Low-temperature Sintering And Performance Control Of Low Dielectric Constant Microwave Dielectric Ceramics

With the rapid development of communication technology,miniaturization and light-duty microwave devices have been paid much more attention.Due to the low temperature co-firing ceramic(LTCC)technology which can effectively decrease the size of the device,realize the device integration has become a research hotspot in recent years.In addition to the excellent microwave dielectric properties,which the most important characteristic for LTCC materials is lower sintering temperature(<960?).Magnesium silicate(Mg2SiO4)ceramic and magnesium lithium niobate(Li3Mg2NbO6)ceramic with high quality factor of microwave dielectric properties,have received the much attention.However the high sintering temperature,make it difficult to widely used in practice.Therefore,it is necessary to lower the sintering temperature.In this paper,the advantages of high energy ball-milling,that is,by reducing the grain size of powders,and reduce the sintering temperature of ceramics,magnesium silicate(Mg2SiO4)ceramic and magnesium lithium niobate(Li3Mg2NbO6)ceramic were prepared at low sintering temperature.The influence of the ceramics on sintering behavior,phase structure,and microwave dielectric properties by high energy ball-milling were investigated,systematically.The research results are shown as follows:(1)In order to adjust the temperature coefficient of resonant frequency and lower the sintering temperature of Mg2SiO4 ceramics,the(1-x)Mg2SiO4-xCa0.9Sr0.1TiO3(0.1 ?x ? 0.15)composite ceramics added with 4 wt%LiF were prepared by using nanopowders(?200nm)of Mg2SiO4 and Ca0.9Sr0.1TiO3 were derived from high energy ball milling(HEBM),are well sintered at 850 ?-950 ?.As the amount of Ca0.9Sr0.1TiO3 increased,the temperature coefficient of resonant frequency(?f)was adjusted.LiF addition successfully lowered the sintering temperature of ceramics to 900 0C.It is found that Ca0.9Sr0.iTiO3 and Mg2SiO4 co-exist in all composite ceramics and a minor of SiO2 phase was detected from the XRD patterns.The well-dense microstructure was obtained for all samples.The decrease of the average grains size with increase of Ca0.9Sr0.1TiO3,owning to the low-melting point of LiF(845?)and Ca0.9Sr0.iTiO3 phase which also inhibits the grain growth of Mg2SiO4.The average grain size of samples at the range of 1.64?1.13 ?m,with increase addition of Ca0.9Sr0.1TiO3,?r gradually increased from 8.9 to 10.4.The 4 wt%LiF doped 0.86 Mg2SiO4-0.14 Ca0.9Sr0.1TiO3 ceramics showed optimized microwave dielectric properties with ?r = 10.0,Q×f = 61,000 GHz,and ?f=-5.96 ppm/?.With low sintering temperature and good dielectric properties,this material is compatible with Ag electrodes and suitable for the low-temperature co-fired ceramics(LTCC)applications.(2)Li3Mg2NbO6 ceramics were prepared by high energy ball-milling at 900-1000 0C for the first time.The average grain size of Li3Mg2NbO6 nanopowders is 341 nm by high energy ball-milling.The XRD shows that Li3Mg2NbO6 ceramics has a well-dense microstructure accompanying a minor of the second Nb2O5 phase.Because the existence of Nb2O5,worsening the microwave dielectric properties of the ceramic,and the density of ceramics is 3.72 g/cm3 at 950?.Excellent microwave dielectric properties of Li3Mg2NbO6 ceramics are:?r= 16.85,Q × f= 25,400GHz,?f=-21.96 ppm/0C.The XRD patterns of Li3Mg2NbO6 ceramics co-fired with 10 wt%Ag at 950? for 2 h indicates that no chemical reaction has taken place between the main phase and Ag,indicating that the low-temperature sintering ceramics are suitable for the LTCC application.(3)Low-fired Li2O-MgO-SnO2 ceramics doped with 4 wt%LiF were prepared by a conventional solid-state reaction method.It is significant that LiF addition lowered the sintering temperature to below 900 ?.The XRD patterns display that the specimens are constituted of Li4MgSn2O7 and Li2SnO3 below 1200 ?,above which,the samples are made up of Li2SnO3 and Li2Mg3SnO6 phases.This phenomenon can be explained by the fact that the Li4MgSn2O7 is unstable at higher sintering temperature(1200 ?).The ?f values of Li2O-MgO-SnO2 ceramics did not change remarkably with increasing sintering temperature and remained stable at the range of-8.5?4.7 ppm/?.4.0 wt%LiF-doped Li2O-MgO-SnO2 ceramics were obtained at 900 ? and exhibited microwave dielectric properties of ?r,14.2,Q×f=23,120 GHz and ?f 4.0 ppm/?.This material is compatible with Ag powders,and is an attractive candidate for LTCC applications.