An Investigation of Dielectric, Piezoelectric Properties and Microstructures of Bi_0.5Na_0.5TiO₃-BaTiO₃-Bi_0.5K_0.5TiO₃ Lead-Free Piezoelectric Ceramics Doped with K₂AlNbO₅ Compound

The effect of K₂AlNbO₅ compound acting as both donor and accepter on the phase, microstructures and electrical properties of the 0.9362(Bi_0.5Na_0.5)TiO₃-0.0637BaTiO₃-0.02(Bi_0.5K_0.5)TiO₃ [(1−x)(0.9163BNT-0.0637BT-0.02BKT)-x(K₂AlNbO₅)] (BNKBT-1000xKAN) ternary lead-free piezoelectric ceramics was systematically investigated. When doping content of K₂AlNbO₅ was varied from 0 to 0.009, the BNKBT-1000xKAN ceramics showed a single perovskite structure, and the phase structure transferred from a rhombohedral-tetragonal coexistent morphotropic phase boundaries zone to a tetragonal zone. The x-ray photoelectron spectroscopy analysis indicated that the chemical valence of the Nb and Al element are 5+ and 3+, respectively. Strong relaxor characteristics were revealed by the temperature-dependent dielectric properties of the ceramics. Typical square polarization-electric field (P-E) hysteresis loops were observed in the samples with doping content lower than 0.005. However, with further increasing the doping content (x = 0.007 and 0.009), round P-E hysteresis loops were observed due to the high conductivity of these samples. Moreover, when the doping content was less than 0.005, the ceramic samples exhibited good piezoelectric properties. Specially, when the doping content was 0.001, the piezoelectric constant d₃₃ and electromechanical coupling coefficient k_p of the sample were 197 pC/N and 22%, respectively. However, further addition would deteriorate both the dielectric and piezoelectric properties.