The effect of pre-milling/pre-synthesis process and excess Ba on the microstructure and dielectric/piezoelectric properties of nano-sized 0.94[(Bi0.5Na0.5)TiO3]–0.06[Ba(1+x)TiO3]

Lead-free piezoelectric ceramic specimens of 0.94[(Bi0.5Na0.5)TiO3]–0.06[Ba(1+x)TiO3] (where 0 ≤ x ≤ 0.03) (abbreviated as nano-sized BNBT6(x)) compositions containing excess Ba were synthesized by a modified mixed oxide method. In this modified process (Bi0.5Na0.5)TiO3 and Ba(1+x)TiO3 were separately prepared by pre-milling the starting powders in high energy mill (HEM) in order to obtain nano-particle size. BNBT6(x) specimens were also prepared by the conventional process to be compared with the former one. The pre-milling of the raw materials lowered the calcination temperatures of (Bi0.5Na0.5)TiO3 and Ba(1+x)TiO3 by 110 and 200 °C, respectively, as compared with the conventional process. High energy milling improved the reaction activity and homogeneity of the materials used throughout the process, enhanced the sintering density and grain uniformity, and decreased the grain size. The effects of excess Ba on the characteristic of nano-sized BNBT6(x) specimens were systematically investigated. The piezoelectric and dielectric properties of BNBT6(x) specimens containing various amounts of excess Ba show maximum values of the planar electromechanical coupling factor (kp) of 38% and the piezoelectric constant (d33) of 198 pC/N with Ba excess amount of 0.02 mol [BNBT6(0.02)]. The d33 then decreases with increasing excess Ba content to 0.03 mol, whereas the relative dielectric permittivity (KT33) steadily increases with increasing excess Ba and reaches the maximum value of 785 for this composition. Besides, the depolarization temperature (Td) slightly decreased within the range of x = 0–0.01 mol and then tends to rapidly decrease with the excess Ba of 0.02 mol. In addition to this, the Td remains unchanged with the higher excess Ba of 0.03 mol. The modified mixing and milling method were considered to be a new and promising process for lead-free piezoelectric ceramics owing to their excellent piezoelectric/dielectric properties.