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Chiral (mirror) molecules give relaxor ferroelectrics their amazing properties. Credit: MRI/Penn State

A new fundamental understanding of the behavior of polymeric relaxor ferroelectrics could lead to advances in flexible electronics, actuators and transducers, energy storage, piezoelectric sensors and electrocaloric cooling, according to a team of researchers at Penn State and North Carolina State.

The theory behind the mechanism of relaxor ferroelectrics has been debated for more than 50 years, says Qing Wang, professor of materials science and engineering, Penn State. While relaxor ferroelectrics are well-recognized fundamentally fascinating and technologically useful materials, they have been commented as heterogeneous, hopeless messes by Nature in 2006. Without a fundamental understanding of the mechanism, little progress has been made in designing new relaxor ferroelectric materials. The new understanding, which relies on both experiment and theoretical modeling, shows that relaxor ferroelectricity in polymers comes from chain conformation disorders induced by chirality. Chirality is a feature of many organic materials in which molecules are mirror images of each other, but not exactly the same. The relaxor mechanism of polymers is found to be vastly different from the mechanism proposed for ceramics whose relaxor behavior originates from chemical disorders.

“Different from ferroelectrics, relaxors exhibit no long-range large ferroelectric domains but disordered local polar domains,” Wang explains. “The research in relaxor polymeric materials has been challenging owing to the presence of multiple phases such as crystalline, amorphous and crystalline-amorphous interfacial area in polymers.”

In energy storage capacitors, relaxors can deliver a much higher energy density than normal ferroelectrics, which have high ferroelectric loss that turns into waste heat. In addition, relaxors can generate larger strain under the applied electric fields and have a much better efficiency of energy conversion than normal ferroelectrics, which makes them preferred materials for actuators and sensors.

Penn State has a long history of discovery in ferroelectric materials. The first relaxor ferroelectric polymer was discovered by Penn State electrical engineering professor Qiming Zhang in 1998, when he used an electron beam to irradiate a ferroelectric polymer and found it had become a relaxor. Zhang along with Qing Wang also made seminal discoveries in the electrocaloric effect using relaxor polymers, which allows for solid state cooling without the use of noxious gases and uses much less energy than conventional refrigeration.

“The new understanding of relaxor behavior would open up unprecedented opportunities for us to design relaxor ferroelectric polymers for a range of energy storage and conversion applications”, says Wang.

Their work, titled “Chirality-induced relaxor properties in ferroelectric polymers,” appears today 06/29/20 in the journal Nature Materials. The lead author is Yang Liu, a postdoctoral scholar in Wang’s group. Co-authors Wenhan Xu and Aziguli Haibibu are former graduate students in Wang’s group. Zhubing Han is Wang’s current graduate student. Bing Zhang is a graduate student in Professor J. Berholc’s group at NC State. And Wenchang Lu is a research associate in Berholc’s group.

This research was funded by the US Air Force Office of Scientific Research and the US Office of Naval Research. The supercomputer time at the National Center for Supercomputing Applications was provided by the National Science Foundation.