Piezoelectric materials convert mechanical stress into electricity, or vice versa, and can be useful in sensors, actuators, and many other applications. But implementing piezoelectrics in polymers — materials made up of molecular chains and commonly used in plastics, drugs and more — can be tricky, according to Qiming Zhang, a distinguished professor of electrical engineering.
Zhang and a team of interdisciplinary researchers led by Penn State have developed a polymer with robust piezoelectric efficiency, resulting in 60% more efficient power generation than previous iterations. They published their results today (March 25) in Science.
“Historically, the electromechanical coupling of polymers has been very weak,” Zhang said. “We set out to improve on this because the relative softness of polymers makes them excellent candidates for soft sensors and actuators in a variety of fields, including biosensing, sonar, artificial muscles and more.”
To create the material, the researchers deliberately implemented chemical impurities into the polymer. This process, known as doping, allows researchers to tune a material’s properties to generate desirable effects, provided they incorporate the correct number of impurities. Adding too little dopant could prevent the desired effect from being triggered, while adding too much could introduce unwanted traits that hinder the material’s function.
Doping distorts the spacing between positive and negative charges in the structural components of the polymer. Distortion separates opposing charges, allowing components to accumulate external electrical charge more efficiently. This buildup improves the transfer of electricity through the polymer when it is deformed, Zhang said.
To reinforce the doping effect and ensure the alignment of the molecular chains, the researchers stretched the polymer. This alignment, according to Zhang, favors an electromechanical response more than from a polymer with randomly aligned chains.
“The power generation efficiency of the polymer has been significantly increased,” Zhang said. “With this process, we achieved 70% efficiency – a huge improvement from 10% efficiency before.”
This robust electromechanical performance, which is more common in rigid ceramic materials, could enable a variety of applications for the flexible polymer. Because the polymer exhibits similar resistance to sound waves as water and human tissue, it could be applied for use in medical imaging, underwater hydrophones, or pressure sensors. Polymers also tend to be lighter and more configurable than ceramics, so this polymer could provide opportunities to explore improvements in imaging, robotics and more, Zhang said.
Other contributors to this work include Xin Chen, Department of Materials Science and Engineering, Penn State College of Earth and Mineral Sciences; Hancheng Qin, Bing Zhang, Wenchang Lu, and J. Bernholc of North Carolina State University; Xiaoshi Qian with Shanhai Jiao Tong University in China; Wenyi Zhu with Penn State School of Electrical Engineering and Computer Science; Bo Li and Shihai Zhang of PolyK Technologies at State College; Ruipeng Li with Brookhaven National Laboratory; Lei Zhu with Case Western Reserve University; and Fabrice Domingues Dos Santos with Arkema in France. Qiming Zhang is also affiliated with the Materials Research Institute at Penn State.
The United States Office of Naval Research supported this work.
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Relaxor ferroelectric polymer exhibits ultra-high electromechanical coupling at low electric field
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