Electrophoretic impregnation of porous anodic aluminum oxide film by silica nanoparticles

To link to this article : DOI:10.1016/j.colsurfa.2012.09.011

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Fori, Benoit and Taberna, Pierre-Louis and Arurault, Laurent and Bonino,

Jean-Pierre and Gazeau, Céline and Bares, Jean-Pierre Electrophoretic impregnation of

porous anodic aluminum oxide film by silica nanoparticles. (2012) Colloids and

Surfaces A: Physicochemical and Engineering Aspects, vol. 415. pp. 187-194.

ISSN 0927-7757

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Electrophoretic

impregnation

of

porous

anodic

aluminum

oxide

film

by

silica

nanoparticles

Benoit

Fori

_,

_Pierre-Louis

_Taberna

_,

_Laurent

_Arurault

_,

_Jean-Pierre

_Bonino

_,

Céline

Gazeau

_,

_Pierre

_Bares

a_{UniversitédeToulouse,CIRIMATUPS-CNRS,118routedeNarbonne,31062ToulouseCedex9,France} b_{MecaprotecIndustries,34BoulevardJoffrery,31605MuretCedex,France}

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◮ Poresofananodicfilmwere com-pletely filled by electrophoretic impregnation.

◮ Electric field and dispersion con-ductivity control the impregnation depth.

◮ Progressivefillingfromthebottomof porestothetop.

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Inthispaper,itisproposedtostudythedepositionofnanoparticlesbyelectrophoreticdeposition(EPD) insideaporousanodicaluminumoxidefilm.Despitethepresenceofahighlyresistivebarrierlayeratthe metal-anodicfilminterface,porousanodicfilmsonAA1050Aweresuccessfullyfilledby16-nm,surface modifiedsilicaparticles.Duringthisstudyitwasshownthatboththecolloidalsuspensionconductivity andtheappliedelectricfielddrivethepenetrationintotheporousfilm.FEG-SEMobservationsshowed thatlarge(130-nmdiameter),linearporesof10mminlengthcanbecompletelyfilledin1min.These resultsattestthatporousanodicfilmscanbeefficientlyfilledwithnanoparticlesbyEPDdespitethe presenceofthebarrierlayer.

1. Introduction

Nanomaterials in the form of nanoparticles, nanowires and nanotubesare themaincomponents intheburgeoningfield of nanotechnology. Duetotheirphysical andchemical properties, thesenanoelementshavethepotentialforuseinapplicationssuch asmemorydevices[1],chemicalsensors[2],medicalscience[3] andenergystorage[4].Onepromisingtechniquetoassemblethese

∗ Correspondingauthor.Tel.:+33561556106;fax:+33561556163. E-mailaddress:taberna@chimie.ups-tlse.fr(P.-L.Taberna).

nanoelementsovermacroscopiclengthscalesistheelectrophoretic deposition(EPD)[5].EPDisawell-established,versatile,low-cost methodtodepositceramicandmetalfilmsonaconducting sub-strate. This process deposits directly theend product avoiding chemicalreactionsatthesurfaceofthesubstrate.However,theuse ofEPDinwaterisveryrestrictedduetotheelectrolysisofwater whichoccursforlowvoltagesandcausestheevolutionofhydrogen andoxygengasesattheelectrodeswhichcouldaffectthequality ofthedepositsformed[6].

AnotheradvantageisthatEPDcanbeusedaswellonflatsurface asoncomplexsurface.Forexample,Bazinetal.depositedsilica nanoparticlesbyEPDonnanostructuredcopperelectrodestoform

sensors[7].Likewisenumerousresearchers[8–10]havestudiedthe synthesisandthepreparationofoxidenanorodsandnanotubesby EPDusingananodicaluminumoxidemembraneastemplateand fixedontoametalfoil.Thistechniqueconsiststofillporesofthe membranebyEPDandthentochemicallyremovethemembrane. Theresultingmaterialsofferasignificantlylargersurfaceareathan thatoffilmsorbulkmaterialandthusfinddiverseapplicationin nanotechnology(sensors,batteries,SOFC,etc.).

