Hydrothermal Preparation Of Sodium Potassium Niobate Based Lead Free Piezoelectric Powders

The pure perovskite powders including KNbO3, NaNbO3, NaTaO3, (K,Na)NbO3 and Na(Nb,Ta)O3 were synthesized by Hydrothermal method in the low temperature. And the effect of reaction parameters on the phase structure and morphology of the powders was analyzed using X-ray diffraction (XRD) and scanning electron microscope (SEM).KNbO3 powder was fabricated using a reaction temperature of 200℃. It was found that hydrothermal reaction was not complete and a lot of hydration niobate existed in the KNbO3 powder prepared using a low alkali concentration (7.5 mol/L) of solution. With the increased alkali concentration, the pure KNbO3 powder was obtained. And the morphology of the synthesized powder was also influenced by the alkali concentration. While the reaction temperature increased to 210 ℃, the pure KNbO3 powder could be fabricated under low alkali concentration (7.5 mol/L); simultaneously, the NaNbO powder was synthesized in a similar way. The prepared KNbO3 and NaNbO3 powders were used as raw materials to fabricate the K0.5Na0.5NbO3 lead-free piezoelectric ceramics, sintering temperature from 1040 ℃ to 1100 ℃. The piezoelectric ceramics showed an extraordinary structure and high quality. The increased sintering temperature improved their microstructure and performance. The Ko.5Nao.5Nb03 piezoelectric sintered at 1100 ℃ showed the density of 4.38 g/cm3, d33 of 110.8 pc/n and the dielectric constant of 420. The Nb2O5 powder and the mixed solution of NaOH and KOH were used to fabricate the (K,Na)NbO3 powders. Sodium dodecyl benzene sulfonate and lauryl sodium sulfate were added to the reaction solution as surface active agents. Thus the plate-like hydrated niobate particles whose morphology was related to the reaction temperature were obtained. Pure perovskite (K,Na)NbO3 powders could be fabricated after the hydrated niobate particles were calcined at 450℃, which provided a method to produce plate-like piezoelectric template. Moreover, the prepared (K,Na)NbO3 powders were used as raw materials to fabricated a porous piezoelectric ceramic with a uniform layered structure.In order to improve the properties of the K0.5Na0.5NbO3 piezoelectric ceramic by doping, the NaTaO3 powder was synthesized by Hydrothermal method at the tenmperature of 140℃. Experimental results showed when the alkali concentration was 0.6mol/L, the reaction time to obtain the pure NaTaO3 powder with a quadrature phase was 21 hours. While the alkali concentration increased to 0.75 mol/L, the pure quadrature phase powder could be fabricated in 12 hours.The NaNb1-.xTax03 powders were also synthesized by the hydrothermal reaction of NaOH solution and the mixtures of Nb2O5 and Ta2O5. And the prepared doping powders and KNbO3 powders were mixed and pressed into tablets. A single perovskite solid solution with the more uniform and compact struture than that of the undoped samples after the Ta-doped sample were sintered at the temperature of 1100 ℃. And the Ta-doped piezoelectric ceramic had the similar value of d33 with the undoped samples.