Researchers from Tsinghua University, the Beijing Institute of Technology, the University of Wollongong (Australia), and the Hefei Institutes of Physical Science of the Chinese Academy of Sciences, have achieved an ultrahigh electrostrain of 1.9% in (K,Na)NbO₃ (KNN) lead-free piezoelectric ceramics. 

The breakthrough, facilitated by the electron spin resonance (ESR) spectrometer at the Steady High Magnetic Field Experimental Facility (SHMFF), marks a significant advancement in piezoelectric material performance. 

The findings were published in Nature Materials.

Piezoelectric actuators, known for their rapid response and precision, dominate the global market, with performance largely dictated by the electrostrain figure of merit. Traditional piezoceramics typically exhibit electrostrain below 1%, but in 2022, defect-engineered KNN ceramics reached 1.04% at room temperature, igniting interest in lead-free alternatives. Recent advances in asymmetric electrostrain responses have further pushed performance boundaries, driving mechanistic studies to decode their origins.

In this study, the researchers explored hot-pressing sintered and annealed KNN ceramics, uncovering a unique thickness-dependent electrostrain response. Under room temperature, a 3 kV/mm electric field, and 1 Hz conditions, the material demonstrated an unprecedented effective piezoelectric coefficient exceeding 6,300 pm/V.

Using a combination of in situ characterization techniques and ESR analysis, the team identified the driving mechanism behind the high electrostrain: heterogeneously distributed oxygen vacancies under an electric field cause significant volume variations in surface unit cells, enhancing electrostrain performance.

To explain this phenomenon, the researchers introduced the concept of the "chemopiezoelectric effect," which describes the interplay between piezoelectric effects, ferroelectric domain switching, electrostriction, and short-range oxygen vacancy migration. This theoretical framework provides new insights into high performance piezoelectric actuators.

Furthermore, the KNN piezoceramic exhibits exceptional frequency/temperature stability and fatigue resistance, showcasing potential for application in multilayer piezoelectric actuators via laminated stacking technology. 

These findings provide critical guidance for advancing research in smart materials, flexible electronics, and micro/nano-actuation systems.

Ultrahigh electrostrain in KNN ceramics (Image by CHEN Feng)