The The Bi <sub> 2 </ Sub> (zn <sub> 1/3 </ Sub> Nb <sub> 2/3 </ Sub>) <sub> 2 </ Sub> O <sub> 7 </ Sub > Preparation Of Microwave Dielectric Ceramics And Dielectric Performance

Recently, as a kind of new electronic materials, microwave dielectric ceramics had been widely used in many components, such as oscillators, filters, dielectric antennas, dielectric substrates, and so on. Those components had been widely used in many fields, such as wireless communication, mobile telephone, military radar and satellite broadcasting, and so on. So nowdays more and more attention had been paid to microwave dielectric ceramics. For the practical application, the microwave dielectric ceramics were required to have high permittivity and quality factor. Besides, the temperature coefficient of resonance frequency should be near zero.Bi₂(Zn_1/3Nb_2/3)₂O₇ (BZN) based ceramics had received more and more attention due to their excellent dielectric properties and low sintering temperature. In this thesis, Bi₂(Zn_1/3Nb_2/3)₂O₇ ceramics were prepared by different methods. After that, the effects of ion substitution and doping element on the ceramics prepared by the optimized method were also studied.Firstly, The BZN ceramics were prepared by using the solid state reaction method, the semichemical method and the molten salt method respectively. Pure monoclinic pyrochlore phase was obtained from the ceramics prepared by three methods; the phase formation mechanisms of these methods were similar. Besides, the powders prepared by the solid state method and the semichemical method showed spherical particles, but the powder obtained by the molten salt method showed slice-like particles. The properties of the sintered samples were listed as follows:ρ= 7.79g/cm³,ε_r = 83, tanδ= 0.0022(solid state method);ρ= 7.61g/cm³,ε_r = 84, tanδ= 0.0029(semichemical method);ρ=7.64g/cm³,ε_r=81, tanδ=0.0051 (molten salt method). From those results, it could be conclude that the solid state method was the most promising method.Then, in order to improve dielectric properties of the ceramics, the Nd³⁺ and Ta⁵⁺ were respectively introduced in the A and B site to substitute Bi³⁺ and Nb⁵⁺ in BZN ceramics. For (Bi_2-xNd_x)(Zn_1/3Nb_2/3)₂O₇ ceramics, the phase structure of samples were still of pure monoclinic pyrochlore when x<0.3. With the Nd³⁺ content increasing, the cubic pyrochlore phase appeared and Bi₂Nd₄O₉ phase was also detected. Besides, with increasing Nd³⁺ content,ρandε_r increased at first, and then decreased, but the variety of tanδshowed the reverse trend. The optimal values were obtained by substituting x=0.2 Nd³⁺ and sintered at 1000℃and they were listed as follows:ρ= 7.76g/cm³,ε_r = 79, tanδ= 0.002(1 MHz). For Bi₂(Zn_1/3Nb_(2/3-y/3)Ta_y/3)₂O₇ ceramics, the phase structure showed a mixture of monoclinic pyrochlore and Bi₃TaO₇ phases. When Ta⁵⁺ completely substituted Nb⁵⁺, the sample showed pure monoclinic pyrochlore phase. Furthermore, with increasing Ta⁵⁺ substitution,ρincreased gradually, butε_r and tanδdecreased gradually. So, no significant effect on the dielectric properties of ceramics could be seen when Nd³⁺ was introduced to the A site to substitute Bi³⁺. However, the dielectric properties of ceramics were degraded when Ta⁵⁺ substituted Nb⁵⁺.Finally, to further decrease the sintering temperature of the BZN ceramics, LiSbO₃ and BiFeO₃ were introduced as the sintering aids. The results indicated that the crystal structure of all LiSbO₃ and BiFeO₃ doped samples remained monoclinic pyrochlore, and no secondary phase was observed. The adding of LiSbO₃ and BiFeO₃ could make the sintering temperature decrease to 920℃. The optimal values were obtained by doping 0.10wt% LiSbO₃ and 0.15wt% BiFeO₃ respectively, which were listed as follows:ρ=8.04g/cm³,ε_r=79, tanδ=0.002(0.10wt% LiSbO₃);ρ=7.87g/cm³,ε_r =79, tanδ=0.00086 (0.15wt% BiFeO₃).