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Compositionandstructurecontrolofultralightgraphenefoamforhigh-performancemicrowaveabsorptionYiZhang,YiHuang*,HonghuiChen,ZhiyuHuang,YangYang,PeishuangXiao,YingZhou,YongshengChen**CentreforNanoscaleScienceandTechnology,KeyLaboratoryofFunctionalPolymerMaterials,CollaborativeInnovationCenterofChemicalScienceandEngineering(Tianjin),SchoolofMaterialsScienceandEngineering,NankaiUniversity,Tianjin,300071,ChinaarticleinfoArticlehistory:Received18February2016Receivedinrevisedform21April2016Accepted27April2016Availableonline28April2016abstractMacroscopiclossyfoamhasbeenexpectedtobethemostpromisingcandidateforlightweighthigh-performancemicrowaveabsorption(MA).
However,inferiorMAbehaviorsofconventionalfoamsre-portedpreviouslyaredisappointing.
Theemerginggraphenefoam(GF)hasbrokenthisparadoxicalstateofaffairs.
Here,seriesofGFswithvariouschemicalcompositionsandphysicalstructureshavebeenpreparedviaafacileandcontrollablemethodandtheirMAperformanceisinvestigatedin2e18GHz.
Thein-depthanalysesoftheGF'scomposition,structureandMApropertydemonstratethattheMAper-formanceoftheGFisstronglycorrelatedwiththeC/Oratio,conjugatedcarbondomainsizeandgra-pheneframework'smicrostructure.
Amaximumabsorptionvalueof34.
0dBaswellas14.
3GHzqualiedbandwidthwithreectionlossbelow10dBisachievedfortheGFwithanultralowbulkdensityof1.
6mg/cm3,ofwhichtheaverageabsorptionintensityandthespecicMAefciencyaremuchhigherthanthoseofthebestavailableMAmaterialsinpreviousliterature.
Thecomposition&structureeperformancerelationshipofMAfoamsisrevealed.
ThebalancebetweensmallinterfacialimpedancegapandhighlosscharacteristichaswideimplicationsinimprovingtheMAperformanceoftheGFandotherporousmaterials.
2016ElsevierLtd.
Allrightsreserved.
1.
IntroductionWiththerapidarisingofinformationtechnology,microwaveabsorptionmaterialsareplayinganincreasinglysignicantroleinelectronicreliability,healthcare,andnationaldefensesecurity[1e4].
Forexample,themicrowaveabsorption(MA)materialsappliedintheemerginghigh-speedcommunicationapparatuslikesatellitescouldimprovethereceiver'ssignalqualitybysuppressingthenoise[5].
Besides,MAmaterialsintheradarstationandtherelaystationcouldprotectinsideworkersfromoverdoseexposuretohigh-powermicrowave[6].
Mostimportantly,withthegradualmaturationofnoveladvancedanti-stealthradarssuchasultrawidebandradar,phasedarrayradar,multi-staticradarandpassiveradar,high-performancecounter-detectionMAmaterialsserveasaveryefcientrouteinincreasingthesurvivabilityofmilitaryunitsviareducingtheirradarcross-section[7].
TheidealMAmaterialsareprimarilyrequiredtoestablishanexcellentdouble-winrelation-shipbetweenintenseabsorptionabilityandbroadabsorptionbandwidth.
Inaddition,MAmaterialswithultralightweightandthinthicknesswillbeadvantageousintheeldsofaerospace,aviation,groundvehiclesandfast-growingnext-generationgreenminiatureelectronics[1,8,9].
Theinterfacialimpedancegapandradiationenergylosschar-acteristicsareconsideredasthetwocoreprinciplesthatdeterminetheMAperformanceofamaterial[10e13].
Themicrowaveprop-agationforatypicalhomogenousmaterial'sMAprocessdependsonseveralfactors,includingdielectricpermittivityε,magneticpermeabilitymandelectricalconductivityd,whichareacompre-hensivereectionofsignicantcomponentandstructuralcharac-teristics[4,14e16].
Fordecades,researchershavemadeconsiderableeffortsto-wardsdesigningandfabricatingvariousMAmaterialsbyadjustingtheelectricalconductivity,dielectricconstantandmagneticpermeabilityinthepursuitoflowinterfacialimpedancegapaswellashighlossratioofincidentmicrowave[10,11,13,17,18].
In*Correspondingauthor.
**Correspondingauthor.
E-mailaddresses:yihuang@nankai.
edu.
cn(Y.
Huang),yschen99@nankai.
edu.
cn(Y.
Chen).
ContentslistsavailableatScienceDirectCarbonjournalhomepage:www.
elsevier.
com/locate/carbonhttp://dx.
doi.
org/10.
1016/j.
carbon.
2016.
04.
0700008-6223/2016ElsevierLtd.
Allrightsreserved.
Carbon105(2016)438e447mostcases,separatesolidparticleabsorbents,suchasferrites[19,20],metalpowders[20,21],ceramics[22],carbonnano/micromaterials[23,24]andtheirhybrids[2,15,25,26],areexten-sivelyadoptedasllersintomicrowave-transparentorganicorinorganicadhesivestofabricateMAcomposites.
BesidesmediocreMAperformance,mostofthemhavealsobeenkeptfarfrompracticalapplicationforsomeshortcomings,suchashighdensity,poorstabilityandlargeloadingcontent[1,12,27].
Ithasbeendemonstrated,forinstance,70wt%ormoremagneticironparti-cleswithaveryhighdensityof8g/cm3arerequiredintypicalMAcomposites[17,20].
Three-dimensional(3D)macroscopicporouslossymaterialshavebeenexpectedtobethemostpromisingcandidateforlight-weighthigh-performancebroadbandMAapplication[28e32].
ComparedwithconventionaluniformsolidMAmaterials,theMAfoam,withsomanyhomogenously-dispersedinternalpores,notonlyshowslowerbulkdensitybutalsogivesmuchsmallereffectivepermittivity,whichmakesitlessresistivetothedetectiveincidentmicrowaveinawidefrequencyrange[16,33].
Untilnow,consid-erableattentionshavebeenpaidtosynthesisandapplicationofporousbulkmaterialsformicrowavesuppression,suchasconductivepolymerfoam[28,34,35],siliconcarbidefoam[36,37],carbonfoam[7,29,38,39]andcarbonnanotubesponge[40].
How-ever,formostMAfoamsreportedpreviously,theirMAbehaviorscouldnotbecomparedwiththoseoftraditionalsolidMAmaterials[7,35,36].
Furthermore,itisstillabigchallengetorevealthecomposition&structureeperformancerelationshipofMAfoamsduetotheircomplicatedirregularstructuresandpreparationtechniques,whichseverelyhinderstheirpracticalapplication.
Recently,signicantprogresstoward3Dmacroscopicinter-connectedgraphenenetworkshasopenedupanewroutefortheexploitationofporousbulkmaterialforlightweightandbroadbandhigh-performanceMAapplication[38,41e49].
Inthepreviouscommunication,wepreliminarilyprovedtheoutstandingmicro-waveabsorbingperformanceofmacroscopicGFs,whichshowedthatGFsmayhavegreatpotentialinMAapplication[32].
However,thereremainsmuchuncertaintyintheGF'sMApropertydepen-denceonitsmorphologyandcomposition.
Therefore,itisverysignicanttodevelopafacileandcontrollablemethodtoprepareadditive-freelarge-sizedGFsandestablishtherelationshipbe-tweentheMApropertyandtheGF'sintrinsicstructureandcomponent,whichisessentialinanin-depthunderstandingofitsMAmechanismandmoreimportantlydevelopingauniversalstrategytoeffectivelyenhancetheMApropertyofbulkporousmaterials.
Herein,wedemonstratedesignandfabricationofvariousGFswithdifferentinternalmorphologiesandcompositionsandinvestigatetheirMAperformancein2e18GHz,whichisinten-sivelyoccupiedforsatellitecommunications,remotesensing,ra-dardetectionsandweaponsguidanceandtracking.
TheMAperformanceoftheGFfoamisfoundtobestronglycorrelatedtotheC/Oratio,sp2carbondomainsizeandgrapheneframeworkmicrostructure.
Amaximumabsorbingvalueof34.
0dBaswellas14.
3GHzqualiedbandwidthcanbeobtainedfortheGFwithanultralowdensityof1.
6mg/cm3,whichisclosetothedensityofambientair(1.
2mg/cm3)andmuchlowerthanthoseofthecar-bonfoam(166mg/cm3)[29]andtheSiCfoam(~256mg/cm3)[37].
Moreimportantly,theGFpresentsthebestaverageabsorptionintensitycomparedwithothertypicalMAmaterialsin2e18GHz.
ThespecicMAefciencyisnearlytwoordersofmagnitudehigherthanthoseofthebestavailableMAmaterialseverreported.
ThemechanismfortheMAperformancedepen-denceonthecompositionandstructureoftheGFisrevealed.
Thewell-matchedinterfacialimpedancecombinedwithhighlossabilitygivesrisetotheenhancedMAperformance.
2.
Experimental2.
1.
SynthesisofGFTherawmaterial,single-layergrapheneoxide(GO),waspre-paredusingamodiedHummersmethodasdescribedelsewhereandhasthelateralsizemainlyabove10mm[42].
TheinitialconcentratedGOethanolsolutionwasdilutedintothreeGOethanolreactionsolutionswithconcentrationsof0.
3,0.
6and0.
9mg/mL,respectively.
Aftersolvothermalreaction,solventex-changeandfreezedrying,threeoriginalGFswithdifferentgra-phenevolumefractionswereobtained.
TheGFsmadefrom0.
3to0.
9mg/mLGOethanolsolutionswereannealedat600Cfor1hinargonatramprateof10Cmin1toobtainthetargetGFslabeledasC0.
3andC0.
9,individually.
TheremainingGFsstartingfrom0.
6mg/mLGOsolutionweredividedintovesmallbatches,fourofwhichwereannealedatdifferenttemperaturesof200,400,600and800Cfor1hinargonatramprateof10Cmin1separatelytoobtainthetargetGFswithdifferentcompositions.
