Study On The Preparation And Properties Of Temperature Stable Solid Solution Microwave Dielectric Ceramics

As the core material of filters,resonators and antennas,microwave dielectric ceramics play an important role in the communication industry.Especially in recent years,the fifth-generation（5G）communication network and wireless systems have gradually integrated into all aspects of people’s lives,resulting in a blowout growth of the microwave dielectric ceramic industry.In order to meet the high throughput demand of5G communication,more and more attention is focused on the research and design of new microwave dielectric components for high frequency band.In this study,the temperature stable 0.95CaSm AlO₄-0.05Sr₂TiO₄（CSAST）ceramics were prepared by the traditional solid-state reaction method,and sintering optimization of CSAST ceramics and commercial ceramics 0.65CaTiO₃-0.35LaAlO₃ was studied.The relationship between processing,phase structure and microstructure and microwave dielectric properties were systematically investigated by XRD,SEM and vector network analyzer.The main contents of this study are listed as follow:Firstly,the （1-x）CaSmAlO₄-xSr₂TiO₄（0.01≤x≤0.06）microwave dielectric ceramics were for the first time prepared by a conventional solid-state method.The phase composition,sintering behavior,microstructure and microwave dielectric properties were investigated as function of sintering temperature and composition.A single-phase solid solution with K₂NiF₄-type tetragonal structure was formed in the range of 0.02≤x≤0.06.All samples are well densified with very little pores.The addition Sr₂TiO₄ greatly improves dielectric properties originally by restraining the secondary phase and then deteriorates Q×f value because of decreasing tolerance factor.Moreover,theτ_f value dominated by the thermal expansion coefficient increases lineally as x rises.Finally,excellent microwave dielectric properties ofε_r～18.1,Q×f～140,433 GHz（8.5 GHz）,and a near-zeroτ_f～+0.05 ppm/℃ can be obtained in the x=0.05 ceramic sintered at 1425 ℃.Subsequently,the addition of LiF can effectively reduce the sintering temperature to1350 ℃ on the basis of maintaining the excellent dielectric properties of 0.95CaSm AlO₄-0.05Sr₂TiO₄ ceramics,and there is no reaction between Li F and CSAST and no induction of other impurity phase.On this basis,SiO₂ is added to further reduce the sintering temperature.single pure phase can still maintained with small amount of SiO₂,but when the doping amount of SiO₂over 0.75 wt%,it will react to generate secondary phase Ca₂SiO₄.The maximum sintering density can be obtained at 1280 ℃ with0.95CaSm AlO₄-0.05Sr₂TiO₄+0.5wt%Li F+0.25wt%SiO₂ composition,and the system has the best dielectric properties（ε_r=18.7,Q×f=113,137GHz,τ_f=-1.46ppm/°C）,while the 0.95CaSm AlO₄-0.05Sr₂TiO₄+0.5wt%Li F+0.50wt%SiO₂ composition sintered dense at 1250 ℃ and still maintain excellent dielectric properties（ε_r=18.5,Q×f=108,060GHz）.Finally,oxides were selected to optimize the sintering of 0.65CaTiO₃-0.35LaAlO₃（CTLA）commercial solid solution ceramics.The results show that CTLA ceramics with a small amount of doped oxides can maintain a single orthogonal perovskite structure,and obtain a high density with uniform grain size after sintering at 1400 ℃.Among the oxides,SnO₂ is the best choice which can maintain a high quality factor and promote the densification of the ceramics.The doped SnO₂ can form a liquid phase at the grain boundary to promote sintering,and at the same time,a small amount of SnO₂ will enter the crystal lattice,which has an effect on the dielectric properties of CTLA ceramics from the intrinsic aspect.When SnO₂ content is 0.25wt%,CTLA ceramics can be sintered at1375 ℃ and maintain excellent dielectric properties（ε_r=46.2,Q×f=43,403GHz,τ_f=1.5ppm/℃）.