Electromechanical strain and bipolar fatigue in Bi(Mg1/2Ti1/2)O3-(Bi1/2K1/2)TiO3-(Bi1/2Na1/2)TiO3 ceramics

Abstract

Lead-free ceramics of composition Bi(Mg1/2Ti1/2)O3-(Bi1/2K1/2)TiO3-(Bi1/2Na1/2)TiO3 were prepared using solid state synthesis techniques. The dielectric spectra showed a Tmax of more than 320 °C for all compositions, and the transitions became increasingly diffuse as the Bi(Mg1/2Ti1/2)O3 content increased. A lower temperature transition, indicating a transformation from an ergodic to a non-ergodic relaxor state, was also seen for all compositions, and this transition temperature decreased as the mole fraction of Bi(Mg1/2Ti1/2)O3 increased. The composition with 1% Bi(Mg1/2Ti1/2)O3 showed characteristic ferroelectric-like polarization and strain hysteresis. However, compositions with increased Bi(Mg1/2Ti1/2)O3 content became increasingly ergodic at room temperature with pinched polarization loops and no negative strain. Among these compositions, the magnitude of d33* increased with Bi(Mg1/2Ti1/2)O3 content, and the composition with 10% Bi(Mg1/2Ti1/2)O3 exhibited a d33* of 422 pm/V. Fatigue measurements were conducted on all compositions and while the 1% Bi(Mg1/2Ti1/2)O3 composition exhibited a measurable, but small loss in maximum strain after a million cycles; all the other compositions from 2.5% to 10% Bi(Mg1/2Ti1/2)O3 were essentially fatigue-free. Lastly, optical and alternating current impedance measurements were employed to identify intrinsic conduction as the dominant conduction mechanism. These compositions were also highly insulating with high resistivities (∼107 Ω-cm) at high temperatures (440 °C).