Study On Borate LTCC Microwave Dielectric Ceramics And Applications

With the rapid development of large computer,artificial satellites,mobile base stations,portable electronic equipments,vehicle electronic systems and medical electronics,etc.,the development of microwave dielectric ceramics has been promoted.With the requirements for high frequency,high integration,miniaturization,high reliability,high performance,and multi-function of electronic devices and systems are becoming higher and higher,low-temperature co-fired ceramic（LTCC）technology has developed from traditional high-temperature sintered ceramic（HTCC）.LTCC technology is an important means to achieve these requirements.LTCC has the characteristics of diversified dielectric constant,low dielectric loss and low sintering temperature.The ability to co-fire with silver with low resistivity is its obvious advantage contrasted to HTCC.Developmenting more LTCCs with extremely low dielectric loss is one of the research directions.However,in practical applications,microwave dielectrics are required to have a near-zero temperature coefficient of resonance frequency（τ_f）.There is currently no universal and effective way to balance dielectric loss andτ_f.Usually the dielectric loss increases significantly after adjustingτ_f.A part of the LTCC is developed from HTCC by adding flux.Developmenting excellent LTCC systems with inherently low sintering temperature and without sintering flux,can reduce the complexity of the preparation of material and the difficulty of converting materials to industrial production.In this paper,the Zn₃B₂O₆ ceramic with an inherent sintering temperature of 925°C is selected as the basic research object.Its microwave dielectric loss has beed further reduced through ion substitution and sintering temperature kept around 900°C.New types of LTCC system that does not require sintering aids and with extremely low dielectric loss has been developed.The chemical compatibility and sintering characteristics etc.between the optimized system and some ceramics with positiveτ_fvalues have been specially studied.Traditional solid-phase reaction method was used to prepared the samples.From the aspects of phase composition,crystal structure,micro morphology,microwave dielectric properties,bond energy and bond value,the relationship between the microwave dielectric properties of LTCC and other characteristics has been study.The following is a brief research content and conclusions:The effect of Li⁺on the microwave dielectric properties of Zn_3-xLi_2xB₂O₆（0≤x≤0.3）low-temperature sintered ceramics was studied.It was found that a proper amount of Li⁺can significantly reduce the microwave dielectric loss and slightly improve the temperature stability of the resonance frequency.But the ceramics produce a new phase Li BO₂ with excesive Li⁺.This phase causes a sharp increase in dielectric loss.The bond energy of the main phase and average B-O bond valence is negatively correlated with theτ_f.Zn_2.99Li_0.02B₂O₆ sintered at 850°C for 3 hours has the characteristics of low loss.Its microwave dielectric properties are:ε_r=6.55,Q×f=122030 GHz,τ_f=-76.86 ppm/°C.Studying the influence of Co²⁺on the microwave dielectric properties of Zn_3-xCo_xB₂O₆（0≤x≤0.25）low-temperature sintered ceramics,it was found that Co²⁺can replace the position of Zn²⁺in the crystal structure and solid solution was formed.An appropriate amount of Co²⁺promoted the grains growing more uniform and compact,and significantly reduce the dielectric loss.the bond energy of the main phase,the average B-O bond valence possessed a positive correlation with theτ_f.Zn_2.925Co_0.075B₂O₆ sintered at875°C for 4 hours has a very low microwave dielectric loss,and its microwave dielectric properties are:ε_r=6.79,Q×f=140402 GHz,τ_f=-87.42 ppm/°C.The effect of Cu²⁺on the microwave dielectric properties of Zn_3-xCu_xB₂O₆（0≤x≤0.12）low-temperature sintered ceramics has been studied.similar to Co²⁺,it was found that Cu²⁺could also replace the position of Zn²⁺in the lattice and solid solution was formed.Cu²⁺could increase the grain size.An appropriate amount of Cu²⁺could significantly reduce the microwave dielectric loss and slightly improve the temperature stability of the resonance frequency.The change trend of bond energy of the main phase was positively correlated with theτ_f.The phase Zn₃（BO₃）₂ has a good chemical compatibility with Ag at low sintering temperature.Zn_2.96Co_0.04B₂O₆ sintered at 850°C for 3 hours has ultra-low microwave dielectric loss,and its microwave dielectric properties are:ε_r=6.64,Q×f=160887 GHz,τ_f=-42.76 ppm/°C.The phase composition,microwave dielectric properties,sintering characteristics,etc.between Zn_2.96Co_0.04B₂O₆ and Ca Ti O₃,Sr_0.8Ca_0.2Ti O₃,Ti O₂ were studied.It was found that the Sr_（1–y）Ca_yTi O₃（0≤y≤1）system was not applicable to adjust theτ_f of the microwave ceramic with the main phase of Zn₃（BO₃）₂.Although combining the Zn_2.96Co_0.04B₂O₆by could effectively adjust theτ_f,the phase composition would be changed and it casused the microwave dielectric loss drastically reduced.So Ti O₂ is also not suitable for adjusting theτ_f of microwave ceramics with a main phase of Zn₃（BO₃）₂.Zn_2.94Co_0.06B₂O₆ was selected as the dielectric layer to design an LTCC bandpass filter with a passband of 2.4～2.5 GHz.The band insertion loss is less than 1.4 dB,and the return loss is greater than 25 dB.