Grain size effect on electrical properties of Mn-modified 0.67BiFeO₃-0.33BaTiO₃ lead-free piezoelectric ceramics

To investigate how grain size affects the dielectric, ferroelectric, and piezoelectric properties of Mn-modified 0.67BiFeO₃-0.33BaTiO₃ ceramics, we prepared samples with a wide variety of grain sizes from 4.1μm to 0.59μm via a conventional solid-state process that use the normal and the two-step sintering methods. Small-signal dielectric measurements show that all the samples exhibit a relaxor-like behavior and that grain size has little influence on the room-temperature dielectric permittivity. For grain sizes below 2μm, the remanent polarization P _r and piezoelectric coefficient d ₃₃ decrease with the grain size, whereas they remain almost constant near P _r = 27μC/cm² and d ₃₃ = 70 pC/N in samples with grain sizes exceeding 2μm. The mechanism underlying the observed grain size effect is discussed in terms of the electric-field-induced formation of macroscopic ferroelectric domains.