Preparation Of Low Temperature Sintering Li₂MgTi₃O₈ Microwave Dielectric Ceramics And Microwave Device Design

The rapid development of the fifth generation（5G）mobile communication system puts forward high performance and miniaturization requirements for filters.Substrate integrated waveguide（SIW）has the advantages of large power capacity,low transmission loss,easy integration,etc.,and can be used to design high-performance,miniaturized filters.Microwave dielectric ceramics,which are the key materials of dielectric filters,are a research hotspot.Li₂MgTi₃O₈ ceramics with cubic spinel structure has attracted extensive attention of many scholars due to its low inherent sintering temperature and good comprehensive microwave dielectric properties.However,in order to be suitable for low-temperature co-fired ceramics（LTCC）technology,Li₂MgTi₃O₈ ceramics have the problems of low quality factor and inherent sintering temperature above 950°C.Therefore,on the basis of Li₂MgTi₃O₈ ceramics,the performance of Li₂MgTi₃O₈ ceramics is adjusted by substituting the B position and adding sintering aids,and the changes in the microwave dielectric properties of the ceramics after modification and addition of sintering aids and the crystal structure and microscopic topography are discussed.Finally,the developed ceramic with low loss and excellent temperature stability was used as the dielectric substrate to design a high-power substrate integrated waveguide filter.In this paper,Li₂Mg(Ti_1-xNb_x)₃O₈（x=0,0.005,0.01,0.015,0.02,0.03）ceramics was prepared by traditional solid-phase method and the influence of substituting B position of Li₂MgTi₃O₈ ceramics for Nb⁵⁺on the microwave dielectric properties was explored.Results have shown that all samples have a pure phase spinel structure.Nb⁵⁺ion replacement can promote the growth of crystal grains and fill the oxygen vacancies in the ceramic lattice,inhibit the conversion of Ti⁴⁺to Ti³⁺,and improve the quality factor of ceramics.Raman spectroscopy further characterizes the relationship between crystal structure and microwave dielectric properties.When x=0.02,ceramics sintered at 1080°C for 4 hours have the best microwave dielectric properties:ε_r=24.96,Q×f=42 079 GHz,TCF=-0.46 ppm/°C.The sintering temperature of Li₂Mg(Ti_0.98Nb_0.02)₃O₈ ceramics is above 950°C,which is not suitable for LTCC technology.Considering this,Li₂Mg(Ti_0.98Nb_0.02)₃O₈+x wt%Li F（x=1,2,4,6）ceramics were prepared by adding Li F sintering aid and the influence of which on the microwave dielectric properties was further explored.Results have shown that the addition of an appropriate amount of Li F can introduce a liquid phase sintering mechanism during the sintering process,reducing the sintering temperature of Li₂Mg(Ti_0.98Nb_0.02)₃O₈ceramics to 850°C,and all samples show a single-phase spinel structure.When the content of Li F is 4 wt%,the best microwave dielectric properties of Li₂Mg(Ti_0.98Nb_0.02)₃O₈ ceramics are obtained after sintering at 850°C for 4 hours:ε_r=25.05,Q×f=39 242 GHz,TCF=-4.3ppm/°C.The addition of an appropriate amount of Li F can effectively reduce the sintering temperature of Li₂Mg(Ti_0.98Nb_0.02)₃O₈ ceramics to 850°C.The developed Li₂Mg(Ti_0.98Nb_0.02)₃O₈+4 wt%Li F ceramics have low loss and excellent temperature stability.Using this material as the dielectric substrate,a complementary split resonant ring etched substrate integrated waveguide resonator filter is designed in this paper.The designed filter has a bandwidth of 176 MHz,a return loss of 37.40 d B,an insertion loss in the passband of 0.344 d B,a remote rejection of 12.7 d B,and a group delay of less than 10 ns and it can work under 10 W power,which satisfies well the design indicators required by 5G base station applications.