Study On The MgTiO₃-based RF Ceramic Materials With High Q-value

RF ceramics with high Q are the first choice of manufacturing high frequencymicrowave components and high frequency multilayer ceramic capacitor. Facing theproblems of scarce spectrum and explosion of information capacity, corresponding to that,material system must satisfy three main criteria: proper dielectric constant, high qualityfactor under high frequency and a near-zero temperature coefficient of resonant frequency（τ）.We focus on the MgTiO₃-based ceramics because it is full of raw materials and with avery high Qf value （16000GHz）. However, it exists some disadvantages of low relativepermittivity（17）, large negative τ value（-55ppm/℃） and high sintering temperature（1380℃） which not only stops it from sintering with low melting point electrode materialslike Cu、Ni, and need high requirement with Ag/Pd electrode. According to previoussituation, in the present dissertation we firstly replace partial Mg by Zn in A site withpartial replacement of Ti by Sn in B site. Using ZnO to improve sintering can reduce theirsintering temperature. And the doping of SnO2could inhibit the rapid growth of somegrains. Sintered at1240℃for4h, affecting by those two factors, the（Mg1-xZnx）(Ti_0.95Sn_0.05)O₃possess good dielectric properties with εr=16.34, Qf=103626GHz（6.967GHz）, τ=-25ppm/℃.Then, after we determine the content of ZnO, Ca0.8Sr0.2TiO₃（εr¹81, τ⁹91ppm/℃）having a much higher εrand positive τ value, was doped to promote the relativepermittivity and adjust the τ value. With the increase of the content of Ca0.8Sr0.2TiO₃, thephase composition was changed, MgTi2O5was founded in the crystalline phase, but grainsget closer, and the dielectric constant of the sample increased, the quality factor becomesmaller, the τ value increased to a maximum, and thereafter it decreased. A dielectricconstant （εr） of19.62, a high Qf value of52030GHz and a temperature coefficient ofresonant frequency （τ） of-3.51ppm/℃were obtained for96MZCST ceramics sintered at 1230℃for4h.Finally, we try to use (La_0.44Sr_0.33)TiO₃to adjust εrand τ value of (Mg_0.95Zn_0.05)TiO₃,investigating the microstructure and dielectric properties of MZLST ceramics in order todevelope a series of new applicable composite materials. The main phases consist of(Mg_0.95Zn_0.05)TiO₃and (La_0.44Sr_0.33)TiO₃, the intensity of a minor phase MgTi2O5increased with rising sintering temperature what make the dielectric loss grow. Withincreasing doped amount of (La_0.44Sr_0.33)TiO₃, the dielectric constant of the sample growup, the quality factor become smaller, the τfvalue rised to a maximum, and thereafter itdecreased. When x=0.10,90MZLST sintered at1285℃for2hours will possess gooddielectric properties with εr=22.17, Q=48471GHz （6.72GHz）, τ=–7.99ppm/℃.