Compositional dependence of piezoelectric properties in NaxK1-xNbO3 lead-free ceramics prepared by spark plasma sintering

Lead-free piezoelectric NaxK1-xNbO3 (x=20-80 mol%) ceramics were fabricated using spark plasma sintering at a low temperature (920 degrees C). All the NaxK1-xNbO3 ceramics showed a similar orthorhombic phase structure, while the corresponding lattice parameters decreased from the KNbO3 side to the NaNbO3 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 NaxK1-xNbO3 ceramics. The sintered density of the NaxK1-xNbO3 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.