Niobate Potassium And Sodium-based Lead-free Piezoelectric Ceramics Were Prepared And Modified

Piezoelectric ceramics are the very important functional materials, and its applications to human beings throughout all aspects of daily life and production. At present, in the development and applications of the piezoelectric ceramics, Pb(Zr, Ti)O₃ (PZT) materials have been dominant, eventhough, the content of PbO is up to 60% in the PZT ceramics. During the sintering process, PbO causes serious pollution on the environment, endangering human health. With the enhanced awareness of environmental protection, lead-free piezoelectric materials are concerned. In this paper, Potassium Sodium Niobate [Na_0.5K_0.5NbO₃(KNN)] ceramics were selected as the experimental system. Considering the Curie temperature, piezoelectric properties and thermal stability, pure Na_0.5K_0.5NbO₃, 0.95(K_0.5Na_0.5)NbO₃-0.05AETiO₃ [KNN-AET] and 1.0 wt.% Li₂CO₃ doped 0.98 (K_0.5Na_0.5)NbO₃-0.02AETiO₃ [KNN-AET-Li] ceramics (AE = Mg, Ca, Sr and Ba) ceramics were synthesized by the conventional solid-state reaction method in air. Their sintering behaviors, phase structures, microstructures, dielectric and piezoelectric properties were investigated.Above all, the pure KNN ceramics were prepared. This paper studied sintering and polarization techniques of KNN, and found the optimal sintering conditions: 1120℃, 5 h, heating rate of 2.5℃/min; the optimal polarization conditions: 120℃, 3 kv / mm, 20min.In order to decrease the orthogonal-tetragonal phase transition temperature T_o-t, the 0.95KNN-0.05AET ceramics were prepared. Effects of different AE ions on the sintering properties, phase structures, microstructures, dielectric and piezoelectric properties of the KNN ceramics were investigated. The experimental results showed that KNN-CaT and KNN-SrT ceramics sintered at 1130℃for 5 h have high relative density (> 96%). The XRD diffraction spectra show that the samples possess perovskite structure. KNN, KNN-MgT ceramics possess orthorhombic phase, but KNN-CaT, KNN-SrT and KNN-BaT possess tetragonal phase at room temperature. Scanning electron micrographs show that KNN-CaT, KNN-SrT ceramics exhibit fine grains with dense surface and small amount of pores. The phase transition temperature T_o-t shifts below room temperature and Curie temperature T_c decreases (≤400℃) for KNN-CaT;KNN-SrT and KNN-BaT ceramics. At room temperature, KNN-CaT, KNN-SrT ceramics exhibit obviously "soft" like behavior and a high piezoelectric coefficient d₃₃ (170 pC/N and 134 pC/N) and electromechanical coupling coefficient k_p (0.27 and 0.25).To abtain a lead-free system of KNN-based ceramics with good temperature stability whilemaintaining a high Curie temperature T_c (≥400℃), 0.98KNN-0.02AET-1.0 wt.%Li lead-free piezoelectric ceramics system were prepared. Effects of Li⁺ ions on the sintering properties, microstructure and electrical properties of the 0.98KNN-0.02AET ceramics were investigated. The experimental results show that Li₂CO₃ doping obviously decreases sintering temperature of the KNN-AET-Li ceramics. KNN-CaT-Li and KNN-SrT-Li ceramics sintered at 1070℃for 5 h have high relative density (> 96%). XRD diffraction spectra show that all the samples possess perovskite structure and both KNN-CaT-Li and KNN-SrT-Li ceramics are to be a coexistence of orthorhombic and tetragonal phases at room temperature. Scanning electron micrographs show that KNN-CaT-Li and KNN-SrT-Li ceramics exhibit fine grain size with high porosity. The phase transition temperature To-t decrease to room temperature and Curie temperature T_c increase up to 400℃for KNN-CaT-Li and KNN-SrT-Li ceramics. At room temperature, KNN-CaT-Li ceramics (d₃₃ = 197 pC/N, k_p = 0.32) and KNN-SrT-Li ceramics (d₃₃ = 243 pC/N, k_p = 0.34) obtain good piezoelectric properties. The changes of piezoelectric properties (k_p and d₃₃) are less than 7.0% within the temperature range of 50-200℃, exhibiting good temperature stability for KNN-SrT-Li ceramics. So KNN-SrT-Li ceramics have practical applications for piezoelectric devices operating at high temperatures.