聚酰亚胺辅助制备高定向石墨烯基全炭泡沫及其在导热聚合物复合材料中的应用.pdf

Cite this:NewCarbonMaterials,2024,39(2):271-282DOI:10.1016/S1872-5805(24)60835-8Polyimide-assisted fabrication of highly oriented graphene-basedall-carbon foams for increasing the thermal conductivityof polymer compositesXIONGKe1,SUNZhi-peng1,HUJi-chen1,MACheng1,WANGJi-tong1,*,GEXiang2,QIAOWen-ming1,*,LINGLi-cheng1（1.State Key Laboratory of Chemical Engineering,East China University of Science and Technology,Shanghai 200237,China;2.Changzhou Fuxi Technology Co,Ltd.,Changzhou 213144,China）Abstract：Grapheneanditsderivativesareoftenpreferentiallyorientedhorizontallyduringprocessingbecauseoftheirtwo-di-mensional(2D)layerstructure.Asaresult,thermalinterfacematerials(TIMs)composedofapolymermatrixandgraphene-derivedfillersoftenhaveahighin-plane(IP)thermalconductivity(K),however,thelowthrough-plane(TP)Kmakesthemunsuitableforpracticaluse.Wereportthedevelopmentofhigh-qualitypolyimide/graphitenanosheets(PG)perpendiculartotheplaneusingadir-ectionalfreezingtechniquethatincreasetheTPKofpolymer-basedcomposites.Graphene-derivednanosheets(GNs)wereobtainedbythecrushingofscrapsofhighlythermallyconductivegraphenefilms.Awater-solublepolyamicacidsaltsolutionwasusedtodis-persethehydrophobicGNsfillertoachievedirectionalfreezing.Thepolyimide,whichfacilitatedthedirectionalalignmentoftheGNs,wasthengraphitized.TheintroductionoftheGNsincreasestheorderanddensityofthePG,thusimprovingthestrengthandheattransferperformanceofitspolydimethylsiloxane(PDMS)composite.TheobtainedPG/PDMScomposite(21.1%PG,massfrac-tion)hasanimpressiveTPKof14.56Wm1K1,81timesthatofpurePDMS.Thissimplepolyimide-assisted2DhydrophobicfillersalignmentmethodprovidesideasforthewidespreadfabricationofanisotropicTIMsandenablesthereuseofscrapsofgraphenefilms.Key words:Graphenefilm；Reutilization；Thermalconductivity；Anisotropicfoam；Thermalinterfacematerials1IntroductionTheperformanceofintegratedcircuitsoftende-gradessignificantlyorfailstooperateduetooverheat-ing.Consequently,effectivethermalmanagementma-terials are vital for the optimal operation of high-power electronic devices13.Polymer-based thermalinterfacematerials(TIMs)arewidelyinvestigatedfortheheatdissipationofelectronicdevicesduetotheirmultiplemeritsoflightweight,chemicalstability,easyprocessability,and cost-effectiveness46.However,polymersgenerallyexhibitlowthermalconductivity(K)(0.5 Wm1K1)due to the disordered struc-tures and entanglements which cause considerablephononscattering78.ToimprovetheKofcompos-ites,thermallyconductivefillersareoftencompoun-dedwithpolymersforthegoalofincreasingthephon-ontransportpath9.Grapheneanditsderivativesareconsideredtobethemostpromisingcandidatesforthermalfillersbe-causeoftheirhighin-plane(IP)K1014.Linetal.ob-tainedpolymer/graphenecompositeswithKof1.431.84Wm1K1byfirstcoatingthepolymerpowderwith graphene and then hot pressing,in which thegraphenecontentwas10%(massfraction)15.Fangetal.preparedpolyvinylbutyral(PVB)/graphenecom-positesbysolutionblending,andtheKofthesamplereached 4.52 Wm1K1 with 30%(mass fraction)graphenenanoplates16.Nevertheless,evenwithhighgrapheneconcentration,theKofcompositesformedbydirectlymixinggraphenesheetswithpolymermat-Received date：2023-09-17；Revised date：2023-12-19Corresponding author：WANGJi-tong,Professor.E-mail:;QIAOWen-ming,Professor.E-mail:Author introduction：XIONGKe.E-mail:Supplementarydataassociatedwiththisarticlecanbefoundintheonlineversion.Homepage:http:/ did not rise as significantly as predicted.It ismostly owing to the disconnected phonon transportchannelbetweenthefillerandpolymermatrixandthelargefiller-fillerinterfacialthermalresistance,whichcanbemitigatedbypre-establishingthree-dimension-al(3D)fillernetworksinthepolymermatrix1722.Baietal.fabricatedgraphenefoamswith3Dinterconnec-tednetworksbychemicalvapordeposition,andtheKof