溶胶凝胶中独立的厚膜.docx

溶胶凝胶中独立的厚膜.docx

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溶胶凝胶中独立的厚膜

Self-StandingThickFilms

INTRODUCTION

Oneofthemostimportanttechnologicalaspectsofsol–gelprocessingisthepreparationofthinfilmsbycommontechniquessuchasdip-orspin-coating.Awidevarietyofmaterialswithspecialfunctionsofopticalandopto-electronicimportancehavebeenproducedtodatebytheapplicationofthesecoatingmethods.However,thedip-andspin-coatingmethodsincludetheexperimentalexperiencethatinorganicfilmsthickerthan1μmarevirtuallyimpossibletodrywithoutcracksregardlessofthedryingrate（BrinkerandScherer,1990）.

Ontheotherhand,therearevariouseconomicaldemandsforoxidefilmswithathicknessoftheorderofseveralmicrometersformedonsubstratematerialsinthefieldofelectrical,opticalandopto-electronicdevices（Kawachi,1990;Moilanen,1994）.Theformationofsuchoxidefilmshavebeenmadebyrepetitivespin-coatingandrapidthermalannealing（SymsandHolmes,1994）,flamehydrolysis（Kawachi,1983）,plasmaCVD（Grand,1990）,screenprinting（Fernandes,1995;Futakuchi,1999;Akiyama,1999）,hydrothermalprocess（Shimomura,1991）,gasdepositionprocess（Ichiki,1997）,electrophoresis（Nishimori,1995）,etc.Buteachprocesshasdrawbackinherenttothemethod.Someofthemaretime-consumingandlesseconomical.Othersrequirehighsinteringtemperatureandsometimesgivethefilmsoflowdensityorpoorflatness.

Recentlyanewtechniqueofformingoxidegelfilmscorrespondingtooxidefilmsofseveralto20μminthicknessbasedoninterfacialpolymerizationwasdevelopedbyaresearchgroupofTokyoInstituteofTechnology（Yamane,1994）.Thetechniqueoriginallydevelopedaimingtheformationofathicksilicaglassfilmonasiliconsubstrateforthepurposeofthefabricationofaplanerwaveguideenlargedtherestrictedfilmthicknessrelatedtoconventionalsol–gelprocessingandwasextendedtotheformationofthickPZTfilmsusedforpiezoelectricdevicesinMEMSsuchasmicro-actuators,sensors,ultrasonictransducers,ultrasonicmotorsandsoon（Tsurumi,2003）.

Theprincipleoftheinterfacialpolymerizationmethodisbasedonthehydrolysisandpolycondensationofprecursorsattheinterfaceoftwoimmiscibleliquidswithoutdirectcontactwiththesubstratesurface.Theevaporationoftheliquidintheupperphaseandthesubsequentcarefuldrainageoftheliquidinthebottomphasecausesagentleplacementofthefreestandinggelfilmontothesubstratesurfacewithoutformationofchemicalbonding,whichisofgreatadvantageoverspin-ordip-coatinginavoidingcracksduringdrying.

Inthisarticle,theoutlineofthemethod,effectsofvariousreactionparametersonthegelfilmpropertieswillbeintroducedinthecaseofboththicksilicaglassfilmandPZTfilmonsiliconsubstrates.

OUTLINEOFGELFILMFORMATIONBYANINTERFACIALPOLYMERIZATION

Theformationofafree-standinggelfilmbyaninterfacialpolymerizationiscarriedoutwithinacylindricalcontainerhavingadrainpipeatitsbottom.First,asubstratematerialonwhichtheformedgelfilmisplacedissetnearthebottomofthecylindricalcontainer.Thenwaterforthehydrolysisofanalkoxideispouredinthecontainertocoverthesubstratetothelevelseveralmillimeterabovethesurface.Next,theprecursorsolutionpreparedbydissolvingthealkoxideinanorganicsolventisgentlyporedontowater.

Figure16-1.Schematicillustrationofgelfilmformationprocessbyaninterfacialpolymerizationmethod.

Thehydrolysisandpolycondensationofthealkoxidetakesplaceattheinterfaceformedbetweentwoimmiscibleliquids.Thereactionproceedsuntiltheintroducedalkoxideisspentandturnsintoagelfilm.Theformedgelfilmseparatesfromthecontainerwallbythecapillaryforceinducedbytheevaporationoftheorganicsolventandshrinkstosomeextentwithoutrestrictionfromthesubstrate.Afterthecompleteevaporationoftheorganicsolvent,theformedgelfilmisplacedonthesubstratebydrainingwaterfromthebottomofthecontainerandsubjectedtodryinginanambientatmosphere.TheschematicillustrationoftheprocessisgiveninFigure16-1.

Thecompositionandtheamountoftheprecursorsolutionandthediameterofthecontainerdeterminethepropertiesofthefilmsuchasporousstructure,thickness,andsoon.Theprecursorsolutionscontainingproperamountoffineparticles,aswellasalkoxides,aresometimesusedinordertoreducetheshrinkageduringdrying.Waterforthehydrolysisofthealkoxideusuallycontainsappropriatecatalyst.

PREPARATIONOFASILICAFILM

PrecursorSolution

Thepreparationoftheprecursorsolutionbeginswiththedissolutionofsiliconalkoxideoritsderivativesinanorganicsolventlikehexanethatmeetstheconditions:

（1）immisciblewithwater,

（2）lowerdensitythanwater,and（3）relativelyhighvaporpressure.Amongcommerciallyavailablesiliconalkoxidesandtheirderivatives,ethylsilicate40（E-40）,partiallyhydrolyzedtetraethoxysilane（TEOS）comprisingthe5-memberedoligomers,turnedouttobethemostsuitablebythesurveyofvariousmaterialsincludingTEOS,tetramethoxysilane（TMOS）anditsderivativemethylsilicate51（Yamane,1994）.

