Effect of cobalt doping on the sintering mechanisms of the lead-free piezoceramic (Bi0.5Na0.5)TiO3

In the search for lead-free piezoelectric ceramics, such as potassium sodium niobate, (K0.5Na0.5)NbO3 (KNN), and bismuth sodium titanate (Bi0.5Na0.5)TiO3 (BNT), high sintering temperatures and the associated volatilization of cations represent a major obstacle to achieve well performing materials. In this study, we investigated the effect of cobalt on the sintering behavior of BNT using in situ thermo-optical dilatometry. The addition of cobalt significantly reduced the sintering temperature at which fully dense ceramic bodies are obtained. This is accomplished by a dual effect of the dopant which facilitates oxygen diffusion: a fraction of the available Ti forms a secondary cobaltous phase. Instead of Ti, some Co is incorporated into BNT at the Ti site, causing oxygen vacancies for charge balancing. To a small degree, the dopant induces liquid phase sintering. At high sintering temperatures, swelling was observed, which was attributed to oxygen release caused by the valence transition from Co3+ to Co2+.