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15 16 tion(atafirsttemperatureandfirstpresure)andexfoliation40 ofagraphiticmaterialtoproduceNGPsandthenrepeating thesupercriticalfluidintercalationandexfoliationstepsfor theresultingNGPs.Theserepeatingstepsinclude(a)subject ingtheNGPmaterialtoasupercriticalfluid(containinga coatingagentdissolvedtherein)atasecondtemperatureand45 asecondpressureforasecondperiodoftimeinapressure veselandthen(b)rapidlyde-presurizingthefluidatafluid releaseratesuficientforefectingfurtherexfoliationofthe NGP material. calfluid,thecoatingagentdifusesintointer-graphenespaces topossiblyexpandorSwellthesespaces.Thestepofdifusing thecoatingagentintothespacesbetweenthegraphenelayers includesmaintainingdifusionforbetweenabout10minutes to24hours(preferably3hours)atsupercriticalconditionsto producetentativelyintercalatedgraphite.Theprocedurefur thercomprisescatastrophicalydepressurizingthetentatively intercalatedgraphitetoprecipitatethecoatingagentfromthe Supercriticalfluid.Duringcatastrophicdepresurization,the Supercriticalfluidexpandsandexfoliatesthegraphitelayers whilethecoatingagentprecipitatesfromtheSupercritical fluidtocoverthelayers. US 8,696,938B2 Inanotherpreferedembodiment,theSupercriticalfluid AssuggestedbyGulari,etal.56,onemaychoosetouse containsaSurfactantordispersingagentdissolvedtherein. Surfactantsordispersingagentsthatcanbeusedinclude anionicSurfactants,non-ionicSurfactants,cationicSurfac tants,amphotericSurfactants,siliconeSurfactants,fluoro-sur factants,andpolymericSurfactants.ParticularlyusefulSur factantsforpracticingthepresentinventionincludeDuPont's Zonylseriesthatentailsanionic,cationic,non-ionic,and fluoro-basedspecies.Otherusefuldispersingagentsinclude Sodiumhexameta-phosphate,SodiumlignoSulphonate(e.g.,10 marketedunderthetradenamesVanisperseCBandMarasp erseCBOS-4fromBorregaardLignoTech),sodiumsulfate, Sodiumphosphate,andsodiumsulfonate.Presumably,aSur factantiscapableofrapidlycoveringthenewsurfacescreated duringthedelaminationorseparationbetweentwographene15 layers,therebypreventingthere-formationofinter-graphene VanderWaalsforces(re-joiningoftwographeneshets).This speculationwasconsistentwithourSurprisingobservation thatthepresenceofasurfactanttendstoresultinmuch thinnerNGPs ascomparedwiththesurfactant-frecase undercomparableprocessingconditions.Itisofsignificance tonotethatthesurfactantisnormallyeasytoremoveafterthe formationofNGPs;e.g.,viaheat-inducedvaporizationor simplewaterrinsing. Again,theprocedureofSupercriticalfluidintercalation25 (presurizingandheating)andexfoliation(depresurization) canberepeatedforatleastanothercycletofurtherreducethe thicknessofNGPs.Thecyclecaninclude(a)subjectingthe NGPmaterialtoasupercriticalfluid(containingasurfactant dissolvedtherein)atasecondtemperatureandasecondpres30 Sureforasecondperiodoftimeinapressureveselandthen (b)rapidlydepresurizingthefluidatafluidreleaserate suficientforefectingfurtherexfoliationoftheNGPmate rial.Again,thesecondtemperaturemaybediferentfromor thesameasthefirsttemperatureandthesecondpressuremay35 bediferentfromorthesameasthefirstpresure. acoatingagentthatcanbesolubilizedintheSupercritical fluidtodifusebetweenthegraphitelayers.Thepurposeof thiscoatingagent,accordingtoGulari,etal.56,wasto allowthecoatingagenttoexpandorSwelltheinterstitial spacesbetweengraphenelayers(toasistinintercalationand exfoliation)and,afterdepresurization,thecoatingagentwil precipitateouttoSurroundandisolatetheexfoliatedgraphene platelets.Thiscoatingagent(e.g.,apolymer)wileventually becomeapart(thematrix)ofacompositematerial.Generaly, thecoatingagentmayincludeapolymer,oligomer,prepoly mer,oramonomer.Inoneembodiment,thecoatingagentis poly-(dimethylsiloxane)(“PDMS)havingaweightaverage molecularweightofpreferablybetweenabout30,000and 200,000g/mole.Othersuitablecoatingagentsincludepoly (tetrafluoroethylene-co-hexafluoropropylene), poly-(per fluoro-propyleneoxide),poly-(diethyl-siloxane),poly-(dim ethylsilicone), poly-(phenylmethylsilicone), perfluoroalkylpolyethers,chloro-trifluoro-ethylene,andbro motrifluoroethylene. ItmaybenotedthatGulari,etal.56 didnotteachabout repeatingtheintercalationandexfoliationprocedureforthe purposeoffurtherreducingtheNGPthicknes. Thegraphiticmaterialparticlesandthecoatingagentmay bedisposedinahighpressureveselthatisisolatablefrom theatmosphere.Inthisembodiment,thegraphiteparticles compriseabout23to83weightpercentandthecoatingagent comprisesabout77to17weightpercentofmaterialplacedin thevesel.Theweightratioofgraphiteparticlestothecoating agentispreferablyatleastabout1:10.Then,thepressure veselissealedofffromtheatmosphere.Thisisfollowedby introducinghigh-presurecarbondioxideintothecompart mentwithCO,beingpresurizedintheveseltopreferably aboveapproximately1,070psig(7.4MPa).Then,thevesel isheatedtoatemperaturepreferablyaboveabout40°C. Theseconditionsdefineasupercriticalconditionofcarbon dioxidewherebythecoatingagentissolubilizedintheSuper criticalcarbondioxide. Inyetanotherpreferedembodimentofthepresentinven tion,theSupercriticalfluidcontainsacoatingagentdissolved therein.TheprocesscomprisesSupercriticalfluidintercala Withthecoatingagentbeingsolubilizedinthesupercriti Thecoatingagentmaycompriseamonomer,aprepolymer50 oroligomer,apolymer,aresin,acuringagent,oracombi nationthereof.Thisprocessisparticularlyusefulforthe Presumably,thelowviscosityandhighdifusivityofthe productionofthinNGP-reinforcedpolymercomposites.For thepreparationofathermosetresincomposite,itisadvanta geoustohaveaSupercriticalfluidcontainingacuringagent55 dissolvedtherein.Thecuringagent,typicalyalowmolecular weightspecies,canpenetrateintotheinter-graphenespaces (alsoreferedtoasinterstitialspaces),alongwiththeSuper criticalfluid.Upon-depressurization,thecuringagentwil precipitateouttocoverthenewlyformedgrapheneSurfaces.60 Inaditiontopossiblyservingtopreventthere-joiningof graphenelayers,thecuringagentalsoactstochangethe grapheneSurfaceproperties,promotingtheSubsequentwet tingofthegrapheneSurfacebyathermosettingresin(e.g., epoxide).Hence,theprocessfurthercomprisesmixingthe 65 curingagent-coveredgraphenematerialwithathermosetting CS1. Althoughthisconventionalrouteisusefulintermsofpro ducingpristineNGPsthatarecoveredwithacoatingagent, onehastoremovethiscoatingagentunlesthecoatingagent Supercriticalfluidallowsthecoatingagentsolubilizedtherein tobecomeintercalatedbetweenthegraphenelayersinthe graphiticmaterialunderSupercriticalconditions,thereby possiblyincreasingtheinterlayerspacing.Upondepressur ization,thesupercriticalfluidresidingintheinterstitial spacesrapidlyexpandandforcethelayerstoexfoliateor delaminatefromeachother,andthecoatingagentpreviously solubilizedinthesupercriticalfluidprecipitatestherefromto depositonthedelaminatedlayers,preventingreformationof thevanderWaalsforcesbetweengraphenelayers.Thatis,the coatingagentprecipitatesfromtheSupercriticalfluidand atachestothegraphenelayers.PDF Image | SCO2 Producing Nanographine Platelets
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