Synthesis Of Potassium Sodium Niobate Tantalate Lead-free Piezoelectric Ceramics Powders By High Temperature Mixing Method Under Hydrothermal Conditions

(K, Na)NbO3lead-free piezoelectric ceramics powders with perovskite structure have been successfully synthesized via the solvothermal synthesis with supercritical isopropanol. Neither Na-rich (K, Na)NbO3nor K-rich (K, Na)NbO3solid solutions under the hydrothermal conditions appear duing the synthesized process, single phase (K0.58Na0.42)Nb03powders are obtained when the ratio of K/(K+Na) in the starting solution is0.5. Furthermore, a possible formation mechanism is proposed in terms of reaction time. The K(Ta, Nb)O3particles with an orthorhombic perovskite structure have been synthesized via a high temperature mixing method (HTMM) under hydrothermal and solvothermal conditions. The influence of alkaline concentration and solvent composition and Ta doping amounts on the phase structure and morphology of the obtained powders are investigated. The results indicate that in comparison with the hydrothermal process, supercritical isopropanol plays an important role in synthesizing K(Ta,Nb)O3particles under milder conditions. The KTao.4Nbo.6O3particles solvothermally synthesized in isopropanol are made of well crystallized and single crystalline particles with a size of about100-200nm.In order to improve the piezoelectric properties of (K, Na)NbO3ceramics, Ta doping are proceeded and single phase (K, Na)(Nb, Ta)O3powders have been synthesized via high temperature mixing method under hydrothermal condition. The influence of Ta doping and K/(K+Na) molar ratios in the starting solution on the resultant powders are investigated. The results indicate that the Ta element was successfully doped into the alkaline niobate structure. Ta-doping promotes the formation of a single crystalline phase rather than the K and Na-rich perovskite phases coexisting in the pure KNN powders without Ta-doping. The composition of the synthesized powder is near the MPB area when the K/(K+Na) equals to0.8and Ta/(Nb+Ta) equals to0.3in the starting solutions. The microstructure, piezoelectric, ferroelectric, and dielectric properties of the sintered (K, Na)(Nb, Ta)O3ceramics from the obtained powders were investigated. The highest density value obtained for the (K, Na)(Nb, Ta)O3ceramics is equivalent to about99.4%of their theoretical density (5.29g/cm3), implying that high densification is realized by normal sintering for the present KNN-based ceramics. The piezoelectric coefficient (d33), electromechanical coupling coefficient (kp), dielectric constant (εr), and remnant polarization (Pr) of the sample sintered at1180℃show optimal values of210pC/N,34.0%,2302, and19.01μC/cm2, respectively.