TheunannealedGFismarkedasT0andtheannealedproductsaremarkedasT200,T400,T600andT800inascendingtemperaturesequences.
Forconvenience,thesampleT600isalsonamedC0.
6intheGF'sstructurecomparison.
TofurtherstudytheGF'sMApropertydependenceonitsinterconnectedconductivenetworkofgraphenesheets,someofthesampleC0.
6wasbrokenintopowdersviamechanicalstirringat1800rpm.
2.
2.
CharacterizationTheRamanspectrumoftheGFwasobtainedonaRenishawinViaRamanspectrometerusinglaserexcitationat514.
5nm.
TheX-Raydiffraction(XRD)measurementoftheGFwascarriedoutonaRigakuD/Max-2500diffractometerwithCuKaradiation.
ThemorphologyoftheGFwasobservedbyScanningElectronMicro-scopy(SEM)(LEO1530VPoperatedat3.
0kV).
Theelectricalcon-ductivityoftheGFwasmeasuredbyahomemadextureaspreviouslyreported.
TheTransmissionelectronmicroscopy(TEM)investigationwasperformedonaFEITecnaiG2F20operatedat200kV.
ThethermogravimetricanalysiswasobtainedusingaNETZSCHSTA409PCanalyzer,withaheatingrateof10Cmin1fromroomtemperatureto850Cintheair.
TheX-rayPhotoelec-tronSpectroscopy(XPS)wasexaminedwithaGENESIS60SX-rayphotoelectronspectrometerusinganAlKa(hn1486.
6eV)radi-ationandthebindingenergieswerecalibratedbyusingthecontainmentcarbonpeak(C1s284.
6eV).
AfterallGFsweredriedinvaccumat75Cfor24h,theirFouriertransforminfrared(FT-IR)spectraandcombustionelementalanalysisweretheninvestigatedatTensor27FT-IRSpectrometer(Bruker,Germany)andVariomicroelementalanalyzer(Elementar,Germany),respectively.
BasedontheArchmethod,theMAperformancewasevaluatedin2e18GHzusinganAngilentHP8757Escalarquantitynetworkanalyzer.
FourGFscutinto90mm90mm10mmwerear-rangedintoacubiccontainerwithinternaldimensionsof180mm180mm15mmformeasurementsinthefrequencybandof2e18GHz.
ThepowdersamplewithamassequivalenttotheMAtestsampleC0.
6waslooselyplacedinthecontainerfortheMAmeasurement.
AllGFswerebackedwithahighlyconductivealuminumplatetoreecttheentireincidentmicrowavebacktothereceivingantenna.
Therelativecomplexpermittivityandpermeabilityweremeasuredinthefrequencyrangeof2e18GHzusinganAngilentHP8722ESvectornetworkanalyzer.
Parafnwasusedasthesup-portingmatrixduetoitsminorcomplexelectromagneticparam-etersapproximatingthoseofair.
Thetoroidaltestsample(3mmi.
d.
,7mmo.
d.
and2mmthickness)wasfabricatedbyvacuum-Y.
Zhangetal.
/Carbon105(2016)438e447439impregnatingtheGFwithparafn.
Theincidentmicrowavedirec-tionwasperpendiculartothetestsample.
3.
Resultsanddiscussion3.
1.
ThedependenceofMAperformanceontheGF'schemicalcompositionThemacroscopicadditive-freeGFforMAtestswaspreparedmainlythroughasolvothermalreaction,followedbysolventremovalandthermalreduction.
Byvaryingannealingtemperaturesinthethermalreductionfromroomtemperatureto800C,vetypesofpie-shapedGFsstartingfromthesameGOconcentrationof0.
6mg/mLwereobtainedtostudytheMAperformancedepen-denceonthechemicalcomposition.
Forconvenience,theunan-nealedGFislabeledasT0andtheotherannealedproductswerelabeledasT200,T400,T600andT800intemperaturesequences.
Duetothepyrolysisofthelabileoxygen-containinggroupsandthecarbondefects,theGFundergoesanobviousweightlossduringthethermalreduction.
AsshowninFig.
1a,theGF'sbulkdensityde-clinesbyover50%withtheannealingtemperaturerisingfromroomtemperatureto800C.
Giventhelowintrinsicdensityofgraphenesheets,theporosityhigherthan99%canbeobtainedforvetypesofGFs,exceedingthoseofmostmacroscopicfoams[29e31,43].
TounderstandchemicalcompositiontransformationoftheGFannealedatdifferenttemperatures,elementalanalysis,XPS,Ramanspectroscopyandthermogravimetricanalysiswereperformedforeverysample.
ElementalanalysisoffersthemostdirectevidencefortheelementcomponentoftheGFsample.
Freeofadditives,alltheGFsamplesaremainlycomposedofcarbonandoxygenassameastheinitialGO.
AsshowninFig.
1b,theunannealedGFT0hasthehighestoxygencontent,indicativeofitsmostseverelydamagedconjugatedcarbonbackbones.
Astheannealingtemperaturerises,thecarboncontentgrowscontinuously,whichisoppositefortheoxygencontentintheGFsample.
EvenfortheGFannealedat800C,theexistenceofoxygenmanifeststhatitservesasthetoughbondingelementinchemicallylinkingadjacentgraphenesheets,ensuringthethermallyreducedGF'srobustmechanicalstrength.
ItshouldbenotedthattheC/Oratioincreasingtrendclearlyaccel-eratesupontheannealingtemperaturerisingto400C,whichmayresultinanabruptchangeintheGF'sMAproperty.
TheFT-IRspectrafortheGFsampleswithvariouschemicalcompositionsareshowninFig.
1c.
Theabsorptionpeaksappearat1730,1570and1220cm1,correspondingtotheC]Ostretchingmode,C]CstretchingvibrationofbenzeneringandbreathingvibrationmodeoftheCeOgroups,respectivelyandthebroadpeakat3400cm1isattributedtotheadsorbedwaterinthesampleduetotheGF'shighporosity[42].
Withtheannealingtemperatureincreasing,theC]Ostretchingvibrationpeakgraduallyattenuatesandnearlyvanishesat800C.
Remarkably,theeverlastingCeObreathingvibrationpeak,ontheonehand,demonstratesthatthemutuallyentangledgraphenesheetsarechemicallyreinforcedbytheCeOeCcovalentbondanalogues.
Ontheotherhand,suchde-fects,consideredasthepolarizationdomains,contributetoimprovingthemicrowave-absorbingabilityoftheGF[11,12].
IntheRamanspectra(Fig.
1d),alltheGFsamplesdisplaytwoevidentpeaksat1351cm1and1588cm1,correspondingtotheDandGbandsrespectively.
TheID/IGratioisassociatedwithdefectconcentrationofgraphiticcarbonmaterials[48].
Itcanbeseenthatwithraisingannealingtemperature,theID/IGratioreducesdramaticallyfrom0.
95forT0to0.
75forT800,whichdemonstratestheenlargementofsp2carbondomain.
Morepreciseanalysesofin-planeconjugatedcarbonstructureswereconductedviaXPSchar-acterization(Fig.
S1).
Besidesthemainsp2carbonpeaklocatingat284.
5eV,thehigh-resolutionC1sregionoftheGFcanbedividedinto4otherttingpeaksat285.
8eV,286.
7eV,287.
7eVand288.
8eV,whichareattributedtocarbonspeciesofsp3carbon,CeOeC,C]OandC(C]O)O,individually[42].
Thesp2conjugatedcarbonskeletonaccountsforthevastmajorityofcarbonspeciesinalloftheGFsamples.
Furthersemi-quantitativeanalyses(Fig.
1e)Fig.
1.
(a)Thebulkdensities,(b)theelementalanalysis,(c)theFT-IRspectra,(d)theRamanspectra,(e)thesemi-quantitativesp2carbondomainsanalysesand(f)thether-mogravimetriccurvesfortheGFsannealedatdifferenttemperatures.
(Acolourversionofthisgurecanbeviewedonline.
)Y.
Zhangetal.
/Carbon105(2016)438e447440provethatwiththereductiontemperatureliftingto800Cthisproportionkeepsrisingfrom66.
7at%to80.
4at%intherangefrom200Cto400Cwhichobviouslygrowsfastest.
Thermogravimetricanalyses(Fig.
1f)wereperformedtoexaminethecompositionthermostabilityofvariousGFsamples.
BothT0andT200appearapparentweightlosswhenthetemper-atureapproaches200C.
However,whentheannealingtempera-turegoesabove400C,theresultingGFscouldmaintainstableinairatoperatingtemperatureofover300C.
Duetothehigherthermostabilityofsp2carbonbackbonesthanthoseofoxygen-containinggroups,theseresultssuggestmoreconjugatedcarbondomainsintheGFsannealedathighertemperature,whichareconsistentwiththeresultsofelementalanalysesandXPSanalyses.
Additionally,theexcellentthermostabilitymakessuchGFssuitableforapplicationsathightemperaturesuchastheskinsofhigh-speedaircraftsandhoodsofvehicleengines[50].
ThequaliedMAintensityformostapplicationsisgenerally10dB[15,26].
Fig.
2exhibitsthereectionloss(RL)curvesforGFsviadifferentthermaltreatmentsintherangeof2e18GHz.
WiththeC/Oratioaslowas5.
4andonly66.
7%sp2carboncontent,theoriginalunannealedGF(T0)exhibitsveryinferiormicrowave-absorbingabilityinthetestfrequencybandowningtoseveredisruptionofin-planeconjugatedstructures.
AlthoughtheC/OratioofT200increasesalittleafterthelow-levelthermalreduction,suchpoorrestorationofitsdamagedconjugatedgraphenenetworkdoesnotmakeitsRLcurvedifferobviouslyfromthatofT0.
Aftertheannealingtemperaturerisingto400C,thereappearsasignicantimprovementintheGF'sMAperformance.
ForT400,thestrongestRLreached28.
4dBat13.
9GHzandthequaliedfrequencyrangesfrom5.
6GHzto16.
9GHz.
ThephenomenondemonstratesthatwiththeC/Oratioandsp2carbondomainover6.
2and74%respectively,theGF'spartiallyrecovered3Dporousconductivenetworkofgraphenesheetsbecomessensitivetotheincidentmicrowave.