its polydimethylsiloxane(PDMS)composite was1.2 times higher than that of non-3D structuredgraphenesheetswiththesamemassloading23.Thesuccessivefillernetworksactashigh-speedchannels,thuseffectivelyreducingphononscatteringwhileen-suringthatmostoftheheatcanbetransportedthroughthe fillers24.However,current polymer/graphenecompositestypicallyexhibitheattransferisotropy,orhighIPK,duetothetendencyofgraphenetoorient-ate horizontally during processing2526.Notably,TIMs are also expected to have high through-plane(TP)K,to minimize the temperature differencebetweentheheatspreadersandtheelectronicsinprac-ticalapplications5,2728.Toachievethisgoal,design-ing anisotropic polymer/graphene composites withhighTPKisofgreatimportance.Directionalfreezingisusedextensivelytovertic-allyalignthefillerswithahighaspectratiooftwo-di-mensional(2D)lamellarstructures,whichobtainananisotropic 3D porous skeleton after freeze-drying,andthecorrespondingpolymerbackfilledcompositesexhibit heat transfer anisotropy4,2931.Wong et al.constructedaligned3Dporousnetworksofboronni-tride/reducedgrapheneoxidebydirectionalfreezingstrategy,whicheffectivelyenhancedtheTPKofnat-uralrubbercomposite30.DirectionalfreezingleadstoanincreaseintheTPKofTIMsbyalteringtheorient-ationofthe2Dfillerinthe3Dthermallyconductivenetworks.Sincetheprocessdependsonthedirection-algrowthoficecrystals,directionalfreezingismostlyadoptedforwetgraphenesystemssuchasgrapheneoxide aqueous solutions or graphene hydrogels29,32.However,finding ways to directly introduce hydro-phobic2Dhigh-qualityfillersintoaqueoussystems,andhencedirectionalfreezingisworthinvestigating.Herein,wedevelopverticallyaligned3Dporouspolyimide/graphitenanosheets(PG)thermally con-ductiveframeworksbydirectionalfreezingandhigh-temperature thermal annealing.On one hand,thegraphene-derived graphite nanosheets(GNs)arepowderscrapsofhighthermallyconductivegraphenefilms,retainingthehighinherentK.IntroducingGNsintoPGfoamsnotonlyrealizesthesecondaryutiliza-tion of the trimmings but also provides morethermallyconductive pathways,thus effectively in-creasingtheKofthepolymer.Ontheotherhand,thewater-solublepolyamicacidsalt(WPAA)solutionisutilizedforthefirsttimetodirectlydispersehydro-phobicGNsfillerswhileenablingthemtoengageinicetemplateassemblyinthewetsystem.Thedisper-sion process is facile without using surfactants ormodifying the GNs.Moreover,the polyamic acid(PAA)hasassistedintheverticalorientationoftheGNs,whilesimultaneouslyactingasthecarbonpre-cursortobeconvertedintohighlythermallyconduct-ivefilleraftergraphitization.Thus,thePGthermallyconductive frameworks entirely consist of verticallyalignedorderedgraphiteskeletons,includingGNsandgraphitized polyimide(PI),and the high-qualitygraphitestructuresfacilitatetheheattransfer.Notably,aftervacuumimpregnationwithPDMS,theTPKoftheobtainedPDMS/PGcompositesissignificantlyin-creased.At21.1%(massfraction)PGcontent,theTPKofPDMS/PGisupto14.56Wm1K1,which81timeshigherthanthatofpurePDMS,withathermallyconductiveenhancementefficiency()of378%.Fur-thermore,theexcellentcompressivepropertiesofPDMS/PGensure a close fit to the solid surface,thus en-ablingtheeffectivefillingofgapsatthesolidinter-face.2Experimental 2.1 MaterialsTheGNswereobtainedbymechanicallyandair-flowcrushingthehighthermallyconductivegraphenefilmscraps,withanaveragelateralsizeoflessthan20m(Fig.S1),whichweremanufacturedbyChang-272新型炭材料（中英文）第39卷zhou Fuxi Tech Co.,Ltd.N,N-dimethylacetamide(DMAc,99%),triethylamine(TEA,99%),andpyro-mellitic dianhydride(PMDA,98.0%)were obtainedfromShanghaiTitanTechCo.,Ltd.4,4-diaminodi-phenyl ether(ODA,98.0%)was purchased fromShanghaiDiboChemicalTechCo.,Ltd.PDMS(Syl-gard184)withcuringagentwaspurchasedfromDowCorning.2.2 Preparation of PDMS/PG