Microwave Dielectric Characteristics And Energy Storage Density Of Ba_6-3xLn_8+2xTi₁₈O₅₄-Based Ceramics （Ln=La,Sm）

In the present thesis, the structures, microwave dielectric properties and high energy storage density were systematically investigated for the typical high-K microwave dielectric ceramics Ba6-3χLn8+2χTi18O54 (Ln=La, Sm). The variation tendency of structure and dielectric characteristics of Ba6-3χSm8+2χTi18O54 ceramics were systematically investigated together with the micro structure dependence of dielectric strength and energy storage density.The upper and lower solubility limits in Ba6-3χSm8+2χTi18O54 tungsten bronze ceramics were determined by the Rietveld refinement of XRD data combined with backscattered electron images, and the variation tendency of microwave dielectric characteristics was also investigated. The upper solubility limit was confirmed as x=2/3, while the lower solubility limit was determined as 1/4 instead of the previously reported one x=3/10. Due to the substitution of 3Ba2+by 2Sm3+with increasing x, the total polarizabilities reduce significantly and hence the dielectric constant of Ba6-3xSm8+2χTi18O54 ceramics decreases monotonically with increasing x. The small temperature coefficient of resonant frequency with complex variation tendency was observed for the compositions 1/2≤χ≤4/5. The QFvalue increases at first, reaches the maximum around x=2/3, and turns to decrease for x≥7/10 The high Qf value at x=2/3 can be interpreted by the lowest crystal distortion and inner stress as a result of the ordering of Sm and Ba ions in AL and A2 sites, respectively.Dielectric strength and energy storage density of Ba6-3χSm8+2χTi18O54 tungsten bronze ceramics were determined, and the relationships among microstructure, dielectric constant, dielectric strength and energy storage density were also investigated. Nearly full densification of Ba6-3χSm8+2χTi18O54 ceramics was achieved at different temperatures for different compositions (relative density values as high as 98%or above). The average grain size increases with the increasing sintering temperature for each composition. Both the weak dependence of the dielectric permittivity on frequency and the low loss tangent are desirable for future dielectric materials. The dielectric strength increases with the increasing x, while the energy storage density values decrease generally with the increase of x. The maximum energy storage density is 0.30 J/cm3 under an electric field of 25.8 kV/mm for x=1/4 at the sintering temperature of 1400℃ Dielectric strength and energy storage density of Ba6-3χLa8＋2χTi18O54 tungsten bronze ceramics were also determined, and the relationships among microstructure, dielectric constant, dielectric strength and energy storage density were also investigated. Nearly full densification of Ba6-3χLa8+2χTi18O54 ceramics was achieved at different temperatures for different compositions (relative density values as high as 98%or above), and all compositions have single phase structure. The average grain size increases with the increasing sintering temperature for each composition. Both the weak dependence of the dielectric permittivity on frequency and the low loss tangent are desirable for future dielectric materials. Totally, the dielectric strength increases at first, reaches the maximum around x=2/3 and turns to decrease for x=3/4, due to the lowest crystal distortion inner stress as a result of the ordering of La and Ba ions in the Al and A2 sites at x=2/3, respectively. The energy storage density values decrease generally with the increase of x, and the maximum energy storage density is 0.38 J/cm3 under an electric field of 26.0 kV/mm for x=1/4 at the sintering temperature of 1375℃...