干胶制备过程.docx
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干胶制备过程.docx
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干胶制备过程
AerogelProcessing
INTRODUCTION
Agelresultsfromacondensationofmoleculesorparticlesinasolvent.Itisconstitutedbytenuousandentangledchainsofsolidwettedbyaliquidwhichoccupiesthewholevolumelocatedbetweensolidchains.Theliquidisamixtureofsolvent,unreactedmoleculesinducinggelationandby-productsofchemicalreactions.Itisobviousthatonlythenetworkisofinterestformaterialapplications.Therearemanywaystoremovetheliquidlocatedwithintheporesofthegel.Adriedgelisnamed“xerogel”(fromtheGreekworkχερροζthatmeansdried).
Dryingisoftenperformedbyagentlesolventevaporationattemperaturesclosetoroomtemperature.Inthecourseofsolventevaporation,theshapeoftheliquid–vaporinterfacechangeswithtime.Thecurvatureradiusofthemeniscusdecreases(Fig.25-1)and,associatedtothiscurvature,capillaryforcestakeplace.Thepressuredifference,∆P,betweenvaporandliquidisgivenbyLaplace’srelation:
(25-1)
whereγLVistheliquid–vaporsurfaceenergyandRisthecurvatureradiusofthemeniscus(hereassumedspherical).Theliquidisconsequentlyunderatensionstressandconverselythesolidnetworkissubmittedtoacompressionstress.Becauseoftheweakmechanicalpropertiesofthegelnetworkashrinkageoccurs.Theporevolumeofthexerogeliswelllowerthanthatofthestartinggel.
Hencepronouncedtexturalmodificationshappen.Thisisthefirstseriousdrawbackthatwemustavoidtopreservetheexpandedtextureofthesolidnetwork.
Thevolumeshrinkageofthegelduringdryinginducesanincreaseofitsstiffness.Atagiventime,thesolidnetworkisnomorecompliantandthemeniscusrecedesinthepores.Atthismoment,thestressismaximumsincethecurvatureradiuscorrespondstothatoftheshrunkpore(assumedcylindrical).Associatedtoevaporation,theliquidflowsfromthecoreofthegeltothesurface.Thisflowishinderedbythesolidarmsofthegel.Agelisbadlypermeablebecausethesizeoftheporesliesmainlywithintherange0.2–10nmindicatingthatagelisamesoporousmaterial.AccordingtotheDarcy’slaw,theliquidflow,J,isrelatedtopermeability,D,bytherelation:
(25-2)
where∇Pisthepressuregradientandηistheliquidviscosity.
Becauseofthestressgradient,thesolidnetworkmaycrack.Thiskineticapproachexplainswhycrackingisrelatedtotheevaporationrate.Indeedtheevaporationratecontrolstheliquidflow.ThedryingofthegelhasbeenverypreciselystudiedbyG.W.SchererinaseriesofpaperslistedinChapter8(Brinker,1990).Ageldriedveryslowlywillproduceafreecrackxerogel.Manyauthorsreportdryingtreatmentsthedurationofwhichisofseveralmonths.Thatisusuallydonebycoveringthegelswithaplasticfilminwhichmanyholesarepunctured.
Figure25-1.Evolutionofthecurvatureofliquid–vapormeniscusatthesurfaceofaporeasafunctionofdryingtime,t.
Crackingofthesolidpartofthegelistheseconddrawbackusuallyencounteredduringdrying.Freezedryingandsupercriticaldryingaretwoprocesseswhichhavebeeninvestigatedtocircumventthesedifficulties.
Freezedryingconsistsofloweringthetemperatureofliquidtoinduceacrystallizationphenomenon.Thesolventisthenremovedfromitsvaporstatebydecreasingthepressure(sublimation).Thisprocessapplieswelltosolventsshowinganappreciablevaporpressureattemperatureslowerthancrystallizationtemperature.Lowmolecularweightalcoholshaveverylowcrystallizationtemperatures(methanol:
–94°C,ethanol:
–117°C).Waterwhichtransformsintoicecrystalshowsanimportantvolumechangeassociatedtothistransformation.Thesolidpartofthegelishighlystressedandusuallybreaksintosmallpieces(Pajonk,1989).Moreoverthesublimationrateisquiteslow.Itisofabout140kg/m2hat15°C.Asolutionwhichmay,inimagination,avoidthelargevolumechangeproducedbycrystallization,istotransformliquidintoglass.Unfortunatelyglassformationdomainoftenoccursneareutecticpointcomposition.AsexemplifiedtheglasstemperatureofmixtureH2O-CH3OHistoolow(–157°C)(Vuillard,1961)toperformthensublimationatappreciablerate.Finally,oneamongthebestliquidsseemstobeterbutanolwhosethemeltingtemperatureis25°Candwhichhasasublimationrateof2800kg/m2hat0°C.Thissolventisnotusualandaprevioussolventexchangeisoftenrequired.Thetexturalpropertiesofthegelsuchastheporevolumeandtheporesizedistributionareapproximatelypreserved.Neverthelessitseemsdifficulttoobtainmonolithicsampleshavingsignificantthickness(higherthan10mm)(DegnEgeberg,1989).AdetailedanalysisofthenucleationandcrystallizationphenomenaoccurringintheliquidwettingthesolidpartofthegelhasbeendonebyScherer(Scherer,1993).Crystallizationstartsfromtheliquidlocatedattheexternalgelsurfaceandthecrystal–liquidinterfacemovesfromthesurfacetothecore.Thusstressesappearasaconsequenceofthesolidcrustwhichformsatthesurfaceandthevolumechangeassociatedtotheliquid–crystaltransformation.
