| With the developing of modern communication technique to high frequency,miniaturization,ntegration,ifunctional and low cost,research and development of high performance with low sintering temperature of microwave dielectric ceramic material has become a current research hot spot.This paper focus on the rock salt structure microwave dielectric material Li2MO3(M=Ti,Sn,Zr),Li2Mg3MO6(M= Ti,Sn,Zr).By researching the sintering property,microstructure and dielectric properties control,a series of LTCC microwave dielectric ceramic materials with adjustable dielectric constant,sintering temperature range wide type,low loss,temperature stability were obtained.The main research results are displayed as following:(1)the crystal band structure,electron density,binding energy and covalent bonds of Li2MO3(M=Ti,Sn,Zr)were calculated by the first principle.The all crystals of Li2MO3 were direct bandgap,insulator and stability material.The forbidden band width was 3.140 eV,3.292 eV and 3.833 eV respectively,and the binding energy was-43.7,-44.5 and-45.1 respectively.The strong covalent interaction between M-O covalent bonds had an important influence on the dielectric properties of Li2MO3 materials.The salt structure of Li2MO3(M= Ti,Sn,Zr)ceramics with the pore structure were prepared by the conventional solid-state route.The formation of the pore structure might be caused by the volatilization of Li element in high temperature.Li2MO3(M = Ti,Sn,Zr)ceramics showed excellent microwave dielectric properties:εr＝17.5、14.7、14.6,Q×f= 51000 GHz、46900 GHz、29000 GHz,τf=28 ppm/℃.25 ppm/℃.17 ppm/℃.The results of αDtheo of Li2M03(M = Ti,Sn,Zr)material was consistent with Dobs.The error of △(%)was less 5%,the minute differences may be due to the extrinsic factor in the process of preparation of ceramic,etc.The M-ion radius,the packing fraction,the M-O bond length had important influence on the Qxfvalues of rock salt structure Li2MO3 material.With decreasing the M-ion radius,the packing fraction and Qxfvalues increased.At the same time,with decreasing the M-O bond,the covalent bond bond and the Qxfvalue of Li2MO3 material increased.(2)Low-fired Li2SnO3 ceramics co-doped with MgO-LiF were fabricated by a conventional solid-state route,and their sinterability,microwave dielectric properties were investigated.With increasing MgO content,a novel phase Li2Mg3SnO6 was formed.A near-zero τf value was obtained at 1325℃ for Li2SnO3-8wt%MgO ceramics.Furthermore,LiF addition not only successfully lowered the sintering temperatures to 880℃ but also promoted formation of Li2Mg3SnO6.The low temperature sintering mechanism may be explained by the following reason.One hand,the substitution of F-with smaller ionic radius for O2-with bigger ionic radius increased the lattice disorder,acted lattice,promoted substance diffusion process.On the other hand,LiF with lower melting point formed liquid in the sintering process,and liquid phase sintering promoting heated transfer and lowered densification sintering temperature of ceramic.Low-fired ceramics inhibited the volatilization of Li in sintering process and improved the density due to reducing the porosity.The microwave dielectric properties of Li2SnO3-8wt%MgO ceramics were further optimized with increasing Li2Mg3SnO6 content.The excellent microwave dielectric properties(εr= 14.7,Qxf= 78400 GHz,and τf=-0.9 ppm/℃)were achieved at 880℃ for Li2SnO3-8wt%MgO-2wt%LiF ceramics,which is compatible with Ag electrodes.(3)Li2MnO3 ceramics was prepared with a well microwave dielectric properties of εr 12.7,Qxf= 43150 GHz,rf=-9.5 ppm/℃ by solid-state reaction.The sintering temperatures of Li2MnO3 ceramics were successfully lowered to 925℃ due to the formation of a LiF liquid phase.Their temperature stability was improved by doping with Ti02.A typical Li2MnO3-2wt%LiF-5wt%TiO2 sample with well-densified microstructures displayed optimum dielectric properties(εr=3.8,Qxf= 23270 GHz,τf= 1.2 ppm/℃),such sample was compatible with Ag electrodes.(4)Using a conventional solid-state reaction Li2Mg3MO6(M = Ti,Sn,Zr)ceramics were prepared and their microwave dielectric properties were investigated.The analysis revealed that cubic Li2Mg3MO6 ceramics with a rock salt structure could be obtained in their respective sintering temperature range.Three promising ceramics Li2Mg3MO6(M= Ti,Sn,Zr)sintered at 1280℃,1360 ℃ and 1380℃ possessed out-bound microwave dielectric properties:εr=15.2,8.8 and 12.6,Qxf= 152000 GHz(at 8.3 GHz),123000 GHz(at 10.7 GHz)and 86000 GHz(at 9.3 GHz),and τf=-39 ppm/℃,-32 ppm/℃ and-36 ppm/℃,respectively.(5)Addition of LiF enhanced the sinterability of Li2Mg3TiO6 ceramics and effectively lowered the sintering temperatures to 950 ℃ without degradation of microwave dielectric properties.The low temperature sintering mechanism may be explained by the following reason.One hand,the substitution of F with smaller ionic radius for O2-with bigger ionic radius increased the lattice disorder,acted lattice,promoted substance diffusion process,and reduced the intrinsic sintering temperature of the material.On the other hand,LiF with lower melting point formed liquid in the sintering process,and liquid phase sintering promoting heated transfer and lowered densification sintering temperature of ceramic.Low-fired ceramics inhibited the volatilization of Li in sintering process and improved the density due to reducing the porosity.A well microwave dielectric properties of εr = 16.2,Q×f= 131000 GHz,τf=-44 ppm/℃ were obtained for 4wt%LiF-doped Li2Mg3TiO6 ceramics.(6)Well-densified Li2Mg3TiO6-SrTiO3 composite ceramics were obtained at 850-950℃.with increasing content of SrTiO3 from 0.05 to 0.15,the Qxf values decreased,and the εr values and of value increased.A near zero of τf value was obtained by tuning SrTiO3 composition.0.9Li2Mg3TiO6-0.1SrTiO3-4wt%LiF ceramics displayed optimum microwave dielectric properties:εr= 19.5,Qxf= 64290 GHz,τf= 6.5 ppm/℃ at 900 ℃.Such samples were compatible with Ag electrodes,which suggests suitability of the developed material for LTCC applications in wireless communication systems. |
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