Preparation And Characterization Of ZnNb₂O₆ Powder

With the continuing proliferation of wireless communications technologies operating at microwave frequencies, there is an ever-increasing demand for cheap, but nonetheless high performance, dielectric ceramics. Microwave ceramics is used for not only insulation patch but also filter and resonator. ZnNb2O6, as a typical microwave dielectric ceramics, has lots of excellent properties and is widely investigated. This paper pays more attention to the research on the manufacture technology of ZnNb2O6 powder, other than a facet of low temperature sintering.In this study, ZnNb2O6 powders are prepared by conventional solid state reaction method and two different molten salt methods including an inorganic salt mixture of KCl-NaCl as a solvent and that with the ZnCl2 addition into the solvent. The process and the condition of three methods are studied in detail. The influence on the synthesis temperature and the powder morphologies with different methods are discussed in emphasis. The samples obtained are characterized by means of X-ray diffraction, scanning electron microscope in order to investigate the characteristic and the factors in the process of preparation and the mechanism of different methods thoroughly.ZnNb2O6 powders are prepared using raw materials of ZnO and Nb2O5 powders as reactants by conventional solid state reaction method in the temperature from 750oC to 800oC. The results of both the XRD and the SEM examinations show that a calcining temperature of 800oC is needed and the obtained ZnNb2O6 particles are equiaxial.The ZnNb2O6 powders are also synthesized in the study using the same raw materials of ZnO and Nb2O5 powders as reactants through molten salt method including an inorganic salt mixture of KCl-NaCl as a solvent and that with the ZnCl2 addition into the solvent in a wide temperature range of 550~800 oC. The XRD and SEM results indicate that the crystal ZnNb2O6 powder with a mixture of blocky and granular particle morphologies could be obtained through this method with KCl-NaCl as a solvent at temperature of 600oC. While in the cases of temperatures above 700oC large amount of impurity of NaNbO3 is present in the products. NaNbO3 developes among the ZnNb2O6 particles, increasing in number with increasing temperature. Then with the ZnCl2 addition into the solvent, both the XRD and the SEM examinations show that the NaNbO3 could be successfully prevented from formation in a wide temperature range of 600oC~800oC and pure ZnNb2O6 are obtained. The ZnNb2O6 particles synthesized are rod-like with particle size of 2 ~ 3μm in length and 0.4 ~ 0.5μm in diameter. The results indicate the effects of ZnCl2 addition are not only a prevention of NaNbO3 from formation but also a noticeable increase of the formed ZnNb2O6 particle size. In addition, the weight ratio of the solvent and the prepared reactant affects the ZnNb2O6 particle morphologies and the growth process. And it has good effect on the process when the weight ratio of the prepared reactant and the solvent is 1:2~1:3. The analysis on the reaction mechanism with different synthesis methods indicates that ZnNb2O6 morphologies are controlled by crystal growth mechanism in essence. The growth process of ZnNb2O6 powders is controlled by diffusion mechanism with the conventional solid state reaction method, and the crystal growth is absolutely the same in different direction, so the powders are equiaxial; while, it is controlled by interface reaction mechanism through two molten salt methods, and the crystal growth velocity lies on the interface reaction velocity, so in the end it results in the anisotropic grain growth and gets the rod-like particle.