Effects of bismuth oxide-doping on properties of potassium sodium niobate lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (Na_0.5K_0.5)_1-3x Bi_xNbO₃(0 ≤ x ≤ 0.05) were synthesized by a traditional solid state reaction. Effects of bismuth oxide (Bi₂O₃)-doping on the crystal structure and piezoelectric properties of (Na_0.5K_0.5)NbO₃ (NKN) were investigated. The results reveal that the pure perovskite structures of NKN-based ceramics were gained when the content of Bi₂O₃ content x < 0.02. The optimum sintering temperature increases with the increase in content of Bi₂O₃, and the density of all samples are greater than that of pure NKN ceramics. Furthermore, the piezoelectric constant (d₃₃), electromechanical planar and thickness coupling coefficients (k_p, k_t) increased firstly and decreased subsequently with the increase in content of Bi₂O₃. The mechanical quality factor (Q_m) increases evidently. Especially, the density of the sample with composition (Na_0.5K_0.5)_0.97 Bi_0.01NbO₃ is 4.42 g/cm³ and it exhibits optimal piezoelectric properties: d₃₃=154 × 10^-6 C/N, k_p=45%, k_t=46%, dielectric dissipation tanδ=3.5%, relative dielectric constant ε_r=598, and Q_m=138.