Study On Preparation, Structure And Dielectric Properties Of Ba_6-xLn_xTi_2+xTa_8-xO₃₀ Ceramics

Microwave Dielectric Ceramics (MWDC) is the key fundamental material for mobile communication technique, and more and more attentions have been paid to MWDC in the world. The trend to miniaturization of microwave telecommunication intensively requires research and development of microwave dielectric ceramics systems with high dielectric constant because the size of microwave resonance takes in inverse proportion to er 1/2. Considering the increased interest in high frequency applications (resonators, filters, arrays, etc.), a full dielectric response up to and above the GHz is needed.Tungsten bronze oxides are one of the most important classes of ferroelectrics next to perovskites. This wide family offers the possibility to tune the material response by changing the composition. Only few studies (owing to synthesis problems) are dedicated to pure tantalates. Tantalates of tetragonal tungsten bronze (TTB) structure have high dielectric constant, low dielectric loss and good stability of frequency as well as the cations in the structure can be easily substituted in a wide range. So,it is possible to find new dielectric materials with good properties in them.The aim of this thesis is to explore and develop new high microwave dielectric materials as well as free lead compensated dielectric ceramics with tungsten bronze (TB) structure (A6B10O30)- According to characteristic of tungsten bronze structure, series of new tantalates Ba6-xLnxTi2+xTa8-xO30 (x=1, 2; Ln=La, Sm, Nd, Y) were designed on the basis of Ba3TiTa4O15 through the substitution of A, B cations with rare-earth elements and titanium, respectively. Furthermore, their structure and dielectric properties were studied.First, Ba6-xLnxTi2+xTa8.x03o (x=l, 2; Ln=La, Sm, Nd, Y) compounds were synthesized by high temperature solid state reaction technique and characterized by chemical analysis, scanning electron microscope (SEM), x-ray powder diffraction (XRD). The results show that the structure of Ba6-xLnxTi2+xTa8-xO3o(x=1,2; Ln=La, Sm, Nd, Y) belongs to tetragonal tungsten bronze structure. Compared with Ba3TiTa4O15, the crystal volumes of Ba6-xLnxTi2+xTa8-xO30 decreased when the rareearth elements were introduced.Furthermore, the dielectric properties of Ba6-xLnxTi2+xTa8.xO3o (x=1,2; Ln=La, Sm, Nd, Y) were studied, such as dielectric constant (î–¸) vs the temperature (T), dielectric constant (tan5) vs frequency (f), dielectric loss (tan5) vs temperature (T) and dielectric loss (tan6) vs frequency (f). Curie temperature (Tc) and structural phases transitions were determined. The experiment results indicated that their dielectric properties are similar with each other. Ba6-xLnxT2+xTa8-xO30 (x=1, 2: Ln=La, Sm, Nd, Y) ceramics undergoes diffuse type of ferroelectric-paraelectric phase transition and has good stability of frequency. Dielectric constants have been obviously increased compared to Ba3TiTa4O15.Finally, the dielectric constants of Ba6-xLnxTi2+xTa8-xO30 were calculated by using Clausius-Mosotti equation with polarizabilities of ions. The calculated values are greatly smaller than the experimental values. The deviations are worthy to further study.