Foundational Issues In Ba_6-3xNd_8+2xTi₁₈O₅₄ Microwave Dielectric Ceramics

Ba_6-3xNd_8+2xTi₁₈O₅₄ solid solutions have important applications in microwave circuits because of their high ε_r, high Qf and small Ï„_f. In this thesis, effects of composition, ionic substitution and process conditions upon the phase constitution, crystal structure, and microwave dielectric properties of Ba_6-3xNd_8+2xTi₁₈O₅₄ ceramics were investigated and the solid solution upper limit was determined. In addition, the effects of Sr²⁺ substitution for Ba²⁺ on the phase constitution and microwave dielectric properties were studied.The upper limit of x in Ba_6-3xNd_8+2xTi₁₈O₅₄ ceramics was determined by investigating the phase constitution as the function of composition and process conditions. For the compositions of x≤0.75, the new tungsten bronze structure was stable and the single-phase structure was easy to be obtained. While, the new tungsten bronze structure became unstable for compositions of x=0.8, 0.9 and 1.0, where the new tungsten bronze major phase was generally detected together with some secondary phases. The excellent microwave dielectric characteristics were not only obtained at x=0.67 but also obtained at 0.8 and 0.9.The Qf values of the (Ba_1-ySr_y)_6-3xNd_8+2xTi₁₈O₅₄(x=0.2) ceramics were improved by substitution Sr²⁺ for Ba²⁺. The highest Qf values of the (Ba_1-ySr_y)_6-3xNd_8+2xTi₁₈O₅₄(x=0.2, y=0.26) ceramic reached 6 886GHz which was more than the three times of the Ba_6-3xNd_8+2xTi₁₈O₅₄ (x=0.2 , y = 0). The crystallographic sites occupancies were calculated from high-resolution x-ray powder diffraction patterns data by Rietveld structural refinement. Compared with the initial sites occupancies, the substituted ones were not changed obviously but the substitution made the A1 sites and A2 sites more ordered. So the stabilization of the new tungsten-bronze structure increased while the internal stress decreased. As a result, the dielectric loss decreased and the Qf value increased. The occupancies in all A1 sites were all the same. It means that there is no special A1 site and all the A1 sites were almost the same.