Fabrication Of ZnO-Al₂O₃-SiO₂-Based Microwave Dielectric Ceramics And Design Of Circularly Polarized Microstrip Antenna

In recent years,communications technology is constantly being updated.From the 2G networks to the 5G networks,people need faster and better communications.The crowding of low frequency spectrum resources makes the microwave communication technology develop rapidly.Dielectric ceramic materials need to be improved in three aspects:low dielectric constant,high quality factor and near-zero resonance frequency temperature coefficient.At the same time,the antenna,as the front-end component,which has the function of sending and receiving signals,is developing to multi-band and circular polarization.In present thesis,the effects of strontium titanate doping on the sintering properties,phase composition and dielectric properties of ZnAl₂O₄-3Zn₂SiO₄ and ZnAl₂O₄-3Zn₂SiO₄-2SiO₂ ceramic systems were systematically studied.A series of high performance microwave dielectric ceramics with low dielectric constant were fabricated.The circular polarization microstrip array antennas at 24GHz based on ceramic substrate were simulated using HFSS software.Strontium titanate doped ZnAl₂O₄-3Zn₂SiO₄ and ZnAl₂O₄-3Zn₂SiO₄-2SiO₂ ceramic were fabricated using the traditional solid phase reaction method in present thesis.The results show as follows:the sintering temperature of ZnAl₂O₄-3Zn₂SiO₄ ceramics can be greatly reduced by doping SrTiO₃.The ZnAl₂O₄-3Zn₂SiO₄-0.2SrTiO₃ ceramics can obtain high density at sintering temperature window of nearly 100 degrees.XRD and SEM?back scattering?results show that ZnAl₂O₄,Zn₂SiO₄ and SrTiO₃ can coexist.At different doping amounts of SrTiO₃,the temperature coefficients of near-zero resonant frequency can be obtained together with a series of dielectric constants.In SrTiO₃ doped ZnAl₂O₄-3Zn₂SiO₄-2SiO₂ ceramics,at first,SrTiO₃reacts with ZnAl₂O₄ and SiO₂ to generate SrAl₂Si₂O₈,TiO₂ and Zn₂SiO₄ phase.With the increasing SrTiO₃ content,ZnAl₂O₄ phase disappeared in ZnAl₂O₄-3Zn₂SiO₄-2SiO₂-SrTiO₃,Zn₂TiO₄ and SrAl₂Si₂O₈ phases were obtained.TiO₂ plays an important role in regulating the temperature coefficient of the resonant frequency of ceramics.The present system has low dielectric constant because of the appearance of strontium feldspar phase and it has also high quality factor.The optimized microwave dielectric properties are obtained as follows:ZnAl₂O₄-3Zn₂SiO₄-0.3SrTiO₃,?_r=8.01,Q×f=19,354 GHz,?_f=6.5 ppm/?;ZnAl₂O₄-3Zn₂SiO₄-2SiO₂-0.6SrTiO₃,?_r=7.16,Q×f=57,837 GHz,?_f=-30 ppm/?.In present thesis,three microstrip circularly polarized array antennas for 24 GHz are designed based on Al₂O₃ ceramics or ZnAl₂O₄-3Zn₂SiO₄ ceramics substrates.All antennas use unequal power division feeding network.Based on Al₂O₃ ceramic with thickness of 0.635 mm,Antenna 1 of 2×2 patch array has properties:bandwidth of 299 MHz,relative bandwidth of1.2%,peak gain of 8.34 dB,main lobe width is 2?_0.5=63?;based on ZnAl₂O₄-3Zn₂SiO₄ ceramic with thickness of 0.635 mm,antenna 2 of 2×2 patch array has properties:bandwidth of 473MHz,relative bandwidth of 1.9%,peak gain of 7.58 dB,main lobe width of 2?_0.5=45?;based on Al₂O₃ ceramic with thickness of 0.254 mm,antenna 3 of 4×4 patch array has properties:bandwidth of 259 MHz,relative bandwidth of 1.1%,peak gain of 13.60 dB,main lobe width of 2?_0.5=33?.Based on antenna 2,the effects of substrate thickness,dielectric constant and dielectric loss on antenna performance parameters are also studied.The conditions of ceramic for antenna stabilization are obtained:the thickness error of substrate is within 0.001 mm,the absolute value of dielectric constant temperature coefficient is less than 30 ppm/?C,and the dielectric loss is less than 0.001.