Predicting optimised piezoelectric properties in BiMeO3 (Me = Al, Ga, Sc, Y, Mg2/3Nb1/3, Zn2/3Nb1/3, Zn1/2Ti1/2) doped lead-free BaTiO3-BiFeO3 based ceramics

A predictive framework is proposed based on electronegativity difference and tolerance factor for BiMeO3 dopants to optimise the piezoelectric and electrostrictive response in BaTiO3-BiFeO3 based ceramics. Compositions in the... [ view full abstract ]

A predictive framework is proposed based on electronegativity difference and tolerance factor for BiMeO₃ dopants to optimise the piezoelectric and electrostrictive response in BaTiO₃-BiFeO₃ based ceramics. Compositions in the series 0.05Bi(Me)O₃-0.25BaTiO₃-0.7BiFeO₃ (BMe-BT-BF, Me: Y, Sc_1/2Y_1/2, Mg_2/3Nb_1/3, Sc, Zn_2/3Nb_1/3, Zn_1/2Ti_1/2, Ga, and Al) were studied and the results used to validate this simple crystallochemical approach. Scanning electron microscopy (SEM) and X-ray diffraction revealed that only Bi(Mg_2/3Nb_1/3)O₃ and BiScO₃ dopants form homogeneous ceramics, free from secondary phases which was reflected in their superior piezoelectric coefficients, e.g. d₃₃ ~ 145 pC/N. All the other BiMeO₃ additions exhibited either grain boundary secondary phases and/or promoted a two phase perovskite matrix driven by immiscibility on cooling in the furnace from the sintering temperature. The promising initial properties of BiMg_2/3Nb_1/3O₃ doped compositions prompted further studies on 0.05Bi Mg_2/3Nb_1/3O₃-(0.95-x)BaTiO₃-(x)BiFeO₃ (BMN-BT-BF, x = 0.55, 0.60, 0.63, 0.65, 0.70, and 0.75) ceramics. SEM revealed a homogeneous microstructure for x<0.75 but there was evidence of a core-shell cation distribution for x = 0.75 the inhomogeneity of which was improved by quenching. As x increased the structure changed from predominantly pseudocubic to rhombohedral, resulting in a transition from a relaxor-like to ferroelectric response. The largest strain was 0.41% for x = 0.63 under 10 kV/mm at the crossover from relaxor to ferroelectric behaviour. BMN-BT-BF with x = 0.63 showed 0.30% strain under 6 kV/mm at 175 ºC, demonstrating its potential for high temperature actuation. Moreover, manufacturing of multilayer actuator (MLA) for x = 0.63 was successfully demonstrated. The strain and the PE hysteresis loop were similar to those of bulk ceramics. These results strongly support BMN-BT-BF as a suitable candidate for commercialisation should exemptions for Pb-based piezoelectrics be revoked.