Preparation And Properties Research Of Ca₃B₂O₆ Microwave Ceramics

Fifth-generation（5G）communication technology has recently been developing at high speed,which has greatly stimulated the industry’s demand for microwave devices and components.High frequency communication needs low loss,low cost and low dielectric constant material.In this thesis,the microwave dielectric performance of Ca₃B₂O₆ ceramic is reported for the frst time,and we investigate their modification by ion substitution.The synthesis of Ca₃B₂O₆ ceramics were systematically investigated,and the phase composition of different Ca/B ratio samples was analyzed by XRD,the sample with Ca/B=1.4 had a single phase Ca₃B₂O₆ when sintered at 1250°C.The sample with Ca/B=1.4was selected as the basic formulation of Ca₃B₂O₆-based ceramics for subsequent doping experiments.The microwave dielectric properties of Ca₃B₂O₆ ceramics at 1250°C were:ε_r=6.58,Q×f=31,142 GHz,τ_f=-40.84 ppm/°C.Next,The effect of Li doping on the synthesis conditions and properties of ceramics was studied,and the results showed that the doping of Li⁺changes the bond ionicity f_i of the B-O bonds as well as the lattice energy U_{t B-O}:B-O bonds contribute more to the total ionicity of the crystals,while the changes in theεr values of Ca_3-xB₂O_6-δ:x Li⁺（0.3≤x≤0.8）ceramic samples that occur with the Li⁺doping concentration can be attributed to the change in the ionicity of the B-O bond.From the calculations,it is concluded that the lattice energy of B-O bonds U_{t B-O} occupies a major part of the total lattice energy,and the Q×f values of ceramic samples are closely related to U_{t B-O}.B-O bonds exist in Ca₃B₂O₆crystals mainly in the form of[BO₃]groups,which play the role of bridging calcium-oxygen polyhedra.Ca_2.3B₂O_6-δ:0.7Li⁺samples sintered at 900°C exhibit excellent microwave properties:ε_r=7.62,Q×f=40,426 GHz,τ_f=-53.12 ppm/°C.Then Ca_3-xSr_xB₂O₆（0.06≤x≤0.16）microwave ceramics were prepared by ion substitution.Raman tests and stacking rate calculations on the samples showed that the sample with x=0.15 has the narrowest Raman half-height width,while the sample has the highest atomic stacking rate,the weakest internal vibrations and the lowest intrinsic loss.The Ca_2.85Sr_0.15B₂O₆ sample sintered at 1250°C has excellent microwave dielectric properties:ε_r=4.78,Q×f=47,488 GHz,τ_f=-30.9 ppm/°C.Finally,Ca_3-xBa_xB₂O₆（0.01≤x≤0.05）microwave dielectric ceramics were prepared by ion substitution.The SEM and EDS energy analysis shows that when the doping amount is less than 0.05,Ba replaces Ca to form a replacement solid solution.When x=0.05,part of Ba enters the Ca₃B₂O₆ lattice and occupies Ca sites,while the rest of Ba precipitates at the grain boundaries in the form of second phase.The intrinsic loss of the ceramic samples was quantified by Raman diffraction pattern and stacking rate.The sample with x=0.04 had the narrowest Raman peak half-height width and larger atomic stacking rate,and the intrinsic loss of the crystal was the smallest with the largest Q×f value.The Ca_2.96Ba_0.04B₂O₆ sample prepared at 1250°C has good dielectric properties:ε_r=5.69,Q×f=51,666 GHz,andτ_f=-34.29 ppm/°C.