Piezoelectric Properties of CuO-Doped (K,Na)NbO3 Lead-Free Ceramics Synthesized with Hydrothermal Powders

We report the piezoelectric properties of CuO-doped hydrothermal (K,Na)NbO 3 ceramics that can be applied as hard-type lead-free piezoelectric ceramics. To date, we have succeeded in synthesizing high-quality KNbO 3 and NaNbO 3 powders by the hydrothermal method, which is based on an ionic reaction at high temperature (around 210 °C) and pressure. Increasing both the piezoelectric constant $d$ and the mechanical quality factor ($Q_{\text{m}}$) is important for resonance-type piezoelectric devices, such as ultrasonic motors and transformers. CuO doping into hydrothermal (K,Na)NbO 3 ceramics was examined to realize hard-type lead-free piezoelectric ceramics. By doping with 1.2 mol % CuO, $Q_{\text{m}}$ was increased and the dielectric loss ($\tan\delta$) was decreased to 0.5%. The grain size was also influenced by the amount of CuO doping, which indicates that $Q_{\text{m}}$ is related to the density. To achieve a higher $Q_{\text{m}}$ value, the grain size is required to be less than 5 μm; however, excessive CuO doping leads to anomalous grain growth. Optimal piezoelectric properties were obtained for 1.2 mol % CuO-doped (K,Na)NbO 3 ; $k_{31} = 0.32$, $d_{31} = -44$ pC/N, $Q_{\text{m}}\ (\text{radial}) = 959$, and $\tan\delta = 0.5$%. These characteristics showed that CuO doping with hydrothermal powders is effective for obtaining hard-type ceramics, and the mechanical quality factor is more than ten times higher than that of nondoped hydrothermal (K,Na)NbO 3 ceramics. Therefore, compared with the conventional solid-state method, we could succeed in obtaining hard-type ceramics by a simple and short process.