BaTiO3-BiFeO3 based lead-free ceramics for actuator applications

A crystallochemical framework is proposed based on electronegativity difference (en) and tolerance factor (t) for BiMeO3 dopants to optimise the piezoelectric and electrostrictive response in BaTiO3-BiFeO3 based ceramics. Compositions in the series 0.05Bi(Me)O3-0.25BaTiO3-0.70BiFeO3 (BMe-BT-BF, Me: Y, Sc1/2Y1/2, Mg2/3Nb1/3, Sc, Zn2/3Nb1/3, Zn1/2Ti1/2, Ga, and Al) were fabricated using solid state synthesis. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) revealed that only Bi(Mg2/3Nb1/3)O3 and BiScO3 dopants, which lie in a narrow range of en vs. t, form homogeneous ceramics, free from secondary phases reflected in their superior piezoelectric coefficients (d33 ~ 145 pC/N). The promising initial properties of Bi(Mg2/3Nb1/3)O3 and BiScO3 doped compositions prompted further studies on 0.05Bi(Mg2/3Nb1/3)O3-(0.95-x)BaTiO3-(x)BiFeO3 (BMN-BT-BF, x = 0.55, 0.60, 0.63, 0.65, 0.70, and 0.75) and 0.05BiScO3-(0.95-x)BaTiO3-(x)BiFeO3 (BS-BT-BF, x = 0.55, 0.60, 0.625, 0.65, and 0.70) ceramics. For Me = Mg2/3Nb1/3, SEM revealed a homogeneous microstructure for x< 0.75. XRD suggested a gradual structural transition from pseudocubic to rhombohedral for 0.63<x<0.70, characterised by the coexistence of phases. The temperature dependence of relative permittivity, polarisation-electric field hysteresis loops, bipolar strain-electric field curves revealed that BMN-BT-BF transformed from relaxor-like to ferroelectric behaviour with an increase in x and the largest effective piezoelectric coefficient (d33*) was 544 pm/V at 5 kV/mm for x = 0.63 at 5 kV/mm, at the point of crossover from relaxor to ferroelectric which facilitates a macroscopic field induced transition to a ferroelectric state. For Me = Sc, the trends were similar but Sc-doped samples retained ferroelectric behavior to slightly lower values of x than Mg2/3Nb1/3 samples. The potential for BMN-BT-BF compositions to be used as high strain actuators is demonstrated by the fabrication of a prototype multilayer which achieved similar % strain to monolithic ceramics and up to 3 μm displacement with good temperature stability up to 150oC.