Hardening of electromechanical properties in piezoceramics using a composite approach

Piezoelectric applications such as ultrasonic motors, transformers and therapeutic ultrasonics demand high power generation with low losses, which is facilitated by “hard” ferroelectrics. Hardening of piezoelectric properties, characterized by high mechanical quality factor (Q_m), is usually achieved by doping with lower valence elements, thereby tailoring the domain wall dynamics. In the present study, we demonstrate a hardening mechanism by developing composites of 0.94(Na_1/2Bi_1/2)TiO₃-0.06BaTiO₃ (NBT-6BT) with ZnO inclusions, as an alternative to chemical modifications. A decrease in the saturation polarization and total strain, higher internal bias fields, lower hysteretic losses and a two-fold increase in Q_m are observed in comparison to NBT-6BT. The composite with 0.1 mole ratio of ZnO exhibits the highest Q_m of 320 with d₃₃ = 125 pC/N and K_p = 0.29. A one-to-one correspondence between the increase in Q_m and the decrease in the domain wall mobility is established from the ac field dependence of permittivity, in the framework of the Rayleigh law. A further increase in ZnO content beyond a mole ratio of 0.1 reduces Q_m, but retains it at a higher level, as compared to NBT-6BT. The results are explained based on the poling-induced strain incompatibility between the matrix and the hard ZnO phase. This composite approach is therefore considered a generic hardening concept and can be extended to other ferroelectric systems.