compositesFig.1depictsaschematicdiagramoftheprepar-ationofPDMS/PGcomposites.Itismainlydividedinto3parts:(1)fabricationofhighlyanisotropicpor-ousPAA/GNsfoamsbydirectedfreezingapproach;(2)high-temperaturethermalannealingofPAA/GNstoformhigh-qualityall-carbonPGthermallyconduct-iveframeworks;(3)PDMSbackfilledwithPGfoamstoconstructthethermallyconductivePDMS/PGcom-posites.TheWPAA solutions were obtained by refer-ence to the previously reported methods3334.First,PAA solid was prepared.ODA was mixed withDMAcinanicebathunderanitrogenatmosphere.Aftersoliddissolution,PMDAwasaddedtothesolu-tionandstirredfor1htoobtainthePAAsolution.ThenthePAAsolutionwasaddedtoexcessivedeion-izedwaterandtheresultingprecipitateswerefreeze-driedtoobtainPAAsolid.Second,theWPAAsolu-tionwasprepared.Typically,5gPAAwasaddedto92.6gdeionizedwater,then2.4gTEAwasaddedandstirredcontinuouslyfor4hatroomtemperature(RT)toobtainaWPAAsolutionwithamassfractionof 5%.Similarly,further 7.5%(mass fraction)and10%(massfraction)of WPAA solutions were pre-pared,respectively.Third,PAAx/GNsy foams werefabricated.Specifically,GNs were added to theWPAAsolution,mixedandstirredfor6h,andthenvacuumdefoamed for 0.5 h.The mixture was sub-sequently placed in a silicone mold for directionalfreezing.This procedure used copper plates as thecoldsourceandliquidnitrogenastherefrigerant.Thefrozenblocksweredriedinafreezedryer(60C,5 Pa)for 72 h to obtain the ordered PAAx/GNsyfoams.Where x is the mass fraction of the WPAAsolution(%)and y is the mass ratio of GNs to theWPAAsolution(%).Finally,the PAAx/GNsy foam was imidized(300C,3h)andfurtherthermallyannealed(1000C,2h;2000C,1h;2800C,0.5h)togivegraphitedPIx/GNsy(PxGy)block.In addition,the mixture wasalsopre-frozeninarefrigerator(4C)for12htoob-tain disordered PxGy(dPxGy)foams by the samemethod.PDMSandcuringagentweremixedinthemassratio of 101 and stirred uniformly.The PxGy orOOOOH2NH2ONH2NOHOOCHOOCOOOOOOOOOR3+NOOCR3+NOOCOOCN+R3NHR=CH2CH3OOOOOOOOLyophilizationThermal annealinglmpregnationPDMSImidizationDirectional freezingONPIIn-planeThrough-planeNHNNHnnnHNCOOHOOOPMDAPAAWPAAODAFig.1SchematicillustrationforthefabricationofPDMS/PGcomposite第2期XIONGKeetal:Polyimide-assistedfabricationofhighlyorientedgraphene-basedall-carbon273dPxGyfoamswereimmersedintheabovesolutionandimpregnatedunderavacuumfor3h.Thentheblocksweretransferredtoanovenat80Candcuredfor12htoobtainPDMS/PxGyorPDMS/dPxGycompos-ites.2.3 CharacterizationsField emission scanning electron microscopy(FE-SEM,Nova Nano SEM 450)was employed toobserve the micromorphology of the foams and thecomposites.TheporosityofPGfoamsiscalculatedfrom the density and the true density of graphite(2.26 gcm3)27,35.The X-ray diffractometer(XRD,BrukerD8Advance)wasappliedtoobtainX-raydif-fraction patterns of the samples,using Cu K radi-ation(=1.540).Ramanmappingimageswerere-cordedusingaRenishawinViaRamanmicroscopesystem(532 nm laser source)with a scan area of40m40minstepsof1m.Thermogravimet-ricanalysis(TGA,TAQ600)wasusedforthethermalstability analysis of samples with a heating rate of10Cmin1.Thethermaldiffusivity()ofthePDMScom-posite was measured by a Nano Flash instrument(NetzschLFA427)andtheKwascalculatedaccord-ingtothefollowingequation:K=Cp(1)whereisthedensity,andCpisthespecificheatca-pacity,CpwasalsomeasuredbyNetzschLFA427.Thesamplesforthermaldiffusivitytestinghaveadia-meterof12.6mmandathicknessofaround2.5mm.Themechanicalpropertiesofblocksamples(14mm14mm5mm)weremeasuredonanelectronicuni-versaltestingmachine(Instron3367)atacompres-sionrateof1mmmin1.Athermalinfraredimager(E4,FLIR)was employed to characterize the heattransferperformanceofthecomposites.3Resultsanddiscussion 3.1 Dispersibility of GNs in the WPAA