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 d(31) and d(33) 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 d(15) of 207 pC/N, and a high k(15) of 0.72. The highest revolution speed of 486 rpm was achieved at 34.5 kHz with the input voltage of approximately 180 Vp-p (peak to peak).