Structures and properties of Li-Ta-Modified KNN lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1-x)K_0.48Na_0.52NbO_3-x Li(Ta_0.5Nb_0.5)O₃ have been fabricated by the conventional solid phase reaction method. The effects of Li(Ta_0.5Nb_0.5)O₃ (LTN) on crystal structure and piezoelectric properties were investigated. The results showed that the perovskite structures of material transformed from the orthorhombic phase to the tetragonal phase as LTN content increased. The morphotropic phase boundary was formed at 0.06<x<0.08 and the second phase K₃Li₂Nb₅O₁₅ with tungsten bronze structure appeared at x>0.08 at beginning and then became dominant with the content of LTN increasing. The ferroelectric phase transitional point of "orthorhombic to tetragonal" ceramic T_O-T shifted to the lower temperature while the Curie temperature (T_c) shifted to the higher temperature because of LTN content increasing. The dielectric constant (ε_r), the piezoelectric constant (d₃₃), the electromechanical coupling coefficient (k_p) increased at beginning and then decreased with the LTN content increasing. For the ceramic with x=0.07, the ceramic material had good piezoelectric properties with ε_r= 856(1 kHz), d₃₃=140 pC/N and kp=0.33. The mechanical quality factor (Q_m) was 161 and the Curie temperature (T_c) was 440°C.