Enhanced electromechanical strain response in (Fe0.5Nb0.5)4+-modified Bi0.5(Na0.8K0.2)0.5TiO3 lead-free piezoelectric ceramics

By the traditional solid-state reaction method, Bi 0.5 (Na 0.8 K 0.2 ) 0.5 Ti 1− x (Fe 0.5 Nb 0.5 ) x O 3 (marked as BNKT- x FN; x  = 0.00, 0.01, 0.02, 0.03 and 0.04) piezoelectric ceramics were fabricated, and the effects of FN substitution on the dielectric, ferroelectric, piezoelectric and electric field-induced strain performance were investigated. The relative dielectric permittivity and loss tangent reveal that the phase transition temperature between ferroelectric and ergodic relaxor phase is reduced from 90 °C to room temperature (RT), even below RT, with increasing FN content. Temperature dependence of polarization–electric field loops and strain-electric field curves exhibits that the ferroelectric order is disturbed gradually, and the ergodic relaxor phase forms with increasing FN content. A large unipolar strain of 0.42% and corresponding d 33 * (=  S max / E max ) of 642 pm/V for the sample with x  = 0.04 are obtained under 6.5 kV/mm due to the phase transition from ergodic relaxor to ferroelectric. These results indicate that the (Fe 0.5 Nb 0.5 ) 4+ complex ion-modified BNKT-based ceramics would have great potentials for lead-free electromechanical actuator applications.