Piezoelectric properties and unipolar fatigue behavior of KNN-based Pb-free piezoceramics

Single-phase perovskite ceramics were synthesized using a Pb-free (K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.10Sb_0.04)O₃ (LF4) piezoelectric material both with and without CuO as a dopant additive. Bipolar hysteresis measurements showed a relatively high remanent polarization (20 μC/cm², 16 μC/ cm²) and maximum polarization (25 μC/cm², 21 μC/cm²) values were found for undoped LF4 and 0.2 wt% CuO-doped LF4, respectively. Unipolar fatigue behavior for this system was found to decrease strongly with increased applied testing fields and increased test durations. For undoped LF4, the maximum polarization values were stable after 10 cycles after testing for 100,000 cycles duration. For the CuO-doped samples, increasing the testing field to 3E_C resulted in twice the decrease in P_MAX (-32%) compared with the 2E_C tests at 10⁶ cycles (-17%). At 2E_C testing for CuO doping, polarization decreases continually through 10⁷ cycles, with the decreases in P_MAX and d₃₃ both reaching a maximum (-22% and -30%, respectively) after 10⁷ cycles. In the CuO-doped samples, the fatigue is exacerbated because of the influence of space charge on the increased number of defects present.