Study On The Dielectric Mechanism Of Ca-Base Complex Perovskite Microwave Dielectric Ceramics

As a kind of information functional material, microwave dielectric ceramic (MWDC) is widely used in the field of microwave communication application. It has become a focus of worldwide functional ceramic researching, after nearly 70 years of research and development. B-site complex perovskite is the most attractive system of MWDC. The high performance B-site complex perovskite MWDC can be obtained by adjusting the properties of it throughs appropriate B-site substitution and doping. However, the dielectric mechanism of this MWDC system is unclear. The study on the influnce of structure and element on the dielectric properties is still empirical and remaining in the stage of trying.Ca-base complex perovskite MWDC was studied in this paper. In terms of structural distortion, element substitution, ordering structure and defect, the electronic structure of the system was investigated by first-principles calculation based on the Density-Functional Theory (DFT). The effect of electronic structure on the dielectric properties was analyzed based on the bond vibration. And the experimental study was also carried out, and combined with the analytical results and prediction of dielectric properties to reveal the relationship between pervoskite structure character and dielectric properties. In the present dissertation, we have conducted the investigation from the following respects:By modeling CaTiO3 structure with various BO6 octahedron tilting, the effect of octahedron tilting on the dielectric properties has been investigated, which can't be achieved by conventional experimental method. The main conclusion is that tilting leads to energy level splitting due to the Jahn-Teller effect, which will form new bond and enhance the covalency. These results imply that tilting will improve the dielectric properties of the perovskite MWDC.B-site complex Ca(Zn1/3Nb2/3)O3 ceramic has been investigated by the calculation method, with element substitution models and ordering structure models respectively. The main conclusions are the following:Elements with empty d orbit, such as Nb, is easy to form d-p bond with oxygen, and the substitution of these elements will improve the dielectric properties of the perovskite MWDC. The result of corresponding experiment has confirmed the prediction of calculation. When the B-site ions is arranging in ordering, the ionic size difference will lead to the octahedron tilting, which has above positive effect on the dielectric properties.Ca(Fe1/2Nb1/2)O3 and Ca(Al1/2Nb1/2)O3 B-site complex perovskite ceramics were modeling in ordering and disordering structure and calculated, to investigate the difference in B-site ionic ordering ability. The results have shown that B'ion with bigger size difference to Nb ion and no empty d orbit will tend to form ordering structure, which has been comfirmed by experiment. This implies that the dielectric properties improvement can't be achived in both structural way and compositional way simultaneously. The electronic structure research on Ca(Fe1/2Nb1/2)O3 suggests that forming of new Ca-O bond increased the free energy of the system and reduced the stability of it. Hence, the B-site complex system, of which B'has empty d orbit, will difficult to form ordering structure. So, the structural and compositional optimization may be implemented simultaneously by A-site doping to prevent the forming of high energy A-O bond.Two CaTiO2.875 supercell model with oxygen vacancy at different site were calculated respectively, and compared with CaTiO3 supercell model in the same size, to investigate the effect of oxygen vacancy on the dielectric properties of perovskite MWDC. The presence of oxygen vacancy destroyed the BO6 octahedron, which led to the forming of a new Ti-O bond at the bottom of conduction band, increasing the conductivity and decreasing the dielectric properties of the system. But on the other hand, the local inhomogeneity due to the oxygen vacancy will be the driving force of ordering, which will improve the dielectric properties in some extent. Both calculation and experimental results are shown that above intrinsic effects of defect are local and limited, the extrinsic effect of defect plays the primary role in practice by adsorbing space charge. So, grain boundary doping may be applied to eliminate space charge, improving the dielectric properties of perovskite MWDC.