Illustration of the polar directions in relaxor-ferroelectric solid solutions where a small amount of polar nanoregions embedded in a long-range ferroelectric domain leads to dramatically enhanced piezoelectric and dielectric properties. (Credit: Xiaoxing Cheng/ Penn State)
Piezoelectricity is a property present in ferroelectric materials where an applied electrical field can elicit a mechanical response and an applied mechanical force can produce an electrical current. Ferroelectric materials are employed in a wide range of industrial applications, from vibration sensors, transducers, capacitors and ultrasensitive infrared cameras through to ultrasound and sonar.
Now, an international team of researchers may have solved the long-standing riddle of why extremely strong piezoelectric responses are exhibited by certain ferroelectric crystals. The study was headed by
In 1997, Thomas R. Shrout, now a senior scientist and professor of materials science and engineering at Penn State, and the late Seung-Eek Park reported a relaxor-ferroelectric solid solution crystal with the highest known piezoelectric response. This crystal possesses a piezoelectric response that is much higher than any other known ferroelectric material.
There have been a number of mechanisms proposed to explain its ultrahigh piezoelectric responses, but none of them offer a satisfactory explanation for all the experimental observations and measurements associated with the high response. Without a firm understanding of the underlying mechanism, it would be difficult to design new materials with even higher piezoelectric response.
Fei Li, Postdoctoral Scholar in Materials Science and Engineering, Penn State
Li is a lead author of a recent article in the journal Nature Communications trying to elucidate the phenomenon.
However, a general consensus has been reached by the scientific community that something called polar nanoregions were responsible for the high piezo response of relaxor crystals, said Li.
A polar nanoregion refers to a spatial region present inside a crystal. It features a net electric polarization and has a nanoscale size (5 - 10 nm). There are a lot of such small regions that are arbitrarily distributed in space in a relaxor crystal.
Lead zirconate titanate (PZT) is another popular piezoelectric material that lacks polar nanoregions, but has much larger ferroelectric domains where the polarization is uniform. The researchers set out to prove that the polar nanoregions actually contributed to the high piezo responses, and more significantly, to establish the mechanism through which they help to create such high responses.
The experiments were performed at ultralow cryogenic temperatures (50 - 150 K), enabling the team to isolate the responses from the polar nanoregions, which continued to stay active within that temperature range, from those high piezoelectric responses that generally occur close to a ferroelectric phase transition.
“We experimentally observed a significant enhancement of piezoelectric response of relaxor-ferroelectric crystals in the temperature range of 50 - 150 K. This enhancement accounts for 50 - 80% of room-temperature piezoelectricity,” said Shujun Zhang, a senior author and professor of materials science and engineering at Penn State (now at University of Wollongong).
We attributed the experimentally observed enhancement to the existence of the polar nanoregions. Using phase-field modeling, we first proved that this significant enhancement originated from the polar nanoregions, i.e., the enhancement is absent without the presence of these polar nanoregions, and then demonstrated how the polar nanoregions help generate ultrahigh responses. Our proposed mechanism is able to successfully explain all the experimental measurements and observations associated with the high responses. This work is an important step in realizing the dream of discovering new piezoelectric materials by design.
Long-Qing Chen, Senior Author and Donald Hamer professor of Materials Science and Engineering, Penn State
A Note of Caution
“However, it should be noted that our proposed model is a mesoscale model, which is an intermediate scale. The atomistic origin of PNRs is still an open question, so further in-depth research is still required to clarify the contribution of polar nanoregions at the atomic scale. And in fact, our ongoing work is focused on understanding the atomic-scale mechanisms of polar nanoregions in piezoelectric responses,” said Chen.
The international group that contributed to the Nature Communications paper, titled
“The Origin of Ultrahigh Piezoelectricity in Relaxor-Ferroelectric Solid Solution Crystals,” includes scientists from the Penn State Materials Research Institute and the Penn State Department of Materials Science and Engineering; University of Wollongong, Australia; Xi’an Jiaotong University, China; Center for High Pressure Science and Technology Advanced Research, China; Carnegie Institute of Washington, USA; TRS Technologies, USA; and Simon Fraser University, Canada.
The study was supported by the National Science Foundation, The U.S. Office of Naval Research, Office of China Postdoctoral Council, Natural Science Foundation of Shaanxi province, the National Natural Science Foundation of China, 111 Project, and the U.S. Department of Energy.