Study On Low Temperature Sintering And Modification Of CaWO₄-based Microwave Dielectric Ceramics

With the rapid development of information technology,especially the advent of the 5G era,communication devices are necessary to have miniaturization,integration,and high performance.Microwave dielectric ceramics,which are key materials in a variety of communication devices,have been widely used in satellite communications,global positioning systems（GPS）,and mobile communications.Low temperature co-fired microwave dielectric ceramics（LTCC）are important materials,which can be used in substrates and devices.Besides low sintering temperature（<950℃）and chemical compatibility with Ag electrodes,LTCC materials are also necessary to have a low dielectric constant（ε_r<20）,a high quality factor（Q×f）,and a near-zero resonant frequency temperature coefficient（τ_f）.CaWO₄with scheelite structure has good microwave dielectric properties（ε_r=9.5,Q×f=80000 GHz,τ_f=-60 ppm/℃）and relatively low sintering temperatures（～1150℃）.Therefore,it has great potential in LTCC applications.In this work,ceramic samples were synthesized by solid state sintering method.The influence ofτ_fcompensator on the microwave dielectric properties of CaWO₄ceramics and the co-firing effect of the material with Ag electrode were studied.Based on the results of the studies mentioned above,sintering additives were introduced to promote the densification of the ceramic samples,and Li₂Ce O₃which has both high Q value and low melting point was further introduced to optimize the microwave dielectric properties of CaWO₄-based ceramics.The main work and corresponding results are as follows:1.CaWO₄ceramics and（1-x）CaWO₄-x(Li_0.5Sm_0.5)WO₄ceramics were fabricated by a solid state sintering method.The effects of(Li_0.5Sm_0.5)WO₄on the phase compositions,relative density,microstructures,and microwave dielectric properties of CaWO₄ceramics and the co-firing with silver were studied.The results show that CaWO₄and(Li_0.5Sm_0.5)WO₄are both in a scheeite structure and can form a solid solution,without impurity phases.Additionally,the material showed no apparent reaction with silver during the co-firing process,which proved that the CaWO₄-based system is suitable for LTCC applications.The optimal sintering temperature of 0.86CaWO₄-0.14(Li_0.5Sm_0.5)WO₄is 900℃and microwave dielectric properties are:ε_r=10.8,Q×f=28754 GHz,andτ_f=-0.5 ppm/℃.2.In order to further reduce the sintering temperature and improve the sintering behavior,H₃BO₃-Li₂CO₃sintering aids were introduced on the basis of the foregoing research to study the effects of sintering aids on the sintering behavior,phase composition,microstructure,relative density,and microwave dielectric properties of materials.The results show that the introduction of sintering aids improve the sintering behavior of the ceramic system and there is no second phase formation.With the increase of the doping amount,the quality factor increases first and then decreases,which is consistent with the change of relative density trend.The best microwave dielectric properties of 0.84CaWO₄-0.16(Li_0.5Sm_0.5)WO₄+0.2wt.%2H₃BO₃-Li₂CO₃ceramics is:ε_r=10.4,Q×f=32601 GHz,andτ_f=-0.8 ppm/℃,and the sintering temperature is 900℃.3.In order to improve the quality factor of the material,Li₂Ce O₃with high Q and low melting point is selected for recombination.The effects of the amount of Li₂Ce O₃addition on the phase composition,microstructure,relative density,and microwave dielectric properties of CaWO₄ceramics were studied.The research results show that the introduction of Li₂Ce O₃can effectively reduce the sintering temperature and maintain a high quality factor.