Study On The Preparation And Dielectric Properties Of Li₂Zn₂Mo₃O₁₂-based Microwave Dielectric Ceramics

In order to meet the requirements of the low-temperature cofired ceramic（LTCC）technology,it is an effective way to develop the new ceramic system with low intrinsic sintering temperature.Li₂Zn₂Mo₃O₁₂ is a typical dielectric ceramic with low intrinsic sintering temperature,low dielectric constant（εr）and high quality factor（Q×f）.It also has good chemical compatibility with Ag and Al,and is the potential ceramic for LTCC technology.In this thesis,the Li₂Zn₂Mo₃O₁₂ ceramics were prepared by the conventional solid state reaction method,then the effects of sintering process,ion substitution and the addition of second phase on phase compositions,microstructures and microwave dielectric properties of Li₂Zn₂Mo₃O₁₂ ceramics were systematically studied by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,Energy Dispersive Spectrometer（EDS）and vector network analyzer,etc.Firstly,the sintering process parameters（calcining temperature,sintering temperature and holding time）of Li₂Zn₂Mo₃O₁₂ ceramics were analyzed.The results showed that the proper sintering process is beneficial to obtain compact ceramics with uniform microstructures,which have the high εr and Q×f value.The phase compositions and temperature coefficient of resonance frequency（τf）slightly changed with the different sintering process.The optimal calcining temperature,sintering temperature and holding time were 540℃,630℃ and 2h,respectively.Li₂Zn₂Mo₃O₁₂ ceramics prepared via this sintering process showed the high relative density of 95.6%,and exhibited the best microwave dielectric properties: εr = 10.6,Q×f= 57,893 GHz,and τf =-82 ppm/°C.The effects of the Co²⁺ substitution for Zn²⁺ on the phase compositions,microstructures and microwave dielectric properties of Li₂Zn₂Mo₃O₁₂ ceramics were investigated.The results showed that the substitution of Co²⁺ improved the sinterability significantly,thereby decreased the sintering temperatures and increased the relative densities of ceramics.The solid solution of Li₂(Zn_1-xCo_x)₂Mo₃O₁₂ was formed,when the Co²⁺ substituted for Zn²⁺.The cell volume first increased and then decreased with the increasing amount of Co²⁺ substitution,which is mainly related to the position of the Co²⁺.The small amount of Co²⁺ substitution obviously improved the Q×f value of Li₂Zn₂Mo₃O₁₂ ceramics,and the dielectric properties of the ceramic are deteriorated when the substitution amount is too large.The sintering temperature of Li₂(Zn_0.9Co_0.1)₂Mo₃O₁₂ ceramics reduced to 570℃,the relative density increased to 97.88%,and exhibited good dielectric properties of εr=10.85,Q×f=65,031 GHz,τf =-73ppm/°C.The effects of SrTiO₃,CaTiO₃ and TiO₂ on the sinterability,phase compositions,microstructure and microwave dielectric properties of Li₂Zn₂Mo₃O₁₂ ceramics were investigated.The results showed that three of them will increase the sintering temperature of the composite ceramics due to their high intrinsic sintering temperature.The τf value of ceramics were not adjusted to near zero because of SrTiO₃ and CaTiO₃ can react with Li₂Zn₂Mo₃O₁₂ during the sintering process.The addition of SrTiO₃ can decreased the Q×f value of Li₂Zn₂Mo₃O₁₂ ceramics,while the addition of CaTiO₃ improved the Q×f value remarkably.TiO₂ adjusted the τf of Li₂Zn₂Mo₃O₁₂ ceramics effectively,also reduced the Q×f value.The composite ceramic with 60% molar fraction of TiO₂ exhibited good dielectric properties of εr = 15.08,Q×f=28736GHz and τf=-6.3ppm/℃.