The Potential of ZnO Nanotubes
Today, Sniper Sensors; Tomorrow, Cheap Power?
Iraqi snipers have gotten better since the U.S. invasion, but so has countersniper technology.
Acoustic detectors, laser spotters and infrared sensors have improved U.S. troops' ability to zero in on enemy sharpshooters. Now nanotechnology promises to produce a generation of much more sensitive sniper-spotters.
The new technology uses zinc oxide nanowires, which are extremely sensitive to the ultraviolet light in the muzzle flash of a firearm.
"It's very exciting material," Ashok Sood said of the zinc oxide (ZnO) nanowires.
Invented in 2001, the nanowires are microscopically tiny hollow tubes about one-thousandth the diameter of a human hair. They exhibit some extraordinary properties: They can create electricity out of light; they can create light out of electricity; they can generate electricity from motion; they can be used to detect and identify gases.
Sood, who is president of Magnolia Optical Technologies, plans to use them to build battlefield sensors for detecting the ultraviolet light generated by gunshots in order to pinpoint the location of snipers.
In April, Magnolia was awarded a $500,000 contract by the U.S. Army and the Defense Advanced Research Projects Agency (DARPA) to continue work on the sniper sensors. So far, the small, high-tech firm in Woburn, Mass., has received $1.25 million in DARPA research contracts for the effort.
Gunfire emits a flash of ultraviolet light that cannot be seen by the human eye, but can be detected by ZnO nanowires.
When struck by ultraviolet photons, the nanowires convert energy from the photons into tiny electrical signals, Sood said. A computer processes the signals to tell soldiers where the sniper is.
Sensors that use ZnO nano-wires may be thousands of times more sensitive than infrared sensors, said Zhong Wang, a leading nanotechnology researcher and professor at Georgia Tech's Center for Nanostructure Characterization.
Wang is working with Magnolia Optical on the sensors. Sood said he hopes to have a prototype ready for the Army as early as a year from now.
Besides spotting snipers, such sensors are expected to prove valuable for detecting other battlefield sources of ultraviolet light such as missiles and jet aircraft.
As they burn fuel, rocket and jet engines emit invisible but detectable plumes of ultraviolet light. Sensors built on ZnO nano-wire technology are expected to be more sensitive than current UV sensors.
Their ability to convert light into electricity also makes ZnO nanowires an attractive material for use in solar cells, Sood said.
The tubular shape of the wires gives them much greater surface area for absorbing sunlight, and the nanowires provide a more efficient means to shuttle electrons to the solar cell's electrodes as light is converted into electricity.
ZnO nanowires can also turn electricity into light.
Sood said that when electricity is applied to them, the nanowires can be turned into light-emitting diodes - LEDs. They emit "very bright ultraviolet light." One use might be for battlefield water purification, he said. Exposing water to the ultraviolet light for about five minutes kills the bacteria in it.
In a similar manner, ZnO nano-wires may be used to produce lasers.
Jia Grace Lu, a researcher at the University of Southern California, says the nanowires might also be used to make sensors for detecting dangerous chemicals.
ZnO nanowires are semiconductors; that is, their ability to conduct changes under certain conditions. When exposed to different chemicals, their conductivity increases or decreases depending on the substance.
In the presence of carbon mon-oxide, for example, ZnO nano-wires become more conductive. In the presence of nitrogen dioxide, however, their ability to conduct electricity decreases.
According to Lu, ZnO nano-wires respond to the presence of different chemicals "with exquisite precision."
So by measuring the changes in conductivity, it should be possible to determine what chemicals are present.
In the most eye-catching recent development, Georgia Tech announced in February that Wang's research group had developed textile fibers that contain ZnO nanowires, and that clothing made from the fibers would turn motion from the wearer into electricity.
"If we can combine many of these fibers in double or triple layers in clothing, we could provide a flexible, foldable and wearable power source that, for example, would allow people to generate their own electrical current while walking," Wang said at the time.
With uniforms that incorporate ZnO nanowires, soldiers might recharge batteries or power small electronic devices.
The nanowires possess "piezoelectric" properties. That is, bending them creates electricity. As each wire is bent, there is a positive charge on one side and a negative charge on the other. When straightened, a tiny electrical current is produced.
Wang dubbed the current-producing nanowires "nanogenerators."
But power-producing clothing is not in the stores or on the battlefield yet. They're "very new. We still have a lot of work to do" to prepare them for real-world applications, Wang said. ■
E-mail: bmatthews@defensenews.com.
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