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17 18 US 8,696,938B2 isdesiredforanintendedaplication(e.g.,forthepreparation equippedwithsecurityclampsandringsthatenableisolation ofapolymermatrixcompositewiththecoatingagentbeing themonomerorpolymerforthismatrix).Forthisparticular purpose,itisadvantageoustohaveaSupercriticalfluidcon oftheveselinteriorfromtheatmosphere.Theveselwasin fluidcommunicationwithhigh-pressureCO bywayofpip ingmeansandcontroledorregulatedbyvalves.A heating tainingacuringagentdissolvedtherein.Thecuringagent,5 jacketwaswrappedaroundtheveseltoachieveandmaintain typicalyalowmolecularweightspecies,canpenetrateinto theinter-graphenespaces,alongwiththeSupercriticalfluid. Upon-depressurization,thecuringagentwilprecipitateout tocoverthenewlyformedgrapheneSurfaces.Inaditionto possiblyactingtopreventthere-joiningofgraphenelayers, thecuringagentalsoservestochangethegrapheneSurface10 properties,promotingtheSubsequentwettingofthegraphene Surfacebyathermosettingresin(e.g.,epoxide).Hence,the processfurthercomprisesmixingthecuringagent-covered graphenematerialwithathermosettingresin. InSummary,afteranextensivestudy,wehaveSurprisingly15 observedthat: (1)Supercriticalfluidscontainingnocoatingagentareat leastasefectiveasthosecontainingacoatingagentfor intercalatingandexfoliatingnaturalgraphite.Thereis nomajordiferenceinthesupercriticalfluidtempera ture,presure,time,andde-presurizationconditions betweenthetwospecies(onewithandtheotherwithout acoatingagent); (2)Supercriticalfluids,withorwithoutacoatingagent dissolvedtherein,areefectiveinintercalatingandexfo 25 liatingawidevarietyofgraphiticmaterials,including (inaditiontonaturalgraphite)artificialgraphite(e.g., highly oriented pyrolytic graphite, HOPG), graphite oxide,graphitefluoride,graphitefiber,carbonfiber,car bon nano-fiber, carbon nano-tube, mesophase carbon 30 micro-bead(MCMB),graphitizedsoftcarbon,andhard carbon.Previousstudiesonsupercriticalfluiddelami nationofgraphitehavebeen esentialylimitedto chemicallypre-intercalatednaturalgraphite Ref.55 andnaturalflakegraphiteonly Ref.56. (3)Withproperconditionsselectedforsupercriticalfluid35 intercalationandexfoliation,onecouldreadilyobtain ultra-thinNGPs withathicknesslessthan1nm.With otherlesfavorableconditions(e.g.,alowerdepressur izationrateorgasdischargerate),somewhatthicker NGPswereobtained.However,thesethickerNGPs40 couldbesubjectedtoanothercycleofsupercriticalfluid intercalationandexfoliation,preferablyinthesame pressurechamber,toyieldmuch thinerNGPs. By repeatingthecycleoneortwotimeswecouldreadily obtainsubstantialysingle-layerNGPs. thecriticaltemperatureofcarbondioxide.High-pressurecar bondioxidewasintroducedintothevesselandmaintainedat approximately1,100psig(7.58MPa).Subsequently,theves selwasheatedtoabout70°C.atwhichthesupercritical conditionsofcarbondioxidewereachievedandmaintained forabout3hours,allowingCO todifuseintointer-graphene spaces. Then, the vesel was immediately depressurized “catastrophicalyatarateofabout3mililiterspersecond. Thiswasaccomplishedbyopeningaconnectedblow-off valveofthevesel.Asaresult,delaminatedorexfoliated graphene layers were formed, which were identified as Sample1-A.ThissamplewasfoundtocontainpristineNGPs withanaveragethicknessjustunder10nm. A smallamountofNGPswasmixedwithwaterandultra Sonicatedfor15minutestoobtainasuspension,whichwas thencastontoaglasSurfacetoproduceathinfilmof approximately89nm inthicknes.Basedonafour-point probeapproach,theelectricalconductivityoftheNGPfilm was found tobe 889 S/cm. EXAMPLE 2 RepeatedIntercalationandExfoliationwithCO, SupercriticalFluids Approximately two-thirds of Sample 1-A was removed fromthepresurevesel.TheremainingNGPsweresub jectedtoanothercycleofsupercriticalCO intercalationand de-presurizationtreatments(i.e.,theaboveprocedureswere repeatedwithapressureof7.5MPaandtemperature65°C.), yieldingmuchthinerNGPswithanaveragethicknesof2.1 nm (identifiedasSample2).ThespecificSurfacearea,as measuredbytheBETmethod,wasaproximately430m/g. TEMandAFMexaminationsindicatedthatthereweremany single-layergrapheneshetsinthissample.Theelectrical conductivityofathinfilmmadefromtheseultra-thinNGPs was1,450S/cm. EXAMPLE 3 PreparationofPristineNGPsfromNaturalGraphite UsingCO SupercriticalFluidsContaininga Surfactant Sample3waspreparedunderesentialyidenticalSuper criticalCOconditionsasSample1-A,withtheexception thatasmallamountofSurfactant(approximately0.05grams ofZonyl(R)FSO)wasmixedwith5gramsofnaturalgraphite beforethemixturewassealedinthepressurevesel.The resultingNGPshaveaSurprisinglylowaveragethicknes,4.1 nm.Afterthepresurizationandde-presurizationproce dureswererepeatedfortwomorecycles,theresultingNGPs hadanaveragethicknesslesthan1.0nm,indicatingthata majority of the NGPs were single-layer or double-layer shets.ThespecificSurfaceareaofthissampleaftera repeatedcyclewasaproximately900m/g.Itmaybenoted thatasampleofalsingle-layergrapheneshetsshould exhibitaspecificsurfaceareaof>2.670m/g.Itisclearthat thepresenceofaSurfactantordispersingagentpromotes separationofgraphenelayers,perhapsbypreventingtheref ormationofvanderWaalsforcesbetweengrapheneshets onceseparated. 45 50 (6)Afunctionalgroupcanbeconvenientlyimpartedtothe resultingNGPsifareactivechemicalgroupisintro ducedintothepressureveseltocontacttheNGPs55 thereinbefore,during,orafterNGPsareformed. (4)Supercriticalfluidscontainingasurfactantdissolved thereinaremoreefectivethantheircounterpartscon tainingacoatingagent(e.g.,polymer,monomer,andoil, asdefinedbyGulari,etal.56)orthosecontainingno Surfactantandnocoatingagent. (5)Thepresentlyinventedprocessisfastandenvironmen talybenign. Thefollowingexamplesservetoprovidethebestmodesof practiceforthepresentinventionandshouldnotbeconstrued aslimitingthescopeoftheinvention. EXAMPLE1 PreparationofPristineNGPsfromNaturalGraphite UsingCO SupercriticalFluids Anaturalgraphitesample(approximately5grams)was placedina100mililiterhigh-pressurevesel.Theveselwas 60 65PDF Image | SCO2 Producing Nanographine Platelets
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