Study On The Processing And Properties Of Lithium Based Microwave Dielectric Ceramics

Microwave dielectric ceramic has been applied in microwave frequency circuits?300 MHz-300 GHz?as dielectric material to serve multiphe functions.With the rapid development of modern microwave mobile communication technology,microwave dielectric ceramics have become the hot topic in the field of dielectric materials research in recent years.In this paper,Li₂Mg₃BO₆?B=Zr,Sn?ceramics with good microwave dielectric properties were prepared via a conventional solid state method.Their sintering behavior and properties were further improved by the methods of atmosphere-controlled sintering,non-stoichiometry,adding sintering aids,and composite process.The relationship between the process,phase compositions,microstructure and the microwave dielectric properties of the ceramics has been investigated by XRD,SEM,EDS and network analyzer,etc.The main contents are listed as follows:In the first part,single-phase Li₂Mg₃ZrO₆ ceramics were successfully prepared in non-stoichiometric Li_2?1+x?Mg₃ZrO₆ ceramics by the control of Li-rich atmosphere.This technique played an effective role in suppressing the lithium volatilization to eliminate the secondary phase,and further promoting the formation of the liquid phase to reduce the sintering temperature.The results of XRD,Rietveld refinement,and SEM indicated that the secondary phase in matrix decreased obviously,and denser microstructure could be obtained in current ceramics together with outstanding Q×f values of 150000-300000 GHz?@9.84-9.88 GHz?.Particularly,the x=0.06 sample exhibited excellent microwave dielectric properties of ?r=12.8,Q×f=307319 GHz?@9.88 GHz?,?_f=-35 ppm/? as it was sintered at 1275 ? for 6 h.This result demonstrated that a family of Li₂Mg₃BO₆?B= Zr,Sn?might be suitable for low temperature co-firing ceramic?LTCC?technology based on its ultra-low loss.The effects of sintering aid LiF on the sintering behavior and microwave dielectric properties of Li₂Mg₃SnO₆ ceramics were systematically studied in the second part of the paper.Li₂Mg₃SnO_6-x wt.% LiF ceramics with excellent microwave dielectric properties were successfully prepared.The results showed that Li F could effectively reduce the sintering temperature through the liquid sintering mechanism,accompanying the improvement in sintering and microstructure of Li₂Mg₃SnO₆ ceramics.The XRD results displayed that a single-cubic Li₂Mg₃SnO₆ ceramic was obtained in the temperature range of 875-1100 ?.When the sintering temperature was lower than 850 ?,a phase transition from Li₂Mg₃SnO₆ to Li4MgSn2O7 occurred.Another secondary phase Mg2SnO4 appeared as the temperature was above 1150 ?.Single-phase Li₂Mg₃SnO₆ ceramics with er=11-12,Q×f=120000-240000 GHz?@9.99-11.60 GHz?,?_f^-40 ppm/? were achieved by adding 2-7 wt.% LiF in the sintering temperature range of 875 ?-1100 ?,which proves that Li₂Mg₃SnO₆ ceramic has a great potential in the application of LTCC technology.The optimal microwave dielectric properties of er=10.7,Q×f=330461 GHz?@10.40GHz?,?_f=-42 ppm/? was yielded in Li₂Mg₃SnO₆ ceramics with 2 wt% LiF at 1150 ?,and when sintered at 875 ? for 6 h,the x=4 sample owned a microwave dielectric properties of er=11.6,Q×f=155424 GHz?@9.98 GHz?,tf=-35.5 ppm/?.Finally,the temperature coefficient of Li₂Mg₃SnO₆ was adjusted by forming Ba₃?VO₄?₂-Li₂Mg₃SnO₆-3 wt.% Li F composite ceramics.The results of XRD,SEM,and EDS indicated that Ba₃?VO₄?₂ and Li₂Mg₃SnO₆ could coexist in the sintered body with uniform and compact microstructures.Moreover,the composite ceramics had a good compatibility with silver.Near-zero ?_f values were obtained and could be finely tuned by changing Ba₃?VO₄?₂ content,within the scope of the designed composition Li₂Mg₃SnO₆-3 wt.% LiF ceramics with 34-42 vol.% Ba₃?VO₄?₂.Besides,Ba₃?VO₄?₂ could also effectively improve the material density and reduce the sintering temperature of Li₂Mg₃SnO₆ though it slightly deteriorated the quality factor of the matrix.These composite ceramics also exhibited excellent microwave dielectric properties of er=12.6-13,Q×f=79761-99230 GHz?@12.67-12.94GHz?,which makes them suitable for the application of LTCC technology.