Optimization Of Microwave Dielectric Properties Of Fluoride-based Ceramics And Application Of 5G Antennas

In the past decades,in order to meet the growing demand for large-scale traffic communication,the 5G communication frequency band has gradually expanded to the millimeter wave field,which has put forward higher requirements for electronic components in 5G communication.As the critical components of passive microwave devices such as resonators,filters,and antennas in 5G communication,develop low dielectric constant（ε_r）,low dielectric loss（high quality factor Q×f）and near zero resonant frequency temperature coefficient with（τf）microwave dielectric ceramics can meet the needs of wireless communication development.In this paper,several fluoride ceramics with low dielectric constant have been prepared by cold sintering and standard solid state reaction method,and the feasibility of fabricating antennas based on fluoride ceramics in the 5G frequency band has been verified.The specific research contents are as follows:（1）SrF₂ is a promising low-ε_r fluoride with excellent microwave dielectric properties,but it is challenging to achieve densification and sintering of SrF₂ ceramics by traditional thermal sintering（TTS）method.SrF₂ microwave dielectric ceramics with density ranging from 93.4%to 97.2%were prepared by cold sintering（300 MPa-900 MPa,150℃,1 h）and post-annealing（950℃,3h）,indicating that pretreated cold sintering process is conducive to achieve microstructure optimization.The best microwave dielectric property of SrF₂ ceramics is obtained at 750MPa:ε_r=5.94,Q×f=62,037 GHz,τf=-78.26 ppm/℃.The calculated dielectric constant and dielectric loss of ceramics based on the fitted infrared spectrum data are basically consistent with the measured results.The good chemical compatibility between SrF₂ ceramics and silver indicates that it has broad application prospects in LTCC technology.In addition,the lowε_r and high Q×f value of SrF₂ ceramics optimized indicate that it has great potential in 5G millimeter wave antenna system.A SrF₂ceramic based dielectric resonator antenna is designed and fabricated based on HFSS software.The antenna resonates at 24.50 GHz in the millimeter wave frequency band,exhibiting an excellent S₁₁ of-43.95d B and a broad bandwidth of 4.51 GHz.（2）MgF₂ single phase ceramics were prepared by standard solid state reaction method,and its microwave dielectric properties were reported for the first time.Through the analysis of the measured data,the dielectric constant（ε_r）and quality factor（Q×f）of the Mg F₂ ceramic are closely related to porosity and lattice energy,respectively.Optimum microwave dielectric properties（ε_r=4.67,Q×f=92,233 GHz,τf=-67.1 ppm/℃）are obtained when sintered at 1100℃.The ultra-low dielectric constant and high Q×f value make Mg F₂ ceramics become the perfect substitute for the common polymer based FR4 epoxy in the future 5G/6G era.An X-band microstrip patch antenna is fabricated using Mg F₂ ceramic as the substrate,with a resonant frequency of 8.25 GHz,a return loss(S₁₁)of-23.39 d B,and an impedance bandwidth of 318 MHz.This proves the feasibility of using Mg F₂ceramic as an antenna substrate.（3）A new core-shell structure is proposed and proved to be effective in realizing temperature stable Li F-Ca Ti O₃ microwave dielectric ceramics.Firstly,dense Ca Ti O₃ ceramics were prepared by solid state reaction method,and then pre-pressed and wrapped in Li F powders.The assembled green pellets were pre-densified by cold sintering process（150℃,350 MPa,60 min）,and then post-annealed at 800℃to obtain dense Li F-xwt%Ca Ti O₃ ceramics with core-shell structure（x=3,5,10,15）.SEM images confirmed the core-shell microstructure,XRD and EDS results revealed the excellent chemical compatibility between Li F and Ca Ti O₃ components.The large negative resonant frequency temperature coefficient（-138 ppm/℃）of Li F ceramics can be effectively adjusted to 2.89ppm/℃when x=3,the Q×f value is 26,580 GHz,ε_r=10.89.