Grain size effect on phase transition behavior and electrical properties of (Bi_1/2K_1/2)TiO₃ piezoelectric ceramics

Dense and phase-pure (Bi_1/2K_1/2)TiO₃ (BKT) ceramics with various grain sizes from 0.18 to 1.01μm were prepared by conventional sintering of a hydrothermally synthesized fine powder. The decrease in grain size resulted in the reductions in tetragonality, remanent polarization, and the piezoelectric d33 coefficient, whereas the room-temperature dielectric permittivity slightly increased with decreasing grain size. The measurement of the temperature dependence of permittivity revealed that BKT exhibited the spontaneous relaxor-to-normal ferroelectric (R-nFE) phase transition. It was also found that the maximum permittivity was decreased and the R-nFE transition was inhibited by the reduction in grain size. In this paper, on the basis of the observed grain-size-dependent phase transition behaviors, microstructural models are proposed for both coarseand fine-grained BKT ceramics, and the mechanism underlying the grain size effect on the electrical properties is discussed.