The Study On High-ε Microwave Ceramic Material

In this paper, the BaO-Nd₂O₃-TiO₂ system(BNT system for short) is studied and improved. The optimazation of BNT system dielectric properties is achieved by adjusting its crystal and micro- structure through the substitution for A site or B site or composite phases.1. Bi₂O₃ for additive. It is adopted Ba_6-3x(Nd_1-yBiy)_8+2xTi₁₈O₅₄(x=3/2) as molecular formula. The dielectric properties and micro-structure, also the relationship between them are studied. It is indicated that the dielectric constant increases greatly whereas Q value decreases with the increase of y value. The second phase Bi4Ti3O12 having highεwhich appears after doping with Bi₂O₃ contributes the most to the increase ofε. Bi³⁺ whose ion radius is similar to Nd³⁺ will substitute for part of Nd³⁺ or occupy A1 sites and A2 sites which have larger space, which makes BNT system doping with Bi₂O₃ change to full-filled tungsten bronze-like structure. Furthermore, the cell volume will increase and the spontaneous polarization of Ti4+ ions will be strengthened when the content of BiO₂ increases. Besides, the modification of structure coefficient inside enhances the inner electric field. So the dielectric constant shows linear increase with Bi₂O₃'s content increase. Why the dissipation factor increases and Q value decreases can be explained from two aspects. On the one hand, the strengthened polarization costs more energy. On the other hand, the few composite phases formed during sintering procedure cause the attenuation factorγincreases then Q value decreases. Temperature coefficientαc changes towards positive direction firstly then negative direction with the increase of Bi₂O₃. It is because Bi³⁺'s substitution for Nd³⁺ will cause worse distortion which makesαc increase. However, when Bi₂O₃'s content reaches to a certain level(y equals to 0.25 around),αc decreased, this phenomenon should be related to the second phase and the liquid phase.2. Ta2O5 for additive. It is started with the molecular formulas of Ba4Nd2Ti4Ta6O30 and Ba5NdTi3Ta7O30. The effect of Ta2O5 and TiO₂ on the system is researched. The dielectric constant increases greatly while the dissipation factor changes a little around 5×10-4 along with the content of Ta2O5 increases, whereas the temperature coefficient changes towards negative direction.When the content of TiO₂ increase from 0.3 to 0.8(molecule mole ratio),εdecreases firstly then increases, and then dereases again, which shows dereasing trend as a whole. Dielctric dissipation and temperature coefficient changes not too much. All the phenomen should have something to do with the full-filled tungsten bronze structure of Ba5NdTi3Ta7O30 and the tetragonal phase of TiO₂ which has low dielectric constant.During the whole experiment, the microwave dielectric ceramic materials having excellent dielectric properties are prepared, which main dielectric pamameters are listed below:ε≈120, tgδ=2.36x10-4(f=1MHz),Q≈4600 (f=1GHz),αc=-24ppm/℃,(Bi₂O₃ for additive)。ε≈96, tgδ=0.69x10-4(f=1MHz),αc =5.6723ppm/℃(Bi₂O₃ for additive)。ε=103,tgδ=5.32x10-4(f=1MHz),αc =-760.763ppm/℃(Ta2O5 for additve)。...