29 July 2009

Harvesting electricity under pressure

Materials are capable of some pretty remarkable things when they are under pressure; they may even produce electricity.

Driven by a self-propelled vision, researchers want to investigate the potential of piezoelectrics, which naturally produce electricity when subjected to strain. The goal is to use piezoelectrics to create nanodevices that can power electronics, such as cell phones, MP3 players, and even biomedical implants.

“Nanodevices using piezoelectric materials will be light, environmentally friendly, and draw on inexhaustible energy supplies,” said University of Houston (UH) Associate Professor Pradeep Sharma. “Imagine a sensor on the wing of a plane or a satellite. Do we really want to change its battery every time its power source gets exhausted? Hard-to-access devices could be self-powered.”

Piezoelectric materials convert mechanical energy into electrical energy, Sharma said.

“Indeed, gas lighters used in most homes are based on this,” he said. “These future piezoelectric nanodevices will also generate an electrical current in response to mechanical stimuli. Then, the energy will be stored in batteries or, even better, in nanocapacitors for use when needed.”

Although piezoelectrics have been around for years, Sharma’s team is exploring new possibilities by beefing up the effect in natural piezoelectrics. Doing so requires understanding the phenomenon that spurs piezoelectricity, known as “flexoelectricity.”

“Flexoelectricity, at the nanoscale, allows you to coax ordinary material to behave like a piezoelectric one. Perhaps more importantly, this phenomenon exists in materials that are already piezoelectric. You can make the effect even larger,” Sharma said.

In one case, the piezoelectricity in barium titanate can increase by 300% when the material reduces down to a 2-nanometer-beam and researchers then apply pressure. “Thus, you’ll take an ordinary piezoelectric material and really give it some juice,” he said.

Sharma underscores the flexoelectric effect is a function of size—and the smaller the better, at least for generating piezoelectric power. Materials with nanoscale features, such as nanoscale thin plates stacked on each other or materials with particles or holes the size of a few nanometers, exhibit a much larger flexoelectric effect, he said.

Ramanan Krishnamoorti, chairman of the UH department of chemical and biomolecular engineering, is working with Sharma to embed classes of nanostructures in polymers to create unusual types of piezoelectrics.

Meanwhile, Sharma and UH Professor Ken White also found the electrical activity caused by flexoelectricity affects a material’s resiliency. They tested their theory that elasticity of a material would be altered by flexoelectricity-caused electrical activity by poking the material with a sophisticated needle.

“We basically predicted that when you poke it, because of this electrical activity, depending upon how big a crater you create, your elastic behavior will change,” Sharma said. “It’s not supposed to. Ordinarily, whether you make a big crater or small crater, if you calculate how stiff it is or soft it is, it’ll give you the same answer—a constant.”

White and Sharma conducted several experiments on single crystals of materials. “By monitoring the stiffness of the material as the crater became larger and larger,” White said, “we discovered a change in elasticity relative to size, which could only be explained by flexoelectric effect.”

Though research on piezoelectrics has been ongoing, the fabrication of piezoelectric nanostructures remains challenging, White said. So far, the amount of power researchers can harvest is still too low to actually power wearable devices, he said. However, once the innovation process ratchets up a bit, they may find more efficient electric storage solutions, like nanocapacitors.

Sharma said he would like to harvest wasted energy from a variety of sources.

“In principle, any human activities—for example, walking, jumping, swimming—will produce a certain amount of energy,” he said, and that could convert to electricity by piezoelectric nanostructures in shoes or in backpacks.

For related information, go to www.isa.org/manufacturing_automation.