Hard piezoceramics are widely used in high-power applications such as ultrasonic motors and transducers. These materials are typically driven at or near their resonance frequencies to achieve maximum oscillating strain with relatively low AC electric field application. While this approach allows for high power output, it also exposes the resonator to harsh conditions. Therefore, the piezoelectric resonator must possess specific characteristics to avoid issues such as self-heating, depolarization, and, in the worst case, failure. The most important requisite is a high mechanical quality factor (Qm). Additionally, it must exhibit the thermal stability of the piezoelectric properties in a broad temperature range.
The material most commonly used for high-power applications is lead zirconate titanate (PZT). However, because of its harmful effects on health and the environment, legislative regulations require using lead-free alternatives. Sodium Potassium Niobate (KNN) is one of the most investigated lead-free piezoelectric materials as a potential substitute. It is particularly appealing due to its high Curie Temperature of 420 °C. Hardening of KNN is most often achieved with chemical doping. Specifically, KNN modified with CuO has significantly improved the mechanical quality factor of this system, which, in small-signal measurements, has reached comparable levels of commercial PZT 1–3. However, there has been no systematic study of their behaviour in high-power mode.
To address this, we investigated the effect of different amounts of CuO doping on the hardening behaviour of KNN samples driven at high-vibration velocities. KNN-xCuO (x = 0, 0.5, 1.0 mol%) ceramics were measured using the pulse drive method with a burst excitation on a customized setup at temperatures ranging from -40 °C to 140 °C, which is the typical operating temperature range for most high-power applications. The results of the experiments indicated that the Qm of KNN-CuO doped samples behaves similarly to that of commercial hard-PZT ones. However, the Qm of KNN-CuO doped ceramics decreases at a lower rate when the vibration velocity increases. Furthermore, while PZT stops vibrating at around 1 m/s, KNN-CuO doped ceramics continue to vibrate at velocities above 2 m/s. Finally, it was observed that the 1.0Cu-doped composition had the highest Qm at room temperature, while the 0.5Cu-doped one exhibited better thermal stability.




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