Effects of Na/K ratio on the phase structure and electrical properties of Na_xK_(1-x)NbO_3 lead-free

Lead-free piezoelectric Na_xK_(1-x)NbO_3 (x = 0.3-0.8) (NKN) ceramics were fabricated by normal sintering at 1060℃ for 2 h. Microstructures and electrical properties of the ceramics were investigated with a special emphasis on the influence of Na content. The grain size of the produced dense ceramic was decreased by increasing Na content. A discontinuous change in the space distance was found at the composition close to Na_(0.7)K_(0.3)NbO_3 ceramic, which indicates the presence of a transitional composition between two different orthorhombic phases, which is similar to the behavior of morphotropic phase boundary (MPB) in Na_xKi_(1-x)NbO_3 ceramics. Such MPB-like behavior contributes to the enhanced piezoelectric coefficient d_(33) of 122 pC/N, planar-mode electromechanical coupling coefficient k_p of 28.6%, and dielectric constant ε_r of 703, respectively for the Na_(0.7)K_(0.3)NbO_3 ceramic. Cubic temperature T_c and the transitional temperature T_(O-t) from orthorhombic to tetragonal phase are observed at around 420℃ and 200℃, respectively.