Preparation And Microwave Properties Of Germanium-based Garnet Dielectric Ceramics

Microwave dielectric ceramics（MWDC）is a key component of passive devices,mainly used as filters,resonators,antennas,frequency discriminators,etc.In addition,with the vigorous development of millimeter wave and 5G communication,industry and commerce have put forward a new round of requirements for the performance of microwave dielectric ceramics.Microwave dielectric ceramics with appropriate dielectric constant,low dielectric loss and near-zero temperature coefficient of resonant frequency have become the focus of scholars.The general structural formula of garnet compound is A₃B₂C₃O₁₂,which has gradually attracted people’s attention due to its diverse composition and excellent performance in the field of microwave dielectric ceramics.In this paper,a series of germanium-based garnet-type microwave dielectric ceramics Ca₃Mg BGe₃O₁₂（B=Zr,Sn）,Ca₃BTi Ge₃O₁₂（B=Mg,Zn）and Na₂Ca Ti₂Ge₃O₁₂ with lower sintering temperature than aluminate have been prepared by solid phase method based on ion occupation,structural stability,and realization of low-temperature co fired ceramic technology.The internal and external factors responsible for the differences in microwave dielectric properties were analyzed in detail using characterization methods such as X-ray diffraction（XRD）,scanning electron microscopy（SEM）,and vibration spectroscopy.（1）Under the guidance of the tolerance factor and the rule of ion occupation in the garnet structure,the ceramic Ca₃Mg BGe₃O₁₂（B=Zr,Sn）was successfully synthesized by introducing a positive tetravalent cation at the B site.Both samples belong to the cubic garnet structure,and the space group is Ia-3d.XRD patterns and Rietveld refinement results show that the single-phase components are obtained at the sintering temperature of 1300℃,and the structure is stable within the sintering temperature range.Ca₃Mg Zr Ge₃O₁₂ sample has the best compactness at the sintering temperature of 1400℃,at which time its theoretical density is 99.5%,and its microwave dielectric property isε_r～10.80±0.2,Q×f～79,600±1000 GHz,τ_f～-66.8±1 ppm/℃.The optimum density temperature of Ca₃Mg Sn Ge₃O₁₂ ceramics is 1420℃,the theoretical density is 97.8%,and the microwave dielectric property isε_r～9.68±0.2,Q×f～83,400±1000 GHz,τ_f～-57.9±1 ppm/℃.（2）In raw materials,Ti O₂ with a lower melting point was selected to replace Zr O₂and Sn O₂,and Ca₃BTi Ge₃O₁₂ ceramics were prepared in the ratio of Ti:B=1:1（B=Mg,Zn）in the oxygen octahedron,achieving a reduction in sintering temperature.XRD and Rietveld refined results show that the two samples belong to the cubic garnet structure,and the space group is Ia-3d.The sintering temperature and density of the two ceramics are related to the sintering characteristics of the raw materials.The best microwave dielectric properties are obtained at the sintering temperature with the best compactness.The optimum sintering temperature of Ca₃Mg Ti Ge₃O₁₂ ceramics is1280℃,and the microwave dielectric properties areε_r～12.97±0.03,Q×f～84,000±3000 GHz,τ_f～-29.4±1.5 ppm/℃.The optimum sintering temperature of Ca₃Zn Ti Ge₃O₁₂ is 1180℃,and all performance parameters areε_r～12.84±0.03,Q×f～45,000±2000GHz,τ_f～-33.19±1.6 ppm/℃.The difference of microwave dielectric properties was analyzed by the theory of ionic polarizability,stacking rate and bond valence.（3）A germanium-based garnet-type microwave dielectric ceramic Na₂Ca Ti₂Ge₃O₁₂ containing sodium was prepared by solid-state method.XRD,SEM,Raman and other characterization methods showed that the second phase of the ceramic would be produced after the sintering temperature exceeded 980℃,and the quality factor would be seriously deteriorated.The best microwave dielectric property is obtained at the sintering temperature of 960℃,namelyε_r～15.3±0.2,Q×f～55,900±2000 GHz,τ_f～-15.7±0.5 ppm/℃.The results of XRD,SEM and EDS show that the matrix ceramic and metal electrode have good chemical compatibility after co-firing,and can be used as a candidate material for practical application of LTCC.