Low Temperature Sintering And Property Optimization Of Ultralow-loss Microwave Dielectric Composite Ceramics

With the continuous upgrading of mobile communication technology,microwave electronic components are developing towards high frequency,miniaturization,ultra-low loss and temperature stability,as an important part of mobile communication base station,aerospace and Global Positioning System.Therefore,the new microwave dielectric ceramic materials need to have ultra-low dielectric loss in order to meet the requirements of microwave electronic equipment in millimeter wave frequency band,as well as the low dielectric constant and near zero resonant frequency temperature coefficient.In this paper,the temperature stable Sr₂Ce_0.665Ti_0.335O₄（SCT）composite ceramics and the novel low-loss（1-x）Ca₂SnO_4-xCaTiO₃ composite ceramics were prepared by the traditional solid-state reaction method.The relationship between the sintering behavior,phase composition,microstructure and microwave dielectric properties of the ceramics was systematically investigated by introducing A-site ion substitution,improving the low temperature sintering properties together with XRD,SEM,EDS and vector network analyzer.The main research contents of this paper are as follows:Firstly,SCT microwave dielectric composite ceramics with Sr₂CeO₄ and Sr₂TiO₄ as main phases were synthesized by traditional solid phase method.Then,Ca²⁺was introduced into SCT ceramics for A position substitution,and the effect of Ca²⁺on the microwave dielectric properties of the matrix was studied.The XRD and EDS results showed that there was a stable two-phase structure in the sintered ceramics,and Ca²⁺was dissolved into the matrix lattice.The substitution did not cause the change of phase composition.SEM results showed that the substitution of Ca²⁺promoted the grain growth and microstructure densification.In addition,a small amount of Ca²⁺helped to stabilize the crystal structure,and Q×f values of the sintered samples were effectively improved.When 0.25 mol.%Ca²⁺was added to SCT ceramics,the optimum sintering properties were obtained for ceramics sintered at 1150℃ for 4 h:ε_r=20.9,Q×f=168,580 GHz,τ_f=-0.8 ppm/℃,showing great potential in millimeter wave application.Subsequently,a small amount of Li₂O-B₂O₃-SiO₂（LBS）glass was added to the SCT ceramics,and the sintering temperature decreased from 1400℃ to 1050℃.XRD results showed that the addition of LBS glass could cause a small amount of SrCeO₃ impurity phase,resulting in the increase of dielectric loss.Meanwhile,a small amount of LBS glass could promote the densification of SCT ceramics,improve the microstructure,reduce the number of pores,and then maintain good microwave dielectric properties.However,excessive glass could precipitate at grain boundaries,which hindered its densification and worsening its microwave dielectric properties.The sample with 1.5 wt.%LBS sintered at1050℃ for 4 h exhibites optimum properties:ε_r=19.4,Q×f=63,010 GHz,τ_f=-4.8ppm/℃.Finally,a novel low-loss（1-x）Ca₂SnO_4-xCaTiO₃ composite ceramics was prepared,and the effects of the relative content of two phases on the phase structure and microwave dielectric properties of the ceramics were investigated.XRD and SEM results showed that the Ca₂SnO₄ and CaTiO₃ phases could exist in the sintered body between 1350℃ and 1475℃,and no other heterophase was formed.The uniform microstructure with clear grain boundary and packed closely grains were observed.By adjusting the relative content of the two phases,the multiphase ceramic material with a near-zero resonance frequency temperature coefficient can be obtained.When x=0.12,0.88Ca₂SnO₄-0.12CaTiO₃ multiphase ceramics samples sintered at 1425℃ for 4 h have good dielectric properties:ε_r=13.2,Q×f=44,510 GHz,τ_f=-1.3 ppm/℃.