Hydrothermal Synthesis Of Potassium Sodium Niobate Based Lead-free Piezoceramic Powders

(K, Na)NbO₃ Lead-free piezoelectric ceramic powders with perovskite structure have been successfully synthesized at 240 oC via isopropanol-assisted hyarothermal method or two-step hydrothermal method. Effects of K/(K+Na) and (K+Na)/Nb molar ratio in the starting solutions on the phase structure and morphology have been investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM) technology. The results indicate that K-rich or Na-rich (K, Na)NbO₃ solid solutions are easy to be prepared by isopropanol-assisted hydrothermal method. But a mixture of K-rich and Na-rich (K, Na)NbO₃ solid solutions is acquired in process of synthesizing single-phase (K, Na)NbO₃ solid solution near the composition K0.5Na0.5NbO₃. However, the K/Na molar ratio in the products has been controlled successfully via two-step hydrothermal method by adjusting the K/Na molar ratio in the second step. The mixture of K-rich and Na-rich (K, Na)NbO₃ solid solutions was also annealed under different temperature. The XRD results present that the mixture has changed to single-phase (K, Na)NbO₃ solid solution annealed at 900 oC for 2 hours.In order to improve the piezoelectric properties of (K, Na)NbO₃, Sb-doping was proceeded and single-phase (K_0.7Na_0.3)(Nb_0.95Sb_0.05)O₃ powders have been synthesized. Effects of Sb-doping on the phase structure and morphology of the powders were also investigated via XRD, Raman scattering and SEM method. With the increasing of Sb content, the resultant powders is a single-phase (K, Na)(Nb, Sb)O₃ solid solution, rather than a mixture of K-rich and Na-rich (K, Na)NbO₃ solid solutions. Besides, the morphology change from irregular aggregation to hexagonal shape as the Sb content increasing. Furthermore, the phase structure and morphology of the (K, Na)(Nb, Sb)O₃ powders hydrothermally synthersized at 240 oC in terms of reaction time were studied. Based on the results obtained, a possible formation mechanism was proposed and how Sb-doping affected the phase structure and morphology was discussed.Using the hydrothermally synthesized K(Nb_0.95Sb_0.05)O₃ and Na(Nb_0.95Sb_0.05)O₃ powders as raw powders, (K0.5Na0.5)(Nb_0.95Sb_0.05)O₃ piezoelectric ceramics were fabricated via traditional sintering process. Comparing with the solid-state reaction method, the (K0.5Na0.5)(Nb_0.95Sb_0.05)O₃ ceramics sintered from hydrothermal powders feature lower porosity and higher density. So the strength of the crystal boundary was increased and transgranular fracture occurred. Dielectric, piezoelectric and ferroelectric properties were measured by impedance analyzer and TF analyzer. The piezoelectric constant d33, electromechanical coefficient kp, mechanical quality factor Qm, relative dielectric constantεr, dielectric loss tanδand remnant polarization Pr, reached about 146pC/N, 42%, 63, 750, 0.03 and 30μC/cm², respectively.