Low-temperature Sintering Of Ba₅Nb₄O₁₅-based Microwave Dielectric Ceramics And Gel-casting Of MCT Ceramics

In this thesis, Ba₅Nb₄O₁₅ ceramics were prepared via conventional mixing oxide route, and the effects of sintering aid, ionic substitutions and processing factors on chemical structure and dielectric properties of the samples were systematically investigated. Dielectric mechanisms of the ceramic were also discussed in this thesis. In addition, gel-casting forming process was used to fabricate microwave components with complex shapes.B₂O₃, LiF and B₂O₃-Bi₂O₃ were used as sintering additives to lower the sintering temperature for Ba₅Nb₄O₁₅ ceramics. Sintering behavior, microstructure and microwave properties were investigated by XRD, SEM, EDS and network analyzer. The results showed that a few amount of B₂O₃ or LiF could effectively lower the sintering temperature of Ba₅Nb₄O₁₅ ceramics into about 920℃. However, for the additions of B₂O₃-Bi₂O₃, the sintering temperature of Ba₅Nb₄O₁₅ ceramics was firstly decreased and then increased as the amount of Bi₂O₃ increased. The temperature coefficients of resonant frequency (Ï„f) for Ba₅Nb₄O₁₅ ceramics was tuned effectively by the addition of B₂O₃, LiF and B₂O₃-Bi₂O₃, respectively. However, the Q×f values of the sample were decreased by those additions.The effects of B-site substitution with V⁵⁺ ions or Sb⁵⁺ ions for Nb⁵⁺ ions on the structure and properties of Ba₅Nb₄O₁₅ ceramics were systematically studied. The results showed that with a small amount of substitution with V⁵⁺ ions or Sb⁵⁺ ions for Nb⁵⁺ ions, the Ba₅Nb₄O₁₅ ceramics could be sintered at about 900℃, and theÏ„f value of ceramics could be effectively tuned toward to zero, especially for the substitution of Sb⁵⁺ ions for Nb⁵⁺ ions. The Q×f values of Ba₅Nb₄O₁₅ ceramics firstly increased slightly and then decreased for the substitution of V⁵⁺ ions for Nb⁵⁺ ions, and, increased firstly from 22,890GHz to 34,630GHz and then decreased with a increasing in the substitution of Sb⁵⁺ ions for Nb⁵⁺ ions. Best dielectric properties of Ba₅(Nb_1-xSb_x)₄O₁₅ ceramics were obtained with a formula of Ba₅(Nb_1-xSb_x)₄O₁₅ (x=0.15). In order to improve the dielectric properties of Ba₅(Nb_1-xSb_x)₄O₁₅ further, the orthogonal design of experiment, L9(34), was used, in which the amount of H3BO3 and the processing parameters were considered as factors. The result showed Ba₅(Nb_1-xSb_x)₄O₁₅ (x=0.15) ceramic added with 0.8wt% H3BO3 had a optimal dielectric properties ofεr=30.02, Q×f=15,780GHz,Ï„f=10ppm/℃, in addition to good chemical compatibility with silver electrodes.Meanwhile, gel-casting forming process was successfully used to fabricate microwave components with complex shapes. The dielectric properties of ceramics prepared by gel-casting were compared with those by die-pressing. In addition, one kind of microwave antenna was also developed with the gel-casting forming process.In the end of this thesis, dielectric polarization mechanism of Ba₅Nb₄O₁₅ ceramics was also discussed and useful hints for adjusting permittivity and temperature coefficient of resonant frequency for the ceramics were present.