Piezoelectricity: press to power

An acoustic violin producing music without strings? Science fiction, you will scoff. Science fact, if the piezoelectric effect is deployed.

A company in the US has produced a 3D-printed violin based on the piezoelectric effect, wherein pressure applied on the violin’s surface by the bow gets converted into an electric signal which results in sound.

The violin is just one among many applications of the piezoelectric effect, first discovered in 1880 by physicists Jacques and Pierre Curie. They found that in crystals such as quartz and tourmaline, pressure generates an electrical charge. This they called the piezoelectric effect (piezo in Greek means to press or squeeze). In simple terms, piezoelectric effect is the generation of electrical energy when mechanical stress is applied to certain kinds of crystals. The reverse is also true: which means that a mechanical action is generated when a material is subjected to an electric charge.

Materials displaying a piezoelectric effect have come a long way since 1880. Till the 1940s, natural crystals such as quartz were used in a range of applications. Then came piezoelectric ceramics as they were found to be way more effective than natural crystals. Between 1965 and 1980, Japan made huge strides in developing new piezo-ceramic materials, which, in turn, led to whole new applications and processes.

Today, technology forms a part of our lives without us even realizing its importance. Lighters, for instance, or wristwatches, even ultrasound used for medical purposes work on the principles of piezoelectricity.

With fossil fuel reserves running low and the quest for clean energy sources gathering pace, piezoelectricity as a pollution-free solution is gaining ground. Intense research is on across the world to come up with new products that use the piezoelectric effect, to cut down the dependence on traditional energy sources. For instance, scientists are working on shoes fitted with piezo cells that can enable the wearers to generate enough power to charge gadgets such as mobile phones or music players even as they walk.

“The piezoelectric effect has been harnessed for the development of power-harvesting devices which scavenge energy from ambient vibrations or mechanical disturbances. Such piezoelectric power-harvesting devices find applications in wearable electronics without the need for replacing batteries and other finite sources of power,” said M.C. Ray, a professor at the Department of Mechanical Engineering at the Indian Institute of Technology, Kharagpur. To drive home his point, he said a piezoelectric power harvester mounted inside a shoe could generate as much as 8.4 watts of power.

Beyond energy, piezoelectric materials have, over the past few decades, resulted in a range of products and applications cutting across diverse areas. They are used by the automobile industry for ensuring keyless entry, as seat-belt buzzers and as airbag sensors, in consumer products such as lighters and depth finders, in medical devices and by armed forces in sonars, among others.

The principle can be used to generate energy from cars stuck in a traffic jam. Pratyush Maini, a Gurgaon-based student, was among the winners of the annual national science competition organized by the ministry of science and technology for this application. His model involves placing piezoelectric generators beneath intersections which would convert the force exerted by cars to electricity.

But there are limits to piezoelectricity and thus researchers are looking to improve the properties of piezoelectric materials. One issue that is being addressed by them is the use of piezoelectric materials in harsh environments.

Ray said his research had revealed that properties of carbon nanotubes (CNT) could be exploited to improve the piezoelectric properties of the existing composites. “It is found that CNTs used as fibre reinforcement and radially grown straight and wavy CNTs on the surface of piezoelectric fibres significantly improve the piezoelectric properties of existing piezoelectric composites,” he added.

The professor also had a note for the future. “Most recently, extensive research on harnessing flexoelectricity for developing nano sensors, nano actuators and nano generators is on. Flexoelectric effect is also an electromechanical coupling effect. A dramatic enhancement of harvested power by a piezoelectric cantilever nanobeam has reportedly been achieved. The flexoelectric effect has revealed the possibility of developing nano generators from piezoelectric materials,” Ray explained.

Besides traditional uses, piezoelectricity technology is being tried in several innovative ways across the world. The technology is being deployed by authorities in Tokyo at a subway station to generate power from movement of people using it. Similarly, a dance club in the Netherlands used it in dance floor to generate power. One of the latest applications is in a lawn tennis racquet wherein it reduces transmission of shock vibration to arms of players.

Wearables are also among the next big things that could be the result of piezoelectric applications.

Scientists are working on piezoelectric fibres which are not only flexible and strong but breathable, too. They are working to integrate them into textiles so that they can be then used in a wide range of health monitoring and other applications. There are many futuristic uses of piezoelectric effect which scientists are trying to harness in order to move towards an infinite source of energy. Researchers are also working on developing materials to try to generate energy from movement of wind or waves under the ocean.