezoelectric effect. The charge builds at the point of contact between the probe and the wire. When the wire is relaxed, an electrical current can be detected. Although the wire continues to vibrate, current only gets discharged at the moment the strain gets relieved.

"If you continuously bent the nanowire, electricity can be generated in each cycle of the bending," Wang wrote in an e-mail. "Due to the small size, the wires are very robust. They are much tougher than bulk ceramics."

Zinc oxide works because it is both piezoelectric and a semiconductor. Other compounds also exhibit these properties, but zinc oxide has the added benefit of being nontoxic to humans, he noted.

Around 30 percent of the mechanical energy gets converted into electricity. The small size of the nanowires--which measure only 200 to 500 nanometers in length and 20 to 40 nanometers wide (a nanometer is a billionth of a meter)--means that the corresponding electrical charge produced by a single wire, or even a few million, is miniscule. Nonetheless, because of their small size they could be inserted into a variety of devices and harvest mechanical energy from walking, muscle stretching, blood pressure or even the flow of liquids. Vibration from a wire could also provide energy for these devices.

Right now, the wires are in the experimental phase. Wang next wants to build an array measuring 10 microns square containing millions of nanowires to judge how effectively energy can be generated. Wang and his group will also study ways for fabricating the wires.

Military equipment using Wang's technology could appear in three to five years, he said.

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10 comments

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Highly Misleading
These are NOT perpetual motion devices. The writer says so himself... so why on earth the misleading headline?
Posted by cryforlife (17 comments )
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Posted by baswwe (299 comments )
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He's preparing to write for those Tabloid newspapers!
nm
Posted by baswwe (299 comments )
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Doesn't know what they are
Some people don't know what perpetual motion machines would be
if they could exist - which they can't.

:-D
Posted by ewelch (767 comments )
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Headline changed
The writer does make this point as well, referring to perpetual motion: But from a practical point of view, these devices come close to that ideal because they can survive on energy that otherwise would be unused and they get it on their own.

And the headline was a question, not a statement.

But your larger point is well taken. We've changed the headline to fit the story's focus on sensors and their energy consumption.
Posted by Jon Skillings (249 comments )
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Because&
It is CNet
Posted by DeusExMachina (516 comments )
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Devices like that already in use
Many cigarette lighters no longer use a flint, but have a piezoelectric crystal imbedded in them to generate the spark that lights the flame. One watch maker created a watch that runs on a photovoltaic (also doubles as the faceplate). It stored the charge for when you were indoors or at night. I've also seen a mechanical watch that used the movement on your wrist to rewind itself. The warning lights on the school zone signs in my neighborhood are powered by solar cells which stores the charge in batteries.
Posted by Seaspray0 (9714 comments )
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