| In this dissertation, the effect of phase composition and ionic substitution on the rock-salt structured ceramics of Li3Mg2NbO6 and Li2TiO3 was investigated systematically. The ceramics prepared by a traditional solid reaction method, and the results demonstrated that the microwave dielectric properties of Li3Mg2Nb06 and Li2TiO3 largely depended on the phase composition and ion substitution.The effect of various content of Zn2+ substitution on the Li3Mg2NbO6 ceramic was firstly investigated. The results revealed that the substitution of an appropriate amount of Zn2+ benefited to the improvement of Q×f value, owing to the distortion of crystal lattice caused by larger ionic radii of Zn2+ when compared with Mg2+. As the substitution content of Zn2+ was 8mol%, the ceramics obtained excellent microwave dielectric properties of ?r=17.2, Q×f=142331 GHz, ?f=-23.2 ppm/? when sintered at 1120?. After 0.5 wt% 0.17Li2O-0.83V2O5 low-melting additive was added, the as-prepared ceramics could be sintered well at 925? and achieved good microwave perperties of ?r=14, Q×f=83395 GHz, and ?f=-37.2 ppm/?. Meanwhile, this ceramic has a good chemical compatibility with Ag electrode.Based on results above mentioned, we further investigated the influence of on the sintering behavior and dielectric characteristics of b06 ceramics due to its large negnative ?f value. The XRD results showed that the Li3(Mg0.92Zn0.08)2Nb06 and Ba3(VO4)2 ceramics formed a diphasic composite ceramic in the whole composition range due to their different crystal structure. Because of the lower ?r and Q×f values of Ba3(VO4)2, the variation of the composite ceramics generally presented a decreasing trend in ?r and Q×f. By adding Ba3(VO4)2 phase, the ?f value of the composite ceramic moved toward the positive direction, and the sintering temperature decreased obviously as a result of the partial diffusion of V5+ into the main crystal phase. As a result, the composite ceramics achieved excellent microwave dielectric properties of ?r=16.3,Q×f=50084 GHz, and ?f =1.5 ppm/? when 30 vol% Ba3(VO4)2 was added, which could be sintered compactly at 950?.This composite ceramics also does not react with Ag electrode under a co-firing condition.Furthermore, the effect of (Al0.5Nb0.5)4+ substitution on the microstructure and microwave dielectric properties of Li2TiO3 ceramic was explored. The results indicated that the complex substitution of Al3+ and Nb5+ produced a significant effect on the microstructural morphology. Both microcrack healing and grain growth contributed to the obviously enhanced Q×f values in comparison with the decrease of ?r and ?f values in the solution were ascribed to the ionic polarizability and the cell volume, respectively. Excellent microwave dielectric properties of ?r=21.2,Q×f=181800 GHz and ?f=12.8 ppm/? were achieved in the x=0.15 sample when sintered at 1150?. After 6 wt% BaCu(B2O5) additive was introduced, it can be well sintered at 950? and exhibits good microwave dielectric properties of ?r=20.4, Q×f=53290 GHz and ?f=3.6 ppm/? as well. The co-firing test of the low-sintering sample with Ag powder proves its good chemical stability during high temperature, which enables it to be a promising candidate material for the application of low temperature cofired ceramics (LTCC). |
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