Incontrast,veryfewstudieshavebeendevotedtothe depo-sition ofnanoparticles intounmodified porousanodic filmsi.e. includingporousandcompactlayersstillsupportedonaluminum substrate.Seoetal.[11]reportedthatporescanbepartiallyfilled bynanoparticlesusingthedip-coatingprocess.Butonlyaslight amountofnanoparticlesweredepositedatthebottomofpores withthis method.Asaresultthequantityofnanoparticlesinto poresistoolowtofillthemcompletely.Becausethemigrationof chargedparticlestowardtheporebottomispromotedbyapplying anelectricfield,theEPDtechniqueshouldincreasethedensityof particlesincomparisonwiththedip-coatingprocess.

Todemonstratethefeasibilityofsuchprocess,silica nanopar-ticleswerechosenasmodelchargedparticles.EPDofsilicaisan easyprocessbecausesilicasuspensionsarecharacterizedby spher-icalparticlesandahighnegativezetapotential,providing good stabilityofthesuspensionandefficientmigration[7,12].Another advantageisthatsilicasurfacecanbefunctionalizedwithvarious organicmolecules,providingnewpropertiestoparticles[13].

To our knowledge, Kamada et al. [14] studied for the first time theinsertionof SiO2 nanoparticlesintoporesof anodized

aluminumbyelectrophoreticdeposition.However,theyusedan aqueoussilicasolutionandtheircharacterizations(e.g.EPMA pro-files)showedanuncompletedfillinginthepores.

Inthispaperweproposedtodeeplystudytheelectrophoretic deposition of silica nanoparticles into pores of an unmodified anodicfilmof10mmofthickness.Itisgenerallyassumedthatthe presenceof aconductingsurface increasestheefficiencyofthe EPD.Completelyfillingporousanodicfilmappearsthereforeasa challengebecauseoftheresistivebarrierlayerbelowthepores. Furthermoreone ofourwills in thisworkis theelucidation of themechanismofparticlemigrationanddepositionintothepores whichisveryfewdiscussedintotheliterature.Thusinafirststep, analuminumporousanodicfilmsuitabletotheelectrophoretic impregnationwillbeprepared.Inasecondsteptheinfluenceof theappliedvoltageandthesuspensionconductivityonthepores fillingbygraftedSiO2particlesusingEPDwillbestudied.

2. Materialandmethods 2.1. Preparationoftheanodicfilm

Tofacilitatetheinsertionofparticlesinsidepores,theporosity oftheanodicfilmshastoexhibitlowtortuosityandpore diam-eterlargerthanthesilicaparticleone(15nm).1050Aaluminum alloy(chemicalcompositioninweightpercent:99.5%Al,<0.40% Fe,<0.25%Si,<0.05%Cu)wasusedasasubstratetogetlinearpores perpendiculartotheinitialmetalsurface[15].Sincelargepores (averageporediameter>100nm)arerequired,aphosphoricacid basedelectrolytewaschosenasanodizingbath[16].

Firstthealloysheet(20mm×20mm×1mm)wasdegreased withethanol.SecondlythesamplewasetchedinNaOH(0.5gL−1₎

aqueoussolutionat40◦_{Cfor5minandthenneutralizedinHNO} 3

(25vol%)atroomtemperaturefor2min;waterusedtomakethese solutionsexhibitaresistivityof10cm−1_{.Thirdlythealuminum}

sheetwasusedasanodeandaleadplate(3mm×40mm×40mm) ascounter-electrodeintheelectrochemicalcell.Theanodizingwas runinthegalvanostaticmode(TDK-LambdaGEN300-5)usinga

temperaturecontrolledphosphoricacidsolution(0.4molL−1_).The

sampleswererinsedindeionizedwater(10kcm−1_)immediately

attheendofeachstep.

