| As the base materials of electronic information system,microwave dielectric ceramics have been used extensively in Internet of things,5G communication,radar detection and automotive electronics applications.The explosive growth of information data transmission rate have triggered an increasingly urgent need for integration,high-frequency and lightweight electronic devices.Therefore,low temperature co-fired ceramic(LTCC)has drawn much attention recently due to its excellent integration.In this dissertation,Li2Mg3BO6(B=Sn,Ti,Zr)systems with low loss were selected as the research objects.Ions substitution and nonstoichiometric ratio were employed to optimize their microwave dielectric properties.Based on refinement result and chemical bond theory,the relationship among structure characteristics and dielectric properties were investigated.Then,the sintering temperature and?f value were adjusted by introducing sintering aid and composite ceramics.As a result,microwave ceramics to meet the technological requirements of LTCC have been developed.Furthermore,a filter was designed and processed to verify the feasibility of materials.The main results are displayed as follows:The influences of (Al1/2Nb1/2)4+substitution on the structure,morphology and microwave dielectric properties of Li2Mg3TiO6 ceramics were investigated.Considering the similar ionic radius with Ti4+,(Al1/2Nb1/2)4+substitution formed the Li2Mg3Ti1-x(Al1/2Nb1/2)xO6 solid solutions.With increasing(Al1/2Nb1/2)4+content,the densification increased gradually while ionic polarizability decreased gradually.The obviously opposite trends led to the nonlinear variation in the dielectric constant(?r).The improvement in the quality factor(Q×f)was ascribed to the enhanced packing fraction and lattice energy.Meanwhile,the bond energy increased gradually with(Al1/2Nb1/2)4+substitution,resulting in a positive shift of?f value.Li2Mg3Ti0.92(Al1/2Nb1/2)0.08O6 ceramic sintered at 1250 ? exhibited optimized microwave dielectric properties:?r=15.26,Q×f=174300 GHz and?f=-25.14 ppm/°C.Then,LiF was introduced as sintering aid to fabricate Li2Mg3TiO6 at low sintering temperature.The results indicated that moderate amount LiF additive could optimize the sintering characteristic and densification,whereas excessive LiF would deteriorate the dielectric properties owing to the second phase.The Li2Mg3TiO6-4wt.%LiF ceramic sintered at 900 ? possessed dense microstructure and excellent microwave dielectric properties:?r=15.57,Q×f=124200 GHz and?f=-30.32 ppm/°C.Additionally,to achieve a near-zero?f,composite ceramics were introduced by two different methods:(1)According to the formula Li2Mg3SrxTi1+xO6,Li2Mg3TiO6-x SrTiO3composite ceramics were prepared by one-step synthesis process.As the SrTiO3content increased,the?fincreased rapidly from-30.32 ppm/? to+13.16 ppm/?.Particularly,a near-zero?f could be achieved for the Li2Mg3TiO6-0.09SrTiO3-4 wt.%LiF sintered at 900 ?:?r=17.47,Q×f=80200 GHz and?f=+1.85 ppm/°C.(2)The composite additives Sr F2+LiF were adopted to regulate the phase compositions and microwave dielectric properties of Li2Mg3TiO6 ceramics.The results indicate that main phases of all specimens were Li2Mg3TiO6and SrTiO3.The?f monotonically increased from-30.32 ppm/? to+7.41ppm/? with increasing Sr F2+LiF content.At x=5 wt.