Preparation And Properties Of Novel Molybdate Microwave Dielectric Ceramics With Low Dielectric Constant

With the continuous development of communication technology to high frequency and the increasing demand for miniaturization and multifunction of communication equipment,materials with low dielectric constant,high quality factor,excellent temperature stability and low temperature sintering have become the focus of attention in the field of microwave dielectric ceramics.Among them,molybdate ceramics can not only meet these needs,but also take into account the cost and environmental protection.It is a kind of microwave dielectric ceramic material with great development prospects.Therefore,in this study,a series of novel molybdate microwave dielectric ceramics were prepared by solid-state reaction method,and their phase structure,sintering behavior and microwave dielectric properties were systematically investigated.（1）Novel Ca Ln₄Mo₃O₁₆（CLn M）（Ln=La,Nd,Sm）microwave dielectric ceramics were prepared by the solid-state reaction method.XRD and Rietveld refinement showed that these series of ceramics were fluorite-like structure,but the system was always accompanied by a small amount of second phase Ca Mo O₄.The formation of intermediate products was confirmed by DSC-TG and XRD,and the insufficient reaction between intermediate products was the main reason for the appearance of the second phase.The study of dielectric properties showed that the appropriate grain size,high relative density and packing fraction were the key factors affecting theε_rand Q×?values of the ceramics.The best sintering charectestics and microwave dielectric properties of these systems were:CLM ceramics sintered at1375℃ hadε_r=13.60,Q×?=54182 GHz,τ_?=-26.0 ppm/℃;CNM ceramics sintered at 1400℃ hadε_r=13.84,Q×?=44560 GHz,τ_?=-42.8 ppm/℃;CSM ceramics sintered at 1400℃ hadε_r=14.57,Q×?=61920 GHz,τ_?=-40.2 ppm/℃.（2）In order to reduce the sintering temperature of ceramics,low melting point Na CO₃was added to the above system materials,and a new type of Na Ca La（Mo O₄）₃（NCLM）microwave dielectric ceramics was prepared by the solid-state reaction method.XRD and Rietveld refinement showed that the sample was a single-phase scheelite structure,and the A-site in the structure was occupied equally by three ions,Na⁺,Ca²⁺and La³⁺.The effects of non-intrinsic factors such as grain size and relative density,as well as intrinsic factors such as polarization rate,cation ordering and stacking fraction on the microwave dielectric properties were investigated in depth.The results showed that the NCLM ceramics hadε_r=10.72,Q×?=49495 GHz,τ_?=-49 ppm/℃when sintered at 825℃.On this basis,Na Ca Ln（Mo O₄）₃（NCLn M）（Ln=Nd,Sm）ceramics with the same structure were prepared by replacing La³⁺with Nd³⁺and Sm³⁺,respectively.Among them,NCNM ceramics sintered at 875℃ had the best dielectric properties ofε_r=11.06,Q×?=58442 GHz,τ_?=-37.72 ppm/℃.These series of ceramics had lower sintering temperature than A-site single-ion scheelite ceramics（such as:Ca Mo O₄,S.T.=1100℃）,and exhibited better dielectric properties than A-site double-ion scheelite ceramics(such as:(Na_0.5Ln_0.5)Mo O₄,Q×?=20000～25000 GHz).In addition,this series of ceramics could be co-fired with Ag.（3）Na Y（Mo O₄）₂（NYM）microwave dielectric ceramic was also designed and synthesized by the solid-state reaction method.The best microwave dielectric properties of ceramics sintered at 900℃ were obtained:ε_r=10.69,Q×?=53080.73GHz,τ_?=-46.84 ppm/℃.Subsequently,Na_1-xLi_xY（Mo O₄）₂(N_1-xL_xYM)solid solution ceramics were prepared by partially replacing Na⁺with Li⁺to adjust the excessive negativeτ_?of NYM ceramics.The results showed that the substitution of Li⁺could reduce the cell volume,the Na/Li/Y-O bond is stretched,and the Mo-O bond is compressed.The study of dielectric properties showed that the addition of Li⁺effectively adjusts the value ofτ_?to zero,but increased theε_rvalue and decreased the Q×?value.The calculation of P-V-L theory showed that Na/Li/Y-O bond was closely related to dielectric properties.When x=0.25,the N_0.75L_0.25YM ceramic sintered at 875℃had the closest zeroτ_?value of-6.35 ppm/℃,ε_rand Q×?values of 12.18 and 30373.95 GHz,respectively.In addition,N_0.75L_0.25YM ceramics could be co-fired with Ag.