The Processing Technique And Its Influence On The Properties For Piezoelectric Ceramics (Na_0.5K_0.5)NbO₃

Lead-free and bismuth-free piezoelectric ceramics is a kind of "green" materials which does no harm to environment. It mainly includes: Lead-free piezoelectric ceramics based on BaTiO₃ , niobate compounds. The electrical properties of doped and textured (Na_0.5K_0.5) NbO₃(NKN) ceramics is comparable to the conventional piezoelectric ceramics Pb(Zr_0.5Ti_0.5))O₃(PZT). Now, the technique of preparing high properties Lead-free and bismuth-free piezoelectric ceramics becomes an important research theme.This work emphasized the syntheses of perovskite NKN. NKN was prepared by Molten Salt Synthesis (MSS). Scanning Electron Microscopy (SEM) and X-ray Diffraction patterns (XRD) revealed that there was no serious aggregation with NKN powders and spherical particles of NKN with size of 100nm 150nm was obtained by MSS when the proportion of the salt and the raw materials was 1:1. As the proportion of the salt and the raw materials increases to 2:1, the shape of the powders is cubic with size of about 1 μm. Reacting temperature had much effect on the phase of the end product;while reacting time above 850℃ had little effect on the product purity. The growth process of NKN powders is controlled by diffusion mechanism at low salt content;while, it is controlled by interface reaction mechanism at high salt content.On the base of foregoing research, this paper studied the sintering and polarization techniques of NKN ceramics, and found the optimal sintering condition: 1080℃, 40min, heating rate of 250℃/h;the optimal polarization condition: 100℃, 3kv/mm, 10min.In the present paper, doped NKN was studied. As the doping content of LiTaO₃ (LT) is 5mol%⁸mol%, the morphology phase boundary (MPB) between orthorhombic and tetragonal existed in NKN. The NKN-6%LT samples sintering at 1130℃ for 20 min behave the good properties: density p=4.38g/cm3, dielectric constant ε_r=672, piezoelectric constant d₃₃ =123pc/N, remnant polarization Pr=32μc/ cm², coercive electric field Vc=23kv/ cm. As the content of Ba(Zr_0.5Ti_0.5)O₃ (BZT) is less than 8mol%, a complete solid solution is obtained. And the sintering temperature is 1230℃;The content of BZT has little effect on the piezoelectric properties. Dielectric constant increases with the increase of BZT, when x=8mol%,ε_r =2760;remnant polarization reaches the highest point of 7.35μc/cm² as x=1mol%, and coercive electric field Vc³2.2 kV/cm are observed. Experiment indicates that the effect of adding ZnO to NKN-6%LT is obvious. Adding 0.5wt% of ZnO, the density of the sample is 4.47g/cm³, piezoelectric constant is 112pc/N, and remnant polarization reaches 13μc/cm².