Structure-porperty Relationships For New Perovskite-related Microwave Dielectric Ceramics

Microwave dielectric ceramics with new perovskite-related structures have received much attention in recent years. This dissertation summarizes a series of investigation on the structure-property relationships for such materials, including double perovskites, octahedral perovskites and hexagonal perovskites. Two type of hexagonal perovskites are systematical investigated, focusing on the correlations between their temperature coefficient of resonant frequency and structure variations.Firstly, the relationships between structure and microwave dielectric properties of A₂MeWO₆ (A = Sr, Ba; Me = Co, Ni, Zn) double perovskite ceramics are investigated. In this system, the concept of B-site cation ordering is introduced, on which the Q×f values show strong dependence. Meanwhile, From the application point of view, the modification of octahedral perovskite structure with general formula A_nB_nO_3n+2 is performed. It is found that the octahedra in this structure are markedly affected by A-site ion size, and the microwave dielectric properties are closely related with the connection of the octahedra. In this system, a series of new candidate materials with excellent microwave dielectric properties are obtained.Secondly, the relationship between the temperature dependence of resonant frequency and the hexagonal perovskite structure with general formula A_nB_n-1O_3n is thoroughly discussed. It is found that key difference between every single hexagonal perovskite compound is that, the thickness of its filled octahedral layers and that of its vacant octahedral layer is much of diversity. Based on this, a new concept named octahedral-layer aberrance in the hexagonal perovskite is raised, which can then be characterized by the parameter of octahedral thickness variance. The relationship between the temperature dependence of resonant frequency and the octahedral thicknessvariance, the degree of octahedral-layer aberrance in the hexagonal perovskite, is finally extracted. Its physical mechanism is also discussed. Furthermore, this concept is applied on the intergrowth compounds between the hexagonal perovskites, explain- ing the variation of their temperature dependence of resonant frequency. It suggests that, from the crystal structure point of view, the temperature stability of certain ma-terials can be improved by forming intergrowth structure, due to the reduction of their octahedral thickness variance.Finally, the h-BaTiO₃-type filled hexagonal perovskites as microwave dielectric ceramics are explored in (1-x)Ba(Ni_1/2W_1/2)O₃-xBaTiO₃ system. A series of temperature-stable and high Q materials are found. By using Rietveld refinement, XPS and HRTEM etc., the stability of this type of hexagonal perovskites is proved to be mainly due to the selective occupation of B-site ions. Besides, a new dielectric mate-rial with giant dielectric constant is found in this system and the physical mechanism is discussed on the basis of the dielectric and impedance measurement results. This finding is of significance due to its high dielectric constant and interesting physical mechanism.