Dielectric and piezoelectric properties of BiFeO3 modified Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3 lead-free piezoelectric ceramics

The (0.82 − x)Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3–xBiFeO3 (x = 0–0.07) lead-free piezoelectric ceramics were fabricated by a conventional solid-state reaction method and the effect of BiFeO3 addition on microstructure and electrical properties of the ceramics was investigated. The specimens with x ≤ 0.05 maintained a rhombohedral–tetragonal phase coexistence and changed into a rhombohedral phase when x > 0.05 in crystal structure. The addition of BiFeO3 caused a promoted grain growth. All the specimens reveal a low-frequency dielectric dispersion in the frequency range of 40–1 MHz. The piezoelectric constant d33 and the electromechanical coupling factor kp show an obvious improvement by the addition of small amount of BiFeO3, which shows optimum values of d33 = 170 pC/N and kp = 0.366 at x = 0.03. Contrary to the enhancement of piezoelectric properties, Qm decreases with increasing BiFeO3 content. The mechanisms of intrinsic and extrinsic contributions to the dielectric and piezoelectric responses have been proposed. Intrinsic contributions are from the relative ion/cation shift that preserves the ferroelectric crystal structure. The remaining extrinsic contributions are from the domain-wall motion and point defects.