Investigation On The Structure Modification And Microwave Dielectric Properties Of Cordierite-based Compounds

In this thesis, the investigation mainly focused on the cordierite microwave dielectric ceramics, which aimed at proposing corresponding solutions for the problems of high but narrow densification sintering temperatures, low quality factor(Qf) and high temperature coefficient of resonance frequency(?f) of them. The researches covered a wide field of phase composition, sintering behaviors, microtopography, lattice parameters, Si/Al order, [(Si/Al)O4] tetrahedrons, and [(Si4Al2)O18] rings structure change on the microwave dielectric properties of cordierite ceramics, and successfully obtained excellent cordierite-based microwave dielectric ceramics. The main conclusions are as follows:1) In the modification research of the(Mg1-x Lax)2Al4Si5O18+x(0≤x≤0.35) ceramics, La3+ fully substituted Mg2+ ions to form the cordierite solid solution. As 0≤x≤0.05, external [(Mg1-x Lax)6O36] rings distorted, and then compressed and improved the symmetry propertry of the inner [(Si4Al2)O18] rings. However, the preferential substitution advantages gradually reduced and the compress stress obtained a balance as 0.05≤x≤0.10. At last, the(Mg1-xLax)2Al4Si5O18+x(x=0.05) ceramics achieved an optimum microwave dielectric property: ?r = 6.7, Qf = 56, 500 GHz, ?f =-20.5 ppm/°C.2) In the modification research of the(Mg1-x Yx)2Al4Si5O18+x(0≤x≤0.25) ceramics, Y3+-substitution could only form microscale cordierite solid solution. The relative dielectric constant of the(Mg1-x Yx)2Al4Si5O18+x ceramics presented a slight increase as 0<x<0.05. However, as 0.05≤x≤0.25, the secondary Y2Si2O7 phase appeared, and the ceramics exhibited the Mg2Al4Si5O18/Y2Si2O7 composite phases and the microwave dielectric property met with mixture laws. As x=0.05, the microwave dielectric properties of(Mg1–x Yx)2Al4Si5O18+x was: ?r = 6.3,Qf = 38, 900 GHz,?f =-28 ppm/°C.3) In the modification research of the(1-x)Mg2Al4Si5O18-x Ti O2(0≤x≤0.35) ceramics, it presented single cordierite solid solution as 0≤x≤0.10. As 0.10≤x≤0.35, the ceramics presented the Mg2Al4Si5O18/Ti O2 composite phases. Hence, the(1-x)Mg2Al4Si5O18-x Ti O2(x=0.10) cordierite-based solid solution achieved optimum quality factor value: Qf = 69, 800 GHz, ?r = 6.3,?f =-21 ppm/°C. While the(1-x)Mg2Al4Si5O18-x Ti O2(x=0.25) composite ceramics obtained a near-zero temperature coefficient of resonance frequency: ?f =-0.2 ppm/°C, ?r = 6.8, Qf = 47, 600 GHz,.4) In the modification research of the(1-x)Mg2Al4Si5O18-x SCTi O3(SC= Sr, Ca; 0≤x≤0.35) ceramics, it presented cordierite solid solution as 0<x≤0.10. However, as 0.15≤x≤0.35, the feldspar SCAl2Si2O8 liquid phase appeared apart from slight SCTi O3 deposited in the cordierite grains. Last, the(1-x)Mg2Al4Si5O18-x Sr Ti O3(x=0.10) cordierite-based ceramics possessed optimal microwave dielectric properties: ?r = 6.53, Qf = 80, 600 GHz, ?f =-18 ppm/°C. Since the Ca Al2Si2O8 phase owned the lower quality factors Qf than that of Sr Al2Si2O8 feldspar. All in all, the microwave dielectric properties of(1-x)Mg2Al4Si5O18-x Ca Ti O3(x=0.05) solution were: εr = 6.27, Qf = 40, 300 GHz, τf =-31 ppm/°C.