Preparation And Properties Of Silicon-Based Low-Permittivity Microwave Dielectric Ceramics

With the rapid and great applications of artificial intelligence,Internet of Things,big data and other emerging technologies.Microwave communication equipment tends to be highly integrated,miniaturized,high stability and low cost.In order to meet the growing needs of microwave circuits for modern communication,mobile communication and military applications,microwave dielectric ceramics with low dielectric constant（ε_r）,high quality factor（Q×f）and near-zero temperature coefficient of resonant frequency（τ_f）have higher requirements.Silicon-based microwave dielectric ceramics have been widely studied because of their advantages such as low dielectric constant,high quality factor,easy access to raw materials,abundant resource and low cost.In order to explore novel low-permittivity microwave dielectric ceramics for practical application,in this paper we synthesized the orthorhombic structure Ca₃M₂Si₃O₁₂（M=Yb,Y）and Ca₂B₂CO₇（B=Al,Ga;C=Si,Ge）ceramcis.The phase composition,crystal structure,microstructure and microwave dielectric properties of the ceramic samples were investigated by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,packing fraction,bond valence theory and far-infrared reflectance spectroscopy.（1）Firstly,Ca₃M₂Si₃O₁₂（M=Yb,Y）ceramics with orthogonal structure（pnma）were obtained by traditional solid phase synthesis method.The relative density of Ca₃Yb₂Si₃O₁₂ ceramic sintered at 1420℃is 86.5%,and the microwave dielectric properties areε_r=7.2,Q×f=21,037 GHz andτ_f=-64.1 ppm/℃.The relative density of Ca₃Y₂Si₃O₁₂ ceramic with sintering temperature of 1400℃is 87.2%,the microwave dielectric properties areε_r=7.4,Q×f=12,255 GHz andτ_f=-68.7 ppm/℃。In order to improve the density of the ceramic,the high-energy ball milling process was used.The average particle size of ceramic powders obtained by high-energy ball milling is smaller than that obtained by ordinary ball milling process.Under this process,the optimum sintering temperature of Ca₃Yb₂Si₃O₁₂ ceramics is reduced to 1260℃,the relative density and microwave dielectric properties are increased to 98.36%,ε_r=9.2,Q×f=56,400 GHz andτ_f=-77.5 ppm/℃;the optimum sintering temperature of Ca₃Y₂Si₃O₁₂ceramics is reduced to 1240℃,the relative density and microwave dielectric properties are enhanced to 94.6%,ε_r=8.7,Q×f=29,094 GHz,τ_f=-76.9 ppm/℃.According to the fitted infrared reflectivity spectra,the extrapolated intrinsic permittivity of Ca₃Yb₂Si₃O₁₂ and Ca₃Yb₂Si₃O₁₂ ceramics is 8.9 and 9.0,and the extrapolated Q×f is79,815 Ghz and 33,470 GHz,respectively.In the low frequency range of 50-400 cm^-1,the contribution of dielectric polarization is 5.57 and 6.32,which account for 85.3%and 89%of the extrapolation of dielectric constant in the wave number of 50-1000 cm^-1,respectively.（2）Since Ca₃Yb₂Si₃O₁₂ ceramic has a large negativeτ_f value,which hinders its practical application in microwave devices.At present,the methods of adjusting the temperature coefficient of resonance frequency are mainly divided into:changing the type and proportion of ions,forming solid solution and forming composite with two materials with positive and negativeτ_f value.In this thesis,a series of（1-x）Ca₃Yb₂Si₃O₁₂-x Ca TiO₃ composite ceramics with differentτ_f were obtained by adjusting the ratio of Ca₃Yb₂Si₃O₁₂ and Ca TiO₃ by the most commonly used two-phase composite method.The microwave dielectric properties of 0.91Ca₃Yb₂Si₃O₁₂‐0.09Ca TiO₃ceramics areε_r=12.93、Q×f=26,729 GHz andτ_f=+2.9 ppm/℃.（3）A series of Ca₂B₂SiO₇（B=Al,Ga）ceramics with tetragonal melilite structure were prepared by the traditional solid-phase reaction method.The relative density of Ca₂Ga₂SiO₇ ceramics sintered at 1360°C is up to 98.5%,and the microwave dielectric properties withε_r=10.3,Q×f=30,413 GHz andτ_f=-12.1 ppm/℃were obtained.The intrinsic dielectric properties of Ca₂Ga₂SiO₇ ceramics extrapolated from far infrared reflectance Spectra areε_r=10.735,Q×f=34,722 GHz.The contribution of dielectric polarization in the range of 50-400cm^-1 is 5.235,which accounts for 81.8%of the extrapolated value of dielectric constant in the range of 50-1000 cm^-1,indicating that the dielectric polarization of Ca₂Ga₂SiO₇ microwave dielectric ceramics is related to the absorption of phonons in the far infrared region.Ca₂Al₂SiO₇ ceramics also have high relative density（98.2%）at 1340°C with microwave dielectric propertiesε_r=7.7,Q×f=17,600 GHz,andτ_f==-51.06 ppm/℃.In order to further reduce the sintering temperature of Ca₂B₂SiO₇ ceramics,the Ca₂Ga₂Ge O₇ ceramics with the same melilite structure were obtained at 1280°C by means of the traditional solid-phase synthesis method through the equivalent substitution of ions.By completely replacing Si⁴⁺with Ge⁴⁺,the relative density of Ca₂Ga₂Ge O₇ ceramics is only 86%,the dielectric properties of microwave are also deteriorated:ε_r=5.86 and Q×f=5,153 GHz.