Piezoelectric Properties of CuO-Doped (K,Na)NbO sub(3) Lead-Free Ceramics Synthesized with Hydrothermal Powders

We report the piezoelectric properties of CuO-doped hydrothermal (K,Na)NbO sub(3) ceramics that can be applied as hard-type lead-free piezoelectric ceramics. To date, we have succeeded in synthesizing high-quality KNbO and NaNbO powders by the hydrothermal method, which is based on an ionic reaction at high temperature (around 210 [derees]C) and pressure. Increasing both the piezoelectric constant d and the mechanical quality factor (Q sub(m)) is important for resonance-type piezoelectric devices, such as ultrasonic motors and transformers. CuO doping into hydrothermal (K,Na)NbO sub(3) ceramics was examined to realize hard-type lead-free piezoelectric ceramics. By doping with 1.2 mol % CuO, Q sub(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 sub(m) is related to the density. To achieve a higher Q sub(m) value, the grain size is required to be less than 5 [mu]m; however, excessive CuO doping leads to anomalous grain growth. Optimal piezoelectric properties were obtained for 1.2 mol % CuO-doped (K,Na)NbO sub(3); k sub(31) = 0.32, d sub(31) = -44 pC/N, Q sub(m) (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 sub(3) ceramics. Therefore, compared with the conventional solid-state method, we could succeed in obtaining hard-type ceramics by a simple and short process.