%,Li2Mg3TiO6-x Sr F2+LiF sample sintered at 900 ? possessed dielectric properties of?r=17.36,Q×f=74900 GHz and?f=-3.68 ppm/°C.The impurity phase Mg2SnO4 and porous morphology have worsened the dielectric properties of Li2Mg3SnO6 ceramic.To reduce the extrinsic losses,the influences of non-stoichiometry ratio on phase compositions,microstructure and microwave dielectric properties of Li2Mg3SnO6were investigated.The results indicated that non-stoichiometry ratio could provide lithium-rich atmosphere,eliminate the impurity phase Mg2SnO4 and dramatically reduce the porosity,thereby leading to enhanced microwave dielectric properties of Li2Mg3-2xSn1-xO6 ceramics.When the impurity phase disappeared,the declining trends of?r and Q×f were associated with the reductions in relative density,dielectric polarizability and packing fraction.As Mg2SnO4content decreased,the?f shifted toward the positive direction owing to the large negative?f value of Mg2SnO4.Li2Mg3-2xSn1-xO6ceramic sintered at 1350 ? exhibited optimal dielectric properties:?r=11.23,Q×f=142000 GHz and?f=-27.64 ppm/°C.Then,to reduce the sintering temperature,LiF was selected as sintering aid to form liquid phase,which could facilitate the densification process.As a result,Li2Mg2.92Sn0.96O6 ceramic with compact microstructure could be obtained at low sintering temperature.Typically,Li2Mg2.92Sn0.96O6-3wt.%LiF ceramic sintered at 925 ? possessed excellent dielectric properties:?r=11.33,Q×f=113300 GHz and?f=-31.79 ppm/°C.In addition,the(1-x)Li2Mg2.92Sn0.96O6-x Ba3(VO4)2-3 wt.%LiF composite ceramics were successfully fabricated by introducing Ba CO3and V2O5 to improve temperature stability.At x=42wt.%,the composite ceramics displayed a near-zero?f:?r=12.61,Q×f=75100 GHz and?f=-3.01 ppm/?.The high sintering temperature makes it difficult to fabricate Li2Mg3Zr O6ceramics with single phase and dense microstructure.To optimize the microwave dielectric properties,the non-stoichiometry ratio was used to adjust the phase compositions and microstructure of Li2Mg3Zr1-xO6 samples.The results indicated that moderate Zr-deficiency suppressed the formation of impurity phases and remarkably enhanced the densification of Li2Mg3Zr1-xO6 samples,which results in the enhancement of?r and Q×f.Meanwhile,the?f moved to the negative direction due to the decrement of bond energy and impurity phases.Particularly,the Li2Mg3Zr0.94O6 ceramic sintered at 1375 ? displayed optimal dielectric properties:?r=13.13,Q×f=112400 GHz and?f=-26.3ppm/°C.Given the single cubic phase of Li2Mg3Zr0.94O6 calcined powder,the LiF was used as sintering aid to lower the sintering temperature,which could eliminate the impurity phases by suppressing lithium volatilization during the sintering process.This method could reduced the extrinsic loss associated with impurity phases.Li2Mg3Zr0.94O6-4 wt.%LiF sample sintered at 900 ? exhibited excellent dielectric properties:?r=13.66,Q×f=99800 GHz and?f=-27.65 ppm/°C.Subsequently,Li2TiO3was adopted to adjust the?f of Li2Mg3Zr0.94O6-4 wt.%LiF ceramic.The results indicated that solid solutions between Li2TiO3 and Li2Mg3Zr0.94O6 were formed,where the structure changed from ordered monoclinic phase to disordered cubic phase with decreasing Li2TiO3 content.Accompanied with the phase transition,the?f decreased considerably from+17.05 ppm/? to-10.16 ppm/?.Especially,at x=6 wt.%,x Li2Mg3Zr0.94O6-(1-x)Li2TiO3-4 wt.%LiF sample sintered at 925 ? possessed the optimal dielectric properties of?r=21.44,Q×f=43600 GHz and?f=-4.11 ppm/°C.Based on Li2Mg3TiO6-0.09SrTiO3-4 wt.%LiF ceramics,a filter was designed and fabricated.The results dispalyed a center frequency 4.9 GHz with the bandwidth of 200MHz,the interpolation losses lower than 3.0 d B,while the return losses larger than 15d B.The test results were in good agreement with the simulation results,implying the reliability of ceramics in microwave devices. |
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