CeramicsInternational42(2016)9636–9639ContentslistsavailableatScienceDirectCeramicsInternationaljournalhomepage:www.elsevier.com/locate/ceramintSynthesisandelectromagneticwavereflectivityofSi3N4ceramicwithgradientFe3O4distributionXiangmingLia,n,MingjunGaoa,YunJiangbabSchoolofEnvironmentandMaterialsEngineering,YantaiUniversity,Yantai,Shandong264005,PRChinaDepartmentofForeignLanguages,NorthwestA&FUniversity,Yangling,Shaanxi712100,PRChinaarticleinfoArticlehistory:Received2February2016Accepted7March2016Availableonline8March2016Keywords:Si3N4Fe3O4GradientdistributionChemicalprecipitationElectromagneticpropertiesabstractASi3N4ceramicwithgradientdistributionoftri–irontetroxide(Gradient–Si3N4–Fe3O4)wasfabricatedwithacombinedtechniqueofchemicalprecipitationanddirectionalinfiltration.ElectromagneticwavecouldenterGradient–Si3N4–Fe3O4withlittlereflectionbecauseofweakimpedancemismatchatitssurface.AlsotheelectromagneticwaveenteringtheGradient–Si3N4–Fe3O4propagatedwithsmallre-flectionduetocontinuousandgradualchangeofimpedanceresultingfromthegradientFe3O4dis-tribution,andwasabsorbedcompletelybyFe3O4ofthegradientstructure.&2016ElsevierLtdandTechnaGroupS.r.l.Allrightsreserved.1.IntroductionManystudieshavedemonstratedthatabsorptionismoreuse-fulthanreflectioninshieldingelectromagneticwavethoughab-sorptionismoredifficulttoachievethanreflection[1–11].Inthesestudies,carbonandferritearetwotypicalelectromagneticwaveabsorbers,whichareusuallyaddedinamaterialtoenhanceitselectromagneticwaveabsorption.However,thesurfaceim-pedancemismatchofthematerialworsenswithcarbonorferriteinclusion,whichpresentsadifficultyinabsorbingelectromagneticwave.Therefore,theelectromagneticwaveabsorptionofamate-rialcouldhardlybeenhancedsimplybyaddingcarbonorferrite[1–3].Forexample,Si3N4ceramicwithuniformdistributionofpyr-olyticcarbon(PyC–Si3N4)demonstratesstrongsurfaceimpedancemismatch,sothereisalargepartofincidentelectromagneticwavereflectedonthesurfaceofPyC–Si3N4[2].However,duetoabsenceofPyCatitssurface,Si3N4ceramicwithgradientdistributionofPyC(Gradient–PyC–Si3N4)notonlyshowsstrongattenuationofelectromagneticwavebutalsodemonstratesweaksurfaceim-pedancemismatch.Thus,mostofincidentelectromagneticwavecouldenterGradient–PyC–Si3N4andbeabsorbed[3].Asdemon-stratedbyourpreviouswork[3],theelectromagneticwaveab-sorptionofthematerialcouldbeenhancedeffectivelybythein-troductionoftheabsorberwithagradientdistribution.Tri–irontetroxide(Fe3O4)isoneoftheferriteseriesnelectromagneticwaveabsorbers,whichisgaininginterestforitshighwave–absorbingstrength.However,thenarrowwaveab-sorbingbandofFe3O4limitsitsapplicationasagoodelectro-magneticwaveabsorber[1,5–11].SomestudieshavebeencarriedouttofabricateFe3O4nanoparticleswithdifferentmorphology[12–14],especiallyurchin–like[12],dendrite–like[13],hollowspherical[14],etc.,towidenthewaveabsorbingbandofFe3O4.TheelectromagneticwaveabsorptionofFe3O4nanoparticleswithspecialmorphologyisenhancedsignificantlybecauseoflargespecificsurfaceareaofFe3O4nanoparticles.However,thematerialwithuniformdistributionofFe3O4nanoparticlesalsoshowspoorelectromagneticwaveabsorptionduetoitsstrongsurfaceim-pedancemismatch.Inthiswork,aSi3N4ceramicwithgradientdistributionofFe3O4(Gradient–Si3N4–Fe3O4)isfabricatedwithacombinedtechniqueofchemicalprecipitationanddirectionalinfiltration.MicrostructureobservationandphaseidentificationofFe3O4arecarriedout.TheeffectofinfiltrationpressureonFe3O4distributioninGradient–Si3N4–Fe3O4isinvestigated.TheelectromagneticwavereflectivityofGradient–Si3N4–Fe3O4withdifferentpatternsofFe3O4distributionismeasuredanddiscussed.2.ExperimentalprocedureTheporousSi3N4ceramicfabricatedinourpreviouswork[15]wasmachinedintopreformwithadimensionof180mmÂ180mmÂ5mm,andthenwasassembledinadevice(Fig.1)toinfiltrateFe3O4nanoparticlesdirectionally.Beforeinfiltrationprocess,asolutionofFeCl3(0.024mol/L)andFeCl2(0.012mol/L)Correspondingauthor.E-mailaddress:li_xiangming@yahoo.com(X.Li).http://dx.doi.org/10.1016/j.ceramint.2016.03.0490272-8842/&2016ElsevierLtdandTechnaGroupS.r.l.Allrightsreserved.X.Lietal./CeramicsInternational42(2016)9636–96399637Fig.1.SchematicoftheprocessofGradient–Si3N4–Fe3O4fabrication.waspreparedbymixingFeCl3Á6H2OandFeCl2Á4H2Owithdis-tilledwater.AsshowninFig.1,anammoniawaterwithcon-centrationof5–6mol/Lwaspouredintothechamberabovethepreform,andthenthemixedsolutiondrippedslowlyintotheammoniawater.Duringtheadditionprocess,theammoniawaterwasstirredrapidlyandabigpowerfulmagnetwasplacedbeneaththepreformtospeedtheinfiltrationofreaction–derivedFe3O4nanoparticlesintothepreform.Whentherewasnoliquidseepingoutfromthelowersurface,thepreformwastakenoutanddriedat90°Cfor5hinair.ThedistributionofFe3O4inthepreformcouldbecontrolledandadjustedbychangingthepressureinthechamberatthebottomofthedevice(Fig.1).Thesuitablepressurerangewas0.9–1.05timesatmosphericpressure.Forthecon-venienceofthefollowingdiscussion,theGradient–Si3N4–Fe3O4preparedwithmtimesatmosphericpressurewasdenotedasGradient–Si3N4–Fe3O4–m.Themicrostructurewasobservedwithascanningelectronmicroscopy(SEM,S–4800,Hitachi,Japan).PhaseanalyseswereconductedbyX–raydiffraction(XRD,X'PertPro,Philips,Nether-lands).TheFe3O4distributionwasanalyzedwithanenergydis-persiveX–rayspectrometer(EDS,GenesisXM2,EDAX,USA)dur-ingSEManalysis.TheelectromagneticwavereflectivitywasmeasuredwithaNavalResearchLaboratory(NRL)testingsystem[16,17].3.ResultsanddiscussionDuringinfiltration,asthemixedsolutiondrippedslowlyintoammoniawater,moreandmoreFe2þandFe3þionsinammoniawaterreactwithOHÀtoproduceFe3O4accordingtothefollowingreactionequation.Fe2þþ2Fe3þþ8OHÀ-Fe3O4þ4H2O(1)Asknownfromourpreviouswork[15],therearelotsofwell–connectedporesformedbybondingtherod–likeSi3N4particleswitheachotherinporousSi3N4ceramic.Atthebeginningofin-filtrationprocess,withthehelpofpowerfulmagnetbeneaththepreform,thereaction–derivedFe3O4nanoparticlesinammoniawaterenterthepreformalongthewell–connectedporesandde-positintheporesgradually.Asinfiltrationprocessgoeson,theporesinthepreformgetsmallerandsmallerduetocontinuousdepositionofFe3O4nanoparticles.TheFe3O4nanoparticlescouldhardlyarriveatdeeperplaceinthepreformbutdepositintheporesneartheuppersideofthepreform.Finally,Fe3O4nano-particlescouldonlydepositontheuppersurfaceofthepreformwhentheporesneartheuppersideofthepreformarestuffed.Fig.2(a)and(b)showthemicrostructuresattheupperandlowersurfacesofGradient–Si3N4–Fe3O4respectively.Aspredicted,theporesamongSi3N4particlesattheuppersurfaceofGradient–Fig.2.Microstructuresatthe(a)upperand(b)lowersurfacesofGradient–Si3N4–Fe3O4.Fig.3.High–magnificationmicrographoftheFe3O4nanoparticles.Si3N4–Fe3O4arefilledwithFe3O4nanoparticles,whilethelowersurfaceofGradient–Si3N4–Fe3O4isstillporouswithnoFe3O4na-noparticlesdetected.Accordingly,itisinferredthatthereisagradientdistributionofFe3O4inGradient–Si3N4–Fe3O4.Inaddi-tion,Fig.3showsthehigh–magnificationmicrographoftheFe3O4nanoparticlesintheporesattheuppersurfaceofGradient–Si3N4–Fe3O4.TheFe3O4nanoparticlesstackcloselywitheachotherandhaveuniformdiametersofabout15–20nm.Afterinfiltration,thereisanumberofFe3O4nanoparticlesdepositedontheuppersurfaceofGradient–Si3N4–Fe3O4.Fig.49638X.Lietal./CeramicsInternational42(2016)9636–9639Fig.4.XRDpatternofFe3O4nanoparticles.showstheXRDpatternofFe3O4powdercollectedwithawirebrushfromtheuppersurfaceofGradient–Si3N4–Fe3O4.ComparingtheXRDpatternwiththepowderdiffractionfileofFe3O4(JCPDS19–0629),alldiffractionpeaksintheXRDpatternareconsistentwiththoseinJCPDS19–0629.The2θvalueof(311)crystalplaneis35.422°,thedistancebetween(311)crystalparallelplanescalcu-latedwithXRDis2.532Å,andthelatticeconstantiscalculatedas8.394Åwhichisalmostthesameasthestandardone(8.396Å)listedinJCPDS19–0629.Thus,itisaffirmedthatthereaction–derivedpowderisFe3O4butnotγ–Fe3O4.Inaddition,thestrongpeaksintheXRDpatternmeanahighcrystallinityofFe3O4.Cal-culatedaccordingtoScherrerequation,themeangrainsizeofreaction–derivedFe3O4is22nm,whichisalittlebiggerthanthemeasurementwithSEM.TheinfiltrationpressureinthechamberatthebottomoftheinfiltrationdevicehasgreateffectonthedistributionofFe3O4inGradient–Si3N4–Fe3O4.Fig.5showstheFe3O4distributioninGradient–Si3N4–Fe3O4fabricatedwithinfiltrationpressuresof1.02,1.00,0.96and0.93timesatmosphericpressure,respectively.Ascanbeseen,thereisacontinuousandgradualFe3O4changingbeltinGradient–Si3N4–Fe3O4,andthebeltwidthincreaseswiththedecreaseofinfiltrationpressure.Duringinfiltration,theflow-ingvelocityoffluidinSi3N4preformislowwhentheinfiltrationFig.5.ContentsofFe3O4atdifferentlocationsinGradient–Si3N4–Fe3O4.pressureisrelativelyhigh,sotheFe3O4nanoparticlesenteringthepreformmoveslowlydownwardsanddepositneartheuppersurfaceofthepreform.TakingGradient–Si3N4–Fe3O4–1.02asanexample,theFe3O4contentis45.4vol.