solutionToaccomplishthedirectedfreezingtechnique,ithasbeenestablishedthattheWPAAsolutionmaybeappliedtodirectlydispersehydrophobicGNs.Theini-tialcontactanglesofGNswithdeionizedwaterandWPAAweremeasured,respectively(Fig.2a,b).Thecontact angle between deionized water and GNs is95.95,indicating the hydrophobicity of GNs26,36.WhereasthecontactanglebetweentheWPAAsolu-tionandGNsis78.84,demonstratingthatGNscouldbeinfiltratedbyWPAA.AsseeninFig.2c,attheini-tial stage,GNs are partially dispersed in deionizedwaterwhileuniformlydispersedintheWPAAsolu-tion.After3hofstanding,theGNs/H2OsuspensionshowsobviousseparationandmostoftheGNsprecip-itatesatthebottomofthebottle.ItresultsfromthehydrophobicityoftheGNsmakingthemsubjecttotherepulsive force of water molecules.In contrast,theGNs/WPAAsuspensionisstillwelldispersedwithoutanyseparationorprecipitation(Fig.2d).Itispossiblydueto the-interactions of the linear PAA mo-leculeswiththeGNsthatcontributetotheuniformdispersionoftheGNsintheaqueoussolution.Thus,theWPAA/GNssuspensiondoesnotsuffersignific-ant precipitation or separation during the brief pre-freezing process,allowing the PAA/GNs foam tomaintainintegrityandconsistency.3.2 Morphologic and structural evolution of PGfoamTheWPAA/GNswetdispersionsystemisplacedinadirectionalfreezingunit,wherethedirectionalar-rayofPAA(Fig.S2)inducesverticalalignmentofGNs.As shown in Fig.3a-c,typical PAA10/GNs2.5foam exhibits a highly anisotropic structure.In theverticaldirection,small-sizedGNsareembeddedinthePAAcellwallanduniformlydistributedalongthe(a)(c)(d)(b)GNs/H2OGNs/H2OGNs/WPAAGNs/H2OGNs/WPAAGNs/WPAA3 h0 h95.9578.84Fig.2TheinitialcontactangleofGNswith(a)H2Oand(b)WPAAsolu-tion.ThesuspensionsoftheGNs/H2OandGNs/WPAA(c)beforeand(d)afterstayfor3h274新型炭材料（中英文）第39卷wallextensiondirection.Incontrast,thecross-section-alviewsoftheundirectedfrozendPAA10/GNs2.5foamdisplayarandomlyarrangedstructure(Fig.S3),lead-ing to an increased interfacial thermal resistancebetweenthefillers17.High-temperaturethermalannealingisessentialtoenhancethequalityofthermallyconductiveskelet-ons3738.In this regard,the PAA/GNs blocks weretransformed into oriented graphite frameworks bythermaltreatments including imidization,carboniza-tion and graphitization.The temperature-dependentevolutionofthecross-sectionalmicromorphologyoftypicalPAA10/GNs2.5foamsismonitoredinFig.3andFig.S4.With increasing annealing temperature insteps,thecross-sectionalorientedporousstructureoftheP10G2.5foamremainscompletewithoutsignificantdamageorcollapse.Inaddition,thecellwallspacingshortens due to the shrinkage of the foam after theheattreatment(Fig.3c,f).ThevolumeshrinkageofP10G2.5foamis30.06%and61.54%afterimidizationat300Candcarbonizationat1000C,respectively.However,thischangeisinsignificantabove1000C,whichisconsistentwiththedigitalpictureofP10G2.5foams(Fig.3g).ThevolumeshrinkageaswellasthedensityoftheP10G2.5foamsateachtemperaturestagearedetailedinTableS1.ComparedtoPAA10/GNs2.5(0.22 gcm3),the P10G2.5 foamstill retains a relat-ivelyhighdensity(0.24gcm3)afterthermalanneal-ingduetoshrinkage,andthedensestructurecorres-pondstoahigherheattransferefficiency27.Inaddi-tion,afterhigh-temperaturethermalannealing,thePGfoamsallexhibitmoderatecompressivestrength(0.050.75MPa)andhighporosity(87.6%96.7%)(Ta-bleS2),whichallowsthemtoadequatelybackfillthePDMSwithoutstructuralcollapse(Fig.3g).TheeffectofthermalannealingtemperatureonthestructureofPGfoamswasinvestigatedbyXRD.AsshownintheXRDpatternsinFig.4a,thepolymerPAAexhibitsabroadpeakat19.5,correspondingtoitssemicrystallinenature37.Aftergraphitization,theP10G0showsastrongandsharppeakat26.28,corres-pondingtothe(002)crystalplaneofgraphite,whichissimilartotheGNs.ItindicatesthatPAAactsasthecarbonprecursortobeentirelyconvertedintograph-ite after graphitization.For typical PAA10/GNs2.5,itexhibits2peaksrepresentingthepolymerPAAandGNs,respectively.After graphitization,the w