Effect of Li₂O on the defect polarization in CuO-added (K_0.9Na_0.1)NbO₃ piezoelectric ceramics

CuO-added (Li_xK_0.9−xNa_0.1)NbO₃ [C(L_xK_0.9−xN_0.1)N] ceramics with 0.0≤x≤0.05 were well-sintered at 960°C for 6 hours. The lattice parameters of the specimens decreased with the addition of Li₂O. Defect polarization (P_D) formed between Cu²⁺ions and oxygen vacancies. Double polarization vs electric field (P-E) hysteresis and sprout-shaped strain vs electric field (S-E) curves were observed in these specimens with a large strain of 0.16% at 7.0 kV/mm, possibly owing to the presence of P_D. When the P-E curve was measured at temperatures higher than 75°C, the C(K_0.9N_0.1)N ceramic exhibited a normal P-E hysteresis curve, whereas the C(L_0.04K_0.86N_0.1)N ceramic maintained the double P-E hysteresis curve up to 125°C, indicating that Li₂O increased the thermal stability of P_D. The latter specimen also showed the sprout shaped S-E curve with a strain of 0.15% at 7.0 kV/mm after 10⁴ cycles of a high electric field of 7.0 kV/mm.