Preparation, Structure And Dielectric Properties Of Ba₅LnSn₃Nb₇O₃₀ And (Ba,Sr)₈Ti₃Nb₄O₂₄ Ceramics

The aim of the thesis is to explore and develop promising new niobate dielectric function materials. With the high speed development of modern communication, the trend to miniaturization, high frequency and integration of mobile communication system intensively requires research and development of new dielectric function ceramics.Firstly, on the basis of the characteristic of filled tungsten bronze structure with the general formula A6B10O30, a series of new niobates Ba5LnSn3Nb7O30 (Ln=La,Nd) were designed in the thesis, and prepared by high temperature solid-state reaction technique (both calcined at 1300℃ for 12h and sintered at 1350℃ for 4h). The ceramics were characterized by X-ray powder diffraction (XRD), Scanning Electron Microscope (SEM), density analysis and chemical analysis. The dielectric properties of them show that: Ba5LaSn3Nb7O30 and Ba5NdSn3Nb7O30 ceramics have high dielectric constant of 171 and 182 together with low dielectric loss 1.2×10-3 and 1.8×10-3 at 1MHz, respectively. In comparison with dielectric ceramics with TB structure in BaO-Ln2O3-TiO2-Ta2O5 (Ln=La, Nd) system, the temperature coefficients of the dielectric constant (τε) are significantly reduced, and they are promising to be a new type of free lead compensated dielectric ceramics.Then, according to layered perovskite-like structure with the general formula A8B7O24, Ba8Ti3Nb4O24 was prepared (calcined at 1380℃ for 12h and sintered at 1480℃ or 4h). We substituted Ba for Sr at A place, and Ba8-xSrxTi3Nb4O24(x=1-8) were prepared, but they are not pure phases. The result of microwave dielectric testing show that: with substitution of Sr for Ba, dielectric constant (si) of them varies from 42.14 to 75.6, first increase and then fall down; quality factors (Q·f) reduced from gradually from 1976; the temperature coefficients of resonance frequency (τf) reduced gradually in the range of-70-+75 ppm℃-1.It is hopeful to develop some new microwave dielectric ceramics with middle and high dielectric constants from them, if they are optimized.