A Study On The Microstructure And Microwave Dielectric Properties Adjustment Of Li₂Zn₂Mo₃O₁₂-based Ceramics

Li₂Zn₂Mo₃O₁₂ ceramics is one of the promising candidates for LTCC technology,which has low sintering temperature and can be cofired with the low melting point electrodes such as Ag and Al.In the present thesis,Li₂Zn₂Mo₃O₁₂ ceramics were prepared by the conventional solid-state ceramic method.The crystal structure of Li₂Zn₂Mo₃O₁₂ ceramics was studies by Rietveld refinement,and its symmetry and mode of lattice vibration were classified.Furthermore,the relationships between lattice vibrations and dielectric properties of Li₂Zn₂Mo₃O₁₂ ceramics were investigated in detail.The dielectric constant and Q×f value of the Li₂Zn₂Mo₃O₁₂ ceramics were related to the peak position and FWHM of A_g mode in Raman spectroscopy.In the Raman spectrum,the Ag mode showed red shift,the bond energy of the Mo-O bond reduced,the polarization was easy to occur,thereby reducing the dielectric constant of the ceramic.The decrease of FWHM of Ag mode responded to the decrease of lattice vibration attenuation coefficient,indicating the decrease of intrinsic dielectric loss and the increase of Q×f value.The effects of ion substitution on the microstructures and properties of Li₂Zn₂Mo₃O₁₂ ceramics were investigated.The results showed that Li₂(Zn_1-xNi_x)₂Mo₃O₁₂ solid solutions with lyonsite crystal structure were fromed by the substitution of Ni²⁺for Zn²⁺.The cell volume of the solid solutions decreased with the increase of x.The small amount of Ni ²⁺substitution obviously increased the Q×f values of Li₂Zn₂Mo₃O₁₂ ceramics,and the Q×f values were deteriorated when the amount of Ni²⁺substitution exceed.The ceramic with x=0.08(Li₂(Zn_0.92Ni_0.08)₂Mo₃O₁₂)sintered at 630?exhibited good dielectric properties of?_r=10.82,Q×f=65054GHz,?_f=-62ppm/?.The?_r value of Li₂Zn₂(Mo_1-xWx)₃O₁₂ ceramics increased with the increase of amount of W⁶⁺ionic substitution,and then decreased after reaching the maximum value at x=0.025,which depend on its relative density and molecular polarizability.The Q×f value also increased firstly and then decreased,which was mainly related to its packing fraction and the FWHM of Ag mode.The ceramic with x=0.025(Li₂Zn₂(Mo_0.075W_0.025)₃O₁₂)sintered at 630?exhibited good dielectric properties of?_r=10.75,Q×f=63095GHz,?_f=-65ppm/??The effects of Ba₃?VO₄?₂ and(A_0.5Bi_0.5)MoO₄?A=Li,K?on the sinterability,phase compositions,microstructure and microwave dielectric properties of Li₂Zn₂Mo₃O₁₂ ceramics were investigated.The results showed that the?_f value of ceramics were not adjusted to near zero because Ba₃?VO₄?₂ and Li₂Zn₂Mo₃O₁₂ will react to forming BaMoO₄ phase with negative?_f value during the sintering process.(A_0.5Bi_0.5)MoO₄?A=Li,K?could adjust the?_f value of Li₂Zn₂Mo₃O₁₂ ceramics effectively,and the densification temperature is only 600?.With the increase of x,the?_r and?_f values gradually increased,whereas the Q×f value decreased.Typically,the Li₂Zn₂Mo₃O₁₂-(Li_0.5Bi_0.5)MoO₄ ceramic with x=0.43 exhibited good microwave dielectric properties of?_r=14.08,Q×f=20365GHz,?_f=2.04ppm/?,and the Li₂Zn₂Mo₃O₁₂-(K_0.5Bi_0.5)MoO₄ ceramic with x=0.55 exhibited good microwave dielectric properties of?_r=16.5,Q×f=12859GHz,?_f=-3.8 ppm/?.