The Study Of BaO-Ln₂O₃-TiO₂ Microwave Ceramics Modification By Lanthanide Elements

The researching of microwave dielectric ceramics, especially BaO-Ln₂O₃-TiO₂ was summarized in this thesis. The effective electric field analysis in BaO-Ln₂O₃-TiO₂ indicated that it was electron-ion displacement polarization coupling that caused high permitivity. The contribution to polarization of A position cation was not so big. While the analysis results of intrinsic loss of ion crystal indicated that the ordering distribution of the cations in BaO-Ln₂O₃-TiO₂ and the lean angle increment of the octahedron structure were advantaged to loss reduction and heat stability enhancement. Based on the mentioned two points, the modification of BaO-Ln₂O₃-TiO₂ was mainly focused on the effect of lanthanide ion to crystal structure and microwave dielectric properties. At last the modification system and special case of BaO-Ln₂O₃-TiO₂ were examined to identify the antecedent rules.The main part of BaO-Ln₂O₃-TiO₂(Ln = La, Pr, Nd, Sm, Gd)system, i.e., Ba_6-3xLn_8+2xTi₁₈O₅₄ solid solution, belonged to finite substitution solid solution, and the limits were: when Ln=La, 0.1≤x≤0.8; when Ln=Pr, 0.1≤x≤0.8; when Ln=Nd, 0.2≤x≤0.7;when Ln=Sm, 0.3≤x≤0.7; when Ln=Gd, x=0.5.With the lanthanide cation diameter increment, the solid solubility limit of related system gradually reduced till reaching the only value.In BaO-Ln₂O₃-TiO₂ system, when the lanthanide concentration was determined, with the Ln3+ ion diameter increased, the crystal cell would mainly expand at AB plane that was vertical to C axis, so the change of crystal cell parameters were mainly represented as the increment of a and b, but the change of c was quite few. When the concentration of lanthanide ion increased, the whole crystal cell would shrink. Besides, the decrease of lanthanide cation diameter of Ba_6-3xLn_8+2xTi₁₈O₅₄ solid solution will increase the lean angle of [TiO6] octahedral, while the lean angle increase would also lead to the crystal cell volume shrink.The effect of lanthanide ion diameter, ion content, electronegativity and polarizability to dielectric constant were researched in this thesis. The tungsten-bronze type like structure caused high permittivity of BaO-Ln₂O₃-TiO₂ system, while the exsistence of lanthanide ion resulted in the permittivity change in extent. With the raw material recipe Ba_6-3xLn_8+2xTi₁₈O₅₄, with lanthanide ion diameter, polarizability and electronegativity increased, the dielectric constant showed a increasing trend. When x=2/3,the quality factor of BaO-Ln₂O₃-TiO₂ was the maximum. With the decrease of ion diameter and Ln3+ electronegativity, the maximum quality factor would increase accordingly.The lanthanide ion concentration increment , Ln3+ ion diameter decrease and structure torlerance factor decrease were advantaged to obtain near-zero resonant frequency temperature coefficient.The cell parameter and microwave dielectric properties of BaO-Sm2O3-Nd2O3-TiO2 and BaO-Nd2O3-TiO2 doped by Bi2O3 identify the rule as before. BaO-CeO2-TiO2 microwave dielectric ceramic was the special case in BaO-Ln₂O₃-TiO₂ seriers just because the smaller ion diameter and unstable valence. The sintering character and phase composition were different from other.And the permittivity and quality factor were lower than those of other BaO-Ln₂O₃-TiO₂ ceramics.