Study On The Preparation And Properties Of Sodium Potassium Niobate-based Lead-free Piezoelectric Ceramics

In this paper, (Na_1-xK_x)NbO₃ based ceramics were prepared by conventional solid-state reaction method. The structure, piezoelectric, ferroelectric and dielectric properties were investigated.Firstly, the piezoelectric properties of (Na_1-xK_x)NbO₃ based ceramics around morphotropic phase boundary were studied. The system still has pervoskite structure which means Li-doping has not effects on crystal structure. The (Na_0.95K_0.05)NbO₃ ceramics show the best piezoelectric properties with piezoelectric constant d₃₃=117pc/N,the electromechanical coupling facto k_p=36.4%,the loss tangent tanδ=0.032 and mechanical quality factor Q_m=104.The piezoelectric properties of (K_1-xNax)NbO3: d33, kp and er were evidentlyheightened after the doping Li+ in the A-site and Tas+, Sbs+ in B-site. With the increasingof Li content, the structure of ceramics turns from orthorhombic to tetragonal at roomtemperature in MPB. The (Na_0.52K_0.48-xLi_x)(Nb_0.86Ta_0.10Sb_0.04)O₃ exhibits the best properties in MPB, with the d₃₃=268pC/N, k_p=50.4%, tanδ=0.025 when x=0.04. The P-E hysteresis loops show strong ferroelectricity with remnant polarization P_r=20.1μc.cm^-2 and coercive field E_c=1.16kV/mm when x=0.04 at room temperature. In addition, according to patterns of temperature dependence of dielectric constantε, and dielectric loss tangent tanδ, the Curie temperature of these ceramics gradually rised from 289℃to 343℃with the increase of Li⁺ content.Lastly, SrTiO₃ modified (Na_0.52K_0.48-xLi_x)(Nb_0.86Ta_0.10Sb_0.04)O₃ system were studied. All ceramics samples have perovskite structure at room temperature. The cross-section morphological features of the system show that grain growth of the ceramics was restrained at a certain extent after the addition of SrTiO₃. The d₃₃, kp and tanδdecreased with the increase of the SrTiO₃, whereas the mechanical quality factor. Q_m increased siginificantly after the addition of SrTiO₃, which showed the SrTiO₃ acting as "hard dopant" in the (Na_0.52K_0.48-xLi_x)(Nb_0.86Ta_0.10Sb_0.04)O₃ system. The dielectric property was obviously changed by adding SrTiO₃, the relaxor-like ferroelectric was more evidently with increasing SrTiO₃-doping. Compared to the pure ceramic sample, the hump becomes broader and the T_c decreases simultaneously with increasing of SrTiO₃.