T600showstheoptimalMAperformance.
InadditiontothemaximumRLof34.
0dBat13.
1GHz,itsqualiedfrequencybandwidthreaches14.
3GHz,covering89.
4%oftheentiremeasuredbandwidth,whichismuchwiderthanthoseofmostMAmaterialsreportedpreviously[7,8].
Withannealingtemperaturearisingcontinuously,thechangeoftheGF'sMAperformancedoesn'tmaintainamonotonicevolution.
Onthecontrary,uponthetemperaturesurpassing600C,theGF'sMApropertystartsweakening,whichsuggeststhatwiththeC/Orisingmorethan12.
4,over-restoredconjugatedcarbonframeworkwillincreasethereectionofthemicrowaveandbecomesharmfulfortheGF'sMAproperty.
Therefore,thereexistsanoptimalreductiontemperature,suchas600Cinthiscase,underwhichtheannealedgrapheneskeletonwithcertainC/Oratioandconjugatedcarbondomainsizeexhibitsthebestwave-absorbingability.
3.
2.
ThedependenceofMAperformanceontheGF'sphysicalstructureThematerial'smicrowaveabsorbingpropertiesrelyonnotonlythechemicalcomposition,butalsothephysicalstructure,espe-ciallythebasicabsorbent'smorphologyandtheinterfacialmicro-structure[3,17,18].
AfacileandcontrollablemethodisdevelopedtofabricateGFswithcontrollablemicrostructure.
ViaadjustingGOconcentrationoftheinitialGOconcentrationfrom0.
3to0.
9mg/mL,threekindsofGFsannealedatthesametemperatureof600Cwereprepared,whicharemarkedasC0.
3,C0.
6andC0.
9inascendingGOconcentrationforconvenience.
Fig.
3ashowsthebulkdensitiesofthreeGFswithdifferentphysicalstructures.
WiththeoriginalGOethanolsolutiongettingdenser,theresultingGF'sdensityrisesfrom0.
9to2.
4mg/cm3.
ThehighergraphenecontentresultsintheweakerGF'sliquidabsorp-tioncapability(Fig.
3b),whichisadirectreectionofthedecreaseintheGF'sporosity[38,42].
However,similartothoseofourotherGFs,thecalculatedgraphenevolumefractionforallthethreeGFsamplesstillkeepsbelow1%,whichtosomeextentcanberegardedasastableindividualcharacteristicoftheadditive-free3Dcross-linkednetworkofgraphenesheets.
Fig.
4givestheSEMimagesoftheGFsmadefromtheGOso-lutionswithdifferentinitialconcentration.
Ascanbeseen,thestartingGOconcentrationhasagreatimpactontheinternalporousmorphologyandcellsizeoftheresultingGF.
WhenthestartingsolutioncontainsveryfewGOsheets(0.
3mg/mL),theresultingGFshowsareticulum-likeopencellstructurewiththeporesizerangingfrom30to90mm,inwhichmostgraphenesheetsconcentrateontheedgeandformthestrutswithmanyenclosedwallsincompletely.
UponraisingtheGOconcentrationofthestartingsolutionto0.
6mg/ml,theGFwithnegligiblechangeinitsinternalporesize,experiencesanenormousmorphologicalevo-lutionfromtheopenreticularstructuretothesemi-closedcellularonewithmuchmorecompletecellwalls.
AstheinitialGOcon-centrationkeepsrisingto0.
9mg/mL,theGFinternalporesbeginshrinkingdistinctly,Itisworthnotingthatalthoughitslong-rangenetworkhasbeenbroken,theC0.
6powderstillpresentsanintri-cate3Dinterconnectedgraphenenetworkwithtremendousmicro-sizedpolygonporeslikeotherintactGFs,whichfurtherdemon-stratestherobustmechanicalpropertyofthe3Dmonolithicstructure.
Aswehavepointedout,theinterfacialmicrostructureoftheabsorbentwillexertsignicantimpactonthemicrowavelossypropagationintheMAmaterial[19,32,48].
Therefore,wefurtherinvestigatetheinuenceofgraphenecontentontheGF0cellwallviaTEMcharacterization(Fig.
5aec).
C0.
3'scellwalliscomprisedof1e5layergraphenesheetswithpoorrestacking,whichshouldbemostbenecialtothetransmissionoftheincidentmicrowavethroughthewholefoam.
WiththeincreasingoftheGOcontentinthestartingsolution,theGF'scellwallsgraduallybecomethicker.
UponraisingGOconcentrationto0.
9mg/mL,theGFevenpresentssomeinternalcellwallsconsistingofashighas8e14layergra-phenesheets,overtwicethethicknessoftheGFwiththelowestinitialGOcontent.
TofurthercharacterizeinternalstructureoftheGF,theX-raydiffraction(XRD)examinationofvariousGFsmadefromdifferentGOsolutionswereperformedwithakegraphiteasthecompari-son(Fig.
5d).
Differentfromthestrongsharppeakforgraphiteat24681012141618-40-35-30-25-20-15-10-50ReflectionLoss(dB)Frequency(GHz)T0T200T400T600T800Fig.
2.
TheRLcurvesfortheGFswithdifferentchemicalcompositionsin2e18GHz.
(Acolourversionofthisgurecanbeviewedonline.
)Y.
Zhangetal.
/Carbon105(2016)438e4474412q26.
5,alltheGFsexhibitratherbroadfeeblepeaks(002),indicatingthatthelong-rangerestackingofgraphenesheetsisveryweak.
NotethatastheinitialGOconcentrationmovesup,addi-tionalwideweakpeaksat25.
0and25.
9emergeforC0.
6andC0.
9,respectively.
TheresultprovesthatC0.
6andC0.
9,withmoregrapheneconstituent,possessmorerestackingstructuresthanC0.
3,whichisconsistentwiththeTEMcharacterization.
TheRLcurvesforalltheGFsamplesin2e18GHzareshowninFig.
6.
TheGFmadefromthe0.
3mg/mLGOsolutiondisplaystheweakestwave-absorbingabilitywiththemaximumabsorptionintensityof15.
0dBat18.
0GHzandthequaliedbandwidthof6.
0GHz.
WiththeinitialGOincreasingfrom0.
3mg/cm3to0.
6mg/cm3,amaximumRLof34.
0dBisachievedat13.
1GHz.
Moresignicantly,thequaliedfrequencybandwidth(14.
3GHz)improveddramatically,coveringmostoftheentiremeasuredbandwidth.
WiththefurtherincreasingoftheconcentrationofGOsolution,theGFexhibitsareducedmicrowaveabsorbingability.
TheresultindicatesthatthereexistsanoptimalinitialGOcon-centrationthatmakestheself-assembledGFpossessthebestMAperformanceandthisvalueismostapproximateto0.
6mg/mLinourcase.
ItshouldbenotedthatthevarianceforalltheGFsamplesingraphenevolumefractiondoesnotsurpass1%,whichdemon-stratesthattheGF'sMAperformanceismuchsensitivelyaffectedbyitsphysicalstructure.
Furthermore,comparedtootherunbrokenGFs,theGFpowderexhibitaverypoorMAperformancewiththenarrowqualiedbandwidthof1.
4GHzandlowoptimalabsorptionstrengthof11.
5dB.
Theexperimentalresultindicatesthatthehighlyintricateinterconnectedlong-rangeconductivenetworkisimperativetotheexcellentMApropertyoftheGF.
3.
3.
ComparisonoftheGF'scomprehensiveMAperformancewithotherMAmaterialsThedetailedMAperformanceofsomerepresentativeMAma-terialsreportedbeforeislistedinTableS1forcomparison.
ItcanbeseenthattheoptimizedGFexhibitsmuchmoreexcellentMAperformancebothinthemaximumabsorbingintensityandinthequaliedbandwidththanmostMAmaterials,includingtheMAfoams.
Becauseofthegreatimportanceoftheabsorbingintensitythroughoutquiteabroadfrequencyrangeinthebroadbandhigh-performanceMAresearch,it'snecessarytointroduceanewconceptoftheaverageabsorptionintensity(AAI)whichisanaloguetoExternalQuantumEfciencyinthephotovoltaiceld.
TheAAIisexpressedasAAIdBZfhflRLdffhfl(1)whereflandfhrespondtothelowestmeasuredfrequencyandthehighestmeasuredfrequency,respectively.
ThehighertheAAIvalueis,thestrongerbroadbandmicrowaveabsorbingabilitythemate-rialpresents.
TheAAIin2e18GHzofseveralrepresentativeMAmaterialsinpreviousliteraturewerecalculatedinFig.
7a.
Owningtothestrongmicrowaveabsorbingbehaviorinthewidefrequencyband,theGFshowsaveryexcellentAAI(17.
9dB),whichishigherthanthebestavailableMAmaterialsreportedpreviously.
Furthermore,lightweightaccountsforagiantproportionindesigningandevaluatingmicrowaveattenuationmaterialsappliedinaerospace,aviationandgroundvehicles[9,35].
Therefore,weadoptthespecicMAefciency(SMAE)integratedwithsignicantMAindicatorssuchasthickness,density,qualiedbandwidthandRLvaluesinthemeasuredfrequencybandtoevaluatethemicro-waveabsorption(MA)performancemorecomprehensively[32]TheSMAEisexpressedasSMAEdB$Hz$cm2$g1ZfhflRLdft$rbulk(2)wherefl,fh,tandrbulkrespondtothelowestqualiedfrequency,thehighestqualiedfrequency,theaveragethicknessandtheaveragebulkdensityoftheMAmaterial,respectively.
AmaterialwithhighSMAEisanticipatedtohaveahugepotentialinMAapplication.
ThespecicMAefciencyin2e18GHzofseveralrepresentativeMAmaterialsinopenliteraturewerecalculatedinFig.
7b.
Despiteofthelowbulkdensity,thespecicMAefciencyofthecarbonfoam[7]andSiCfoam[37]don'tgoover100dBHzcm2g1,whichismuchinferiortosometypicalsolidMAmaterial,suchastheSWNT/polyurethanecomposite(~1.
5102dBcm2g1)[8],thereducedGO/NBRcomposite(~2.
