Zhong Lin Wang displays flexible charge pumps. (Credit: Gary Meek, GIT)

Flexible charge pumps that produce alternating current by stretching and relaxing zinc oxide wires. (Credit: GTI, Gary Meek)

Researchers at the Georgia Institute of Technology have created a micro-scale “flexible charge pump” that produces alternating current by utilizing the piezoelectric properties of cyclically stretched and released zinc oxide wires.

“The flexible charge pump offers yet another option for converting mechanical energy into electrical energy,” says project leader Zhong-Lin Wang, director of GIT’s Center for Nanostructure Characterization. Wang reports details of the project in the November issue of Nature Nanotechnology. “This adds to our family of very small-scale generators able to power devices used in medical sensing, environmental monitoring, defense technology and personal electronics,” he says, defining the pump’s significance.

Wang reports that the generator can produce up to 45 millivolts of electricity by converting about seven percent of the mechanical energy applied to the zinc-oxide wires. He notes that arrays of such generators could be used to charge low-power devices like sensors.

On GIT’s website, Wang explains that earlier nanowire generators and microfiber nanogenerators developed by his team required “intermittent contact between vertically-grown zinc oxide nanowires and an electrode or the mechanical scrubbing of nanowire-covered fibers.” Such pumps were difficult to build and had a short lifetime because their need for mechanical contact created wear that eventually wore them out. Additionally, because zinc oxide is soluble in water, so they also had to be protected from moisture.

The design of the new flexible charge pump resolves all of the earlier pumps’ shortcomings, according to Wang.  In the new pump, he says, when the zinc-oxide wires are mechanically stretched and released, their piezo properties cause the material to create a piezoelectric potential that grows and diminishes.

On GIT’s website, he explains that a “Schottky barrier” controls the electrons’ alternating flow, and that the driving force is an electric potential. “The electrons flow in and out, just like AC current,” Wang describes. “The alternating flow of electrons is the power output process.”

The newly developed generator is not comprised of nanometer-scale structures. For ease of fabrication, Wang has chosen to construct it with zinc-oxide piezoelectric fine wires that measure three to five microns in diameter and 200 to 300 microns in length. He notes, however, that the same piezoelectric principles “would apply at the nanoscale.”

The GIT research team used physical vapor deposition to grow the wires at about 600°C. Then they used an optical microscope to bond the wires onto a polyimide film and closed both ends (which serve as electrodes) with silver paste. Polyimide was then used to encapsulate both wires and electrodes to prevent them from becoming worn.

During the testing phase, a motor-driven mechanical arm was used to repeatedly bend the encased wires to measure the electric energy generated. The bending supplied the tensile strain that generated the piezoelectric potential field along the wires. “This, in turn, [drives] a flow of electrons into an external circuit, creating the alternating charge-and-discharge cycle and corresponding current flow,” Wang explains on GIT’s website. He notes that the team controls the amount of electricity produced in both voltage and current by increasing or decreasing the strain rate.

To confirm that they were measuring current produced by the generator, Wang’s team repeated the test under the same conditions with stretched carbon filters and Kevlar fibers coated with polycrystalline zinc oxide. They did not see current flow in these instances.

What does the future hold for such small-scale generators. Wang says he sees them being used in self-powered wireless sensing systems that gather, store and transmit data. “Self-powered nanotechnology could be the basis for a new industry. That’s really the only way to build independent systems,” he concludes.