Influence of A-site deficiency on oxygen-vacancy-related dielectric relaxation, electrical and temperature stability properties of CuO-doped NKN-based piezoelectric ceramics

The lead-free ceramics with composition of (Na0.5K0.5)0.9975−xCa0.0025NbO3+1 mol% CuO (NKN−100x−C−1,where x=0, 0.02, 0.03, 0.04, 0.06, 0.08) were synthesized by the mixed-oxide method at a sintering temperature of 1080 °C. Effects of Na and K deficiency amounts of x on the oxygen vacancies relating to the electrical and temperature stability properties were systematically investigated. Experimental results showed that the bulk densities of composition ceramics increased with increasing x contents due to the sintering aid of TTB formation at the grain boundary. The electrical properties of NKCN−100x−C−1 ceramics exhibited the optimum values: bulk density ρ  ∼ 4.45 g/cm3, dielectric constant (ε33T/ε0)∼255, dielectric loss (tanδ)∼0.003, kp∼ 0.38, kt∼ 0.49, g33∼ 39×10−3 V-m/N, and Qm∼ 2850. Furthermore, the change rate of kp and Qm versus temperature (TCkp and TCQm) could increase by 7% and 35% in the temperature range 20–120 °C, respectively. This may be due to the decrease of mechanical damping. The imaginary part impedance of NKN−100x−C−1 ceramics with the x variation was evaluated, and the activation energy was correspondingly calculated, which disclosed the deficient x amounts correlating with the oxygen vacancy and high Qm value.