%attheuppersurfacewhiledecreasesslowlyfirstandthenfastto0vol%atthelocationof2.8mmfromtheuppersurface.Asinfiltrationpressuredecreases,thefluidinthepreformflowsdownwardsrelativelyfast,soitispossibleforFe3O4nanoparticlestoreachdeeperplaceinthepreformanddepositthere.TakingGradient–Si3N4–Fe3O4–0.93asanexample,theFe3O4contentis40.7vol%attheuppersurfaceanddecreasesgraduallyto0vol%atthelocationof4.4mmfromtheuppersurface.Whenelectromagneticwavespropagatinginairstrikeama-terialwithdifferentimpedancetoair,reflectionwilloccuronthesurfaceofthematerial,andthereflectionstrengthisinfluencedbythedegreeofimpedancemismatchbetweenairandthematerial[18–20].WhenmeasuringthereflectivityofamaterialwithNRL,thetotalreflectioniscodeterminedbytheprimaryreflectiononthesurfaceofthematerialandthesecondaryreflectiononthesurfaceofmetalpanel.ThereisastrongimpedancemismatchattheuppersurfaceofGradient–Si3N4–Fe3O4duetohighcontentofFe3O4,sotheelectromagneticwavecouldhardlyenterGradient–Si3N4–Fe3O4fromitsuppersurface.Asknownfromourpreviouswork,porousSi3N4ceramicisanexcellentelectromagneticwavetransparentmaterialduetoitslowdielectricconstantandloss[21,22].Thus,theelectromagneticwavepropagatinginaircouldenterporousSi3N4ceramicwithalmostnoreflection.Themicro-structurenearthelowersurfaceofGradient–Si3N4–Fe3O4isthesameasthatofporousSi3N4ceramic,soelectromagneticwavecouldenterGradient–Si3N4–Fe3O4fromitslowersurfacealsowithlittlereflection.DuetotheslowandgradualincreaseofFe3O4alongtheFe3O4changingbelt,theimpedanceinGradient–Si3N4–Fe3O4changesgraduallywithslightmismatch.Therefore,mostofelectromagneticwaveenteringGradient–Si3N4–Fe3O4couldpro-pagateforwardalongtheFe3O4changingbeltandsimultaneouslybeabsorbedgraduallybyFe3O4nanoparticleswithlittlereflection.Fig.6showsthereflectivity–versus–frequencycurvesofGra-dient–Si3N4–Fe3O4withdifferentpatternsofFe3O4distribution.ThewidthofFe3O4changingbeltincreaseswiththedecreaseofinfiltrationpressure(Fig.5),sotheimpedancemismatchinGra-dient–Si3N4–Fe3O4becomesweakerduetotheslowerincreaseofFe3O4.Therefore,thereflectivityofGradient–Si3N4–Fe3O4de-creasesnaturallywiththedecreaseofinfiltrationpressure.Therearelotsofabsorbingpeaksofthefourcurvesinthefrequencyrangeof8–18GHz(Fig.6).Themeanreflectivitydecreasesandtheabsorbingpeakswidenasinfiltrationpressuredecreases.AsshowninFig.6,themeanreflectivityofGradient–Si3N4–Fe3O4–1.02isÀ7.9dB(Fig.6(a)).OnlythreenarrowandsharpabsorbingpeaksbelowÀ10dBemergeatthefrequencyof8.4,10.8and14.9GHzrespectively.ThemeanreflectivityofGradient–Si3N4–Fe3O4–1.00isÀ8.5dB(Fig.6(b)).Theabsorbingpeakswidenslightlyandthereappearthreemoreabsorbingpeaksbe-lowÀ10dB.ThemeanreflectivityofGradient–Si3N4–Fe3O4–0.96isÀ9.6dB(Fig.6(c)).Thewideningtendencyofabsorbingpeakscouldbeseen,andmostofabsorbingpeaksisbelowÀ10dB.Gradient–Si3N4–Fe3O4–0.93hasareflectivityaslowasÀ10.8dB(Fig.6(d)),whichmeanstheelectromagneticwaveisabsorbedby92%andonly8%isreflected.TheabsorbingpeaksofGradient–Si3N4–Fe3O4–0.93arewideandthewholecurveisalmostbelowÀ10dB.4.ConclusionsInthisstudy,Gradient–Si3N4–Fe3O4wasfabricatedwithacombinedtechniqueofchemicalprecipitationanddirectionalX.Lietal./CeramicsInternational42(2016)9636–96399639Fig.6.Reflectivity–versus–frequencycurvesofGradient–Si3N4–Fe3O4withdiffer-entpatternsofFe3O4distribution.infiltrationofFe3O4intoporousSi3N4ceramic.ThegradientFe3O4distributionmakesGradient–Si3N4–Fe3O4demonstrateweaksur-faceimpedancemismatchandenhancedelectromagneticwaveabsorption.ElectromagneticwavecouldenterGradient–Si3N4–Fe3O4withlittlesurfacereflection.TheslowincreaseofFe3O4alongtheFe3O4changingbeltmakestheimpedanceinGradient–Si3N4–Fe3O4changegraduallywithslightmismatch.Theelectro-magneticwaveenteringGradient–Si3N4–Fe3O4couldpropagateforwardandsimultaneouslybeabsorbedwithlittlereflection.ThemeanreflectivityofGradient–Si3N4–Fe3O4–0.93isÀ10.8dBinthefrequencyrangeof8–18GHz,whichmakesGradient–Si3N4–Fe3O4–0.93apromisingelectromagneticwaveabsorbingmaterialbecauseabout92%oftheincidentelectromagneticwaveisabsorbed.AcknowledgementsTheauthorsgratefullyacknowledgethefinancialsupportfromtheNationalNaturalScienceFoundationofChina(No.51479172and51209177)andtheWaterConservancyScientificandTechnicalProject(2015slkj–10)fromShaanxiProvince.References[1]L.B.Kong,Z.W.Li,L.Liu,R.Huang,M.Abshinova,Z.H.Yang,Recentprogressinsomecompositematerialsandstructuresforspecificelectromagneticappli-cations,Int.Mater.Rev.58(2013)203–259.[2]X.M.Li,L.T.Zhang,X.W.Yin,Synthesis,electromagneticreflectionlossandoxidationresistanceofpyrolyticcarbon–Si3N4ceramicswithdenseSi3N4coating,J.Eur.Ceram.Soc.32(2012)1485–1489.[3]X.M.Li,L.T.Zhang,X.W.Yin,Electromagneticpropertiesofpyrolyticcarbon–Si3N4ceramicswithgradientPyCdistribution,J.Eur.Ceram.Soc.33(2013)647–651.[4]X.M.Li,L.T.Zhang,X.W.Yin,Synthesisandelectromagneticshieldingpropertyofpyrolyticcarbon–siliconnitrideceramicswithdensesiliconnitridecoating,J.Am.Ceram.Soc.95(2012)1038–1041.[5]S.B.Ni,S.M.Lin,Q.T.Pan,F.Yang,K.Huang,D.Y.He,HydrothermalsynthesisandmicrowaveabsorptionpropertiesofFe3O4nanocrystals,J.Phys.D:Appl.Phys.42(2009)055004.[6]C.P.L.Rubinger,D.X.Gouveia,J.F.Nunes,C.C.M.Salgueiro,J.A.C.Paiva,M.P.F.Graça,P.Andre,L.C.Costa,MicrowavedielectricpropertiesofNiFe2O4na-noparticlesferrites,Microw.Opt.Technol.Lett.49(2007)1341–1343.[7]H.J.Zhang,Z.C.Liu,C.L.Ma,X.Yao,L.Y.Zhang,M.Z.Wu,Complexpermittivity,permeability,andmicrowaveabsorptionofZnandTi–substitutedbariumferritebycitratesol–gelprocess,Mater.Sci.Eng.B96(2002)289–295.[8]P.Toneguzzo,G.Viau,O.Acher,F.F.Vincent,F.Fievet,Monodisperseferro-magneticparticlesformicrowaveapplications,Adv.Mater.10(1998)1032–1034.[9]V.B.Bregar,Advantagesofferromagneticnanoparticlecompositesinmicro-waveabsorbers,IEEE.Trans.Magn.40(2004)1679–1684.[10]M.S.Pinho,M.L.Gregori,R.C.R.Nunes,B.G.Soares,Agingeffectonthere-flectivitymeasurementsofpolychloroprenematricescontainingcarbonblackandcarbonyl–ironpowder,Polym.Degrad.Stabil.73(2001)1–5.[11]S.B.Ni,X.H.Wang,G.Zhou,F.Yang,J.M.Wang,D.Y.He,DesignedsynthesisofwiderangemicrowaveabsorptionFe3O4–carbonspherecomposite,J.Alloy.Compd.489(2010)252–256.[12]G.X.Tong,W.H.Wu,J.G.Guan,H.S.Qian,J.H.Yuan,W.Li,Synthesisandcharacterizationofnanosizedurchin–likeα–Fe2O3andFe3O4:microwaveelectromagneticandabsorbingproperties,J.Alloy.Compd.509(2011)4320–4326.[13]G.B.Sun,B.X.Dong,M.H.Cao,B.Q.Wei,C.W.Hu,Hierarchicaldendrite–likemagneticmaterialsofFe3O4,γ–Fe2O3,andFewithhighperformanceofmi-crowaveabs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