A Shear-Mode Ultrasonic Motor Using Potassium Sodium Niobate-Based Ceramics with High Mechanical Quality Factor

(K,Na)NbO₃–LiNbO₃–CuO lead-free piezoelectric ceramics that show a high mechanical quality factor Q_m were synthesized and used as a drive element of an ultrasonic motor. The Q_m of the (K,Na)NbO₃ ceramic could be enhanced by chemical modification using Li and Cu as well as microstructure control to obtain ceramics with fine grains. The grain size dependence of the Q_m was consistent with a model based on the formation of internal bias field to stabilize the domain structure. A shear mode was used to drive the ultrasonic motor because the piezoelectric d₃₁ and d₃₃ constants of the ceramics were not sufficient for the motor applications. A shear-mode motor driven with four piezoelectric ceramic plates was developed using the lead-free ceramics with a high Q_m of 1400, a high d₁₅ of 207 pC/N, and a high k₁₅ of 0.72. The highest revolution speed of 486 rpm was achieved at 34.5 kHz with the input voltage of approximately 180 V_p–p (peak to peak).