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Flexible Charge Pump: Harvesting Mechanical Energy Through Zinc Oxide Wires

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Researchers from Georgia Institute of Technology developed a new type of small-scale electric power generator, based on stretching and releasing zinc oxide wires encapsulated in a flexible plastic with two ends bonded.

They called their new device “flexible charge pump”, and it is the fourth generation of such inventions harvesting the piezoelectric properties of zinc oxide structures. It is basically a new type of mechanical-to-electricity converting device.

“The flexible charge pump offers yet another option for converting mechanical energy into electrical energy,” said Zhong Lin Wang, Regent’s professor and director of the Center for Nanostructure Characterization at the Georgia Institute of Technology. “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.”

The voltage of this piezoelectric device is not very high, in fact it’s only 45 mV (millivolts – 45 thousandth of a volt). Its uses are in very small devices, such as future nano-robots, or in medical implants. Its efficiency is estimated to about 7%.

Wang’s team had developed before other nanowire nanogenerators and microfiber generators, but they were difficult to construct and the mechanical contact required caused wear that limited how long they could operate. Because zinc oxide is soluble in water, they had to be protected from moisture: “Our new flexible charge pump resolves several key issues with our previous generators,” Wang said. “The new design would be more robust, eliminating the problem of moisture infiltration and the wearing of the structures. From a practical standpoint, this would be a major advantage.”

To boost the current produced, arrays of the flexible charge pumps could be constructed and connected in series. Multiple layers of the generators could also be built up, forming modules that could then be embedded into clothing, flags, building decorations, shoes – or even implanted in the body to power blood pressure or other sensors.

When the modules are mechanically stretched and then released, because of the piezoelectric properties, the zinc oxide material generates a piezoelectric potential that alternately builds up and then is released. A Schottky barrier controls the alternating flow of electrons, and the piezoelectric potential is the driving force of the charge pump.

“The electrons flow in and out, just like AC current,” Wang explained. “The alternating flow of electrons is the power output process.”

Constructed with zinc oxide piezoelectric fine wires with diameters of three to five microns and lengths of 200 to 300 microns, the new generator no longer depends on nanometer-scale structures. The larger size was chosen for easier fabrication, but Wang said the principles could be scaled down to the nanometer scale.

“Nanoscale materials are not required for this to work,” he said. “Larger fibers work better and are easier to work with to fabricate devices. But the same principle would apply at the nanometer scale.”

The wires are grown using a physical vapor deposition method at about 600 degrees Celsius. Using an optical microscope, the wires are then bonded onto a polyimide film and silver paste applied at both ends to serve as electrodes. The wires and electrodes were then encased in polyimide to protect them from wear and environmental degradation.

To measure the electric energy generated, the researchers subjected the substrate and attached zinc oxide wires to periodic mechanical bending created by a motor-driven mechanical arm. The bending induced tensile strain which created a piezoelectric potential field along the laterally-packaged wires. That, in turn, drove a flow of electrons into an external circuit, creating the alternating charge and discharge cycle – and corresponding current flow.

Increasing the strain rate increased the magnitude of the output electricity, both in voltage and current. Wang believes the frequency of the current is limited only by the mechanical properties of the polyimide substrate.

The researchers conducted a number of tests to verify that the current measured was produced by the generator – and not an external measurement artifact. Using the same experimental setup, they stretched carbon fibers and Kevlar fibers coated with polycrystalline zinc oxide, and did not observe current flow. The research team also developed two criteria and eight tests for ruling out experimental artifacts, Wang noted.

The researchers foresee this kind of devices being used in small-scale generators powering wireless sensing systems, that could gather information, store it and transmit the data, without using external power sources.

(Adapted from GAtech press release, via EurekAlert!)

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