Preparation, Structure And Dielectric Properties Of Ba₅LnNiM₉O₃₀ Ceramics

With the development of microwave technique, Microwave Dielectric Ceramics (MWDC) is becoming the key fundamental material for mobile communication technique, and people have showed great interests in their investigation and application. Now mobile communication systems are developing toward high frequency, miniaturization, integration, high reliability, which intensively requires exploration and research of MWDC systems with high dielectric constant.Tungsten bronze oxides are one of the most important classes of dielectric materials. This family offers the possibility to adjust the property of material by changing the composition. Tetragonal tungsten bronze (TTB) structure niobates and tantalates may have high dielectric constant, low dielectric loss and good stability of frequency and temperature because the cations in the structure can be easily substituted in a wide range.The aim of this thesis is to explore new MWDC as well as free lead compensated dielectric ceramics with tungsten bronze (TB) structure. According to the characteristic of tungsten bronze structure, series of new niobates and tantalates Ba6Ni0.67M9.33O30, Ba5LnNiM9O30 (Ln= La, Nd, Sm; M=Nb, Ta) were designed. Furthermore, their structure and dielectric properties were studied.Firstly, Ba6Ni0.67M9.33O30 and Ba5LnNiM9O30 compounds were synthesized by high temperature solid-state reaction technique and characterized by scanning electron microscope (SEM), x-ray powder diffraction (XRD) and chemical analysis. The accurately revised unit cell parameters and indexed XRD data were acquired. The results show that the structures of these compounds belong to tetragonal tungsten bronze structure, and their unit cell parameters vary with the different radii of Ln3+.Secondly, the dielectric properties of Ba6Ni0.67M9.33O30 and Ba5LnNiM9O30 were studied. Through the dependence of dielectric constant vs temperature, Curie temperature (Tc) region and structural phase transitions were determined. The experiment results indicate that the dielectric properties of Ba6Ni0.67M9.33O30 and BasLnNiM9O30 vary with different Ln and different M, but all of them have high dielectric constants.Finally, the dielectric constants of Ba6Nio.67M9.33O30 and Ba5LnNiM9O30 were calculated by using Clausius-Mosotti equation with polarizabilities of ions. The calculated values are much smaller than the experimental ones. The deviations are worthy of further study.