Hydrothermal synthesis and characterization of (1 − x)K0.5Na0.5NbO3-xBi0.5Na0.5TiO3 lead-free ceramics

Lead-free piezoelectric powders (1 − x)(K0.5Na0.5)NbO3-x(Bi0.5Na0.5)TiO3 ((1 − x)KNN-xBNT, x = 0.01-0.08) were firstly synthesized by hydrothermal method and their ceramics were subsequently prepared after sintering and polarizing treatment. The phase structure and the micro-morphology of as-prepared powders and ceramics were characterized by X-ray diffraction and field emission scanning electron microscopy. The effects of preparing conditions such as titanium source, hydrothermal temperature and BNT doping amount (x, mol%) on phase formation and micro-morphology of (1 − x)KNN-xBNT powders were investigated in detail. It is showed that (1 − x)KNN-xBNT (x = 0.05) powders were successfully prepared by using TiO2 as titanium source when the temperatures were controlled from 210 °C to 230 °C. Two phases of K-rich (K, Na)NbO3 and Na-rich (K, Na)NbO3 coexisted, and they gradually transformed into only K-rich (K, Na)NbO3 with the increase of BNT content. After sintering, the (1 − x)KNN-xBNT ceramics exhibited single (1 − x)KNN-xBNT phase, and a morphotropic phase boundary (MPB) between the orthorhombic and tetragonal phases was identified in the composition range of 0.02 ⩽ x ⩽ 0.05 where optimal piezoelectric constant d33 was obtained.