Preparation And Dielectric Properties Of Magnesium Titanate Based Ceramics

As a microwave dielectric ceramic material with high quality factor（Q·f）,Magnesium titanate（MgTiO₃）,is widely used in mobile communication base stations at microwave frequency for the preparation of dielectric resonators and filters.However,the dielectric constant（ε_r）of MgTiO₃ ceramic material is low;The sintering temperature of ceramics prepared by solid-phase method is as high as 1400°C,and then energy loss is large.The resonant frequency temperature coefficient（τ_f）is as low as-50 ppm/°C,which is not conducive to the stability of microwave devices in practice.Therefore,it is a hot issue in the current research to look for a new preparation process to reduce the sintering temperature of MgTiO₃ ceramics,and improve its microwave dielectric properties by combining with other ceramics.In this thesis,MgTiO₃-based nano-ceramic powders were successfully prepared by wet chemical method.The effects of sintering temperature,the adding of BaTiO₃ and CaTiO₃ on phase composition,microstructure,density and dielectric properties of MgTiO₃-based composite ceramics were studied.The main research contents and results of this paper are as follows:Firstly,MgTiO₃ nano-ceramic powders with good crystallization and particle size of 56 nm were synthesized by chemical precipitation method at the concentration of 4 mol/L NaOH and calcination at 900°C for 2 h.The MgTiO₃ ceramic powders sintered at 1200°C with the dielectric constant of 17.74.Compared with the solid phase method,the powders prepared by wet chemical method have smaller particle size,higher surface energy and higher sintering activity,resulting in the lower sintering temperature of ceramics.In order to improve the dielectric constant of MgTiO₃ ceramics,the（1-x）MgTiO₃-x BaTiO₃composite ceramics were prepared by combining BaTiO₃ with highε_r and MgTiO₃.（1-x）MgTiO₃-x BaTiO₃ composite ceramics were prepared by ball milling and sintering with MgTiO₃ nanopowders synthesized by chemical precipitation method and BaTiO₃ nanopowders obtained by hydrothermal method as raw materials.The results show that when x=0.05,0.95MgTiO₃-0.05BaTiO₃ ceramics sintered at 1175°C for 4 h has good microwave dielectric properties:ε_r=23.45,Q·f=11869 GHz,τ_f=-23.51 ppm/°C,indicating that the addition of BaTiO₃ can effectively increase theε_r of MgTiO₃ ceramics.In order to further regulate the microwave dielectric properties of MgTiO₃ ceramics,（1-y）MgTiO₃-y CaTiO₃ composite ceramics were prepared by combining CaTiO₃ with highτ_f and MgTiO₃.Nanosized（1-y）MgTiO₃-y CaTiO₃ powders with different CaTiO₃ quantity were synthesized by chemical precipitation method under the conditions of NaOH concentration of 4mol/L and calcination at 900°C for 2 h.Then,（1-y）MgTiO₃-y CaTiO₃ composite ceramics were prepared with the nanopowders obtained above.The results show that when y=0.07,0.93MgTiO₃-0.07CaTiO₃ ceramics sintered at 1175°C for 4 h has good microwave dielectric properties:ε_r=20.94,Q·f=46690 GHz,τ_f=-3.26 ppm/°C,indicating that the addition of CaTiO₃ effectively regulates the τ_f value of MgTiO₃ ceramics near zero.