Investigation On Low Temperature Sintering And Performance Optimization Of Low-Loss Niobate Microwave Dieletric Materials

With the rise of emerging technologies such as digital home appliances,5G mobile communications,Internet of Things,and artificial intelligence,the miniaturization,high frequency,integration,and multifunction of electronic devices have been developed.High requirements for microwave dielectric materials have also been proposed and these materials have been designed to meet the requirements of high integration,ultrawide band,and ultralow power consumption for communication systems.Therefore,it is significant to reveal dielectric mechanism,optimize microwave dielectric perforemance,explore performance optimization and internal mechanism,and fabricate high-performance microwave dielectric materials for 5G communications.This work is of theoretical and practical significance.In this dissertation,ZnZrNb₂O₈ and Li₃Mg₂NbO₈ ceramics in a niobate system were chosen as research objects.Their microwave dielectric properties were optimized in terms of ion substitution,nonstoichiometric ratio,and sintering aid.The influence of microstructure on microwave properties was revealed through,structure refinement,Raman vibration analysis and chemical bond theory.Additiaonally,low-fired niobate materials with low loss and pure phase were obtained through sintering aid addition and ion substitution to meet the requirements of low-temperature co-fired ceramic(LTCC)applications.Then,a band-pass filter for 5G communications was designed to verify the feasibility of engineering applications.The main results were presented as follows:First,the effects of Cu²⁺substitution on the crystal structure,microstructure,Raman vibration,and microwave dielectric performance of Zn_1-xCu_xZr Nb₂O₈ ceramics were studied systematically.The results showed that Cu²⁺substitution formed a solid solution as the unit cell volume decreased and contributed to microstructural evolution from a polyhedral shape to a rod shape.As Cu²⁺substitution increased,the Nb–O bond ionicity gradually decreased,resulting in a decreased dielectric constant.While the improvement in Q×f value was attributed to the high lattice energy and low full width at half maximum(FWHM).?_f was correlated with the Nb–O bond energy.Thus,the microwave dielectric performance was achieved for the x=0.06 composition sintered at1175°C,where?_r=27.9,Q×f=73,200 GHz,and?_f=-40 ppm/°C.Second,an LBBS sintering aid was adopted to sinter ZnZrNb₂O₈ ceramics with excellent microwave dielectric properties at low temperatures.An LBBS glass could be used to fabricate pure-phase ZnZrNb₂O₈ ceramics with a low sintering temperature while promoting the grain growth and increasing the densification of ZnZrNb₂O₈ceramics.Furthermore,the unit cell volume,NbO₆ octahedral distortion,Raman shift,and FWHM changed,thereby affecting the microwave dielectric properties.ZnZrNb₂O₈-0.75 wt.%LBBS ceramic sintered at 950°C had a good chemical compatibility with Ag electrode and exhibited excellent microwave dielectric properties:?_r=27.1,Q×f=54,500 GHz,and?_f=-48.7 ppm/°C,providing candidates for LTCC applications.Third,the modification mechanism of the microwave dielectric properties of Li₃Mg₂NbO₆ ceramics was investigated through A/B ion substitution.Nonstoichiometric Li addition could introduce lattice defects and compensate Li volatilization to promote sintering,improve densification,and increase ionic polarization.As a result,the microwave dielectric properties improved.Cu and Ta ions were adopted as substitutes for A and B sites,respectively,to achieve a near-zero?_f.Through Cu²⁺substitution,the unit cell volume gradually increased,but the packing fraction decreased.Consequently,Q×f decreased.The increase in dielectric constant was related to the high ion polarizability of Cu²⁺.Additionally,Cu²⁺substitution changed the NbO₆ octahedral distortion,thereby causing?_f to move toward the positive direction.Li_3+xMg₂NbO₆(x=0.04)ceramics possessed excellent microwave dielectric performance:?_r=15.8,Q×f=150,000 GHz(9.85 GHz),?_f=-29 ppm/°C.Additionally,Ta⁵⁺was used to enhance Q×f and adjust?_f.Through Ta⁵⁺substitution,the unit cell volume decreased,and the packing fraction consequently increased.The optimized microstructure and high packing fraction contributed to the great improvement in Q×f value.Temperature stability also improved because of the variation in the NbO₆octahedral distortion and Nb–O bond valence.The Li₃Mg₂Nb_0.98Ta_0.02O₆ composition displayed the most remarkable improvements in microwave dielectric properties:?_r=15.58,Q×f=113,000 GHz,and?_f=-4.5 ppm/°C,providing a candidate for next-generation microwave and millimeter-wave applications.Furthermore,the effects of Li F addition and V⁵⁺substitution were compared to sinter Li₃Mg₂NbO₆ ceramics at low temperatures.V⁵⁺ions entered the crystal lattice to form a solid solution,which contributed to low-temperature sintering with a pure phase.Notably,the Li₃Mg₂Nb_1-xV_xO₆(x=0.02)ceramics sintered at 900°C exhibited remarkable microwave dielectric performance:?_r=16,Q×f=131,000 GHz,and?_f=-26 ppm/°C with an excellent chemical compatibility with Ag electrode.As such,these ceramics could be considered promising candidates for LTCC applications.Additionally,Li F was used as a sintering aid to reduce the sintering temperature and eliminate the second phase introduced by a low-melting glass with a complicated composition.During liquid phase sintering,the simple Li F could achieve the low-temperature sintering and densification of pure-phase Li₃Mg₂NbO₆ ceramics.When 6 wt.%Li F was added,a near-zero?_f value was obtained:?_r=15.4,Q×f=100,000 GHz,and?_f=-3.1ppm/°C.Lastly,flat and uniform casting tapes were obtained on the basis of Li₃Mg₂NbO₆-6wt.%Li F ceramics by adjusting the proportion of organic additions.A band pass filter was also designed and manufactured.The results showed a center frequency of 3.5 GHz with an interpolation loss of 3 d B,and high attenuations of 40 and 50 d B at 3.5 and 4GHz,respectively.The measurement and simulation results were consistent,thereby verifying the feasibility and practicability of these ceramics in engineering applications.