May 24, 2006 12:00 PM PDT
Sensors: Living off scraps of energy
(continued from previous page)
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.
10 commentsJoin the conversation! Add your comment