凝胶的机械性能醇凝胶气凝胶眼镜.docx

凝胶的机械性能醇凝胶气凝胶眼镜.docx

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凝胶的机械性能醇凝胶气凝胶眼镜

MechanicalPropertiesofGels:

FromAlcogelandAerogelstoGlasses

INTRODUCTION

Themostfascinatingfeaturesofgels（lowsoundvelocity,highspecificsurfacearea,lowthermalandelectricconductivity…）aregenerallyduetotheirverylargeporosity,whichcanbeashighas99%.However,thecounterpartofthishugeporosityispoormechanicalpropertiesandtheconsequenceisthatgelstendtocrackduringdrying.So,oneofthemostdifficultproblemsinsol–gelscienceistomakelargebodiesofdriedgels（xerogelsoraerogels）.

Thedryingstressesareattributedtocapillaryphenomenaanddifferentialstrainwhichresultfromapressuregradientintheporeliquid（Brinker,1990）.Differentwayshavebeenstudiedtosolvethisproblemandforexamplesupercriticaldrying（SD）allowsonetoavoidthecapillarystressesandmonolithicaerogelcanbeobtained（Kistler,1932）.

Thepotentialitiesofthismaterial（Cerenkovdetector,acousticorthermalinsulator,hostmatrixforcatalysts）areincreasedifconsiderednotonlyasanendproduct,butasaprecursor.Byasetofsinteringtreatments,thesilicaaerogelscanbeeasilytransformedintopuresilicaglass（Woignier,1990）andappropriateheattreatmentsleadtoPartiallyDenseAerogel（PDA）whichcanbeusedasahostmatrixforthesynthesisofdopedglassesorcomposites.

Theknowledgeofthemechanicalpropertiesofgelsandaerogelsisofinterestfortechnologicalapplications,butalsofortheoreticalresearch.Gelsareidealmaterials,inthesensethattheevolutionofphysicalpropertiesinrelationwiththestructurecanbeexperimentallystudiedoverthewholerangeofporosity,from0to99%.

Themechanicalbehaviorofgels,xerogelsandaerogels,isgenerallydescribedintermsofbrittleandelasticmaterials,likeglassorceramics（West,1988;Zarzycki,1988;Woignier,1988a）.Duringmechanicaltestingevenforveryporousmaterial（99%）,thestress–straincurveshowsaperfectelasticbehaviorandtheconchoidalfracturemorphologyindicatesthatthematerialisbrittle,likeaconventionalglass.Themaindifference,comparedtosilicaglass,istheorderofmagnitudeoftheelasticandmechanicalmoduliwhichare104timeslower.However,ifthisanalogyispertinentwhengelsareunderatensionstress（bendingtest）,theyexhibitamorecomplicatedresponsewhenthestructureiscompressed（compressiontest）.Thenetworkislinearlyelasticundersmallstrains,thenexhibitsyield,followedbydensificationandplastichardening（Pirard,1995;Scherer,1995;Duffours,1995）.Asaconsequenceoftheplasticshrinkage,itispossibletoeliminatetheporesandstiffenthegelatroomtemperature.Wewillseethattheseoppositebehaviors（elasticandplastic）aresurprisinglyrelatedtothesametwokindsofgelfeatures:

thesilanolcontentandtheporesizedistribution.Differentkindsofgelstructure（fractalornotfractal）havebeensynthesized,throughacontrolofthedifferentstepsoftransformation,suchassinteringandplasticcompactionandwerestudiedasafunctionoftheirstructure.Theinfluenceofthesinteringprocess,comparedtotheplastictransformation,ontheelasticpropertiesandmechanicalbehaviorareexplainedbytheassociatedstructuralchanges.Therelationshipsbetweenstructuralandmechanicalpropertieswillbediscussedintermsofthecellularmodel,percolationtheory,fractalstructureandtheblobsandlinksmodel.

EXPERIMENTALPROCEDURE

Differentfamiliesofgelshavebeenelaboratedforthesestudies.Themainaresilicaalcogelsandaerogels.Thealcogelsarepreparedbyhydrolysisandpolycondensationreactionsoftetramethoxysilane（TMOS）.TheTMOSisdissolvedinvariousamountsofethanol,therebyadjustingtheoxidecontentofthesol（andthefinalbulkdensityofthematerial）.Thesolutionsarehydrolyzedunderneutral,basic（NH4OH,5×10–2N）oracidic（HNO3,10–4N）conditions.Thealcogelsaretransformedintoaerogelsbysupercriticalevacuationofthesolvent.Forseveralsamples,thesupercriticalheattreatmentisnotfollowedbytheevacuationofthesuperfluid,sothesolventinvadesthegelduringcooling.Theinterestofthisprocedureistopreparematerialsfullofsolventforwhichthesolidnetworkhasundergonethesameheattreatmentasforclassicalaerogels.Thesesamplesarecalled“re-wettedaerogels”.ThesamplesarelabeledN,BorA（withrespecttothecatalyst）,followedbytheTMOSweightpercent.

Thedensificationoftheaerogelsisobtainedbyheattreatmentatatemperatureof1050°Cand,asafunctionofthesinteringtime,thebulkdensityincreases.ThesamplesarelabeledPDAxx（PartiallyDenseAerogel）wherexxisthebulkdensityexpresseding/cm3.ThePCAxxsamples（PartiallyCompressedAerogel）correspondtogelswhosedensityhasbeenincreasedbyroomtemperaturecompression.

