Skip to main content
Polymers

Polymers

New ‘flexoelectret’ material could create high voltages when bent

26 Apr 2019
Experimental setup
Flexible terms: the experimental setup showing the flexoelectret material with negative charges shown in red. The material is deformed by pushing down in the middle and a voltage is measured between the top and bottom surfaces. (Courtesy: Xin Wen et al/Phys. Rev. Lett.)

A soft dielectric material that could create a relatively high voltage when bent has been created by physicists in China. Qian Deng and colleagues at Xi’an Jiaotong University describe their material as the first-ever “flexoelectret”. It was made by embedding a charged polymer layer in the middle of a dielectric silicone rubber material. With some improvements, the new material could find a wide range of applications including wearable electronics.

When some materials are deformed non-uniformly, a strain gradient drives positive and negative ions apart to create a voltage across the material. Known as flexoelectricity, this effect is observed in many dielectric materials, including crystals, polymers, and semiconductors.

The effect is usually strongest in brittle ceramic materials, which are unsuitable for practical applications such as stretchable electronics. While far higher strain gradients can be achieved in softer dielectric materials, flexoelectric voltages generated in these materials are typically several orders of magnitude smaller than in ceramics.

Flexible electret

Deng and colleagues have now come up with a way of generating much higher voltages in a deformable dielectric material by adding a layer of permanent negative charges (called an electret) within the material. To test the idea, the physicists embedded a charged polymer layer in the central plane of a 10 cm-long bar of silicone rubber, forming a flexible electret – or flexoelectret.

In its undeformed state, the dielectric material is polarized equally and in opposite directions on either side of the charge layer – which means that there is no voltage between the top and bottom of the bar. However, if the bar is fixed at both ends and a force pushes down on its middle, the regions above and below the charged layer are deformed in different ways. The polarizations are no longer equal and opposite – leading to a voltage across the top and bottom of the bar. Deng and colleagues measured a “flexoelectric coefficient” that is 100 times greater than that of silicone rubber alone. The flexoelectric coefficient is a measure of the electric polarization that occurs when a material develops a strain gradient.

The team says that several improvements will be needed before the flexoelectret becomes a commercial possibility. Perhaps their most significant challenge will be to maintain the charge of the bar’s embedded layer, which tends to leak over time. However, Deng and colleagues hope that this and other challenges could be overcome within the next five years.

With these improvements, the physicists believe the flexoelectret will prove to be biocompatible, environmentally friendly, and suitable for a wide variety of commercial applications. In particular, it could lead to a new generation of flexible sensors, actuators, and energy harvesters that could be used in stretchable and wearable electronics.

The research is described in Physical Review Letters.

  • This article was updated on 1 May to correct an error regarding our misinterpretation of a voltage measurement reported by the researchers.
Copyright © 2024 by IOP Publishing Ltd and individual contributors