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US 8,696,938B2 1. 2 SUPERCRITICAL FLUID PROCESS FOR PRODUCING NANO GRAPHENE PLATELETS FIELD OF THE INVENTION Thepresentinventionrelatesgenerallytothefieldofcar bon-orgraphite-basednanomaterials,andmoreparticularly tonanographeneplatelets(NGPs). BACKGROUND OF THE INVENTION Thepresentdiscusionofthepriorartwilmakereference tothepatentliteratureandtechnicalpaperslistedattheendof thissection. 10 15 obtainedGICorGO isthensubjectedtoexfoliationusing eitherathermalshockexposureorasolution-basedgraphene layerseparationapproach. Technicaly,theacid-treatedgraphiteisactualyoxidized graphiteorgraphiteoxide(GO),ratherthanpristinegraphite. Inthethermalshockexposureapproach,theGICorGO is exposedtoahightemperature(typicaly800-1,050°C.)fora shortperiodoftime(typicaly15to60seconds)toexfoliate thetreatedgraphite.Typicaly,theexfoliatedgraphiteoxideis thensubjectedtoafurthershetorflakeseparationtreatment usingairmiling,mechanicalshearing,orultrasonicationina liquid(e.g.,water). IntheSolution-basedgrapheneseparationapproach,the GOpowderisdispersedinwateroraqueousalcoholsolution, whichissubjectedtoultrasonication.Alternatively,theGO powderdispersedinwaterissubjectedtosomekindofion exchangeorpurificationprocedureinSuchamannerthatthe repulsiveforcesbetweenionsresidingintheinter-planar spacesovercometheinter-grapheneVanderWaalsforces, resultingingraphenelayerseparations. In both the heat- or solution-induced exfoliation approaches,theresultingproductsareGOplateletsthatmust undergoafurtherchemicalreductiontreatmenttoreduce(but normallynoteliminate)theoxygencontent.Typicallyeven afterreduction,theelectricalconductivityofGOplatelets remainsmuchlowerthanthatofpristinegraphene.Further more,thereductionprocedureofteninvolvestheutilizationof undesirablechemicals,suchashydrazine.Insomecasesof solution-basedexfoliation,theseparatedanddriedGOplate letswerere-dispersedinwaterandthencastintothinGO films.Thesefilmswereexposedtoahightemperature,high vacuumenvironmentforde-oxygenation,buttheresulting GOplateletswerenolongerdispersibleinwaterorother Solvents. ExamplesofApproach1arebrieflydiscussedbelow: (a)Bunnell 10-12 developedamethodinlate1988that entailedintercalatinggraphitewithastrongacidto obtainaGIC,thermallyexfoliatingtheGICtoobtain discretelayersofgraphite,andthensubjectingthe graphitelayerstoultrasonicenergy,mechanicalshear forces,orfrezingtoseparatethelayersintodiscrete flakes.Althoughflakesassmallas10nmwerecitedin thereport 12,mostoftheflakespresentedinthe examplesappearedtobethickerthan100nm. (b)Inasimilarmanner,Zaleski,etal.13 usedairmiling to further delaminate thermally exfoliated graphite flakes.Theresultingstructuresexhibitedaspecificsur faceareaof35m/g,corespondingtoanaverageflake thicknessofapproximately25nm. (c)Horiuchi,Hirata,andco-workers 14-16 prepared nano-scaledgraphiteoxide(GO)platelets,whichthey coinedascarbonnano-films.Thesefilmswereprepared byatwo-stepprocess—oxidationofgraphiteandpuri ficationoftheresultinggraphiteoxide.Theoxidationof graphite was conducted using the now well-known Hummer'smethod 17,whichentailedimmersing natural graphite particles in a mixture of H2SO, NaNO,andKMnO toobtainGICsthatactualywere GOs.ByhydrolyzingtheGIC,functionalgroups,such asacidichydroxylgroupsandethergroups,wereintro ducedintotheinter-graphenelayerspaces.Eachofthe graphiteoxidelayersbecameamultiple-chargeanion, havingathicknesofapproximately0.6mm.Whenthe excess Small ions derived from the oxidants (e.g., NaNO, and KMnO) were thoroughly removed by a purificationproces,manylayerstendedtoautomati calyseparatefromeachotherduetointerlayerelectro Thenanoscalegrapheneplatelet(NGP)orgraphenenano shetisanemergingclasofnanomaterials.AnNGPisa nanoscaleplateletcomposedofoneormorelayersofa grapheneplane,withaplateletthicknessfromlesthan0.34 nmto100nm.Inagrapheneplane,carbonatomsoccupya 2-Dhexagonallaticeinwhichcarbonatomsarebonded togetherthroughstrongin-planecovalentbonds.Inthec-axis or thickness direction, several graphene planes may be weaklybondedtogetherthroughvanderWaalsforcestoform amulti-layerNGP.AnNGPmaybeviewedasaflatened25 shetofacarbonnano-tube(CNT),withasingle-layerNGP correspondingtoasingle-walCNTandamulti-layerNGP correspondingtoamulti-walCNT. Formorethansixdecades,Scientistshavepresumedthata single-layergraphenesheet(oneatomthick)couldnotexist30 initsfrestatebasedonthereasoningthatitsplanarstructure wouldbethermodynamicallyunstable.Somewhatsurpris ingly,severalgroupsworldwidehaverecentlySucceededin obtainingisolatedgrapheneshetsRefs.1-9).NGPsare predictedtohavearangeofunusualphysical,chemical,and mechanicalproperties.Severaluniquepropertiesassociated withthese2-Dcrystalshavebeendiscovered.Inaditionto single graphene shets, double-layer or multiple-layer grapheneshetsalsoexhibituniqueandusefulbehaviors.In40 thepresentcontext,single-layerandmultiple-layergraphene shet structures are colectively refered to as NGPs. Grapheneplateletsmaybeoxidizedtovariousextentsduring theirpreparation,resultingingraphiteoxide(GO)platelets. Hence,althoughNGPspreferablyorprimarilyrefertothose 45 containingnoorlowoxygencontent,theycanincludeGO nanoplateletsofvariousoxygencontents. Although practical electronic device aplications for graphenearenotenvisionedtooccurwithinthenext5-10 years,itsaplicationasananofilerinacompositematerialis50 imminent.However,theavailabilityofprocessablegraphene shetsinlargequantitiesisesentialtotheSuccessinexploit ingcompositeand otheraplicationsforgraphene.The presentpatentaplicationaddressesisuesrelatedtothepro ductionofprocessableordispersibleNGPs. TheprocessesforproducingNGPsandNGPnanocompos iteshavebeenrecentlyreviewedbytheaplicants,Jangand Zhamu Ref.9.Basicaly,therearefourdiferentapproaches thathavebeenfollowedtoproduceNGPs.Theiradvantages andshortcomingsarebrieflySummarizedasfolows: 60 Approach 1: Formation and Reduction of Graphite Oxide (GO)Platelets Thefirstapproachentailstreatingalaminargraphitemate rial(e.g.,inmostcases,naturalgraphitepowder)withan intercalantandanoxidant(e.g.,concentratedSulfuricacid andnitricacid,respectively)toobtainagraphiteintercalation compound (GIC) or,actualy,graphiteoxide (GO). 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