Effects of B-site substitution on microstructure and piezoelectric properties of (NaK)_0.5Bi_0.5TiO₃-based ceramics

The lead-free piezoelectric 0.80Na_0.5Bi_0.5TiO₃- 0.20K_0.5Bi_0.5TiO₃ (NKBT) ceramics were synthesized by the conventional solid state reaction method. The effect of high valence ions (Sb⁵⁺, Nb⁵⁺, W⁶⁺) B-site substitution on the microstructure and piezoelectric properties of NKBT-based ceramics has been investigated. The results showed that the main phase of NKBT-based ceramics was still the perovskite phase. With Sb³⁺ and Nb⁵⁺ doping, there were a small amount of the cubic phase of Sb₆O₁₃ and monoclinic phase Nb₂O₅ which coexisted with the perovskite phase in the samples respectively. The doping ions with larger atomic weight could make the ceramics obtain higher piezoelectric constant of d₃₃. The W⁶⁺ was the optimal doping ion. The main phase of NKBT-based ceramics with different content of W⁶⁺ was the perovskite phase. However, when the content of W⁶⁺ reached to 8 mol%, the pyrochlore phase of Bi₁₄W₂O₂₇ appeared. The solubility limit of W⁶⁺ existed at 4mol% of W⁶⁺content. The dielectric constant (ε), the piezoelectric constant (d₃₃) and the Curie temperature (T_c) decreased and the dielectric loss increased with the content of W⁶⁺increasing. When the mole fraction of W⁶⁺ was 1 mol%, the properties of the NKBT-based ceramic were optimal, in which d₃₃, ε_r, tan δ and T_c were 123 pC/N, 1352, 0.0429 and 318°C respectively.