Modification And Some Foundational Issues Of Ba_6-3xLn_8+2xTi₁₈O₅₄-Based Microwave Dielectric Ceramics

solid solutions have important applications in microwave circuits because of their high ε_r, high Qf and small Xf. In this thesis, effects of ionic substitution and sintering temperature upon the crystal structure, microstructures and microwave dielectric properties of Ba_6-3xLn_8+2xTi₁₈O₅₄ (Ln=La, Nd and Sm) ceramics were investigated, and an abnormal phase transition was observed in Ba_6-3x(Sm₍1-y(La_y)_6+2xTi₁₈O₅₄ (x=0.5) ceramics. The physical properties including thermal expansion and infrared reflectance data were measured and analyzed.Modification of Ba_6-3xSm_6+2xTi₁₈O₅₄ (x=0.5, 2/3 and 0.75) microwave dielectric ceramics was comprehensively investigated by La³⁺ substitution for Sm³⁺. Dense Ba_6-3x(Sm_1-yLa_y)_6+2xTi₁₈O₅₄ ceramics were obtained by sintering at 1320¹350℃. XRD and SEM analysis indicate that Ba_6-3x(Sm_1-yLa_y)_6+2xTi₁₈O₅₄ ceramics take the single phase solid solution form, which is identified with the orthorhombic tungsten bronze-type structure. La substitution is effective in optimizing the dielectric properties of Ba_6-3x(Sm_1-yLa_y)_6+2xTi₁₈O₅₄. Excellent microwave dielectric characteristics are obtained at composition y=0.1 (ε_r=81.5, Qf=9512GHz, Ï„_f=+8ppm/℃) when x=2/3.Co-substitution of La³⁺ and Bi³⁺ on A-site is a promising method to modify the microwave dielectric properties of Ba_6-3xSm_8+2xTi₁₈O₅₄ ceramics. Bi³⁺ substitution is effective in increasing dielectric constant, but leads to a decreased Qf value and an increased negative Ï„_f. The co-substitution by La³⁺ and Bi³⁺ exhibits the advantage in improving ε_r and compensating Ï„_f. A single-phase solid solution with new tungsten bronze-type structure is formed in low Bi³⁺ substituted region and a good combination of large ε_r, high Qf value and small Ï„_f is obtained.Effects of microstructures on the microwave dielectric characteristics of Ba_6-3xSm_8+2xTi₁₈O₅₄ (x=2/3) ceramics were investigated by controlling the sintering temperature and the post densification annealing. The dielectric constant is sensitive to the porosity in ceramics, but insensitive to the annealing process. Qf value varies with both the annealing atmosphere and the sintering temperature, which indicates the strong reliance of dielectric loss on defects and grain boundaries. Ï„_f alue exhibits a complex dependence on the sintering temperature. The orientation of grains is responsible for the variation of Ï„_f.An abnormal phase transition from the tungsten bronze-type to thewas observed in Ba^Smi^La^gTiisC^ ceramics in the La-rich region. Compared with the tungsten bronze-type structure, the cation distribution in the Bai6Ce32Ti64Oi92-type structure is more ordered with the tetragonal and pentagonal sites fully occupied by Ln3+ and Ba2+, respectively. The stability of the Bai6Ce32Ti64Oi92-type structure depends sensitively on the ordering of Ln3+ and Ba2+ and the average radius of Ln3+ ions. The phase transition was observed only in sintered pellets but not in the loose powders. The inner stress caused by the shrinkage during sintering might be necessary for the phase transition. Though the Bai6Ce32Ti64Oi92-type solid solution is not suitable for microwave applications because of its poor Qf value, it is potentially useful for high frequency applications for its high dielectric constant (fi^-176) and low dielectric loss (tanS¹0⁴) at frequencies lower than 108Hz.Thermal expansion coefficients of Ba6-3.*Ln8+2*Tii8O54 (Ln=La, Nd and Sm) ceramics were measured by a push-rod dilatometer. A first-order phase transition is observed by the measurement at about 1380°C. As indicated by step-scan XRD analysis on both the quenched and the slowly cooled samples, a little amount of secondary phase is formed in the quenched sample and the crystal structure expands in b- and c- axis in the magnitude of ⁰.1%, which is similar to the relative change in sample size. The phenomenon is related to a structural phase transition at high temperature, and a meta-stable phase is temporarily formed.Infrared reflection spectra were measured for Ba6-3JrSm8+2xTiigO54 ceramics and analyzed by a standard Kramers-Kroning procedure. The permittivity was extrapolated down to the microwave range using the classical oscillator model for fitting the dielectric function. According to the similar structural units contained in both the new tungsten bronze-type structure and the perovskite structure, the two structures exhibit similar infrared reflection spectra. The phonons corresponding to A-TiO6 translation and B-O6 stretching modes dominate the dielectric properties of tungsten bronze-type solid solutions, and are sensitive to the ordering of A-site cations. Due to the error in reflectance produced by the random scattering on sample surfaces, the calculated dielectric constant is smaller than the measured value together with a relatively large loss tangent.