| With the wide application and rapid development of wireless communication technologies in substrates,resonators,filters,and global positioning system(GPS),researching and exploitating microwave dielectric ceramic materials with low sintering temperature and high performance has become a hot direction of current research.Mg2SiO4 and Li2Mg3TiO6 ceramics have acquired considerable attention because of their high quality factors and low dielectric constant.Improving sintering behavior and microwave dielectric properties of ceramics to be a new generation LTCC materials become an irresistible trend.In this paper,Mg2SiO4 and Li2Mg3TiO6 ceramics which were prepared by a conventional solid-state route were investigated.Ionic substitution,adding sintering aids and synthesizing composite were used to improve the sintering behavior and microwave dielectric properties of the ceramics.The sinterability,phase constitution,microstructure and microwave dielectric properties of the ceramics were investigated systematically.And the effects of sintering temperatues on the ceramics were also explored.The specific research contents and results are as follows:(1)[Mg1-x(Li0.5Bi0.5)x]2SiO4(MLBS,0 ? x ? 0.06)ceramics were fabricated via a conventional solid-state reaction method.The effects of the substitution of Li and Bi for Mg on the sintering characteristics,microstructures,crystal compositions and microwave dielectric properties of[Mg1-x(Li0.5Bi0.5)x]2SiO4 ceramics were investigated systematically.The results showed that the introduction of Li and Bi lowers the sintering temperature of Mg2SiO4 ceramics from 1540 ? to 1230?1430 0C.Within the sintering range,[Mg1-x(Li0.5Bi0.5)x]2SiO4(0.02? x ?0.06)ceramics obtained a stable bulk density and microwave dielectric properties.In the same time,the average grain sizes increased distinctly with increasing sintering temperatues and x.The[Mg0.98(Li0.5Bi0.5)0.02]2SiO4 ceramics sintered at 1350 ? showed good microwave dielectric properties:?r = 7.4,Q×f= 70,000 GHz,and ?f=-57 ppm/?.(2)The(1-y)[Mg0.98(Li0.5Bi0.5)0.02]2SiO4-yCa0.8Sm0.4/3TiO3(MLBS2-CST,0.05 ?y? 0.2)composite ceramics added with 4.0 wt%LiF were synthesized through a conventional solid-state reaction method.The addition of LiF successfully lowered the sintering temperature of[Mg0.98(Li0.5Bi0.5)0.02]2SiO4 ceramics in the range of 850?950 ?.The low sintering temperature may on account of the two following reasons.LiF with low melting point(845 ?)formed liquid in the sintering process,which promote the transfer of the substance.On the other hand,the ionic substitution of F-for O2-facilitated substance spread.With increasing y composition,the composite ceramics obtained a near zero temperature coefficient of resonant frequency(?f).High performance of microwave dielectric properties could be obtained in the 0.85[Mg0,98(Li0.5Bi0.5)0.02]2SiO4-0.15Ca0.8Sm0.4/3TiO3-4.0 wt%LiF composite ceramic sintered at 875 ? for 4 h with a permittivity value of 11.9,a Q×fvalue of 12,300 GHz,and a ?f value of 4.9 ppm/?.The XRD result,SEM image and EDS analyses proved that 0.85[Mg0.98(Li0.5Bi0.5)0.02]2SiO4-0.15Ca0.8Sm0,4/3TiO3-4.0 wt%LiF composite ceramics were co-fired well with silver(Ag)powders at 875 ?,indicating the composite ceramic is a suitable candidate for LTCC application.(3)The Li2Mg3TiO6(LMT)-x wt%CaF2(2 ? x ? 8)ceramics were prepared by a conventional solid-state reaction method.The effects of CaF2 addition on the sinterability,phase constitution,microstructure and microwave dielectric properties of LMT ceramics were investigated systematically.The results showed that CaF2 lowers the sintering temperature of LMT ceramics from 1280 ? to 900 ? and broadens the sintering range of LMT ceramics due to the formation of LiF and 0.8LiF-0.2CaF2 as liquid sintering agent.Formation of CaTiO3 tuned the temperature coefficient of resonant frequency(?f)of LMT from-39 ppm/? to 1.5 ppm/?.The LMT ceramics doped with 6 wt%CaF2 sintered at 1200 ? possess excellent microwave dielectric properties:?r,17.2,Q×f= 76,800 GHz,and ?f= 1.5 ppm/?.(4)The Li2(1+x)Mg3TiO6 ceramics(x =-0.04,0,0.04,0.08,0.12)were prepared via a conventional solid-state route.The effects of the Li content on the sintering characteristics,microstructures,crystal compositions and microwave dielectric properties of Li2(1+x)Mg3TiO6 ceramics were investigated systematically.The XRD results showed that the pure Li2Mg3TiO6 phase could be obtained in all composition.The SEM images manifested that there are pores on surface of the ceramics due to the lithium evaporates at high sintering temperatures.As inceasing Li content x,the grain size increased and grain boundary was clearer.The measurement of microwave dielectric properties of the Li2(1+x)Mg3TiO6 ceramics suggested that the variation in ?r,values and Q×f values with sintering temperatures and x were similar to that of bulk density,indicating that the bulk density is the major factor to control ?r values and Q×f values in Li2(1+x)Mg3TiO6 systems.Excellent microwave dielectric properties with ?r ?15.4,Q×f= 150,000 GHz and ?f=-40 ppm/? was obtained when the ceramics sintered at 1300 ? at x = 0.04 composition. |
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