Design,Preparationand Performance Optimization Of Low Dielectric Microwave Dielectric Ceramics Systems

With the rapid development of modern communication technology,microwave technology development towards a higher frequency direction,and microwave electronic components have been characterized by high frequency,integration,the microwave dielectric materials put forward higher requirements.In this thesis,a new series of low dielectric constant microwave dielectric ceramics with the composition of Li4xZn3（1-x）Al6（1-x）Ti5xO12、Li4x/3Mg2-2xTi1+2x/3O4 、 Li_4x/3Zn_2-2xTi_1+2x/3O₄ 、 Ba₄CuTi₁₁O₂₇ were researched.The low temperature sintering,processing optimization,sintering characteristicsand performance adjustment of Li₂ZnTi₃O₈、Li₂MgTi₃O₈ ceramic were also researched.Finally,the chemical compatibility between these compounds and Ag electrodes were also discussed.（1）（1-x）ZnAl2O4-x Li4/3Ti5/3O4（ZALT）（x=0.2,0.4,0.6,0.8）microwave dielectric ceramics were prepared by a solid state reactionmethod.X-ray diffraction（XRD）data shown that the Zn Al2O4 cannot form a solid solution with Li₄Ti₅O₁₂.With increasing x from 0.2 to 0.8,the phase compositions of ZALT ceramics changed as（ZnAl2O4 and Li₂ZnTi₃O₈,x=0.2,0.4）→(ZnAl2O4,Li₂ZnTi₃O₈ and Li₄Ti₅O₁₂,x=0.6)→(Li₂ZnTi₃O₈ and Li₄Ti₅O₁₂,x=0.8).And the main phase changed from ZnAl2O4（x=0.2,0.4）to Li₂ZnTi₃O₈（x=0.6,0.8）.With increasing x values,the sintering temperature was reduced from 1275 ℃ to 1075 ℃.ZALT ceramics exhibit microwave dielectric properties with ε_rof 11.0²9.0,Q×f valuesof 6,090⁶5,580 GHz and τ_fvalues of-62.9^-5.7 ppm/°C.（2）Li_4x/3Zn_2-2xTi_1+2x/3O₄（0.2≤x≤0.8）（LZT）and Li_4x/3Mg_2-2xTi_1+2x/3O₄（0.2≤x≤0.8）（LMT）ceramics were prepared by a solid state reaction method.XRD analysis showed that the solid solution with Zn2TiO4、Mg2TiO4 structure are formed in the composition of 0.2≤x≤0.4.As 0.6≤x≤0.8,the solid solution of structure changed to Li₂ZnTi₃O₈ 、 Li₂MgTi₃O₈.With increasing x values,the sintering temperature was reduced,and the τ_f values ceramics were increased from large negative value to near zero value.LZT and LMT ceramics presented an excellent comprehensive performance（LZT: ε_r¹2.6 26.3,Q×f⁷930～130560 GHz,τf ^-49.0 ^-14.0 ppm /°C;LMTε_r¹1.6 29.3,Q×f²2000～172670 GHz,τ_f ^-48.9 ^-3.9 ppm /°C）。Moreover,Li_4x/3Zn_2-2xTi_1+2x/3O₄（x = 0.8）、 Li4x/3Mg2-2xTi1+2x/3O4（x = 0.8）could be well co-fired with Ag powders at 925℃、950 ℃ for 2h,indicating that they are potential material for LTCC application.（3）The influence of raw material pretreatment to the microstructure,sintering characteristics and microwave dielectric properties of Li₂ZnTi₃O₈ and Li₂MgTi₃O₈ ceramics was researched.Proper raw material pretreatment made ceramics to obtain high quality factor at a lower temperature range（Li₂ZnTi₃O₈:950¹050 ℃,Li₂MgTi₃O₈:925¹075 ℃）,improved the sintering properties of ceramics,reduced the sintering temperature,and to a certain extent,also has carried on the adjustment to the τ_f value.Li₂ZnTi₃O₈ ceramics sintered at 950 ℃ and Li₂MgTi₃O₈ ceramics sintered at 925 ℃ exhibited good microwave dielectric performance:ε_r =25.8,Q×f = 74,200 GHz,τ_f =-13 ppm/℃（Li₂ZnTi₃O₈）;ε_r =27.0,Q×f= 58,480 GHz,τ_f = 0.45ppm/℃（Li₂MgTi₃O₈）,and were found to be chemically compatible with Ag.The results indicate that the pretreatment of raw materials is a relatively simple and particularly effective method to obtain glass-free LTCC ceramics of Li₂ZnTi₃O₈ and Li₂MgTi₃O₈.（4）A novel low-fired microwave dielectric ceramic with compositionof Ba₄CuTi₁₁O₂₇ was prepared by a conventional solid-state method.A refinement using X-ray powder diffraction data was carried out in the Rietveld method using the parameters of Ba4Ti12O27 as a starting model.At a sintering temperature of 975 ℃,Ba₄CuTi₁₁O₂₇ has a monoclinic structure（C12/m1）with lattice parameters of a=19.8061（4）?,b=11.4456（2）?,c=9.9131（2）?,β=108.8988（15）?,V=2126.08（8）?3,Z=4 The Ba₄CuTi₁₁O₂₇ ceramics are featured with a low sintering temperature（ 975℃）and excellent microwave dielectric properties with a high quality factor Q×f = 15,040 GHz,a low relative permittivity ε_r = 36.3 and a low temperature coefficient of resonant frequency τ_f = 11.9 ppm/℃.More importantly,the microwave dielectrics also demonstrated chemical inertness in the presence of Ag when sintered at 950?C,keeping good microwave dielectric properties,indicating the applicability of Ba₄CuTi₁₁O₂₇ in LTCC devices.