A Study On The Dielectric Properties Of Nonstoichiometric LiMgPO₄-based Microwave Dielectric Ceramics

LiMgPO₄ microwave dielectric ceramic exhibits low intrinsic sintering temperature,low dielectric constant,low dielectric loss,good chemical compatibility with electrode materials,low bulk density and low cost,which is a potential candidate material for Low temperature co-fired ceramics（LTCC）technology.However,lithium loss during sintering process results in the deviation of stoichiometry,which can induce the lattice defects and change the microwave dielectric properties.Besides,LiMgPO₄ microwave dielectric ceramic has a large absolute value of temperature coefficient of resonance frequency,which limits its practical application as microwave components.Therefore,the LiMgPO₄ ceramics were chosen as research subject in this thesis,and the effects of nonstoichiometry,ion substitution and the second phase doping on sintering characteristics,phase compositions,microstructures and microwave dielectric properties of LiMgPO₄ ceramics have been systematically studied by X-ray diffraction（XRD）,scanning electron microscopy（SEM）and vector network analyzer,etc.Firstly,the effects of nonstoichiometry on the sintering characteristics,phase compositions,microstructures and microwave dielectric properties of LiMgPO₄ ceramics were investigated.With the increase of x,the optimum sintering temperature of Li_1+xMgPO₄ ceramics increased and the relative density increased firstly and then decreased.When Li excess less than 0.05,Li_1+xMgPO₄ceramics formed a solid solution with orthorhombic structure.A small amount of Li₃PO₄ and MgO impurities appeared,when Li excess more than 0.075.With the increase of x,theε_r and Q×f values of Li_1+xMgPO₄ ceramics firstly increased and then decreased,while the variation of theτ_f value changed very little.When x=0.0125,the Li_1.0125MgPO₄ ceramics sintered at 950℃for 2h exhibited the best microwave dielectric properties ofε_r=7.14,Q×f=95039GHz,τ_f=-56.1ppm/℃.For LiMg_1-xPO₄ ceramics,the optimum sintering temperature and relative density both decreased with the increase of x.When Mg deficiency below 0.03,only LiMgPO₄ single phase existed in the ceramic.When x=0.04,a small amount of LiMg（PO₃）₃ phase was observed.With the increase of x,theε_r and Q×f values of LiMg_1-xPO₄ ceramics both showed a decreasing trend,which was related to the decrease of the relative densities of ceramics.The variation of theτ_f values did not change obviously.The effects of Co²⁺and Mn²⁺substitution on the phase compositions,microstructures and microwave dielectric properties of Li_1.0125MgPO₄ ceramics were investigated,respectively.The sintering temperature of ceramics decreased obviously by the substitution of Co²⁺,and the optimum relative densities of ceramics were all about 98%.The Li_1.0125(Mg_1-xCo_x)₂PO₄ solid solution was formed by the substitution of Co²⁺.With the increase of Co²⁺ion substitution,theε_r values of Li_1.0125(Mg_1-xCo_x)PO₄ ceramics firstly increased and then decreased,which was depend on the relative density.And,the Q×f values of Li_1.0125(Mg_1-xCo_x)PO₄ ceramics decreases gradually with increasing Co content,which was closely related to the packing fraction of the atoms.Theτ_f values did not change obviously with increasing Co content.The sintering temperature of Li_1.0125(Mg_1-xMn_x)PO₄ ceramics decreased by substitution of Mn²⁺.However,the relative densities of all ceramics decreased.When Mn content less than 0.05,the Li_1.0125(Mg_1-xMn_x)PO₄ solid solution was formed.The second phase of Li_1.126Mn_2.252O₄ appeared in the ceramics,when Mn content more than0.075.With the increase of x,theε_r values of Li_1.0125(Mg_1-xMn_x)PO₄ ceramics increased due to the increase of the molecular polarizability and appearance of the secong phase.The Q×f values of Li_1.0125(Mg_1-xMn_x)PO₄ ceramics decreased significantly with increasing Mn content,mainly due to the decrease of the packing fraction of the atoms and appearance of the second phase.Theτ_f values became improved with increasing Mn content.Ba₃（VO₄）₂ and LiCa₂Mg₂V₃O₁₂ were chosen to tailor theτ_f value of Li_1.0125MgPO₄ ceramic,respectively.The results showed that the Ba₃（VO₄）₂ and LiCa₂Mg₂V₃O₁₂ doping could decrease the sintering temperatures of Li_1.0125MgPO₄ ceramics.Both Ba₃（VO₄）₂ and LiCa₂Mg₂V₃O₁₂ did not react with Li_1.0125MgPO₄ phase,which can effectively adjust theτ_f of Li_1.0125MgPO₄ ceramics.With increasing the content of Ba₃（VO₄）₂ and LiCa₂Mg₂V₃O₁₂,theε_r and dielectric loss of ceramics both increased.The composite ceramic with 50wt%Ba₃（VO₄）₂ sintered at 875℃for 2h exhibited best dielectric properties ofε_r=9.72,Q×f=57347GHz andτ_f=-1.9ppm/℃.The composite ceramic with30wt%LiCa₂Mg₂V₃O₁₂ sintered at 900℃for 2h exhibited best dielectric properties ofε_r=8.07,Q×f=27780GHz andτ_f=-3.1ppm/℃.