Penn State team supercharges piezoelectric tech

Researchers at Penn State University have developed a device for the wrist that uses an increased amount of piezoelectric material to boost energy harvesting performance.

Created in collaboration with the University of Utah, the device captures energy from the movement of the arm and converts it into enough electricity to power a personal health device. The researchers claim the work is a significant improvement on existing energy harvesting technology that functions in a similar way and could have a range of applications in wearables and IoT.

“The devices we make using our optimised materials run somewhere between five and 50 times better than anything else that’s been reported,” said Susan Trolier-McKinstry, the Steward S Flaschen Professor of Materials Science and Engineering and Electrical Engineering at Penn State.

Trolier-McKinstry and her former doctoral student, Hong Goo Yeo, used a well-known piezoelectric material, PZT (lead zirconate titanate), and coated it on both sides of a flexible metal foil to a thickness four or five times greater than in previous devices. Increasing the volume of the active material in this way resulted in the generation of more power. The researchers also oriented the film’s crystal structure to optimise polarisation, meaning energy harvesting performance was increased. According to Trolier-McKinstry, the compressive stresses that were created in the film as it was grown on the flexible metal foils allowed the PZT films to sustain high strains without cracking, making for more robust devices suitable for wearable electronics.

“There were some good materials science challenges,” she said. “The first was how to get the film thickness high on a flexible metal foil. Then we needed to get the proper crystal orientation in order to get the strongest piezoelectric effect.”

To demonstrate the technology, the team designed a wristwatch-like device incorporating the PZT/metal foil materials. It uses a freely rotating, eccentric brass rotor with an embedded magnet, as well as multiple PZT beams with a magnet on each beam. When the magnet on the rotor approaches one of the beams, the magnets repel each other and deflect the beam. This essentially plucks the beam in a process that is referred to as frequency up-conversion, whereby the slow frequency of a rotating wrist is converted into a higher frequency oscillation. The researchers claim the design of the device is more efficient than a standard electromagnetic harvester like those used in self-powered watches.

The work is published in Advanced Functional Materials.