Low Temperature Sintered Zn₂SiO₄ Microwave Ceramics And Fabrication Of A LTCC Coupler

With the rapid development of microwave communication technology,the demand for electronic components for various types of terminal equipment is increasing,and the functional requirements are constantly changing.Plane miniaturization,high frequency,multi-function and integration have become the development trend of electronic components.Low temperature co-fired ceramic（LTCC）technology has the advantages of high design flexibility,good reliability,low cost,and suitability for mass production,making it one of the mainstream processes for preparing passive components.Since this process uses conductive Ag as the wiring material,functional ceramics are required to be densified at low temperatures（typically below the Ag melting point of 961℃）.Considering the requirements of miniaturization and high performance of components,the requirements for ceramic materials are different,such as coupler,ceramic substrates,and balun require materials with low dielectric constants,low losses,and temperature coefficients close to zero at resonant frequencies.Therefore,starting from the design requirements of the coupler,this thesis focuses on the Zn₂SiO₄ microwave ceramic material with low dielectric constant and low loss.The dielectric constant of Zn₂SiO₄ is between 5 and 7,and the Q*f value can reach 219000GHz,but its sintering temperature is too high（1350℃）to use Ag as a conductor for device fabrication;in addition,the τf of this material is a large negative value,so there is also the problem that the performance stability of the device changes with temperature during operation.Based on the above difficulties in engineering applications,it is necessary to reduce the sintering temperature of Zn₂SiO₄ and optimize the microwave dielectric properties of the material.In this thesis,the sintering temperature was mainly reduced by adding an appropriate amount of B₂O₃ as a sintering aid during the sintering process,while the Zndeficient formulation was used to suppress the generation of impurities.In order to improve the temperature stability,the related doping optimization and low-temperature sintering characteristics of Ba₃（VO₄）₂ low-dielectric microwave ceramic materials with positive τf value were studied and a new type of temperature-compensated ceramic material was obtained.And the two groups of optimal formulas were compounded according to a certain ratio,so as to achieve the goal of adjusting the value of τf close to zero.First,the effects of sintering aid B₂O₃ and different Zn contents on the sintering characteristics and microwave dielectric properties of Zn₂SiO₄ were studied.Experiment results showed that the Zn₂SiO₄ ceramics were able to densify when sintered at 950℃,indicating that B₂O₃ can effectively reduce the sintering temperature of ceramics.Meanwhile,with the continuous decrease of Zn content,the phase of SiO2 gradually disappeared,but the second phase Zn（BO2）2 appeared during the sintering process,and the Zn（BO2）2 content gradually decreased as the Zn ratio decreased.At x=0.2,the Zn2xSiO4-x show the best microwave dielectric properties with εr=5.79,Q*f=47522.8 GHz and τf=-23.8 ppm/℃ when the sample was sintered at 950℃.Secondly,to optimize the τf value of Zn₂SiO₄,Ba₃（VO₄）₂ with positive τf value was investigated.The purpose of replacing Ba²⁺with Co²⁺is to improve the sintering and microwave dielectric properties of Ba₃（VO₄）₂.The experimental results showed that the densification temperature of the ceramics was declined to 925℃ by an appropriate amount of Co²⁺substitution.At the same time,with the increasing amount of Co²⁺,the second phase of Ba2V2O7 also showed an upward trend.At x=0.3,the Ba3-xCox（VO4）2 obtained the best microwave dielectric properties:εr=13.2,Q*f=54,063 GHz,τf=+18.7 ppm/℃.The results of co-fired with silver revealed that no reaction or diffusion occurs between this ceramic material and Ag.In order to obtain a microwave dielectric ceramic material with a near-zero τf value,20wt%B₂O₃+Zn1.8SiO3.8 and Ba2.7Co0.3（VO4）3 with the best performance were selected for composite experiments based on the above study.The experimental results show that the composite ceramics can be densified at 850℃,but new impurity phases generated during the sintering process,which led to a certain degree of the deterioration in the material dielectric loss.When the content of Ba2.7Co0.3（VO4）3 was 60wt.%,the τf value of the ceramic was adjusted to near zero,and the microwave dielectric properties were as follows:εr=9.85,Q*f=24532GHz,τf=+4.3ppm/℃.Finally,to verify the practical application value of the material,a 1.7GHz-2GHz coupler was designed and fabricated by LTCC process.After the test,the coupler has coupling degree S41=10±0.4dB in the passband,the isolation degree S31>25dB,the insertion loss S21<0.5dB,and the voltage standing wave ratio of each port VSWR<1.2.