Progress in lead-free piezoelectric ceramics

Piezoelectric ceramics is a functional material which can convert mechanical energy into electrical energy. It can find wide applications ranging from our daily life to high-tech techniques and dominates a billion-dollar market. For half a century, the working horse of the field is the polycrystalline PbZr_1-xTi_xO₃ (PZT), which is now globally resisted for containing the toxic element lead. Thus, the research of lead-free piezoelectric ceramic with high performance has been a hot topic all over the world. It is believed that the morphotropic phase boundary (MPB) in phase diagram is the key to achieve high piezoelectric performance; great efforts have been made to build the MPB in lead-free system and reveal the physical nature of MPB. In the present paper, we review the recent progress, both on the experimental and theoretical, of the most promising lead-free ceramic systems, i.e. BaTiO₃, (KNa)NbO₃ and (BiNa)TiO₃ systems.