Effect of Li content on the microstructure and properties of lead-free piezoelectric(K_0.5Na_0.5)_1-xLi_xNbO₃ ceramics prepared by SPS

Lead-free piezoelectric (K_0.5Na_0.5)_1-xLi_xNbO₃ (x = 0at%-20at%) ceramics were synthesized by spark plasma sintering (SPS) at low temperature and the effects of LiNbO₃ addition on its crystal structure and properties were also studied. When the Li content was less than 6at%, a single proveskite phase with the similar structure of (K_0.5Na_0.5)NbO₃ was formed; and a secondary phase with K₃Li₂Nb₅O₁₅ structure was observed in the 6at% < x < 20at% compositional range. Furthermore, LiNbO₃ existed as the third phase when the Li content was higher than 8at%. The grain sizes increased from 200-500 nm to 5-8 μm when the K₃Li₂Nb₅O₁₅ and LiNbO₃ like phases were formed. With increasing Li content, the relative density of the ceramics first decreased from 97% to 93% and then kept constant. The piezoelectric coefficient d₃₃, dielectric constant, and planner electromechanical coupling factor exhibited a decreasing tendency with increasing Li content because of the decrease in density and the formation of the secondary phase such as K₃Li₂Nb₅O₁₅ and LiNbO₃. The formation of dense microstructure with a single phase is necessary in improving the properties of the (K_0.5Na_0.5)_1-xLi_xNbO₃ ceramics.