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 (Na_0.5K _0.5)_0.9975-xCa_0.0025NbO₃+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 (ε₃₃^T/ε₀)∼255, dielectric loss (tanδ)∼0.003, k_p∼ 0.38, k _t∼ 0.49, g₃₃∼ 39×10^-3 V-m/N, and Q_m∼ 2850. Furthermore, the change rate of k_p and Q_m versus temperature (TCk_p and TCQ_m) 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 Q_m value.