Theelasticmoduliandthemodulusofruptureofthesamplesweremeasuredbythe3-pointsbendingtechnique,usinganInstrontestingmachinewitha20Nloadcellandbysoundvelocity.ThefracturetoughnesswasmeasuredusingtheSingleEdgeNotchedBeam（SENB）methodinthethreepointbending.Generally,thebeambendingdataallowcalculationoftheYoung’smodulus;E.Thatistrueforaerogels,but,inthecaseofthealcogelsamples,thebeambendingtestyieldstheshearmodulus.Duetothelowpermeabilityofthealcogels,thefluidforcesthenetworktobehaveasifitwereincompressible.Thenthevolumeofthesampleisunchangedbythedeformationandthefluidexertsatransversestressonthegelnetwork,providingameasurementoftheshearmodulusG（Scherer,1988）.Inordertocomparetheelasticpropertiesoftheaerogelsandthealcogels,themeasurementshavebeenmadeonalcogelsandre-wettedaerogels.

Theirstructurewillbecharacterizedbysmallanglescatteringtechniques（SANSandSAXS）.Scatteringvectorsq,rangingfrom0.0018to0.3Å–1,wereexploredtoallowadeterminationofthedensity–densitycorrelationfunctioninthelengthscalefrom3to500Å.

MECHANICALBEHAVIOROFALCOGELSANDAEROGELSElasticModuliandMechanicalRupture

Whateverthegoaloftheaerogelsynthesis,itisimportanttoknowhowthesupercriticaldrying（SD）canmodifythephysicalandchemicalfeaturesoftheparentalcogel.Theobjectofthispartistocharacterizetheevolutionofthemechanicalproperties.Wewillstudytheinfluenceofsynthesisparameterssuchastheconcentrationofgelifyingprecursors,pHofthehydrolysissolutionandaging,onthephysico-chemicaltransformationsobservedduringthesupercriticalfluidextraction.Intheliterature,mechanicaltestinghasbeenmadeeitheronalcogels（West,1988;Zarzycki,1988;Scherer,1988）,oronaerogels（Calemczuk,1987;Dumas,1990;Woignier,1988b;Gross,1992）andcomparisonofthetwosetsofdatasuggeststhatsupercriticaldryingenhancesthemechanicalfeaturesofthesamples.Theapplicationoflinearelasticfracturemechanicstowetgelsisquestionable,becauseitisnecessarytojustifythatthematerialhasanelasticbehaviorandcanbetreatedasacontinuum.Scherer（1992）hasdiscussedthisproblemandhasshownthatitseemsreasonabletoapplyfracturemechanics,becausetheelasticregionnearthetipofthecrackismuchlargerthantheplasticdeformationatthecracktip.Aspreviouslymentioned,thebeambendingtestyieldsdifferentelasticmoduliforalcogels（G）andforaerogels（E）.IfwewanttofollowtheinfluenceofSDonthemechanicalfeatures,theshearmodulushasbeenmeasuredonalcogelsandre-wettedaerogels.GcanalsobecalculatedfromEandthePoisson’sratiov.TheshearmodulusisrelatedtoEbyE=2（1+v）G.ThePoisson’sratio,measuredbyBrillouinscatteringandsoundvelocitypropagation,iscloseto0.2overthewholerangeofaerogeldensityandfordifferentcatalysts.InfactGcalculatedfromEandvandGmeasuredonre-wettedaerogels,givesimilarresults（Woignier,1992）.

Figure14-1showstheinfluenceofSDonthemechanicalproperties,Gandthemechanicalstrengthσofaneutralsetofsamples.ItisclearthatGandσincreasebyafactorof10–20.TheeffectofconcentrationisalsoshownandobviouslythemechanicalpropertiesimprovewiththeTMOSconcentration,overalmost2ordersofmagnitude.However,duringSD,ashrinkageisobservedandthechangeofthemechanicalpropertiescouldbeattributedtothisshrinkage,whichincreasestheloadbearingfractionofsolid.InFigure14-2,theshearmodulusofthedifferentsetsofsamples（neutral,basicandacid）havebeenplottedasafunctionofthefractionofsolids.Gvaluesofre-wettedaerogelsarestillhigherbyafactor4–5.Thisfigureshowsalsothat,iftheacidicandneutralsampleshavenearlythesamemechanicalproperties,forthebasicset,thevaluesofGarelower.

Figure14-1.EvolutionoftheelasticandmechanicalpropertiesGandσasafunctionoftheTMOScontentfortheneutralsetsofalcogelsandre-wettedgels

Figure14-2.EvolutionofGasafunctionofthebulkdensityforneutral,acidandbasicsetsofalcogelsandre-wettedgels.

ToexplainthestrengtheningofthematerialsduringSD,wecaninvoketwokindsofprocessesoccurringintheautoclave.Thefirstmustberelatedtotheformationofsiloxanebondsbetween“danglingbonds”inthealcogel.Thesebondscontributetothemass,butnottotheconnectivityofthenetwork.Whentwobranchescomeintocontact,condensationreactionsofsilanolgroupstakeplace,increasingtheconnectivity.Thisprocesswouldincreasethestiffnessandthestrength,butwouldalsoimposestressesonthealcogelnetworkwhichcouldexplaintheshrinkage.Thesecondmechanismofstrengtheningisduetothegrowthofthenecksbetweenparticles.Thisgro