Large Strain in CuO-added (Na_0.2K_0.8)NbO₃ Ceramic for Use in Piezoelectric Multilayer Actuators

In this study, 1.0 mol% CuO-added (Na_xK_1-x)NbO₃ (CN_xK_1-xN) ceramics with 0.0 ≤ x ≤ 0.5 were sintered at a relatively low temperature of 960°C. A large Q_m value (≥1750) was obtained for the specimens with x ≤ 0.2 owing to the hardening effect of Cu²⁺ ions, indicating that a large number of Cu²⁺ ions had replaced Nb⁵⁺ ions in these ceramics. The double polarization versus electric field (P-E) hysteresis loop was observed for the specimens with x ≤ 0.2, and was explained by the defect polarization (P_d) formed between Cu²⁺ ions and O^2- vacancies. These specimens also exhibited a sprout-shaped strain versus electric field (S-E) curves with a large strain of 0.14%-0.15% at 8.0 kV/mm because of the internal bias field produced by P_d. For the CN_0.2K_0.8N ceramic, a double P-E hysteresis loop was maintained at 125°C without remnant polarization. Moreover, this ceramic showed a large strain of 0.175% after 10⁴ cycles when a high electric field of 8.0 kV/mm was continuously applied to the specimens during each cycles. Therefore, CN_0.2K_0.8N is a useful lead-free piezoelectric ceramic for multilayer actuators.