Structural and piezoelectric properties of MnO₂-doped PZT-PSN ceramics for ultrasonic vibrator

For use in ultrasonic actuators, we investigated the structural and piezoelectric properties of (1 - x)Pb(Zr_0.515 Ti_0.485)O₃ - xPb(Sb_1/2 Nb_1/2)O₃ + 0.5 wt‰ MnO₂ [(1 - x)PZT - xPSN + MnO₂] ceramics with a variation of x (x = 0.02, 0.04, 0.06, 0.08). All the ceramics, which were sintered at 1250°C for 2 h, showed a typical perovskite structure, implying that they were well synthesized. A homogeneous micro structure was also developed for the specimens, and their average grain size was slightly decreased to 1.3 μm by increasing x to 0.8. Moreover, a second phase with a pyrochlore structure appeared when x was above 0.06, which resulted in the deterioration of their piezoelectric properties. However, the 0.96PZT-0.04PSN + MnO₂ ceramics, which corresponds with a morphotropic phase boundary (MPB) composition in the (1 - x)PZT - xPSN + MnO₂ system, exhibited good piezoelectric properties: a piezoelectric constant (d₃₃) of 325 pC/N, an electromechanical coupling factor (k_p) of 70.8%, and a mechanical quality factor (Q_m) of 1779. The specimens with a relatively high curie temperature (T_c) of 305°C also showed a significantly high dielectric constant (ε_r) value of 1109. Therefore, the 0.96PZT - 0.04PSN + MnO₂ ceramics are suitable for use in ultrasonic vibrators.