2102dBHzcm2g1)[23]andthea-Feencapsulatedwithincarbonnanotube/epoxycomposite(~2.
0103dBHzcm2g1)[15].
Remarkably,theexcellentMAperformancecombinedwiththeultralowdensity(1.
6mgcm3)givestheoptimizedGFasuperiorspecicMAefciencyaround1.
7105dBcm2g1,nearlytwoordersofmagnitudehigherthanthoseofthebestavailableMAmaterialsreportedbefore.
C0.
3C0.
6C0.
90.
81.
21.
62.
02.
4BulkDensity(mg/cm3)GFSamplesBulkDensityC0.
3C0.
6C0.
9520560600640680WeightGainGFSamplesPetroleumetherEthanolbaFig.
3.
(a)Thebulkdensityand(b)theliquidabsorptioncapabilitiesoftheGFs.
(Acolourversionofthisgurecanbeviewedonline.
)Y.
Zhangetal.
/Carbon105(2016)438e4474423.
4.
ThemechanismfortheMAperformancedependenceonthecompositionandstructureoftheGFBasedonthegeneralMAprinciple,theinterfacialimpedancegapandradiationenergylossratioareregardedasthetwocriticalfactorsthatdeterminethematerial'sMAperformance[8,18,37,52].
Toatypicalsingle-layerdielectricMAmaterial,suchasgraphene/polymercomposites,theincidentwavepropagationfromfreespaceintothematerialismainlyaffectedbymaterial'sdielectricpermittivityεandelectricalconductivityd[4,37,39,53].
TobetterunderstandtheMAperformancedependenceontheircompositionandmorphology,weinvestigatedvariousGFs'bulkelectricalcon-ductivitiesandrelativepermittivitiesin2e18GHz.
TheelectricalconductivitiesforalltheGFsareshowninFig.
S2.
ItcanbeseenthattheriseineitherofthegraphenevolumefractionandtheannealingtemperaturewillresultinthebulkelectricalconductivityincreaseoftheGF.
Thelong-rangeinducedcurrentsdecayontheconductiveskeletonplaysakeyroleinthesubstan-tiallyenhancedMA.
TheimprovedconductivegrapheneframeworkoftheGFshouldcouplewithmoretime-varyingelectromagneticeldswithinbroaderfrequencybandsandthusconsumemoreradiationenergy[32,37].
It'sworthnotingthatevenforC0.
9andT800,theirbulkconductivitiesstillstaybelow1.
0104S/m,whichcontributestothegoodinterfacialimpedancematchingandthusweakeningadversebackreectionofmicrowave[9,22].
Therealpermittivityε0respondstothestoragecapabilityoftheFig.
4.
Thecross-sectionalSEMimagesof(a,b)C0.
3,(c,d)C0.
6,(e,f)C0.
9,(g,h)C0.
6powders.
Y.
Zhangetal.
/Carbon105(2016)438e447443electriceldinsidetheabsorber[10,54],andasmallerε0shortenstheinterfacialimpedancegap,thusdecreasingthereectionco-efcientoftheabsorber[55].
Inaddition,thedielectriclosstangenttandeε00=ε0representsthematerial'sconversionofthemicro-waveradiationintootherenergyforms[8,10].
Thehigherdielectriclosstangent,themoreelectromagneticwaveenergygetsabsorbed[9,11].
Theinclinationtoanysidewouldn'tgiverisetoagoodMAbehavior[10,35].
Fig.
8givesrealpermittivitiesandrelativedielectriclosstangentvaluesforalltheGFs.
BothT0(Fig.
8a)andC0.
3(Fig.
8b)showverylowrealpermittivitiesanddielectriclosstangents,whichareevenbelowthoseofsinglelow-losspolymerssuchaspoly(dimethylsiloxane)[51]andpoly(ethyleneoxide)[56].
It'sclearlyseenthatboththerealpermittivityanddielectriclosstangentoftheGFgrowbiggermonotonicallywitheitherthegra-phenevolumefractionorreductiontemperatureincreasing.
Theresult,however,isnotconsistentwiththeMAperformancechange,whichdemonstratestheimportanceofthebalancebetweenthelowpermittivityandhighdielectriclosstoasatisfyingmicrowave-absorbingability.
InallGFsamples,theT800andC0.
9,thoughpossessingthestrongenergyconversionabilities,exhibitoverhighrealpermittivityvalues,whichgoesagainstthewell-matchedimpedancebridgeandthusresultsinanattenuatedMApropertyoftheGF.
Bycontrast,overlowdielectriclosstangentsoftheGFsincludingC0.
3,T0andT200duetosparseconductivenetworksandexcessivedefectsofgraphenesheets,revealtheirpoormicrowavedissipationperformanceduringthewavepropagationfrompene-tratingthefoamtoreturningtothereceivingantenna.
TheGFT600orC0.
6,withproperpermittivityandhighdielectriclosstangent,successfullybuildawinewinrelationshipbetweentheinterfacialmatchingandlossproperty,thusexhibitingthebestMAperfor-mance.
Itshouldbenotedthatdespiteofultralowrealpermittivity,theinferiordielectriclosstangentoftheC0.
6powdersuggeststhatboththeultrahighporosityandtremendouslong-rangecross-linkedconductivegraphenenetworkareimperativetotheexcel-lentMAperformanceoftheGF.
ToreecttheintrinsicMAmechanismmoredirectly,wethensimulatedtheRLcurvesforalltheGFs.
Accordingtothetransmission-linetheory,theRLofelectromagneticwaveradiation,R(dB),undernormalwaveincidenceonametal-backedsingle-layerMAmaterialiscorrelatedwiththeincidentimpedanceZinas[8,56].
RdB20logZinZ0ZinZ0(3)whereZ0isthecharacteristicimpedanceoffreespace(377U).
ZinistheinputimpedanceattheinterfaceoffreespaceandtheMAFig.
5.
TheTEMimagesofthebrokenGFcellwallof(a)C0.
3,(b)C0.
6and(c)C0.
9aftersonication.
(d)TheXRDresultsoftheGFsmadefromdifferentinitialGOsolutionsandakegraphiteforcomparison.
TheinsetgivesdetailedpatternsofthreeGFsamples.
(Acolourversionofthisgurecanbeviewedonline.
)24681012141618-40-35-30-25-20-15-10-505ReflectionLoss(dB)Frequency(GHz)C0.
9C0.
6C0.
3C0.
6PowderFig.
6.
TheRLcurvesfortheGFswithdifferentphysicalstructuresareshownin2e18GHz.
(Acolourversionofthisgurecanbeviewedonline.
)Y.
Zhangetal.
/Carbon105(2016)438e447444material,givenas[20,35].
ZinZ0mrεrrtanh2pfdmrm0εrε0pj(4)wherefisthemicrowavefrequency,disthematerialthickness,εristherelativepermittivityandmristherelativepermeability.
ThesimulatedMAcurvesin2e18GHzforalltheGFsareshowninFig.
S3.
ThetheoreticalMAcurvesagreewellwiththeexperi-mentalresultsintheaspectoftheMAperformancetrendwithinitialGOconcentrationandthermalreductiontemperature.
Forexample,thecalculatedmicrowaveabsorbingpropertyoftheGFundergoesthesamerstrisingandthenfallingchangewiththeannealingtemperatureelevatingfromroomtemperatureto800C.
However,thetheoreticalcurvedoesnottexactlyonthemeasuredcurveintheabsorptionperformanceandsuchdiscrepancycouldbeattributedtohighlynonuniformporousstructureinsidetheGF[57]andtheedgeeffectoftheGFjunctionintheMAtest[21].
4.
ConclusionInsummary,wehavepreparedseriesofGFswithvariouschemicalcompositionsandphysicalstructuresbycontrollingtheGOconcentrationoftheinitialsolutionandthermal-reductiontemperature.
TheanalysesoftheGFs'compositions,structuresandelectromagneticpropertiessuggestthattheMAperformanceoftheGFisstronglycorrelatedwiththeC/Oratio,conjugatedcarboncontentandthegrapheneskeletonmicrostructure.
Amaximumabsorptionvalueof34.
0dBaswellas14.
3GHzFig.
7.
Comparisonof(a)AAIand(b)SMAEvaluesfortheGF(lightgreypatternedcolumn)inthisworkandtherepresentativematerials(darkgreycolumns).
MoredetaileddatahavebeenlistedinTableS1intheSupportingInformation.
(Acolourversionofthisgurecanbeviewedonline.
)Fig.
8.
Realpartsofthecomplexpermittivitiesof(a)theGFswithdifferentchemicalcompositionsand(b)theGFswithdifferentphysicalstructuresin2e18GHz.
DielectriclosstangentsoftheGFsof(c)theGFswithdifferentchemicalcompositionsand(d)theGFswithdifferentphysicalstructuresin2e18GHz.
(Acolourversionofthisgurecanbeviewedonline.
)Y.
Zhangetal.
/Carbon105(2016)438e447445qualiedbandwidthcanbeachievedfortheGFwithanultralowbulkdensityof1.
6mg/cm3,whichisclosetothedensityofambientair(1.
2mg/cm3).
Particularly,theGFpresentthebestaverageab-sorptionintensitycomparedwithothertypicalMAmaterialsin2e18GHz.
TheoutstandingMAperformancecombinedwithanultralowbulkdensitygivestheGFasuperiorspecicMAefciencynearlytwoordersofmagnitudehigherthanthoseofthebestavailableMAmaterialsreportedbefore.
ThemechanismfortheMAperformancedependenceonthecompositionandstructurerevealsthattheGFwithproperchemicalcompositionandphysicalstruc-turemakingthebalancebetweenexcellentimpedancematchingandhighlosscharacteristiccoulddeliveranexcellentMAproperty.
WiththefacileandcontrollablesynthesisoftheGF,anewapproachtoefcientlyoptimizeandregulateitsMAbehaviorisnowpossible.
Moreimportantly,theprocessingecomposition&structur-eepropertyrelationshipsoftheGFhasopenedupanewstrategytodesignrationallymacroscopicporousmaterialsforlightweighthigh-performanceandbroadbandMAapplicationsuchasthenew-generationultralightheat-resistantMAskinforhigh-speedaircrafts.
