High strain (0.4%) Bi(Mg2/3Nb1/3)O3-BaTiO3-BiFeO3 lead-free piezoelectric ceramics and multilayers

The relationship between the piezoelectric properties and the structure/microstructure for 0.05Bi(Mg2/3Nb1/3)O3-(0.95-x)BaTiO3-xBiFeO3 (BBFT, x = 0.55, 0.60, 0.63, 0.65, 0.70, and 0.75) ceramics has been investigated. Scanning electron microscopy revealed a homogeneous microstructure for x < 0.75 but there was evidence of a core-shell cation distribution for x = 0.75 which could be suppressed in part through quenching from the sintering temperature. X-ray diffraction (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 BBFT transformed from relaxor-like to ferroelectric behaviour with an increase in x, consistent with changes in the phase assemblage and domain structure. The largest strain was 0.41 % for x = 0.63 at 10 kV/mm. The largest effective piezoelectric coefficient (d33*) was 544 pm/V for x = 0.63 at 5 kV/mm but the largest Berlincourt d33 (148 pC/N) was obtained for x = 0.70. We propose that d33* is optimised at the point of crossover from relaxor to ferroelectric which facilitates a macroscopic field induced transition to a ferroelectric state but that d33 is optimised in the ferroelectric, rhombohedral phase. Unipolar strain was measured as a function of temperature for x = 0.63 with strains of 0.30% achieved at 175oC, accompanied by a significant decrease in hysteresis with respect to room temperature measurements. The potential for BBFT compositions to be used as high strain actuators is demonstrated by the fabrication of a prototype multilayer which achieved 3 m displacement at 150 oC.