Research On Preparation And Optimization Of Properties Of MgNb₂O₆ Based Microwave Dielectric Ceramics

With the accelerated development of wireless communication technology toward millimeter wave,there is an urgent need to study microwave dielectric ceramic materials with low dielectric constant（ε_r≈20）,very high quality factor（Q×f≥80,000 GHz）and near-zero temperature coefficient(|τ_f|≤10×10^-6/℃).MgNb₂O₆with niobium iron ore structure is a microwave dielectric ceramic material with low dielectric constant and high quality factor,but its large negative temperature frequency coefficient and high sintering temperature seriously limit its practical application.Therefore,in this paper,the MgNb₂O₆-based microwave dielectric ceramics are studied to obtain MgNb₂O₆-based composite microwave dielectric ceramic materials with suitable dielectric constants,high quality factors and near-zero temperature frequency coefficients by exploring the best preparation process and selecting suitable materials for their composite or doping modification studies.（1）The preparation process of MgNb₂O₆ceramics was explored by investigating the effects of prefiring temperature and sintering temperature on the physical phase structure and microwave dielectric properties of ceramics.The results showed that MgNb₂O₆ceramics with good crystal structure could be synthesized at a prefiring temperature of1200℃ and had good microwave dielectric properties at 1300℃,after sintering for 4h:ε_r=21.06,Q×f=103000GHz,τ_f=-70.1×10^-6/℃.（2）In order to adjust the temperature frequency coefficients of MgNb₂O₆ceramics,（1-x）MgNb₂O_6-xCaTiO₃（x=0,0.02,0.04,0.08,0.12,0.16）microwave dielectric ceramics were prepared by the solid-phase method based on the use of the best synthesis process.The effects of CaTiO₃addition on the physical phase structure,sintering characteristics,microscopic morphology and microwave dielectric properties of MgNb₂O₆ceramics were investigated.The results show that MgNb₂O₆reacts with CaTiO₃at high temperature to form Ca Nb₂O₆and Ti₂Nb₁₀O₂₉,Ti₈O₁₅,Mg₅Nb₄O₁₅.The addition of CaTiO₃not only decreases the sintering temperature and slightly increases the dielectric constant of the composite,but also regulates the temperature coefficient,so that the temperature coefficient of MgNb₂O₆composite tends to nearly to zero.The0.84MgNb₂O₆-0.16CaTiO₃exhibited the best microwave dielectric properties at 1250℃ and 4 h holding time:ε_r=21.60,Q×f=86601GHz,τ_f=-7.3×10^-6/℃.（3）To further improve the quality factors of 0.84MgNb₂O₆-0.16CaTiO₃ceramics.The 0.84MgNb_2-x(Ti_1/2W_1/2)x O₆-0.16CaTiO₃（x=0,0.03,0.05,0.07,0.09,0.11）microwave dielectric ceramics were prepared by the solid-phase method.The effects of different doping amounts of(Ti_1/2W_1/2)⁵⁺on the physical structure,sintering characteristics,microscopic morphology and microwave dielectric properties of0.84MgNb₂O₆-0.16CaTiO₃ceramics were investigated.The results show that(Ti_1/2W_1/2)⁵⁺enters the 0.84MgNb₂O₆-0.16CaTiO₃lattice at high temperature without producing other additional heterogeneous phases.With the gradual increase of the addition of(Ti_1/2W_1/2)⁵⁺,the bulk density of 0.84MgNb_2-x(Ti_1/2W_1/2)x O₆-0.16CaTiO₃（x=0,0.03,0.05,0.07,0.09,0.11）ceramics increases,the sintering temperature does not change significantly,and the cell volume of ceramics decreases,while decreasing the material polarization rate,filling rate,so that the dielectric constant of the ceramic decreases,the quality factor is increases,and the temperature coefficient moves to positive values.The dielectric properties of 0.84MgNb_1.95(Ti_1/2W_1/2)_0.05O₆-0.16CaTiO₃ceramics at x=0.05,sintering temperature 1250℃ and holding time 4 h are:ε_r=21.52,Q×f=96983 GHz,τ_f=-3.8×10^-6/℃.（4）To reduce the sintering temperature of 0.84MgNb1.95(Ti_1/2W_1/2)_0.05O₆-0.16CaTiO₃ceramics.The 0.84MgNb_1.95(Ti_1/2W_1/2)_0.05O₆-0.16CaTiO₃-xwt%LiF-V₂O₅-Fe₂O₃（x=0,0.5,1,1.5,2.0,2.5）microwave dielectric ceramics were prepared by the solid-phase method.The effects of different doping amounts of LiF-V₂O₅-Fe₂O₃on the physical structure,sintering characteristics,microscopic morphology and microwave dielectric properties of 0.84MgNb_1.95(Ti_1/2W_1/2)_0.05O₆-0.16CaTiO₃ceramics were investigated.The results show that LiF-V₂O₅-Fe₂O₃does not react chemically with0.84MgNb_1.95(Ti_1/2W_1/2)_0.05O₆-0.16CaTiO₃at high temperature and does not precipitate the crystalline phase,and finally exists in the ceramic matrix in the glassy state.At542.1℃,LiF-V₂O₅-Fe₂O₃reacts to form Fe₂V₄O₁₃,and Fe₂V₄O₁₃melts at 666℃ in the presence of LiF to form the liquid phase,which promotes the sintering of the matrix.The sintering temperature of 0.84MgNb1.95(Ti_1/2W_1/2)_0.05O₆-0.16CaTiO₃-x wt%LiF-V₂O₅-Fe₂O₃（x=0,0.5,1,1.5,2,2.5）ceramics was reduced from 1250℃to 1125℃.With the gradual increase of LiF-V₂O₅-Fe₂O₃addition,the bulk density of 0.84MgNb_1.95-(Ti_1/2W_1/2)_0.05O₆-0.16CaTiO₃ceramics decreases,the porosity of the matrix increases,the dielectric constant and quality factor of the ceramics decrease,and the temperature coefficient moves to negative values.When the addition amount of LiF-V₂O₅-Fe₂O₃x=2.5 sintering temperature 1125℃ is held for 4 h,0.84MgNb_1.95(Ti_1/2W_1/2)_0.05O₆-0.16CaTiO₃-2.5 wt%LiF-V₂O₅-Fe₂O₃ceramics have better dielectric properties:ε_r=20.15 Q×f=58478 GHz,τ_f=-9.6×10^-6/℃.