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11 12 US 8,696,938B2 Thegraphiticmaterialisnotlimitedtonaturalgraphite;it maybeselectedfromthegroupconsistingofnaturalgraphite, artificialgraphite,graphiteoxide,graphitefluoride,graphite Upon-depressurization,thecuringagentwilprecipitateout fiber, carbon fiber, carbon nano-fiber, carbon nano-tube, mesophasecarbonmicro-bead(MCMB),softcarbon,hard carbon,andcombinationsthereof.Technicaly,MCMBs or CMS areusualyobtainedfromapetroleumheavyoilor pitch,coaltarpitch,orpolynuclearhydrocarbonmaterial (highly aromatic molecules). When Such a precursor pitch materialiscarbonizedbyheattreatmentat400°to550°C. 10 micro-crystalscaledmesophasemicro-spheresareformedin Inmanycases,theNGPsproducedinourstudieshavea anon-crystalinepitchmatrix.Thesemesophasemicro specificsurfaceareaintherangeofaproximately300m/g spheres,afterbeingisolatedfromthepitchmatrix(whichis typicalysolubleinselectedsolvents),areoftenreferedtoas meso-carbonmicro-beads(MCMB).TheMCMBscommer 15 cialyavailablearethosethathavebeensubjectedtoafurther heattreatmentatatemperatureintherangeof2,000°C.and 3,000°C. to2,600m/g.TheNGPsobtainedwiththepresentlyinvented processtendtocontainasignificantproportionofsingle layergraphene(withathicknessof0.34-0.4nm)orgraphene offewlayers(<2nm). TheNGPmaterialobtainedwiththisproces,whenformed intoathinfilmwithathicknesnogreaterthan100nm, exhibitsanelectricalconductivityoftypicalyatleast100 Theprocessmayfurthercompriseaprocedurethat involvesesentialyrepeatingthecycleconsistingofstep(i) andstep(i).Inessence,thisprocedureincludes(a)Subject S/cmandquiteoftenhigherthan1,000S/cm. ingtheNGP materialtoasupercriticalfluidatasecond temperatureandasecondpressureforasecondperiodoftime inapressureveselandthen(b)rapidlyde-presurizingthe fluidatafluidreleaseratesuficientforefectingfurther25 exfoliationoftheNGPmaterial.Thesecondtemperaturemay bediferentfromorthesameasthefirsttemperature.The secondpressuremaybediferentfromorthesameasthefirst pressure. ThepresentlyinventedprocessisSuperiortomanypriorart processes(e.g.,thosebelongingtotheaforementioned Approach1andApproach2)inseveralaspects: (1)Forinstance,asdiscussedearlier,Aksay,McAlister,and co-workers Refs.7-9,45 usedthermalexfoliationof completelyoxidizedgraphite(GO)toobtainexfoliated graphiteoxideplatelets.Theprocessinvolvedexposingthe GOtoagasenvironmentat1,050°C.for30secondsorin apropanetorchforlesthan15seconds.Suchathermal Inanotherpreferedembodiment,theSupercriticalfluid30 containsaSurfactantordispersingagentdissolvedtherein.A shockexposuretypicalyproducesgraphiteoxideplatelets surfactantmaybecapableofrapidlycoveringthenewsur (ratherthannanographene)thataretypicalynotelectri facescreatedduringthedelaminationorseparationbetween calyconducting. byStankovichetal.32andHirataetal.16,graphitewas heavilyoxidizedtoobtaingraphiteoxide,whichwasthen mixedwithwater.TheresultingSuspensionwasthenSub jectedtoultrasonicationforanextendedperiodoftimeto producecoloidaldispersionsofGOplatelets.Thegraphite oxidedispersionwasthenreducedwithhydrazine,inthe presenceofpoly(sodium4-styrenesulfonate).Thispro cessledtotheformationofastableaqueousdispersionof polymer-coatedgrapheneplatelets.Insomeaplications,a polymercoatingmaybeundesirable(puregraphenebeing prefered).Furthermore,thereducingagent,hydrazine,isa