Stretching wires produces power

Researchers have developed a type of small-scale electric power generator able to produce alternating current through the cyclical stretching and releasing of zinc oxide wires encapsulated in a flexible plastic substrate with two ends bonded.

The flexible charge pump generator is the fourth generation of devices designed to produce electrical current by using the piezoelectric properties of zinc oxide structures to harvest mechanical energy from the environment.

“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 Centre 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, defence and personal electronics.”

The generator can produce an oscillating output of up to 45 mV, converting nearly 7% of the mechanical energy applied directly to the zinc oxide wires into electricity.

The research has been supported by the US Department of Energy, the National Science Foundation, the Air Force Office of Scientific Research and the Emory-Georgia Tech Centre for Cancer Nanotechnology Excellence.

Earlier nanowire nanogenerators and microfibre nanogenerators developed by Wang and his research team depended on intermittent contact between vertically grown zinc oxide nanowires and an electrode, or the mechanical scrubbing of nanowire-covered fibres.

These devices were difficult to construct, and the mechanical contact required caused wear that limited how long they could operate. And because zinc oxide is soluble in water, they had to be protected from moisture.

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 generator no longer depends on nanometre-scale structures. The larger size was chosen for easier fabrication, but Wang said the principles could be scaled down to the nanometre scale.

The wires are grown using a physical vapour deposition method at about 600°C. 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 are then encased in polyimide to protect them from wear and environmental degradation.