2.2. Electrophoreticimpregnation

A commercial colloidal suspension of silica nanoparticles (15nm)inisopropylalcohol(ABCR,Germany)wasused.This sus-pensionwasdilutedwithisopropylalcohol(CarloErba,Italy)to obtaina concentrationof about15gL−1 _{and vigorouslystirred.}

FunctionalizationofsilicawasachievedfollowingCousiniéetal. [17]. 3ml of aminopropyltrimethoxysilane (APTMS) was added dropwisein100mLof thedilutedsuspension.Themixturewas then vigorously stirred for three days and then diluted 100 timeswithisopropylalcoholleadingtoaconcentrationofabout 0.15gL−1_{.1–15mLofanI}

2–acetonemixture(6gL−1)wasadded

totheas-preparedsuspension[18].Inordertocarryoutthe elec-trophoretic deposition the anodized aluminum was set as the cathode;theleadfoil,astheanode.Thesubstratewasdriedat ambienttemperatureafterexperiment.

2.3. Characterizations

Afieldemissiongunscanningelectronmicroscope(FEG-SEM, JEOLJSM6700F)wasusedtoobservethemicrostructureofthe coatings.Theaverageimpregnationdepthwasmeasuredonthe FEG-SEMviewsusingthefreesoftwareImageJ.

MALVERNNANOSIZERZS90wasusedforzetapotential mea-surements and Stokes particle diameter measurements using dynamiclightscattering(DLS).

3. Resultsanddiscussion 3.1. Tuningofthefilmporosity

Thediameterofthecommercialsilicananoparticlesusedfor EPDisannouncedtobeequalto15nm.Itseemsobviousthatthe porehastoexhibitalargerdiameterthanparticlestoallowtheir insertion.Thereforeananodicfilmwithporelargerthan100nm shouldbesuitablefortheelectrophoreticimpregnation.So oper-atingparameters(temperature,currentdensity,duration)ofthe anodizingarestudiedinordertoadequatelytunethe characteris-ticsoftheanodicfilmonthealuminiumsubstrate.

3.1.1. Influenceofthetemperature

Tostudytheinfluenceoftheelectrolytetemperature,the cur-rentdensitywasfixedat1.5Adm−2_{andtheanodizationtimeat}

2400s.Thesampleswereanodizedat5◦_C,15◦_C,25◦_Cand35◦_C.

Fig.1ashowstheinfluenceoftheelectrolytetemperatureon theporediameterandtheporedensity.Between5◦_Cand35◦_C,the

porediameterandtheporedensityincreasewithelectrolyte tem-perature.At5◦_{Ctheporeshaveanaveragediameterof110±10nm}

and the pore density is equal to (8.9±5.0)×1012_poresm−2

whereastheporediameteris205±10nmandtheporedensity is(1.7±0.5)×1013_poresm−2_at35◦_C.

Theincreaseoftheelectrolytetemperaturetypicallyincreases reactionkinetics.Theanodicfilmgrowthoccursthroughtwomain oppositeprocesses:analuminumelectro-oxidationaccordingto reaction(1);achemicaldissolution(reaction(2))oftheanodicfilm bytheacidicelectrolyte.

2Al+3H2O→ Al2O3·xH2O+6H++6e− (1)

Al2O3·xH2O+6H+→2Al3++(3+x)H2O (2)

So,theincreaseoftheelectrolytetemperatureincreasesthe dis-solutionrateandleadstotheformationofwiderpores[19].Also,

Fig.1.Porediameter(N),poredensity(),experimentalthickness()andtheoreticalthickness( )versusthetemperature(a,b)andthecurrentdensity(c,d)(duration: 2400s).

theporedensitiesofthis studyareclosetothevalues obtained undersimilaroperatingconditionsandpresentedinotherworks [20,21].

