Pairing High Piezoelectric Coefficients, d(33), with High Curie Temperature (T-C) in Lead-Free (K,Na)NbO3

Abstract

The largest piezoelectric properties, d(33) = 416 pC/N and 490 pC/N, in KxNa1-xNbO3 ceramics have been reported for compositions close to polymorphic phase transition (PPT); however, they also have Curie temperatures, T-C, of around 217-304 degrees C, considerably lower than those of undoped KNN ceramics (420 degrees C). High d(33) along with high T-C remains the ideal choice for applications but, unfortunately, not attained up to now. Here, we show that using KNN single crystals as seeds for template grain growth (TGG) of KNN ceramics enables dramatic improvements in the electromechanical properties while maintaining a high T-C. The (001)-oriented (K0.5Na0.5)(0.98)Li0.02NbO3 ceramics engineered by TGG using (K0.5Na0.5)NbO3 crystals as templates exhibit a high d(33) of 280 pC/N while maintaining the high T-C of 430 degrees C. Enhanced piezoelectricity is attributed to long-range ordered ferroelectric domain patterns consisting of 90 degrees and 180 degrees domains, similar to single crystals. It is the first time that pairing high d(33) and high T-C in KNN, keeping a high PPT temperature, is achieved. This study is an unequivocal proof that it is possible to maximize d(33), keeping a high T-C in KNN without resorting to heavily doped compositions. This work opens the door to high-performance, rare-earth free, compositionally simple lead-free and low-cost electromechanical compounds, which can largely expand lead-free piezoelectrics applications.