AcknowledgmentsTheauthorsgratefullyacknowledgenancialsupportfromtheMOST(Grants2012CB933401),NSFC(Grants21374050,91433101,51472124and51273093),MOE(B12015),PCSIRT(IRT1257)andNSFofTianjinCity(Grant15JCYBJC17700).
AppendixA.
SupplementarydataSupplementarydatarelatedtothisarticlecanbefoundathttp://dx.
doi.
org/10.
1016/j.
carbon.
2016.
04.
070.
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However,inferiorMAbehaviorsofconventionalfoamsre-portedpreviouslyaredisappointing.
Theemerginggraphenefoam(GF)hasbrokenthisparadoxicalstateofaffairs.
Here,seriesofGFswithvariouschemicalcompositionsandphysicalstructureshavebeenpreparedviaafacileandcontrollablemethodandtheirMAperformanceisinvestigatedin2e18GHz.
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Amaximumabsorptionvalueof34.
0dBaswellas14.
3GHzqualiedbandwidthwithreectionlossbelow10dBisachievedfortheGFwithanultralowbulkdensityof1.
6mg/cm3,ofwhichtheaverageabsorptionintensityandthespecicMAefciencyaremuchhigherthanthoseofthebestavailableMAmaterialsinpreviousliterature.
Thecomposition&structureeperformancerelationshipofMAfoamsisrevealed.
ThebalancebetweensmallinterfacialimpedancegapandhighlosscharacteristichaswideimplicationsinimprovingtheMAperformanceoftheGFandotherporousmaterials.
2016ElsevierLtd.
Allrightsreserved.
1.
IntroductionWiththerapidarisingofinformationtechnology,microwaveabsorptionmaterialsareplayinganincreasinglysignicantroleinelectronicreliability,healthcare,andnationaldefensesecurity[1e4].
Forexample,themicrowaveabsorption(MA)materialsappliedintheemerginghigh-speedcommunicationapparatuslikesatellitescouldimprovethereceiver'ssignalqualitybysuppressingthenoise[5].
Besides,MAmaterialsintheradarstationandtherelaystationcouldprotectinsideworkersfromoverdoseexposuretohigh-powermicrowave[6].
Mostimportantly,withthegradualmaturationofnoveladvancedanti-stealthradarssuchasultrawidebandradar,phasedarrayradar,multi-staticradarandpassiveradar,high-performancecounter-detectionMAmaterialsserveasaveryefcientrouteinincreasingthesurvivabilityofmilitaryunitsviareducingtheirradarcross-section[7].
TheidealMAmaterialsareprimarilyrequiredtoestablishanexcellentdouble-winrelation-shipbetweenintenseabsorptionabilityandbroadabsorptionbandwidth.
Inaddition,MAmaterialswithultralightweightandthinthicknesswillbeadvantageousintheeldsofaerospace,aviation,groundvehiclesandfast-growingnext-generationgreenminiatureelectronics[1,8,9].
Theinterfacialimpedancegapandradiationenergylosschar-acteristicsareconsideredasthetwocoreprinciplesthatdeterminetheMAperformanceofamaterial[10e13].
Themicrowaveprop-agationforatypicalhomogenousmaterial'sMAprocessdependsonseveralfactors,includingdielectricpermittivityε,magneticpermeabilitymandelectricalconductivityd,whichareacompre-hensivereectionofsignicantcomponentandstructuralcharac-teristics[4,14e16].
Fordecades,researchershavemadeconsiderableeffortsto-wardsdesigningandfabricatingvariousMAmaterialsbyadjustingtheelectricalconductivity,dielectricconstantandmagneticpermeabilityinthepursuitoflowinterfacialimpedancegapaswellashighlossratioofincidentmicrowave[10,11,13,17,18].
In*Correspondingauthor.
**Correspondingauthor.
E-mailaddresses:yihuang@nankai.
edu.
cn(Y.
Huang),yschen99@nankai.
edu.
cn(Y.
Chen).
ContentslistsavailableatScienceDirectCarbonjournalhomepage:www.
elsevier.
com/locate/carbonhttp://dx.
doi.
org/10.
1016/j.
carbon.
2016.
04.
0700008-6223/2016ElsevierLtd.
Allrightsreserved.
Carbon105(2016)438e447mostcases,separatesolidparticleabsorbents,suchasferrites[19,20],metalpowders[20,21],ceramics[22],carbonnano/micromaterials[23,24]andtheirhybrids[2,15,25,26],areexten-sivelyadoptedasllersintomicrowave-transparentorganicorinorganicadhesivestofabricateMAcomposites.
BesidesmediocreMAperformance,mostofthemhavealsobeenkeptfarfrompracticalapplicationforsomeshortcomings,suchashighdensity,poorstabilityandlargeloadingcontent[1,12,27].
Ithasbeendemonstrated,forinstance,70wt%ormoremagneticironparti-cleswithaveryhighdensityof8g/cm3arerequiredintypicalMAcomposites[17,20].
Three-dimensional(3D)macroscopicporouslossymaterialshavebeenexpectedtobethemostpromisingcandidateforlight-weighthigh-performancebroadbandMAapplication[28e32].
ComparedwithconventionaluniformsolidMAmaterials,theMAfoam,withsomanyhomogenously-dispersedinternalpores,notonlyshowslowerbulkdensitybutalsogivesmuchsmallereffectivepermittivity,whichmakesitlessresistivetothedetectiveincidentmicrowaveinawidefrequencyrange[16,33].
Untilnow,consid-erableattentionshavebeenpaidtosynthesisandapplicationofporousbulkmaterialsformicrowavesuppression,suchasconductivepolymerfoam[28,34,35],siliconcarbidefoam[36,37],carbonfoam[7,29,38,39]andcarbonnanotubesponge[40].
How-ever,formostMAfoamsreportedpreviously,theirMAbehaviorscouldnotbecomparedwiththoseoftraditionalsolidMAmaterials[7,35,36].
Furthermore,itisstillabigchallengetorevealthecomposition&structureeperformancerelationshipofMAfoamsduetotheircomplicatedirregularstructuresandpreparationtechniques,whichseverelyhinderstheirpracticalapplication.
Recently,signicantprogresstoward3Dmacroscopicinter-connectedgraphenenetworkshasopenedupanewroutefortheexploitationofporousbulkmaterialforlightweightandbroadbandhigh-performanceMAapplication[38,41e49].
Inthepreviouscommunication,wepreliminarilyprovedtheoutstandingmicro-waveabsorbingperformanceofmacroscopicGFs,whichshowedthatGFsmayhavegreatpotentialinMAapplication[32].
However,thereremainsmuchuncertaintyintheGF'sMApropertydepen-denceonitsmorphologyandcomposition.
Therefore,itisverysignicanttodevelopafacileandcontrollablemethodtoprepareadditive-freelarge-sizedGFsandestablishtherelationshipbe-tweentheMApropertyandtheGF'sintrinsicstructureandcomponent,whichisessentialinanin-depthunderstandingofitsMAmechanismandmoreimportantlydevelopingauniversalstrategytoeffectivelyenhancetheMApropertyofbulkporousmaterials.
Herein,wedemonstratedesignandfabricationofvariousGFswithdifferentinternalmorphologiesandcompositionsandinvestigatetheirMAperformancein2e18GHz,whichisinten-sivelyoccupiedforsatellitecommunications,remotesensing,ra-dardetectionsandweaponsguidanceandtracking.
TheMAperformanceoftheGFfoamisfoundtobestronglycorrelatedtotheC/Oratio,sp2carbondomainsizeandgrapheneframeworkmicrostructure.
Amaximumabsorbingvalueof34.
0dBaswellas14.
3GHzqualiedbandwidthcanbeobtainedfortheGFwithanultralowdensityof1.
6mg/cm3,whichisclosetothedensityofambientair(1.
2mg/cm3)andmuchlowerthanthoseofthecar-bonfoam(166mg/cm3)[29]andtheSiCfoam(~256mg/cm3)[37].
Moreimportantly,theGFpresentsthebestaverageabsorptionintensitycomparedwithothertypicalMAmaterialsin2e18GHz.
ThespecicMAefciencyisnearlytwoordersofmagnitudehigherthanthoseofthebestavailableMAmaterialseverreported.
ThemechanismfortheMAperformancedepen-denceonthecompositionandstructureoftheGFisrevealed.
Thewell-matchedinterfacialimpedancecombinedwithhighlossabilitygivesrisetotheenhancedMAperformance.
2.
Experimental2.
1.
SynthesisofGFTherawmaterial,single-layergrapheneoxide(GO),waspre-paredusingamodiedHummersmethodasdescribedelsewhereandhasthelateralsizemainlyabove10mm[42].
TheinitialconcentratedGOethanolsolutionwasdilutedintothreeGOethanolreactionsolutionswithconcentrationsof0.
3,0.
6and0.
9mg/mL,respectively.
Aftersolvothermalreaction,solventex-changeandfreezedrying,threeoriginalGFswithdifferentgra-phenevolumefractionswereobtained.
TheGFsmadefrom0.
3to0.
9mg/mLGOethanolsolutionswereannealedat600Cfor1hinargonatramprateof10Cmin1toobtainthetargetGFslabeledasC0.
3andC0.
9,individually.
TheremainingGFsstartingfrom0.
6mg/mLGOsolutionweredividedintovesmallbatches,fourofwhichwereannealedatdifferenttemperaturesof200,400,600and800Cfor1hinargonatramprateof10Cmin1separatelytoobtainthetargetGFswithdifferentcompositions.
TheunannealedGFismarkedasT0andtheannealedproductsaremarkedasT200,T400,T600andT800inascendingtemperaturesequences.
Forconvenience,thesampleT600isalsonamedC0.
6intheGF'sstructurecomparison.
TofurtherstudytheGF'sMApropertydependenceonitsinterconnectedconductivenetworkofgraphenesheets,someofthesampleC0.
6wasbrokenintopowdersviamechanicalstirringat1800rpm.
2.
2.
CharacterizationTheRamanspectrumoftheGFwasobtainedonaRenishawinViaRamanspectrometerusinglaserexcitationat514.