twographenelayers,therebypreventingthere-formationof (2)Inanothercommonlyusedpriorartapproach,aspracticed inter-graphene Van der Waals forces (re-joining of two 35 grapheneshets).Thisspeculationwasconsistentwithour SurprisingobservationthatthepresenceofaSurfactanttends toresultinmuchthinnerNGPsascomparedwiththesurfac tant-frecaseundercomparableprocessingconditions. Again,theprocedurecanberepeatedtofurtherreducethe40 thicknessofNGPs.Therepeatedcyclecaninclude(a)sub jectingtheNGPmaterialtoaSupercriticalfluid(containinga Surfactantdissolvedtherein)atasecondtemperatureanda secondpressureforasecondperiodoftimeinapressure veselandthen(b)rapidlyde-presurizingthefluidatafluid45 releaseratesuficientforefectingfurtherexfoliationofthe NGPmaterial.Again,thesecondtemperaturemaybediffer toxicSubstance. entfromorthesameasthefirsttemperatureandthesecond (3)Beceril,etal46andWang,etal.47independently pressuremaybediferentfromorthesameasthefirstpres SUC. 50 developedaverysimilarprocessforproducingtranspar ent,yetconductingelectrode.Theelectrodewasmadeby followingaverytediousprocessthatinvolvesoxidationof naturalgraphitetoformGO,repeatedwashing,ultrasoni cation,and15cyclesofimpurityremovalstepsthatinclude centrifugation,discardingSupernatantliquid,andre-sus pendingtheSolidinanaqueousmixtureofsulfuricacidand hydrogenperoxide 46.TheSuspensionwaseventualy spin-coatedonasolidsubstratetoformaGO thinfilm, whichwasthenpartialyreducedbyheatingthefilmina highvacuumatahightemperatureforalongperiodof time.Suchalongprocessdoesnotappeartobeamenable tomassproductionofconductingnanographeneplatelets. Inyetanotherpreferedembodimentofthepresentinven tion,theSupercriticalfluidcontainsacoatingagentdissolved thereinandtheprocessfurthercomprisesaprocedureof repeatingtheSupercriticalfluidintercalationandexfoliation steps.Thesestepsinclude(a)subjectingtheNGP materialto 55 aSupercriticalfluid(containingacoatingagentdissolved therein)atasecondtemperatureandasecondpressurefora secondperiodoftimeinapressureveselandthen(b)rapidly de-presurizingthefluidatafluidreleaseratesuficientfor efectingfurtherexfoliationoftheNGPmaterial.Thecoating 60 agentmaycompriseamonomer,aprepolymeroroligomer,a polymer,aresin,acuringagent,oracombinationthereof. (4)Anotherunexpectedbenefitofthepresentlyinventedpro Thisprocessisparticularlyusefulfortheproductionofthin cesistheobservationthatthepristineNGPsproducedare NGP-reinforcedpolymercomposites.Forthepreparationof relativelydefect-fre,exhibitinganexceptionallyhigh athermosetresincomposite,itisadvantageoustohavea 65 conductivity.Incontrast,theheavilyoxidizedGOplatelets Supercriticalfluidcontainingacuringagentdissolvedtherein. aretypicalyhighlydefectedandcouldnotfulyrecover Thecuringagent,typicalyalowmolecularweightspecies, theperfectgraphenestructureevenafterchemicalreduc canpenetrateintotheinter-graphenespaces(alsoreferedto asinterstitialspaces),alongwiththeSupercriticalfluid. tocoverthenewlyformedgrapheneSurfaces.Inaditionto possiblyservingtopreventthere-joiningofgraphenelayers, thecuringagentalsoactstochangethegrapheneSurface properties,promotingtheSubsequentwettingofthegraphene Surfacebyathermosettingresin(e.g.,epoxide).Hence,the processfurthercomprisesmixingthecuringagent-covered graphenematerialwithathermosettingresin.PDF Image | SCO2 Producing Nanographine Platelets
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