溶胶凝胶中独立的厚膜文档格式.docx

溶胶凝胶中独立的厚膜文档格式.docx

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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×

Na2CO3

10±

2×

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×

3.49×

10–2

–c

4.90×

1.21×

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-