Phase structure, microstructure and piezoelectric properties of perovskite (K_0.5Na_0.5)_0.95Li_0.05NbO₃-Bi_0.5(K_0.15Na_0.85)_0.5TiO₃ lead-free ceramics

Lead-free ceramics (1 - x)(K_0.5Na_0.5)_0.95Li_0.05NbO₃-xBi_0.5(K_0.15Na_0.85)_0.5TiO₃ [(1 - x)KNLN-xBKNT] with x = 0-0.04 were successfully synthesized by conventional solid-state sintering. Effects of Bi_0.5(K_0.15Na_0.85)_0.5TiO₃ (BKNT) on the phase structure, microstructure and electrical properties of the (1 - x)KNLN-xBKNT ceramics were investigated. BKNT strongly affected the sintering behavior, densification and microstructure of the ceramics. X-ray diffraction analysis revealed that all the (1 - x)KNLN-xBKNT ceramics possessed pure perovskite structure, showing symmetries of orthorhombic at x < 0.005, and tetragonal at x ≥ 0.015 at room temperature. Coexistence of orthorhombic and tetragonal phases was identified in the range of 0.005 ≤ x < 0.015. The 0.99KNLN-0.01BKNT ceramics showed the optimum piezoelectric, ferroelectric properties and high Curie temperature, which were as follows: d₃₃ = 240 pC/N, k_p = 0.455, P_r = 20 μC/cm², E_c = 20.8 kV/cm and T_c = 469 °C. The mechanism of the enhanced piezoelectric properties was also discussed. These results indicate that this system looks a promising candidate for lead-free piezoelectric materials.