Sincethemainconsequencesofdryingaretheshrinkageandthebreakage,severalexperimentshavebeenperformedtoovercomethesedrawbacks.Wemustunderlinethatcrackinghasbeenchemicallyavoidedbyaddingtothestartingsolutionsomecompoundswhichgiverisetogelhavinganarrowporesizedistribution(formamide,glycerol,oxalicacid).Chemicaladditivescontrollingthedryingstepworkwellbothwithaqueousgels(Shoup,1988)andthosepreparedfromorganometalliccompounds(Hench,1986).
Theincreaseofthestiffnessofthesolidpartofthegelbyadissolution-redepositioneffectallowstopreservethemonolithicityofthegelwhilereducingtheshrinkage(Mizuno,1988).Itisworthnoticingthatageingthewetgelinasolutioncontainingmonomersgivesanalogousresults(Einarsrud,1998).Analternativewaytoproducecrackfreesamplesistosynthesizegelshavingverysmallporesizes.Duringdrying,nucleationandgrowthofbubblesoccurwithintheliquid.Thiscavitationphenomenoninducesthesegmentationoftheliquidwhichbecomesunderalowertensilestress(Sarkar,1994).Ontheotherhandwemustunderlinethatsometimescrackingcanberegardedasanadvantage.Asanexample,anextensivecrackingisbeneficialinthesynthesisofabrasivepowdersissuedfromsol–gelprocess.
THESUPERCRITICALDRYING-PROCESS
ThesupercriticaldryingprocesshasbeenproposedbyKistler.(Kistler,1932)todry,withouttexturalmodification,verytenuoussolidswettedwithasolvent.
Themainideaistoavoidcapillaryforces,whichoccurduringdrying,byaverypeculiarpressureandtemperaturescheduleappliedtotheliquid.Regardingonlytheliquidphaseofthegel,itisobviousthatonecanmodifyitsstatebychangingthermodynamicparameterssuchasthepressureandthetemperature.
Figure25-2showsatypicalphasediagramforapurecompound.Theparameters,P,T,v(usuallythespecificvolume)arethevariableswhichdeterminethestateequation.
Figures25-3and25-4correspondtosomeprojectionsofthepreviousthree-dimensionaldiagram.
Figure25-2.TypicalP,T,vdiagramofachemicalcompound.
Figure25-3.Pressure-specificvolumediagramissuedfromdiagramFigure25-2.
Figure25-4.Pressure-temperaturediagramshowingthedifferentdomainssolid,liquidandvaporandsupercriticalfluid(SF).
TheprincipleofsupercriticaldryingiseasilyunderstoodowingtoFigure25-4.Thepoint,a,definesthecouplepressure-temperatureatwhichthethreestatesofthecompoundareinequilibrium.Underatmosphericpressure,Pat,theliquidtransformsintovaporatboilingtemperature(TB).Thepoint,c,istheboundaryofthevaporizationcurvecorrespondingtoliquid–vaporseparation.Thepoint,c,isnamedthecriticalpoint.Foragivencompoundthecriticalpointisdeterminedbyassociatedcriticalpressureandtemperaturevalues.Abovethispointthereisacontinuumbetweentheliquidandthevaporwhichcannomorebedistinguished.Inthisdomain,thereisanuniquestatenamedsupercriticalfluid(SF).Thisdomainisnotwelldefined.HoweveracrudeapproximateconsistsinlocatingthesupercriticalfluiddomainbyaP,TareaasindicatedinFigure25-4.
Atroomtemperature(TR)startingwithaliquid(N)andincreasingboththetemperatureandthepressure,thecompoundfollowsthepathN→Q(Fig.25-5).AtQ,thecompoundissupercriticalunderitsfluidstate.Itcanbeobservedthatstartingwiththevaporstateatlowpressure(M)andincreasingagainthetemperatureandthepressure,thecompoundreachesthepointQwhereitisinthesamestatethanthatpreviouslymentioned.Thuswehaveobtainedthesamehomogeneousanduniquestateusingdifferentpaths.Foragivencompound,itspropertiesdependobviouslyonthepressureandtemperaturevaluesandcanbeeasilyvariedaccordingly.
Figure25-5.Differentpathstoreachthesupercriticalfluiddomain.
Itisevidencedthatstartingfromtheliquidstate(pointN)andincreasingthetemperatureandpressureuptothesupercriticalfluidstate(pointQ),anadequatedecreaseinthetemperatureandpressure(seefullarrow)willleadtothevaporstate(pointM).Theneteffectofthesesuccessivestepsresultsinthetransformationofliquidintovapor.Adryingstephasbeencarriedout.Thechangefromtheliquidtothevaporfollowsapaththatavoidsthevaporizationcurve(ac).Duringheating,thesurfaceenergyassociatedtotheinterfaceliquid–gasprogressivelydecreasesandvanisheswhenthesuperfluidstateisattained.Consequentlycapillaryforces(seeequation
(1))arenomoreactingandthesolidpartoftheg
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