Large Strain in CuO‐added (Na 0.2 K 0.8 )NbO 3 Ceramic for Use in Piezoelectric Multilayer Actuators

In this study, 1.0 mol% CuO‐added (Na x K 1− x )NbO 3 ( CN x K 1− x N) 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 Cu 2+ ions, indicating that a large number of Cu 2+ ions had replaced Nb 5+ 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 Cu 2+ 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 0.2 K 0.8 N 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 10 4 cycles when a high electric field of 8.0 kV/mm was continuously applied to the specimens during each cycles. Therefore, CN 0.2 K 0.8 N is a useful lead‐free piezoelectric ceramic for multilayer actuators.