UsuallythefilmthicknessispredictedthroughEq.(3),basedon theFaraday’slaw:

h= Mjt

nF (3)

where h is theheight of the anodic film, M the molarweight of the anodic film (M=102gmol−1 _{for Al}

2O3), j the current

density (j=1.5Adm−2 _{in ourconditions),t the anodizing}

dura-tion (t=2400s in our conditions), n the number of electron exchanged during the reaction (n=6), F the Faraday constant (F=96485Cmol−1_{)andthespecificdensityof theanodicfilm}

(=3.97gcm−3_{).Accordingtothisrelation,thepredictedthickness}

shouldbeconstant(hereabout16mm)whatevertheelectrolyte temperature.However,Fig.1b revealsthatthethicknessofthe anodic film,measuredby SEM,increases from10.6±0.1mmto 14.9±0.1mmbetween5◦Cand25◦Candthendecreases.Similar behaviorwasobtainedbyGoueffonetal.[21]andShihandTzou [22].

Duringanodizingacompetitionbetweentheelectrochemical growth of the film (reaction (1)) and its chemical dissolution by theacidic electrolyteoccurs (reaction(2)).Thetemperature influencesachemicalreactionmorethananelectrochemicalone. Sothechangeofthethicknessversustheelectrolytetemperature ismainlyduetothedissolutionreaction,whichisnotconsidered in Faraday’s law. When the temperature is too high the pore diameterissolargethattheporewallsdisappearleadingtopore

interconnection,mechanicalfragilityandfinallytocollapseofthe filmthickness.

Inordertopreparea suitableanodicfilmforelectrophoretic impregnation,anelectrolytetemperatureof25◦_{Cwasused.This}

temperaturegiveslargepores(about150nm)andsimultaneously avoidscollapseofthefilm.

3.1.2. Influenceofthecurrentdensity

This study was performed at 25◦_{C and an anodizing time}

of2400s.Thesampleswereanodizedat1.0Adm−2_,1.2Adm−2_,

1.5Adm−2 _and2.0Adm−2_{.Thevoltagemeasuredinsteady-state}

conditions of the anodizationwas respectively of 121V, 127V, 132Vand141V.

Fig.1cshowstheevolutionoftheporediameterandthepore densityinthe1.0–2.0Adm−2_{range.Theporediameterincreases}

withthecurrentdensity,whereastheporedensityremains con-stant.SimilarbehaviorswereobtainedbyO’SullivanandWood[23] andOnoandMasuko[16].Theincreaseoftheporediametercan beexplainedbyfield-assisteddissolutionoftheanodicfilm.Atthe baseofthepore,thedensityofthecurrentlinesincreasesleading toalocaltemperatureincreasewhichpromotesthedissolutionof theanodicfilmandthustheincreaseoftheporediameter[23].

Fig.1dhighlightsalinearcorrelationbetweenthethicknessand thecurrentdensity.Thestraightlinedoesnotpassbytheoriginin disagreementwithFaraday’slaw.Indeed atlowcurrentdensity (<0.3Adm−2_{)thedissolutionreactiondominateselectrochemical}

filmgrowth.

Acurrentdensityof2.0Adm−2 _{gives largerporesattherisk}

Fig.2. Thicknessoftheanodicfilmversusduration(T=25◦

C,j=1.5Adm−2_).

tonon-homogenousporousoxidelayerincreases.Soinorderto achieveaporousanodicfilmsuitabletotheelectrophoretic impreg-nation,acurrentdensityof1.5Adm−2_wasused.

3.1.3. Characteristicsofthestandardanodicfilm

Thethicknessoftheanodicfilmwasmeasuredversus anodiza-tion duration. It appears that it is proportional to duration as expectedwiththeFaraday’slaw.Inordertostudythemigration oftheparticlesintothepores,it hasbeendecidedtopreparea thickanodicfilmof10mm.AccordingtoFig.2,durationof29min isrequiredtoobtainthisthicknessandconsequentlytoformthe standardanodicfilm.Tosumup,anodizingisperformedonAA 1050inthefollowingoperationalconditions:galvanostaticmode (1.5Adm−2_)in0.4molL−1_{phosphoricacidelectrolyteat25}◦_Cfor

29min.

