Sintering of sub-micron K0.5Na0.5NbO3 powders fabricated by spray pyrolysis

Coarsening and poor densification have hampered the progress to develop lead-free piezoelectric ceramics based on K0.5Na0.5NbO3 (KNN). Here, we present an aqueous spray pyrolysis route to fine (∼100 nm) and phase-pure KNN powders with a narrow particle size distribution. Despite the desirable fine particle size the sintering of this KNN powder was demonstrated to be challenging. The origins of the processes hindering sintering were investigated by systematic changes in the alkali to niobium cation-ratio in the material. In case of alkali-excess, alkali hydroxides and carbonates are formed by reactions with H2O/CO2, leading to formation of a liquid phase during the initial stage of sintering, which further causes growth of cuboid particles that limits further densification. These reactions are also proposed to occur to some extent in stoichiometric KNN, which provide a rationale for the generic sintering challenges of KNN-based systems. Despite these obstacles, 95% dense KNN ceramics were obtained from the spray pyrolysis powders by conventional sintering, and a normalized strain of 180 pm/V was obtained for these materials.