Good vibrations: Ceramic material harvests electricity from waste energy

UNIVERISITY PARK, Pa. — There’s a lion’s share of potential energy in the vibrations produced by footsteps on dance floors, exercise machines in the gym, or the engines of cars, planes or construction equipment. Some tech companies have already begun to harvest electricity from waste vibrations to power lights and recharge batteries using a class of piezoelectric ceramic materials, which emit electrical charges when stepped on or manipulated.

Now, a team led by materials scientists at Penn State has expanded these early efforts of energy harvesting by improving the structure and chemistry of a piezoelectric material made of potassium sodium niobate, or KNN. The improved ceramic samples are thermally stable, fatigue resistant, less dense and perform competitively to existing lead-based piezoelectric materials, the researchers said.

Their work, which was published in the journal Small, could help replace toxic lead-based materials currently used in piezoelectric materials, the team said.

“Mechanical vibrations are everywhere, produced by people or engines,” said first author Aman Nanda, a doctoral student in materials science and engineering at Penn State. “We can place a piezoelectric energy harvester under dance floors and corridors, or under bridges and parking decks, to harvest the energy from those mechanical sources. Because of the lightweight design of our KNN material, we could also include them in aircraft — which wasn’t previously possible with lead-based materials — to harvest the vibrations during flights, even at high altitudes.”

Energy harvesters have a cantilever design, where a stiff element is fixed on one end and unattached on the other, Nanda explained. Since ceramic materials are brittle, special care and device designs are needed to apply them in real applications to handle mechanical stress. 

When pressed, the cantilever vibrates and generates electricity through the piezoelectric effect of the material that converts mechanical energy into power.

To replace lead and produce a more lightweight piezoelectric ceramic, the researchers systematically modified the structure and chemistry of KNN. They first added a magnetic material, manganese, to its chemical composition. Then they adjusted the grain growth, or the size of individual crystals within the microstructure, through heat treatment.

“Aman optimized the material with specific elements to improve the properties,” said co-corresponding author Mike Lanagan, professor of engineering science and mechanics at Penn State. “These materials have been around a while in terms of chemistry, but he has done a lot more work in making the chemistries better by changing the composition and synthesis procedures, such as experimenting with different heat times, temperatures and structures of the material.”  

While grains typically grow randomly in all directions, the researchers used heat and specific fabrication approaches to control the grain growth so that they would all grow in a similar direction.

“With the appropriate synthesis temperature range, we achieved unidirectional grain growth,” Nanda said. “This resulted in enhanced functional properties, such as mechanical strength and toughness, in the direction of the grain alignment, as well as improved piezoelectric response.”

This was the first lead-free piezoelectric material that performed competitively compared to lead-based materials, researchers said, based on a comparison of the amount of voltage generated from a mechanical vibration. In laboratory tests, the improved KNN material harvested a similar amount of energy as a conventional lead-based material.

As a next step, the researchers will test and explore the potential uses of the material. In addition to energy harvesting, the team said, the new material could be used in sensors that detect weight, sound waves, position, air pressure and light.

“Since lead-free materials are biocompatible, our new KNN-based material also opens the possibility to integrate the devices made from these materials in biomedical applications, such as self-powered pacemakers or neural stimulating devices,” said co-corresponding author Bed Poudel, research professor of materials science and engineering at Penn State.

In addition to Nanda, Poudel and Lanagan, the Penn State-affiliated co-authors include Mark Fanton, research professor in the Applied Research Laboratory; and Sumanta Kumar Karan and Shankar Kunwar, postdoctoral scholars in materials science and engineering. For a full list of authors and their affiliations, see Small.

The U.S. National Science Foundation, the University of Minnesota and the Office of Naval Research supported this work.

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