Study On The Preparation And Properties Of Temperature Stable Mid-permittivity Microwave Dielectric Ceramics

With the rapid development of microwave devices towards high frequency,miniaturization,low power consumption and high temperature stability,the microwave dielectric ceramics with medium permittivity,high quality factor and near-zero temperature coefficient have become a new research hotspot in the field of functional ceramic materials.The microwave components based them have been widely used in the global positioning system,mobile base stations,and other fields.In this paper,a conventional solid-state method was employed to prepare temperature stable BaO-ZnO-TiO₂（BZT）and two novel mid-permittivity low loss Ba₂MgTi₅O₁₃/Sr₂CeO₄microwave dielectric ceramics.The temperature coefficient of resonant frequency of Ba₂MgTi₅O₁₃ and Sr₂CeO₄ were optimized via the utilization of ion substitution methods.The effects of processing,phase structure and microstructure on microwave dielectric properties were systematically investigated by XRD,SEM,EDS and vector network analyzer.The main contents of this paper are listed as follow:Firstly,according to the ternary phase diagram of BaO-ZnO-TiO₂,the relative content of each component was determined.Microwave dielectric ceramics with BaTi₄O₉ and BaZn₂Ti₄O₁₁ as main phases were synthesized by solid-state methods.The XRD results indicate that as-sintered BZT samples exhibited two-phase structure.The SEM result exhibits closely packed particles and identifiable grain boundaries.Subsequently,Mn O₂ was added to the matrix ceramics to study its effect on the microwave dielectric properties of BZT ceramics.It was found that the addition of a small amount of Mn O₂ can reduce the loss and improve the Q×f value.It also has a certain influence on the reduction of the sintering temperature of the matrix ceramics and the optimiziation of the microstructure.When the doping amount is 0.5 mol%,BZT,sintered at 1100 ℃ can achieve the best microwave dielectric properties:ε_r=33.9,Q×f=61704 GHz,τ_f=3 ppm/℃,which made it have a good application prospect.Then,a new mid-permittivity microwave dielectric ceramic Ba_2-xCa_xMgTi₅O₁₃（0≤x≤0.3）was successfully synthesized by a solid-state method.The XRD and refinement results indicate that it was a single-phase solid solution ceramic and belonged to a monoclinic system with a space group c2/m.The Ca²⁺with a smaller ionic radius than Ba²⁺was introduced to investigate the effect of A-site substitution on microwave dielectric properties of Ba₂MgTi₅O₁₃.With increasing Ca²⁺contents,the diffraction peaks gradually moved to higher angles,accompanying the increased lattice distortion.Ca²⁺substitution leads to an increase in grain size and an uneven distribution,thereby increasing loss.The bond valance of A-site of Ba_2-xCa_xMgTi₅O₁₃ ceramics increased with increasing x values,which madeτ_f gradually move to the positive direction and finally reach a near-zeroτ_f value.Finally,novel ultra-low loss microwave dielectric ceramics Sr₂Ce_1-xTi_xO₄（0≤x≤0.5）were successfully synthesized with a special focus on the effect of sintering temperature on the phase composition and microwave dielectric properties of Sr₂Ce_1-xTi_xO₄.The XRD results exhibit the Sr₂CeO₄ phase will decompose to SrCeO₃ phase when the sintering temperature is higher than 1400 ℃,which will seriously deteriorate the microwave dielectric properties.Moreover,with an increase of x,the second phase of Sr₂TiO₄ appeared in the ceramic.And the content of Sr₂TiO₄ phase increased with the increase of substitution amount x.Its positiveτ_f value can be used to compensate for the negativeτ_f of the base ceramic.The x=0.35 sample exhibits good microwave dielectric properties of ε_r=20.7,Q×f=115550 GHz,τ_f=-1.8 ppm/℃,showing large application potentials.