Preparation And Structure/Property Regulation Of Low K Microwave Dielectric Materials

In recent years,with the development of high frequency communication,it is very important to ensure the stability of devices at high frequency.Hence,microwave dielectric materials with low dielectric constant（low K）,near zero temperature coefficient of resonant frequency（TCF value）and high quality factor（Q·f value）are required for preparation of high frequency devices,such as dielectric substrates.Zn₂SiO₄ and Mg₂SiO₄ possessed low dielectric constant（ε<15）and high Q·f value,but the negative TCF value limited their application.In this paper,Zn₂SiO₄ and Mg₂SiO₄ ceramics were prepared by the solid-state reaction method.TiO₂ and Li2SnO3 which possessed positive TCF value were added to tailor the TCF value and sintering temperature of Zn₂SiO₄ and Mg₂SiO₄ ceramics.The crystalline structure,microstructure and microwave dielectric properties of the samples were studied.All Zn₂SiO₄-xTiO₂ ceramics were well-densified at 1180℃-1260℃ for 2h.Well-densified ceramic matrix with uniform grains and clear grain boundaries were observed from their SEM patterns.Zn₂SiO₄ and TiO₂ coexisted in all Zn₂SiO₄-xTiO₂ ceramics.No other crystalline phase was detected,which means that Zn₂SiO₄ and TiO₂ are chemical compatible in Zn₂SiO₄-xTiO₂ system.With the addition of TiO₂ from 0.139 to 0.362,the dielectric constant of Zn₂SiO₄-xTiO₂ increased from 7.3 to 9.7,TCF value changed from-40.8ppm/℃ to-1.1ppm/℃,and the Q·f value decreased from 95,800GHz to 74,200GHz.When x=0.362,TCF value was tailored to near zero.Microwave dielectric ceramics with εr=9.7 Q·f=74,200GHz,TCF=-1.1ppm/℃ were obtained.Microwave dielectric ceramics（1-x）Zn₂SiO₄-xLi2SnO3（0.6≤x≤0.9）were prepared by solid-state reaction method.The crystalline structure was studied by XRD,and it is found that Zn₂SiO₄ and Li2SnO3 could not coexist in（1-x）Zn₂SiO₄-xLi2SnO3（0.6≤x≤0.9）system.Li₂ZnSn₂O₆ and Zn₂SnO₄ crystalline phases were detected in XRD patterns.The permittivity of（1-x）Zn₂SiO₄-xLi2SnO3 increased from 10.6 to 11.5 with the addition amount x value of Li2SnO3 increasing from 0.6 to 0.9.The Q·f value decreased from 29,000GHz to 10,700GHz,and the TCF value changed from-142ppm/℃ to-14.9ppm/℃.0.1Zn₂SiO₄-0.9Li2SnO3 ceramics sintered at 1280℃for 2h possessed good microwave dielectric properties of εr=11.5,Q·f=10,700GHz,TCF=-14.9ppm/℃.Mg₂SiO₄-0.193TiO₂ ceramics were prepared by solid-state reaction method and the chemical compatibility between Mg₂SiO₄ and TiO₂ was studied.Mg₂SiO₄ and TiO₂ reacted with each other even when the sintering temperatures were lower than 1300℃.MgTi₂O₅ and MgSiO₃ crystalline phases were generated,and their amount increased with the sintering temperature.So,TiO₂ could not tailor the TCF value of Mg₂SiO₄,because TiO₂ and Mg₂SiO₄ could not coexist in Mg₂SiO₄-TiO₂ system.Mg₂SiO₄-0.193TiO₂ sintered at 1300℃ for 2h showed microwave dielectric properties with εr=7.9,Q·f=69,100GHz,TCF=-65ppm/℃.The effects of sintering additive CuO on chemical compatibility and microwave dielectric properties of Mg₂SiO₄-0.44TiO₂ ceramic were studied.Mg₂SiO₄-0.44TiO₂+xwt%CuO（2≤x≤5）ceramics were densified well at 1140℃-1120℃ for 2h.Although the sintering temperatures were decreased from 1300℃to 1190℃ by addition of CuO,Mg₂SiO₄ and TiO₂ still reacted with each other,and MgTi₂O₅,MgSiO₃ and other unknown crystalline phases were formed.Mg₂SiO₄-0.44TiO₂+4wt%CuO sintered at 1190℃ for 2h possessed microwave dielectric properties of εr=9.3 TCF=-47.2ppm/℃,Q·f=26,300GHz.The effects of MnO₂ and/or MnCO₃ on chemical compatibility and microwave dielectric properties of 0.9Mg₂SiO₄-0.1TiO₂ ceramics were also studied.The introduction of MnO₂ or MnCO₃ reduced the sintering temperature of ceramics,but could not prevent the chemical reaction between Mg₂SiO₄ and TiO₂,and the TCF could not be adjusted to zero.