Application of (K,Na)NbO<sub>3</sub>-based lead-free piezoelectric ceramics to ultrasonic sensors

Ultrasonic sensors fabricated using (K,Na)NbO3–KTiNbO5 (KNN–NTK)-based lead-free piezoelectric ceramics were fabricated, their sensor characteristics were compared with those of sensors fabricated using Pb(Zr,Ti)O3 (PZT)-based piezoelectric ceramics, and effects of material properties on sensor performance were investigated. The sound pressure, sensitivity, temperature increase during driving, and the responsiveness of the KNN–NTK-based sensor equaled those of the PZT-5A-based sensor. PZT-5A is generally used for ultrasonic sensors and has the same mechanical quality factor Qm as the KNN–NTK ceramics. Most sensor characteristics are explained based on the electromechanical coupling factor and Qm. However, the change in frequency characteristics under high applied power was observed that is attributed to the tendency of the nonlinear phenomenon of KNN–NTK-based ceramics to be weaker than that of PZT-based ceramics.