Densification, microstructural evolution and dielectric properties of Ba_6-3x(Sm_1-yNd_y)_8+2x Ti₁₈O₅₄ microwave ceramics

Utilizing different rare-earth cations R³⁺ to the Ba_6-3xR_8+2xTi₁₈O₅₄ compounds is one of effective route to tailor the dielectric constant, quality factor and temperature coefficient of frequency. In this study, densification, microstructural evolution, and microwave dielectric properties of Ba_6-3x(Sm_1-yNd_y) _8+2xTi₁₈O₅₄ compound, with x ranging from 0.3 to 0.7; and y from 0 to 1.00, were investigated. The ceramics with x=0.7 [Ba_3.9(Sm_1-yNd_y) _9.4Ti₁₈O₅₄] has a higher densification compared with others, due to the formation of vacancy, in the perovskite-like tetragonal cavity of the tungsten bronze-type framework structure. Differential thermal analysis and density results show that the densification of Ba_6-3x(Sm_yNd_1-y) _8+2xTi₁₈O₅₄ ceramics during sintering is primarily resulting from the solid state sintering process. The phase homogeneity for the Ba_6-3x(Sm_0.5Nd_o.5) _8+2xTi₁₈O₅₄ system is at least extended in the range of x between 0.3 and 0.7. Combining different rare-earth cations appears not alter the single phase range in tungsten bronze-type Ba_6-3xR_8+2xTi₁₈ O₅₄ ceramics. The size of the columnar-grain in the microstructure increases with increasing the Nd/Sm ratio as well as the x value. Dielectric constant changes from 91.0 to 84.2 as the x increases from 0.3 to 0.7. Variation of the Nd/Sm ratio allows one to control the τ_f value to the nearly 0 ppm/°C.