Enhanced energy harvesting with smart materials

Researchers have found a way to enhance smart materials known as electrostrictive polymers to improve their mechanical energy harvesting performance — that is, the ability for electronic devices to pull ambient energy from their surrounding environment and convert it into electrical energy for stored power.

Conducted by a team from France’s National Institute of Applied Sciences of Lyon (INSA de Lyon), the research has the potential to introduce an alternative power supply for mobile and autonomous wireless electronic devices. It has been published in the journal Applied Physical Letters.

The researchers had been exploring electrostrictive polymers due to their ability to produce field-induced strain when exposed to an applied external electric field. As explained by lead author Xunqian Yin, “This strain has a quadric equation described by the second degree-relationship with the applied electric field.”

The group’s work centres on the piezoelectric effect, which refers to the accumulation of electric charge in certain crystalline solids without a symmetric centre in response to an applied mechanical stress or strain. The problem, according to Yin, was that “the electrostrictive polymers are non-piezoelectric in nature”.

“But a pseudo-piezoelectric effect can be induced for electrostrictive polymers when they’re exposed to a large applied bias DC electric field,” he continued. “As a result, the pseudo-piezoelectric effect was adopted for the mechanical energy harvesting via electrostrictive polymers.”

The group studied the influences on mechanical energy harvesting of a variety of operating conditions, including large applied bias DC electric field, as well as the amplitude and frequency of applied external strain. They discovered that increasing the applied bias provides a way to improve the energy conversion efficiency.

In particular, when they worked with a plasticiser-modified ‘terpolymer’, it offered improved mechanical energy harvesting performance, especially when imposed to the same force level. The researchers noted, “A maximum generated short-circuit current of 3.635 μA (much higher than the leakage current) and a power density of 607 μW/cm³ were observed for modified terpolymer.”

The introduction of the plasticiser resulted in an improvement in the terpolymer’s electromechanical coefficient and, in turn, its pseudo-piezoelectric coefficient. This has led Yin to speculate that “the modified terpolymer thin film can lead to piezoelectric active sensors, such as force sensors”.

“Combining these sensors with advanced fabrication technologies — inkjet or 3D printing — should make it easier to build a network of sensors,” he said.

The group next plans to explore the role that the electrostrictive polymer’s lossy nature plays during the mechanical-to-electrical energy conversion process, in order to establish guidelines for the development of mechanical energy harvesters based on the polymers.

They will also attempt to “find a more efficient plasticiser to modify terpolymer, which can contribute to lower energy losses and also improve its electromechanical performances under a low applied electric field”, said Yin. “The lower the electric field, the safer and more convenient it is for applications.”

Image caption: A schematic illustration of the experimental set-up for energy harvesting via electrostrictive polymers.

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