Preparation, Structure And Dielectric Properties Of Hexagonal Layered Perovskite-like Niobate And Tantalate Ceramics

The aim of this thesis is to explore and develop promising niobate and tantalite materials with perovskite-like structure. The development of information technology has fueled the trend of mobile communication system to high frequency, miniaturization, integration, and high reliability. Microwave Dielectric Ceramics (MWDC) has gradually become the key fundamental material for mobile communication technique due to its excellent properties, and more and more attentions have been paid to MWDC in the world.As for the widely applied BLT materials with high dielectric constant, the value of Q'f is too low. With the increasing of frequency, the Q value decreases and can not meet the requirement of high reliability. While in the systems of BaTi4O9, Ba2Ti9O20, (Zr,Sn)TiO4, their low dielectric constants can not actualize the requirement of more miniaturization under the prerequisite of high Q. The high property MWDC with dielectric constants between 50 and 90, the value of Q f higher than 40000, mainly used in 4-8GHz are highly worth of exploitation not only in science research but also in practical application.It should be noted that not only in the complex perovskite like Ba(X,Y)O3,(X=Mg, Zn, Co, Ni, Mn: Y=Nb, Ta) but also in the compounds with tungsten bronze structure such as Ba6-3xLn8+2XTi18O54, they are consisted of corner-sharing oxygen octahedra and the ratios of ion in B site to ion in A site are usually larger than one. So we think that there may exist new materials with excellent dielectric property in perovskite-like compounds that have such ratio slightly lower than one.Firstly, in this thesis, five new peroviskite-like compounds Ba4LaTiNb3O15, Ba3La2Ti2Nb2O15, Ba4NdTiNb3O15, Ba3Nd2Ti2Nb2O15 and Ba4LaTiTa3O15 were synthesized through solid-state reaction, and characterized by Scanning Electron Microscope(SEM), X-ray powder diffraction(XRD) , EPMA and chemical analysis. The cell parameters and indexed X-ray power diffraction data wereobtained for the first time. The result showed that the structures of above-mentioned compounds belong to hexagonal layered perovskite-like. Secondly, the dielectric properties of Ba4LaTiNb3O15, Ba3La2Ti2Mb2O15, Ba4NdTiNb3O15, Ba3Nd2Ti2Nb2O15 and Ba4LaTiTa3O15 were studied through the dependence of dielectric constant vs temperature and the dependence of dielectric loss vs frequency at room temperature. The results testified their good dielectric capability. Finally, the dielectric constants of these compounds were calculated by using Clausius-Mosotti equation combined with polarizabilities of ions. The obtained result was compared with experiment outcomes.Because both ions in A and B sites of these compounds can be substituted by other ions with different electrovalence and radius in a wide range, their dielectric property is adjustable and can be optimized in a large scale. This implies that new type MWDC with advanced dielectric property may be exploited from these compounds and widely used.