干胶制备过程.docx

干胶制备过程.docx

《干胶制备过程.docx》由会员分享，可在线阅读，更多相关《干胶制备过程.docx（28页珍藏版）》请在冰点文库上搜索。

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