| Potassium niobate (KNbO3) with an perovskite-type structure is a well known ferroelectric compound when it undergoes a series of structural transitions during cooling: from cubic (C) to tetragonal (T) at 435℃, tetragonal to orthorhombic (0) at 225℃and finally from orthorhombic to rhombohedral (R) at-10℃. Except the cubic, others structures all have ferroelectricity. KNbO3 is a promising material for applications such as optical wave-guiding, frequency doubling, holographic storage systems and other technological fields due to the high electro-optic and nonlinear optical coeffcients, and photo-refractive characteristics. Because it has an inspiring electromechanical coupling coefficient that KNbO3 is also interest for surface acoustic wave devices used for signal processing and transducing. As lead free it is, it is an excellent piezoelectric material, which can be an alternative to lead-based materials and they are currently used in majority of ultrasonic applications. KNbO3 even is a good optical catalytor, and thus, has attracted considerable interest among material scientists. KNbO3 has a steady chemical property, unfortunately, it is very difficult to grow bulk KNbO3 crystals and as such, more attention has been paid to growing epitaxial KNbO3 ceramics and films.In this paper, KNbO3 powders are prapared by hydrothermal method and sol-gel/ combustion method. First, the influence of different experimental conditions on the hydrothermal reaction products are studied, furthermore, dicussed the mechanism of hydrothermal synthesized KNbO3 powders. The conclusion as followed:the concentration of OH- is the premise factor to obtain KNbO3 powders, KNbO3 phase appeared when OH-concentration is higher than 6.0 mol/L. The reaction temperature higher than140℃due to the activation energy of Nb=O bonds breaking, or KNbO3 phase wouldn't be obtained. Reaction rate is affected by reaction temperature and reaction time, raction needs shorter time under high temperature. Obtaining pure KNbO3 powders required 24 hours under 150℃, only 5 hours under 180℃when OH- concentration is 8.0 mol/L. Under the same tempereture, the longer action period is, the more complete reation is. The mass of Mb2O5 doesn't affect final product. The preparation process of KNbO3 in KOH-Nb2O5-H2O ternary system coincidented with the Goh-Magrez mechanism, and the crystallization of KNbO3 belong to dissolution-crystallization process.Second, Mb2O5·nH2O is acquired by one step precipitation with KOH and Nb2O5 as the raw materials, then KNbO3 powders are successfully synthesized via the citric acid sol-gel/ combustion process, and the phase formation process of KNbO3 also be analysed when the phosphors is calcined. The experimental results indicate that:The dissolution rate of in pure citric acid solution is slow, the rate becomes rapid with oxalic acid added. But oxalic acid can hinder the formation of sol- gel, there is no gel when the molar ratio of citric acid/oxalic acid is 1/3. KNbO3 nanoparticles with the orthogonal perovkite structure are obtained after phosphors calcined at a temperature higher than 600℃, the powders are cuboid-shaped particles with uniform size, good dispersion. The size of KNbO3 nanoparticles is increased with the calcination temperature. The addition of oxalic acid inhibits the formation of KNbO3 grains, but promotes grain growth. |
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