5nm.
TheX-Raydiffraction(XRD)measurementoftheGFwascarriedoutonaRigakuD/Max-2500diffractometerwithCuKaradiation.
ThemorphologyoftheGFwasobservedbyScanningElectronMicro-scopy(SEM)(LEO1530VPoperatedat3.
0kV).
Theelectricalcon-ductivityoftheGFwasmeasuredbyahomemadextureaspreviouslyreported.
TheTransmissionelectronmicroscopy(TEM)investigationwasperformedonaFEITecnaiG2F20operatedat200kV.
ThethermogravimetricanalysiswasobtainedusingaNETZSCHSTA409PCanalyzer,withaheatingrateof10Cmin1fromroomtemperatureto850Cintheair.
TheX-rayPhotoelec-tronSpectroscopy(XPS)wasexaminedwithaGENESIS60SX-rayphotoelectronspectrometerusinganAlKa(hn1486.
6eV)radi-ationandthebindingenergieswerecalibratedbyusingthecontainmentcarbonpeak(C1s284.
6eV).
AfterallGFsweredriedinvaccumat75Cfor24h,theirFouriertransforminfrared(FT-IR)spectraandcombustionelementalanalysisweretheninvestigatedatTensor27FT-IRSpectrometer(Bruker,Germany)andVariomicroelementalanalyzer(Elementar,Germany),respectively.
BasedontheArchmethod,theMAperformancewasevaluatedin2e18GHzusinganAngilentHP8757Escalarquantitynetworkanalyzer.
FourGFscutinto90mm90mm10mmwerear-rangedintoacubiccontainerwithinternaldimensionsof180mm180mm15mmformeasurementsinthefrequencybandof2e18GHz.
ThepowdersamplewithamassequivalenttotheMAtestsampleC0.
6waslooselyplacedinthecontainerfortheMAmeasurement.
AllGFswerebackedwithahighlyconductivealuminumplatetoreecttheentireincidentmicrowavebacktothereceivingantenna.
Therelativecomplexpermittivityandpermeabilityweremeasuredinthefrequencyrangeof2e18GHzusinganAngilentHP8722ESvectornetworkanalyzer.
Parafnwasusedasthesup-portingmatrixduetoitsminorcomplexelectromagneticparam-etersapproximatingthoseofair.
Thetoroidaltestsample(3mmi.
d.
,7mmo.
d.
and2mmthickness)wasfabricatedbyvacuum-Y.
Zhangetal.
/Carbon105(2016)438e447439impregnatingtheGFwithparafn.
Theincidentmicrowavedirec-tionwasperpendiculartothetestsample.
3.
Resultsanddiscussion3.
1.
ThedependenceofMAperformanceontheGF'schemicalcompositionThemacroscopicadditive-freeGFforMAtestswaspreparedmainlythroughasolvothermalreaction,followedbysolventremovalandthermalreduction.
Byvaryingannealingtemperaturesinthethermalreductionfromroomtemperatureto800C,vetypesofpie-shapedGFsstartingfromthesameGOconcentrationof0.
6mg/mLwereobtainedtostudytheMAperformancedepen-denceonthechemicalcomposition.
Forconvenience,theunan-nealedGFislabeledasT0andtheotherannealedproductswerelabeledasT200,T400,T600andT800intemperaturesequences.
Duetothepyrolysisofthelabileoxygen-containinggroupsandthecarbondefects,theGFundergoesanobviousweightlossduringthethermalreduction.
AsshowninFig.
1a,theGF'sbulkdensityde-clinesbyover50%withtheannealingtemperaturerisingfromroomtemperatureto800C.
Giventhelowintrinsicdensityofgraphenesheets,theporosityhigherthan99%canbeobtainedforvetypesofGFs,exceedingthoseofmostmacroscopicfoams[29e31,43].
TounderstandchemicalcompositiontransformationoftheGFannealedatdifferenttemperatures,elementalanalysis,XPS,Ramanspectroscopyandthermogravimetricanalysiswereperformedforeverysample.
ElementalanalysisoffersthemostdirectevidencefortheelementcomponentoftheGFsample.
Freeofadditives,alltheGFsamplesaremainlycomposedofcarbonandoxygenassameastheinitialGO.
AsshowninFig.
1b,theunannealedGFT0hasthehighestoxygencontent,indicativeofitsmostseverelydamagedconjugatedcarbonbackbones.
Astheannealingtemperaturerises,thecarboncontentgrowscontinuously,whichisoppositefortheoxygencontentintheGFsample.
EvenfortheGFannealedat800C,theexistenceofoxygenmanifeststhatitservesasthetoughbondingelementinchemicallylinkingadjacentgraphenesheets,ensuringthethermallyreducedGF'srobustmechanicalstrength.
ItshouldbenotedthattheC/Oratioincreasingtrendclearlyaccel-eratesupontheannealingtemperaturerisingto400C,whichmayresultinanabruptchangeintheGF'sMAproperty.
TheFT-IRspectrafortheGFsampleswithvariouschemicalcompositionsareshowninFig.
1c.
Theabsorptionpeaksappearat1730,1570and1220cm1,correspondingtotheC]Ostretchingmode,C]CstretchingvibrationofbenzeneringandbreathingvibrationmodeoftheCeOgroups,respectivelyandthebroadpeakat3400cm1isattributedtotheadsorbedwaterinthesampleduetotheGF'shighporosity[42].
Withtheannealingtemperatureincreasing,theC]Ostretchingvibrationpeakgraduallyattenuatesandnearlyvanishesat800C.
Remarkably,theeverlastingCeObreathingvibrationpeak,ontheonehand,demonstratesthatthemutuallyentangledgraphenesheetsarechemicallyreinforcedbytheCeOeCcovalentbondanalogues.
Ontheotherhand,suchde-fects,consideredasthepolarizationdomains,contributetoimprovingthemicrowave-absorbingabilityoftheGF[11,12].
IntheRamanspectra(Fig.
1d),alltheGFsamplesdisplaytwoevidentpeaksat1351cm1and1588cm1,correspondingtotheDandGbandsrespectively.
TheID/IGratioisassociatedwithdefectconcentrationofgraphiticcarbonmaterials[48].
Itcanbeseenthatwithraisingannealingtemperature,theID/IGratioreducesdramaticallyfrom0.
95forT0to0.
75forT800,whichdemonstratestheenlargementofsp2carbondomain.
Morepreciseanalysesofin-planeconjugatedcarbonstructureswereconductedviaXPSchar-acterization(Fig.
S1).
Besidesthemainsp2carbonpeaklocatingat284.
5eV,thehigh-resolutionC1sregionoftheGFcanbedividedinto4otherttingpeaksat285.
8eV,286.
7eV,287.
7eVand288.
8eV,whichareattributedtocarbonspeciesofsp3carbon,CeOeC,C]OandC(C]O)O,individually[42].
Thesp2conjugatedcarbonskeletonaccountsforthevastmajorityofcarbonspeciesinalloftheGFsamples.
Furthersemi-quantitativeanalyses(Fig.
1e)Fig.
1.
(a)Thebulkdensities,(b)theelementalanalysis,(c)theFT-IRspectra,(d)theRamanspectra,(e)thesemi-quantitativesp2carbondomainsanalysesand(f)thether-mogravimetriccurvesfortheGFsannealedatdifferenttemperatures.
(Acolourversionofthisgurecanbeviewedonline.
)Y.
Zhangetal.
/Carbon105(2016)438e447440provethatwiththereductiontemperatureliftingto800Cthisproportionkeepsrisingfrom66.
7at%to80.
4at%intherangefrom200Cto400Cwhichobviouslygrowsfastest.
Thermogravimetricanalyses(Fig.
1f)wereperformedtoexaminethecompositionthermostabilityofvariousGFsamples.
BothT0andT200appearapparentweightlosswhenthetemper-atureapproaches200C.
However,whentheannealingtempera-turegoesabove400C,theresultingGFscouldmaintainstableinairatoperatingtemperatureofover300C.
Duetothehigherthermostabilityofsp2carbonbackbonesthanthoseofoxygen-containinggroups,theseresultssuggestmoreconjugatedcarbondomainsintheGFsannealedathighertemperature,whichareconsistentwiththeresultsofelementalanalysesandXPSanalyses.
Additionally,theexcellentthermostabilitymakessuchGFssuitableforapplicationsathightemperaturesuchastheskinsofhigh-speedaircraftsandhoodsofvehicleengines[50].
ThequaliedMAintensityformostapplicationsisgenerally10dB[15,26].
Fig.
2exhibitsthereectionloss(RL)curvesforGFsviadifferentthermaltreatmentsintherangeof2e18GHz.
WiththeC/Oratioaslowas5.
4andonly66.
7%sp2carboncontent,theoriginalunannealedGF(T0)exhibitsveryinferiormicrowave-absorbingabilityinthetestfrequencybandowningtoseveredisruptionofin-planeconjugatedstructures.
AlthoughtheC/OratioofT200increasesalittleafterthelow-levelthermalreduction,suchpoorrestorationofitsdamagedconjugatedgraphenenetworkdoesnotmakeitsRLcurvedifferobviouslyfromthatofT0.
Aftertheannealingtemperaturerisingto400C,thereappearsasignicantimprovementintheGF'sMAperformance.
ForT400,thestrongestRLreached28.
4dBat13.
9GHzandthequaliedfrequencyrangesfrom5.
6GHzto16.
9GHz.
ThephenomenondemonstratesthatwiththeC/Oratioandsp2carbondomainover6.
2and74%respectively,theGF'spartiallyrecovered3Dporousconductivenetworkofgraphenesheetsbecomessensitivetotheincidentmicrowave.
T600showstheoptimalMAperformance.
InadditiontothemaximumRLof34.
0dBat13.
1GHz,itsqualiedfrequencybandwidthreaches14.
3GHz,covering89.
4%oftheentiremeasuredbandwidth,whichismuchwiderthanthoseofmostMAmaterialsreportedpreviously[7,8].
Withannealingtemperaturearisingcontinuously,thechangeoftheGF'sMAperformancedoesn'tmaintainamonotonicevolution.
