The secret behind high piezoelectric responses
The riddle of why certain ferroelectric crystals exhibit extremely strong piezoelectric responses may have been solved by a team of researchers in the United States.
In 1997, a relaxor-ferroelectric solid solution crystal with the highest known piezoelectric response was reported at Penn State University. It has a piezoelectric response five to ten times 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," said Fei Li, a postdoctoral scholar in materials science and engineering at Penn State.
"Without a firm understanding of the underlying mechanism, it would be difficult to design new materials with even higher piezoelectric response.”
The team set out to prove a theory that polar nanoregions (PNRs), a 5-10 nm region within the crystal with a net electric polarisation, contributed to the high response. From there they hoped to determine the mechanism by which they help to generate such huge responses.
“We experimentally observed a significant enhancement of piezoelectric response of relaxor-ferroelectric crystals in the temperature range of 50-150 K,” said Shujun Zhang, a professor at the Institute for Superconducting and Electronic Materials at the University of Wollongong (formerly from Penn State where he did the research).
"This enhancement accounts for 50-80% of room-temperature piezoelectricity.”
The team says their proposed mechanism is able to successfully explain all the experimental measurements and observations associated with the high responses. However, they did add a note of caution.
“Our proposed model is a mesoscale model, which is an intermediate scale," said Long-Qing Chen from Penn State.
"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.”
[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. Image: Xiaoxing Cheng]