| (1) Single-crystalline orthogonal NaNbO3 powders have been successfully synthesized by a simple hydrothermal method at various temperatures from160℃to 200℃. The effects of NaOH concentrations, hydrothermal temperature and reaction time were investigated. The results show that increasing the reaction temperature would favor the crystallization of NaNbO3 and accelerate the reaction rate as well as reduce the required initial concentration of NaOH. The grain size increased with increasing the synthesis temperature. At the same time, increasing initial NaOH concentration would in favor of the formation of NaNbO3 phase. The effect of hydrothermal reaction time on the phase structure of the product was not obvious. As extending the reaction time, the diffraction peaks became shaper and stronger. Meanwhile, the grain grew more uniform, integrity and the grain size became larger. In the Nb2O5-NaOH-H2O system, the growth mechanism corresponds to the "in situ transformation" mechanism.(2) KNbO3 has been prepared using hydrothermal method, the effects of initial KOH concentrations and the reaction time on the growth and morphology of hydrothermally derived KNbO3 powders were investigated. In the Nb2O5-KOH-H2O system, the growth mechanism corresponds to the " dissolution-crystallization " mechanism.The powders were synthesized at temperature of 200℃from Nb2O5 and 7.5mol/LKOH solution. The KNbO3 phase was not obtained with reaction time less than 12h. The"nanofingers"irregular polyhedron KNbO3 was obtained with reaction time of 12h. As extending the reaction time, the small particles were absorbed on the surface of the large particles to reduce the surface free energy. The morphology of KNbO3 changed to regular cubic particles when reaction time was extended to 36 h. The results reveal that KNbO3 phase was obtained with KOH concentrations higher than 7.5 mol/L at temperature of 200℃for 12 h. The morphology of rhombohedral KNbO3 is an irregular polyhedron with KOH concentrations in the range 7.5~12mol/L. When increasing the KOH concentration to 15mol/L, KNbO3 powders possess orthorhombic phase with morphology of regular cube.(3) KNbO3 powders have been successfully synthesized by ethanol - water mixed solvothermal method. The influnces of the content of ethanol and reaction time on the growth of hydrothermally derived KNbO3 powders were studied. The results exhibit that the addition of ethanol increased the reactor pressure as well as reduced required reaction time for synthesis of KNbO3. In addition, reaction rate is proportional to the amount of ethanol added.(4) NaNbO3 and KNbO3 powders derived from hydrothermal method were mixed fully acroding to molar ratio of 1:1. Conventional sintering was performed at 1040℃~1100℃for 2 h to get KNN ceramics. The crystallinity and morphology of KNN ceramics were characterized, the were also measured. XRD patterns show that the sintered mixtures were single orthorhombic KNN. SEM photoes exhibit that appropriate sintering temperature would favor grain growth and increase density, thus improve the sintering properties of samples. Samples sintered at 1080℃obtained higher density reach to 4.46g/cm3, ceramics achieved the best performance: d33 =96pC/N,kp=0.37,Qm=108,εr=450,tanδ=0.024 and TC=417℃. Due to the powders synthesized by hydrothermal method were uniformly and with higher dispersion, the contact area between the particles were greater, made the reaction more fully, so the piezoelectric properties were improved. Thus, the obtained KNN ceramics were performed better piezoelectric and dielectric properties compared with pure KNN prepared by other methods. The above results show that hydrothermal method one of a very effective technique, which is used to prepared the ceramic powders for improving the KNN lead-free piezoelectric ceramics. |