Onthecontrary,uponthetemperaturesurpassing600C,theGF'sMApropertystartsweakening,whichsuggeststhatwiththeC/Orisingmorethan12.
4,over-restoredconjugatedcarbonframeworkwillincreasethereectionofthemicrowaveandbecomesharmfulfortheGF'sMAproperty.
Therefore,thereexistsanoptimalreductiontemperature,suchas600Cinthiscase,underwhichtheannealedgrapheneskeletonwithcertainC/Oratioandconjugatedcarbondomainsizeexhibitsthebestwave-absorbingability.
3.
2.
ThedependenceofMAperformanceontheGF'sphysicalstructureThematerial'smicrowaveabsorbingpropertiesrelyonnotonlythechemicalcomposition,butalsothephysicalstructure,espe-ciallythebasicabsorbent'smorphologyandtheinterfacialmicro-structure[3,17,18].
AfacileandcontrollablemethodisdevelopedtofabricateGFswithcontrollablemicrostructure.
ViaadjustingGOconcentrationoftheinitialGOconcentrationfrom0.
3to0.
9mg/mL,threekindsofGFsannealedatthesametemperatureof600Cwereprepared,whicharemarkedasC0.
3,C0.
6andC0.
9inascendingGOconcentrationforconvenience.
Fig.
3ashowsthebulkdensitiesofthreeGFswithdifferentphysicalstructures.
WiththeoriginalGOethanolsolutiongettingdenser,theresultingGF'sdensityrisesfrom0.
9to2.
4mg/cm3.
ThehighergraphenecontentresultsintheweakerGF'sliquidabsorp-tioncapability(Fig.
3b),whichisadirectreectionofthedecreaseintheGF'sporosity[38,42].
However,similartothoseofourotherGFs,thecalculatedgraphenevolumefractionforallthethreeGFsamplesstillkeepsbelow1%,whichtosomeextentcanberegardedasastableindividualcharacteristicoftheadditive-free3Dcross-linkednetworkofgraphenesheets.
Fig.
4givestheSEMimagesoftheGFsmadefromtheGOso-lutionswithdifferentinitialconcentration.
Ascanbeseen,thestartingGOconcentrationhasagreatimpactontheinternalporousmorphologyandcellsizeoftheresultingGF.
WhenthestartingsolutioncontainsveryfewGOsheets(0.
3mg/mL),theresultingGFshowsareticulum-likeopencellstructurewiththeporesizerangingfrom30to90mm,inwhichmostgraphenesheetsconcentrateontheedgeandformthestrutswithmanyenclosedwallsincompletely.
UponraisingtheGOconcentrationofthestartingsolutionto0.
6mg/ml,theGFwithnegligiblechangeinitsinternalporesize,experiencesanenormousmorphologicalevo-lutionfromtheopenreticularstructuretothesemi-closedcellularonewithmuchmorecompletecellwalls.
AstheinitialGOcon-centrationkeepsrisingto0.
9mg/mL,theGFinternalporesbeginshrinkingdistinctly,Itisworthnotingthatalthoughitslong-rangenetworkhasbeenbroken,theC0.
6powderstillpresentsanintri-cate3Dinterconnectedgraphenenetworkwithtremendousmicro-sizedpolygonporeslikeotherintactGFs,whichfurtherdemon-stratestherobustmechanicalpropertyofthe3Dmonolithicstructure.
Aswehavepointedout,theinterfacialmicrostructureoftheabsorbentwillexertsignicantimpactonthemicrowavelossypropagationintheMAmaterial[19,32,48].
Therefore,wefurtherinvestigatetheinuenceofgraphenecontentontheGF0cellwallviaTEMcharacterization(Fig.
5aec).
C0.
3'scellwalliscomprisedof1e5layergraphenesheetswithpoorrestacking,whichshouldbemostbenecialtothetransmissionoftheincidentmicrowavethroughthewholefoam.
WiththeincreasingoftheGOcontentinthestartingsolution,theGF'scellwallsgraduallybecomethicker.
UponraisingGOconcentrationto0.
9mg/mL,theGFevenpresentssomeinternalcellwallsconsistingofashighas8e14layergra-phenesheets,overtwicethethicknessoftheGFwiththelowestinitialGOcontent.
TofurthercharacterizeinternalstructureoftheGF,theX-raydiffraction(XRD)examinationofvariousGFsmadefromdifferentGOsolutionswereperformedwithakegraphiteasthecompari-son(Fig.
5d).
Differentfromthestrongsharppeakforgraphiteat24681012141618-40-35-30-25-20-15-10-50ReflectionLoss(dB)Frequency(GHz)T0T200T400T600T800Fig.
2.
TheRLcurvesfortheGFswithdifferentchemicalcompositionsin2e18GHz.
(Acolourversionofthisgurecanbeviewedonline.
)Y.
Zhangetal.
/Carbon105(2016)438e4474412q26.
5,alltheGFsexhibitratherbroadfeeblepeaks(002),indicatingthatthelong-rangerestackingofgraphenesheetsisveryweak.
NotethatastheinitialGOconcentrationmovesup,addi-tionalwideweakpeaksat25.
0and25.
9emergeforC0.
6andC0.
9,respectively.
TheresultprovesthatC0.
6andC0.
9,withmoregrapheneconstituent,possessmorerestackingstructuresthanC0.
3,whichisconsistentwiththeTEMcharacterization.
TheRLcurvesforalltheGFsamplesin2e18GHzareshowninFig.
6.
TheGFmadefromthe0.
3mg/mLGOsolutiondisplaystheweakestwave-absorbingabilitywiththemaximumabsorptionintensityof15.
0dBat18.
0GHzandthequaliedbandwidthof6.
0GHz.
WiththeinitialGOincreasingfrom0.
3mg/cm3to0.
6mg/cm3,amaximumRLof34.
0dBisachievedat13.
1GHz.
Moresignicantly,thequaliedfrequencybandwidth(14.
3GHz)improveddramatically,coveringmostoftheentiremeasuredbandwidth.
WiththefurtherincreasingoftheconcentrationofGOsolution,theGFexhibitsareducedmicrowaveabsorbingability.
TheresultindicatesthatthereexistsanoptimalinitialGOcon-centrationthatmakestheself-assembledGFpossessthebestMAperformanceandthisvalueismostapproximateto0.
6mg/mLinourcase.
ItshouldbenotedthatthevarianceforalltheGFsamplesingraphenevolumefractiondoesnotsurpass1%,whichdemon-stratesthattheGF'sMAperformanceismuchsensitivelyaffectedbyitsphysicalstructure.
Furthermore,comparedtootherunbrokenGFs,theGFpowderexhibitaverypoorMAperformancewiththenarrowqualiedbandwidthof1.
4GHzandlowoptimalabsorptionstrengthof11.
5dB.
Theexperimentalresultindicatesthatthehighlyintricateinterconnectedlong-rangeconductivenetworkisimperativetotheexcellentMApropertyoftheGF.
3.
3.
ComparisonoftheGF'scomprehensiveMAperformancewithotherMAmaterialsThedetailedMAperformanceofsomerepresentativeMAma-terialsreportedbeforeislistedinTableS1forcomparison.
ItcanbeseenthattheoptimizedGFexhibitsmuchmoreexcellentMAperformancebothinthemaximumabsorbingintensityandinthequaliedbandwidththanmostMAmaterials,includingtheMAfoams.
Becauseofthegreatimportanceoftheabsorbingintensitythroughoutquiteabroadfrequencyrangeinthebroadbandhigh-performanceMAresearch,it'snecessarytointroduceanewconceptoftheaverageabsorptionintensity(AAI)whichisanaloguetoExternalQuantumEfciencyinthephotovoltaiceld.
TheAAIisexpressedasAAIdBZfhflRLdffhfl(1)whereflandfhrespondtothelowestmeasuredfrequencyandthehighestmeasuredfrequency,respectively.
ThehighertheAAIvalueis,thestrongerbroadbandmicrowaveabsorbingabilitythemate-rialpresents.
TheAAIin2e18GHzofseveralrepresentativeMAmaterialsinpreviousliteraturewerecalculatedinFig.
7a.
Owningtothestrongmicrowaveabsorbingbehaviorinthewidefrequencyband,theGFshowsaveryexcellentAAI(17.
9dB),whichishigherthanthebestavailableMAmaterialsreportedpreviously.
Furthermore,lightweightaccountsforagiantproportionindesigningandevaluatingmicrowaveattenuationmaterialsappliedinaerospace,aviationandgroundvehicles[9,35].
Therefore,weadoptthespecicMAefciency(SMAE)integratedwithsignicantMAindicatorssuchasthickness,density,qualiedbandwidthandRLvaluesinthemeasuredfrequencybandtoevaluatethemicro-waveabsorption(MA)performancemorecomprehensively[32]TheSMAEisexpressedasSMAEdB$Hz$cm2$g1ZfhflRLdft$rbulk(2)wherefl,fh,tandrbulkrespondtothelowestqualiedfrequency,thehighestqualiedfrequency,theaveragethicknessandtheaveragebulkdensityoftheMAmaterial,respectively.
AmaterialwithhighSMAEisanticipatedtohaveahugepotentialinMAapplication.
ThespecicMAefciencyin2e18GHzofseveralrepresentativeMAmaterialsinopenliteraturewerecalculatedinFig.
7b.
Despiteofthelowbulkdensity,thespecicMAefciencyofthecarbonfoam[7]andSiCfoam[37]don'tgoover100dBHzcm2g1,whichismuchinferiortosometypicalsolidMAmaterial,suchastheSWNT/polyurethanecomposite(~1.
5102dBcm2g1)[8],thereducedGO/NBRcomposite(~2.
2102dBHzcm2g1)[23]andthea-Feencapsulatedwithincarbonnanotube/epoxycomposite(~2.
0103dBHzcm2g1)[15].
Remarkably,theexcellentMAperformancecombinedwiththeultralowdensity(1.
6mgcm3)givestheoptimizedGFasuperiorspecicMAefciencyaround1.
7105dBcm2g1,nearlytwoordersofmagnitudehigherthanthoseofthebestavailableMAmaterialsreportedbefore.
C0.
3C0.
6C0.
90.
81.
21.
