Microwave-Dielectric-Properties-of-Zirconium-Titan

MicrowaveDielectricPropertiesofZirconiumTitanateCeramicsDopedwithMgNb2O6P.R.Khoei,F.Azough,R.FreerManchesterMaterialsScienceCentre,TheuniversityofManchester/UMIST,ManchesterM17HS,UKAbstractThesinteringbehaviour,microstructureandmicrowavedielectricpropertiesoftheZrTiO4dopedwithMgNb2O6wereinvestigatedsystematicallybyX-raydiffractometry(XRD),scanningelectronmicroscopy(SEM)andanetworkanalyseratmicrowavefrequencies.TheXRDanalysisconfirmedamulti-phaseorthorhombicα-PbO2typestructure.Thechosencomposition,0.67ZrTiO4–0.33Mg1/3Nb2/3O2(0.67ZT–0.33MN),wassinteredat1400°Cbothinairandoxygen.Thedensityimprovedsignificantlyforthesamplessinteredinoxygenfrom88%to92%.Subsequently,therelativepermittivity(εr)increasedfrom33to36asthesinteringatmospherechangedfromairtooxygen.Thequalityfactorincreasedfrom27,500GHzinairto37,000GHzinoxygen,whilethetemperaturecoefficientofresonantfrequency(τƒ)for0.67ZrTiO4–0.33Mg1/3Nb2/3O2wasreducedto–12.4ppm/°C,muchlowerthanthatfortheendmemberZrTiO4(+58ppm/°C)andindependentofsinteringatmosphere.Keywords:C.DielectricProperties,D.TiO2,ZrO2,NiobatesIntroductionLowlossdielectricmaterialsareofparticularinteresttowirelesscommunicationtechnology,mainlyduetotheiruseasthebasisforresonatorsandfilters.Therearethreeessentialcharacteristicsofresonatorsinordertooptimizetheoperabilityandviabilityofthecommercialdielectricceramics,andthesearethedielectricconstant(εr),thequalityfactor(Q),andthetemperaturecoefficientofresonantfrequency(τƒ).ThebinarysystemZrO2-TiO2hasbeenstudiedextensivelyinlasttwodecadesbecauseofitsworldwideapplicationthetelecommunicationindustry.Itwasshowedthatthecrystalstructureofthesingle-phaseZr1-xTi1+xO4(0.0≤x≤0.167)undergoesasluggishorder-disorderphasetransformationoncoolingbelow1150°C.Thehigh-temperatureformhastheα-PbO2–typestructure,inwhichthetetravalentcationsrandomlyoccupyeveryoctahedralsite.FurtherstudiesbyMcHaleandRothonthecrystalstructure,aswellasthecompositionofthelow-temperaturephase,showedanindicationoftriplea-axisandhighercontentofTiO2.Therefore,thisphasewasidentifiedasanorderedformofzirconiumtitanatehavingthesamespacegroup(Pbcn)andanidealcompositionZrTi2O6(orZr5Ti7O24).BaseduponthemicrowavedielectricpropertiesofZrTiO4(εr≈43,Q×f≈22,000GHzandτƒ≈+58ppm/°Cat1400°Citisnecessary,forpossibleapplicationsindielectricresonators,thattheseparametersareoptimised.Inordertomodifytheτf,thebinaryniobateceramics,withtheformulaANb2O6whereAisadivalentcation(Ca,Zn,Mg,Co)andisostructuralwiththeorthorhombicmineralcolumbiteandhighlynegativeτƒ,werechosenforthisstudy.Mostniobatecolumbitessinterbetween1100-1300°C,muchlowerthanthatforthezirconiumtitanateandtheniobatespossessamuchhigherqualityfactor.RecentstudiesbyLeeetal.showedthatMgNb2O6,inparticular,exhibitexcellentdielectricproperties;εr≈18.4,Q×f≈93,800GHzandτƒ≈-70ppm/°C.ExperimentalThestartingpowders,ZrO2(MagnesiumElectron,MelE101),TiO2(AlfaAesar,Rutile99.8%),MgO(AlfaAesar,99.95%)andNb2O5(H.C.Stark)weremixedusingsolid-statereactiontoyieldthecomposition0.67ZrTiO4–0.33Mg1/3Nb2/3O2(0.67ZT–0.33MN).Itwascalcinedat900ºCfor4hrsandpressedintocylindricalshapedpellets.Sinteringwasundertakeninaclosedtubefurnace(Vecstar,ModelVTFSP/1500°C)inoxygenandairat1400ºCfor4hrs.Productdensitiesweredeterminedfromweightanddimensionmeasurements.Averagegrainsizesweredeterminedbythelinearinterceptmethod.PhaseidentificationandcrystalstructureanalysisofpolishedproductswereundertakenusingaPhilipsX’PERTX-raydiffractionsystem(PW3710)overthe2θrange20ºto80ºwithastepsizeof0.04º.Beforeanalysis,allsinteredspecimensweregroundusing1200gradeSiCandsubsequentlypolishedon6µm,1µmand0.25µmdiamondpastefollowedbyOPS(colloidalSilicasuspension).Themicrostructuresoftheas-sinteredandpolishedspecimenswereexaminedbyscanningelectronmicroscopy(XL30FEG-SEM).Wavelengthdispersionspectroscopy(WDS,CamecaElectronProbeMicroscopeSX100)wasusedforthequantativechemicalanalysisofthesamples.Themicrowavedielectricproperties(relativepermittivityanddielectricQvalue)weredeterminedat3.5GHzbytheHakkiandColemanmethod,fromtheresonantfrequencyandthepeakwidthoftheTE011resonantmode.Thetemperaturecoefficientofresonantfrequency(τƒ)wasdeterminedbyacavitymethodoverthetemperaturerange-20oCto+60oC.ResultsFigure1showstypicalX-raydiffractionspectraofthe0.67ZT–0.33MNsysteminoxygenat1400°C.Allthereflectionscouldbeindexedsatisfactorilyintheorthorhombicα-PbO2cell(spacegroupPbcn).Second-phasewasalsodetectedbyadding0.33mol%MgNb2O6toZrTiO4.TheCELREFsoftwarewasusedtodeterminethelatticeparametersofthecompound.Theresultsshowednovariationwithsinteringatmosphere.Thereforethefollowingresultscouldbereportedforthecellparameters:a=4.7850,b=5.5298andc=5.0439Å.Themicrostructureoftheas-sinteredsurfaceconfirmedtheformationahomogenousanduniformmatrixalongwithsmallsecond-phasegrains.(Figure2)Thebackscatteredelectronimageofthepolishedsurfaceof0.67ZT–0.33MN,showedclearlytheformationofthesecond-phase,indicatedthatthecompositiontobeequivalenttoMgTi1.6Nb0.2Zr0.2O5basedonWavelengthDispersiveSpectrometry(WDS)results.(Figure3)Table1showstheresultsfortherelativepermittivitya