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Grain size effects on the physical properties of polycrystalline ferroelectrics have been extensively studied for decades; however there are still major controversies regarding the dependence of the piezoelectric and ferroelectric properties on the grain size. Dense BaTiO3 ceramics with different grain sizes were fabricated by either conventional sintering or spark plasma sintering using micro- and nano-sized powders. The results show that the grain size effect on the dielectric permittivity is nearly independent of the sintering method and starting powder used. A peak in the permittivity is observed in all the ceramics with a grain size near 1 μm and can be attributed to a maximum domain wall density and mobility. The piezoelectric coefficient d33 and remnant polarization Pr show diverse grain size effects depending on the particle size of the starting powder and sintering temperature. This suggests that besides domain wall density, other factors such as back fields and point defects, which influence the domain wall mobility, could be responsible for the different grain size dependence observed in the dielectric and piezoelectric/ferroelectric properties. In cases where point defects are not the dominant contributor, the piezoelectric constant d33 and the remnant polarization Pr increase with increasing grain size.

Various lead-free (K x Na 1Àx ) 0.98 Li 0.02 (Nb 0.82Ày Ta 0.18 Sb y )O 3 ceramics with x 5 0.50, y 5 0.00-0.07 or x 5 0.40-0.60, y 5 0.05 were prepared by the conventional solid-state reaction method. Systematic investigation on the microstructures, crystalline structures, and dielectric and piezoelectric properties was carried out. Remarkably strong piezoelectricity has been achieved in (K 0.45 Na 0.55 ) 0.98 Li 0.02 (Nb 0.77 Ta 0.18 Sb 0.05 )O 3 ceramic, which shows the excellent piezoelectric properties of d 33 5 413 pC/N, d 31 5 À153 pC/N, k p 5 0.50, and k 33 5 0.62. It is considered that the observed strong piezoelectricity should be ascribed to several combined decisive factors, such as the phase coexistence due to an orthorhombic-tetragonal polymorphic phase transition near room temperature, the high electronegativity of Sb 51 ions as compared with those of Nb 51 ions and Ta 51 ions, and the relatively ideal microstructure with high density, large average grain size and narrow grain-size distribution.

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