Phase transitions, relaxor behavior, and electrical properties in (1-x)(Bi0.5Na0.5)TiO3-x(K0.5Na0.5)NbO3 lead-free piezoceramics

Phase structures and electrical properties of lead-free piezoelectric (1-x)(Bi0.5Na0.5)TiO3-x(K0.5Na0.5)NbO3 (BNT-xKNN) ceramics with 0.08 <= x <= 0.19 were systematically investigated. Results showed that a phase transition from a tetragonal to a pseudocubic phase occurred in this system, as KNN content increases. The addition of KNN shifted both the depolarization temperature T-d and rhombohedral-tetragonal phase transition temperature TR-T to lower temperatures and tended to enhance the relaxor behavior of the ceramics, which was well explained by the microdomain-macrodomain transition theory with calculating criterion K. At x = 0.10-0.11, T-d reached room temperature (RT), which accordingly induced an enhancement of the unipolar strain that peaks at a value of 0.22% was obtained. Furthermore, as the compositions (x = 0.12-0.15) have T-d below RT, samples exhibited high electrostrictive response with large electrostrictive coefficient Q(33) (0.017-0.019 m(4)/C-2) and good thermostability comparable with that of traditional Pb-based electrostrictors.