62.
02.
4BulkDensity(mg/cm3)GFSamplesBulkDensityC0.
3C0.
6C0.
9520560600640680WeightGainGFSamplesPetroleumetherEthanolbaFig.
3.
(a)Thebulkdensityand(b)theliquidabsorptioncapabilitiesoftheGFs.
(Acolourversionofthisgurecanbeviewedonline.
)Y.
Zhangetal.
/Carbon105(2016)438e4474423.
4.
ThemechanismfortheMAperformancedependenceonthecompositionandstructureoftheGFBasedonthegeneralMAprinciple,theinterfacialimpedancegapandradiationenergylossratioareregardedasthetwocriticalfactorsthatdeterminethematerial'sMAperformance[8,18,37,52].
Toatypicalsingle-layerdielectricMAmaterial,suchasgraphene/polymercomposites,theincidentwavepropagationfromfreespaceintothematerialismainlyaffectedbymaterial'sdielectricpermittivityεandelectricalconductivityd[4,37,39,53].
TobetterunderstandtheMAperformancedependenceontheircompositionandmorphology,weinvestigatedvariousGFs'bulkelectricalcon-ductivitiesandrelativepermittivitiesin2e18GHz.
TheelectricalconductivitiesforalltheGFsareshowninFig.
S2.
ItcanbeseenthattheriseineitherofthegraphenevolumefractionandtheannealingtemperaturewillresultinthebulkelectricalconductivityincreaseoftheGF.
Thelong-rangeinducedcurrentsdecayontheconductiveskeletonplaysakeyroleinthesubstan-tiallyenhancedMA.
TheimprovedconductivegrapheneframeworkoftheGFshouldcouplewithmoretime-varyingelectromagneticeldswithinbroaderfrequencybandsandthusconsumemoreradiationenergy[32,37].
It'sworthnotingthatevenforC0.
9andT800,theirbulkconductivitiesstillstaybelow1.
0104S/m,whichcontributestothegoodinterfacialimpedancematchingandthusweakeningadversebackreectionofmicrowave[9,22].
Therealpermittivityε0respondstothestoragecapabilityoftheFig.
4.
Thecross-sectionalSEMimagesof(a,b)C0.
3,(c,d)C0.
6,(e,f)C0.
9,(g,h)C0.
6powders.
Y.
Zhangetal.
/Carbon105(2016)438e447443electriceldinsidetheabsorber[10,54],andasmallerε0shortenstheinterfacialimpedancegap,thusdecreasingthereectionco-efcientoftheabsorber[55].
Inaddition,thedielectriclosstangenttandeε00=ε0representsthematerial'sconversionofthemicro-waveradiationintootherenergyforms[8,10].
Thehigherdielectriclosstangent,themoreelectromagneticwaveenergygetsabsorbed[9,11].
Theinclinationtoanysidewouldn'tgiverisetoagoodMAbehavior[10,35].
Fig.
8givesrealpermittivitiesandrelativedielectriclosstangentvaluesforalltheGFs.
BothT0(Fig.
8a)andC0.
3(Fig.
8b)showverylowrealpermittivitiesanddielectriclosstangents,whichareevenbelowthoseofsinglelow-losspolymerssuchaspoly(dimethylsiloxane)[51]andpoly(ethyleneoxide)[56].
It'sclearlyseenthatboththerealpermittivityanddielectriclosstangentoftheGFgrowbiggermonotonicallywitheitherthegra-phenevolumefractionorreductiontemperatureincreasing.
Theresult,however,isnotconsistentwiththeMAperformancechange,whichdemonstratestheimportanceofthebalancebetweenthelowpermittivityandhighdielectriclosstoasatisfyingmicrowave-absorbingability.
InallGFsamples,theT800andC0.
9,thoughpossessingthestrongenergyconversionabilities,exhibitoverhighrealpermittivityvalues,whichgoesagainstthewell-matchedimpedancebridgeandthusresultsinanattenuatedMApropertyoftheGF.
Bycontrast,overlowdielectriclosstangentsoftheGFsincludingC0.
3,T0andT200duetosparseconductivenetworksandexcessivedefectsofgraphenesheets,revealtheirpoormicrowavedissipationperformanceduringthewavepropagationfrompene-tratingthefoamtoreturningtothereceivingantenna.
TheGFT600orC0.
6,withproperpermittivityandhighdielectriclosstangent,successfullybuildawinewinrelationshipbetweentheinterfacialmatchingandlossproperty,thusexhibitingthebestMAperfor-mance.
Itshouldbenotedthatdespiteofultralowrealpermittivity,theinferiordielectriclosstangentoftheC0.
6powdersuggeststhatboththeultrahighporosityandtremendouslong-rangecross-linkedconductivegraphenenetworkareimperativetotheexcel-lentMAperformanceoftheGF.
ToreecttheintrinsicMAmechanismmoredirectly,wethensimulatedtheRLcurvesforalltheGFs.
Accordingtothetransmission-linetheory,theRLofelectromagneticwaveradiation,R(dB),undernormalwaveincidenceonametal-backedsingle-layerMAmaterialiscorrelatedwiththeincidentimpedanceZinas[8,56].
RdB20logZinZ0ZinZ0(3)whereZ0isthecharacteristicimpedanceoffreespace(377U).
ZinistheinputimpedanceattheinterfaceoffreespaceandtheMAFig.
5.
TheTEMimagesofthebrokenGFcellwallof(a)C0.
3,(b)C0.
6and(c)C0.
9aftersonication.
(d)TheXRDresultsoftheGFsmadefromdifferentinitialGOsolutionsandakegraphiteforcomparison.
TheinsetgivesdetailedpatternsofthreeGFsamples.
(Acolourversionofthisgurecanbeviewedonline.
)24681012141618-40-35-30-25-20-15-10-505ReflectionLoss(dB)Frequency(GHz)C0.
9C0.
6C0.
3C0.
6PowderFig.
6.
TheRLcurvesfortheGFswithdifferentphysicalstructuresareshownin2e18GHz.
(Acolourversionofthisgurecanbeviewedonline.
)Y.
Zhangetal.
/Carbon105(2016)438e447444material,givenas[20,35].
ZinZ0mrεrrtanh2pfdmrm0εrε0pj(4)wherefisthemicrowavefrequency,disthematerialthickness,εristherelativepermittivityandmristherelativepermeability.
ThesimulatedMAcurvesin2e18GHzforalltheGFsareshowninFig.
S3.
ThetheoreticalMAcurvesagreewellwiththeexperi-mentalresultsintheaspectoftheMAperformancetrendwithinitialGOconcentrationandthermalreductiontemperature.
Forexample,thecalculatedmicrowaveabsorbingpropertyoftheGFundergoesthesamerstrisingandthenfallingchangewiththeannealingtemperatureelevatingfromroomtemperatureto800C.
However,thetheoreticalcurvedoesnottexactlyonthemeasuredcurveintheabsorptionperformanceandsuchdiscrepancycouldbeattributedtohighlynonuniformporousstructureinsidetheGF[57]andtheedgeeffectoftheGFjunctionintheMAtest[21].
4.
ConclusionInsummary,wehavepreparedseriesofGFswithvariouschemicalcompositionsandphysicalstructuresbycontrollingtheGOconcentrationoftheinitialsolutionandthermal-reductiontemperature.
TheanalysesoftheGFs'compositions,structuresandelectromagneticpropertiessuggestthattheMAperformanceoftheGFisstronglycorrelatedwiththeC/Oratio,conjugatedcarboncontentandthegrapheneskeletonmicrostructure.
Amaximumabsorptionvalueof34.
0dBaswellas14.
3GHzFig.
7.
Comparisonof(a)AAIand(b)SMAEvaluesfortheGF(lightgreypatternedcolumn)inthisworkandtherepresentativematerials(darkgreycolumns).
MoredetaileddatahavebeenlistedinTableS1intheSupportingInformation.
(Acolourversionofthisgurecanbeviewedonline.
)Fig.
8.
Realpartsofthecomplexpermittivitiesof(a)theGFswithdifferentchemicalcompositionsand(b)theGFswithdifferentphysicalstructuresin2e18GHz.
DielectriclosstangentsoftheGFsof(c)theGFswithdifferentchemicalcompositionsand(d)theGFswithdifferentphysicalstructuresin2e18GHz.
(Acolourversionofthisgurecanbeviewedonline.
)Y.
Zhangetal.
/Carbon105(2016)438e447445qualiedbandwidthcanbeachievedfortheGFwithanultralowbulkdensityof1.
6mg/cm3,whichisclosetothedensityofambientair(1.
2mg/cm3).
Particularly,theGFpresentthebestaverageab-sorptionintensitycomparedwithothertypicalMAmaterialsin2e18GHz.
TheoutstandingMAperformancecombinedwithanultralowbulkdensitygivestheGFasuperiorspecicMAefciencynearlytwoordersofmagnitudehigherthanthoseofthebestavailableMAmaterialsreportedbefore.
ThemechanismfortheMAperformancedependenceonthecompositionandstructurerevealsthattheGFwithproperchemicalcompositionandphysicalstruc-turemakingthebalancebetweenexcellentimpedancematchingandhighlosscharacteristiccoulddeliveranexcellentMAproperty.
WiththefacileandcontrollablesynthesisoftheGF,anewapproachtoefcientlyoptimizeandregulateitsMAbehaviorisnowpossible.
Moreimportantly,theprocessingecomposition&structur-eepropertyrelationshipsoftheGFhasopenedupanewstrategytodesignrationallymacroscopicporousmaterialsforlightweighthigh-performanceandbroadbandMAapplicationsuchasthenew-generationultralightheat-resistantMAskinforhigh-speedaircrafts.
AcknowledgmentsTheauthorsgratefullyacknowledgenancialsupportfromtheMOST(Grants2012CB933401),NSFC(Grants21374050,91433101,51472124and51273093),MOE(B12015),PCSIRT(IRT1257)andNSFofTianjinCity(Grant15JCYBJC17700).
AppendixA.
SupplementarydataSupplementarydatarelatedtothisarticlecanbefoundathttp://dx.
doi.
org/10.
1016/j.
carbon.
2016.
04.
070.
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