Research On Low Loss Ferroelectric Materials For Tunable Microwave Application

The improved ferroelectric materials which have the superior qualities of low dielectric loss, high tunability and excellent comprehensive dielectric performance can be widely used in phased array antenna systems and microwave communication devices as well. Ferroelectrics of barium strontium titanium oxide (BSTO) have been the focus of new tunable microwave application materials research in recent years, for they possess the highest tunability. However, the high dielectric loss of pure BSTO limits the further application to microwave devices.In order to acquire suitable dielectric performance of BSTO, that's, high tunability, low loss tangent and relatively low dielectric constant, we need develop new method to reach this purpose. Various composites of barium strontium titanium oxide (BSTO) with other low dielectric loss, low dielectric constant dielectric ceramics have been formulated for such uses. The effects of different dopants, different doping concentration and different sintering temperature on the performance of composite ceramics have been discussed in the thesis. The results show that the dielectric constant and the loss tangent of these composites as well as the sintering temperature have been reduced effectively; especially the composites which are combined by BSTO and Zn₂SiO₄ reduce the loss sharply. XRD analyses indicate that the composite ceramics consist of more than two phases by doping BSTO with Mg₂SiO₄, Zn₂SiO₄ and Mg₄Nb₂O9. Through comprehensive consideration the performance of Mg₂SiO₄ doping composites, the doping concentration of Mg₂SiO₄ should be less than 50mol% (₄0wt%); the suitable sintering temperature should be controlled between 1₂₂0 oC ¹₂60 oC according to different doping concentration; dielectric constant of composite ceramics decrease with the increase of doping concentration of Mg₂SiO₄; loss tangent of composites decrease at low doping concentration and increase at higher doping concentration. For the composite ceramics of doping Zn₂SiO₄, the doping concentration of Zn₂SiO₄ should be less than 30mol% (about 30wt%); the suitable sintering temperature is around 1170 oC; loss tangent and dielectric constant decrease with the increase of the doping concentration. For the composite ceramics of doping Mg₄Nb₂O9, the highest sintering temperature change much by different doping concentration; loss tangent and dielectric constant decrease with the increase of the doping concentration at the same sintering temperature; loss tangent and dielectric constant decrease with the increase of sintering temperature at the same doping concentration.Our research work shows that the dielectric performance of composite ceramics has been improved by doping Mg₂SiO₄, Zn₂SiO₄ and Mg₄Nb₂O9 in BSTO. And we can expect they will have better microwave performance. This work lays a solid foundation for the further research about BSTO ferroelectric ceramics in tunable microwave application.