Compositional Dependence of Piezoelectric Properties in Na x K 1− x NbO 3 Lead‐Free Ceramics Prepared by Spark Plasma Sintering

Lead‐free piezoelectric Na x K 1− x NbO 3 ( x =20–80 mol%) ceramics were fabricated using spark plasma sintering at a low temperature (920°C). All the Na x K 1− x NbO 3 ceramics showed a similar orthorhombic phase structure, while the corresponding lattice parameters decreased from the KNbO 3 side to the NaNbO 3 side with increasing Na content. A discontinuous change in lattice parameter close to composition of 60 mol% Na indicated the presence of a transitional area that is similar to the morphotropic phase boundary (MPB) in Na x K 1− x NbO 3 ceramics. The sintered density of the Na x K 1− x NbO 3 ceramics decreased with increasing Na content, from a relative density of 99% for the K‐rich side to 92% for the Na‐rich side. The piezoelectric constant d 33 and planar mode electromechanical coupling coefficient k p showed a maximum value of 148 pC/N and 38.9%, respectively, due to the similar MPB effects in the PZT system.