Figure16-2.DependenceoffilmthicknessontheconcentrationofE-40inhexane（reactionwithammoniawaterofpH=11for24h）.

TheadvantageofusingE-40ratherthanTMOSorTEOSisattributedtothelowhydrolysisrateofsiliconalkoxide.Asitiswidelyknown,thehydrolysisofTMOSorTEOSisenhancedbyacidiccatalyst,whilethepolycondensationratherproceedsunderbase-catalyzedcircumstance.Ontheotherhand,thecatalystthatcanbeaddedinthewaterfortheinterfacialpolymerizationreactionislimitedtoeitheracidorbaseonly.Then,theuseofE-40whichalreadyhadbeenhydrolyzedanddoesnotneedtheassistanceofacidcatalystisobviouslyadvantageousovertheemploymentofTMOSorTEOS.

InthefilmformationfromE-40,itsconcentrationintheprecursorsolutionisobviouslyoneoftheimportantparametersdeterminingthereactiontimetoobtainagelfilmofdesiredthickness.Figure16-2showstherelationbetweentheconcentrationofE-40andtheweightofgelfilmpersquarecentimeterobtainedbythereactionwithammoniawaterofpH=11for24h.Itisknownfromthefigurethatafilmofabout2mg/cm2,whichcorrespondstothethicknessofabout10μmintermsoftheeventualsilicaglassfilm,isobtainedfromtheprecursoroftheconcentration1.2–1.5mol/lundergivenexperimentalconditions.

CatalysttobeUsedfortheReaction

Theeffectsofcatalystsonthegelfilmformationaredifferentdependingonthetypeofcatalystanditsconcentration,i.e.,pHofwater（Shulze-Bergkamen,1995,Yamane,1997）.Forexample,agelfilmisobtainablefromE-40bythereactionwithammoniawaterofpH>10.ThethicknessofthefilmincreaseswiththeincreaseinpHofwaterasitisseeninFigure16-3.ItisalsopossibletoformafilmintheregionpH<4byusingaceticacid,formicacidandcitricacidasacatalyst,althoughthecatalyticeffectisveryweakcomparedwiththatofammonia.Differentfromtheeffectsoftheseweakelectrolytes,however,thecatalyticeffectofstrongelectrolytessuchasNaOH,HCl,orHNO3isnotalmostappreciable.OnlyatraceoffilmisdetectedbythereactionwithaqueoussolutionsoftheseelectrolyteseveninthepHregionssimilartothoseappliedtoammoniaoraceticacid,i.e.,pH>10orpH<4.

AnexampleoftheeffectsofthetypeofbasecatalystsonfilmformationfromE-40isshowninTable16-1alongwiththesolubilityofthesecatalystsinwaterandthegeltimeformonolithformationwiththesamecatalysts.Thereactionconditionswerethesameforallthecases,i.e.,theconcentrationofE-40was2mol/l,pH=10.6,thereactiontemperatureTandtimetwere30°Cand20h,respectively.Itisknownfromthetablethatthethicknessofthegelfilmdecreasesasthesolubilityofthecatalystinwaterincreases,whilethegeltimesformonolithformationfromthesimilarprecursorofpH=9.5areallthesamewithinexperimentalerror.Asthesolubilityofthesecatalystsinanon-polarorganicsolventsuchhexaneincreasesintheorderoppositetothatinwater,i.e.,NH3>Na2CO3>NaOH,thedifferencesintheeffectsshowninthetableisattributedtothedifferenceintheconcentrationofthecatalystsintheupperorganicphase.Thiswasconfirmedbytheexperimentcarriedoutbydirectlydissolvingtri-methylamineinhexane.Therateofgelfilmformationwasdramaticallyincreasedcomparedwithammonia-catalyzedreaction,asitisknownfromTable16-2.Only4hofreactiontimewasenoughtoobtainagelfilmcorrespondingtoaglassfilmof10mminthickness,i.e.,12mg/cm2,evenwithmuchmoredilutedprecursorsolution.ThisshowsthatthereactionproceedsquiteefficientlyifacatalystiscontainedintheupperorganicphasealongwithE-40.

TABLE16-1.EFFECTSOFVARIOUSBASECATALYSTSONFILMFORMATIONANDTHEIRSOLUBILITYDATA（YAMANE,1997）

Catalyst

Filmthickness（gcm–2）a

Geltime（h）b

Ammonia

40±10×10‒4

50–60

NaOH

2±0.5×10‒4

50–60

Na2CO3

10±2×10‒4

50–60

Solubilityparameter（cal/cm3）2

Solubilityinhexane（molefraction）

Solubilityinwater（molefraction）

H2O

23.8

2.55×10–1

–

C6H14

7.25

–

4.41×10–27

NH3

13

4.89×10–1

3.49×10–2

NaOH

–c

–

4.90×10–1

Na2CO3

–

–

1.21×10–2

apH=10.6;CE–40=200mll–1;T=30°C;t=20h.

bE-40/MeOh/H2O=1/1/1（ml）;pH=9.5;T=60°C.

cNoavailabledata.

Figure16-3.DependenceoffilmthicknessonthepHofammoniawater（concentrationofE-40inhexane;1.2mol/l,reactiontime;24h）（Yamaneetal.,1994）.

TABLE16-2.COMPARISONOFTHEEFFECTSOFTRI-