Page last updated at 12:38 GMT, Thursday, 4 December 2008

A step closer to self-powered kit

Batteries (BBC)
Small devices could soon be powered with nothing more than ambient sound

Engineers have doubled the efficiency of piezoelectric devices that harvest energy from movement and vibration.

The trick lies purely in the size of the devices: a narrow range of thicknesses around 5,000 times thinner than a human hair.

The result means that "self-powered" devices, such as phones that charge when you speak into them, are one step closer to reality.

The research was reported in the journal Physical Review B.

The piezoelectric effect occurs in some crystalline and ceramic materials. Stretching or compressing them causes a separation of electric charge across their width, and that sets up a voltage that can be put to use.

Such piezoelectric materials have been in use for years in devices such as electronic lighters and microphones, where pressure from a thumb or even a sound wave is harvested.

More recently, plans to engineer piezoelectrics that collect energy from footsteps or the motion of clothing have made it to the drawing board. Several clubs even incorporate piezoelectrics into their dance floors, recycling a small part of the energy imparted by clubgoers.

Size matters

However, the behaviour of materials in comparatively large devices can change radically when pared down to the nanometre scale.

What Tahir Cagin and his colleagues at Texas A&M University have found is that when piezoelectric materials are made in a narrow size range around 20 nanometres (billionths of a metre), a new effect comes into play.

Dancefloor neon Earth graphic
A piezo layer below this dancefloor makes clubbing more green

This "flexoelectric effect" produces a voltage from twisting and bending, instead of the uniform compression or stretching as in piezoelectricity.

The researchers proved that the effect can be maximised in nano-scale cantilevers - beams like tiny diving boards that generate a voltage - by tailoring the cantilevers' shape.

The theoretical study shows that the effect could as much as triple the amount of electricity available, gram for gram, from piezoelectric materials.

Piezoelectric materials have long been used in microphones and pick-ups for acoustic instruments, with the pressure wave of sound or simple vibration creating an electric signal.

The new results mean that such sounds or vibrations produce significantly more charge that can be gathered up and put to use.

"Even the disturbances in the form of sound waves ... may be harvested for powering nano- and micro-devices of the future if these materials are processed and manufactured appropriately for this purpose," Professor Cagin said.

That means that instead of the tiny electric signal produced in a microphone, piezoelectrics tailored at the nano-scale could directly power small devices or charge a battery.

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