Fig.4. FEG-SEMviewofthesilicananoparticles.

Thestandardfilmshavelargelinearporesmakingthem suit-ableforelectrophoreticimpregnation.FEG-SEMplanviewofthe as-preparedanodicfilmshowsthattheporosityiswellformed,the porediameterbeing130±10nm(Fig.3a).LeCozetal.prepared anodicfilmwithporediameterofthesameorderofmagnitude [24].Thecrosssectionalview(Fig.3b)confirmsthattheporesare orthogonaltotheAAsubstrateandthatthefilmhasathicknessof 10±1mm.Thebarrierlayerthicknessis130±5nm(Fig.3c)which isconsistentwithVrublevskyetal.[25].

3.2. Preparationofthesilicacolloidalsuspension

Electrophoresiscanbeperformedinaqueousororganicsolvent. Sincehigherelectricfieldsarereachableinorganicsolvents, iso-propylalcoholwaschosen.Silicasuspensionswereusedbecause theyhave highstabilityand canbefunctionalizedeasilydueto theirhighsurfacereactivity.IndeedSi–OHgroupsattheparticles

Fig.5. FEG-SEMcrosssectionalviewofthetop(a)andthebottom(b)oftheporousanodicfilmaftertheimmersionintothesuspension.

surfacecanformcovalentbondswithorganicmoleculeshaving alkoxysilaneorchlorosilanegroups.Theorganicmoleculeshave otherorganicfunctionswhosephysicalor/andchemicalproperties canbetransmittedtothesilica particle.Duringelectrophoresis, theanodizedaluminumwasthecathodefortworeasons.Firstly thealuminumfoilcanbeover-oxidizedifusedasananodethusit canmodifyitsmorphology[26].Secondly,byapplyingacathodic polarization,thebarrierlayeroftheanodicfilmactsasann-type semi-conductor, so current can pass through [27]. Electingthe cathodicmodefortheelectrophoreticdeposition,thesilica par-ticlesmusthavepositivesurfacechargeinordertomigratetoward thecathode.

A commercial colloidalsuspension of silica nanoparticles in isopropanol(SiO2-COM)wasused.Thenanoparticlesexhibitan

averagediameterof13nm(Fig.4).Theinitialzetapotentialwas measuredat−40±10mV.Togetapositive potentialweuseda two-step functionalization. Firstlyaminopropyltrimethoxysilane

(APTMS)wasgraftedontosilicasurface.Thisorganicspecieshas apositivelychargedaminegroup[13,17].SecondlyanI2–acetone

mixturewasaddedtothesuspension.Oneoftheadvantagesofthis methodistocreatesomepositiveschargesinorganicmedia with-outaddingwater[18].AcetonegeneratesH+_{ionsbyaketo-enol}

reactioncatalyzedbytheoxidesurface(Eqs.(4)and(5)).

CH3COCH3↔ CH3C(OH)CH2 (4)

CH3C(OH)CH2+I2→CH3COCH2I+H++I− (5)

TheH+ _{soformedareadsorbedonthesilicaparticlesand/or}

reacts with the amino group of the ligands; both of these phenomenamaketheparticlespositivelycharged.Thenthe func-tionalizedsilica suspension(SiO2-FUNC)shows azetapotential

of +30±10mV.The measuredaveragediameterwas16±1nm. Thisdiameterisclosetothediameterofthecommercial suspen-sionconfirmingthatnofurtheraggregationwasnoticed.Moreover

Fig.6.FEG-SEMcrosssectionalviewsoftheporousanodicfilmaftertheelectrophoresisprocessusing33Vcm−1_{(top(a)andbottom(c)ofthepores)and200Vcm}−1_(top

thediameteris wellbelow thealuminum oxidepore diameter (130±10nm).

3.3. Electrophoreticimpregnation 3.3.1. Impregnationwithoutpolarization

Forcomparisonanimpregnationwasrealizedbydip-coating,it istosaywithoutanyelectricfield.Thesubstratewassoakedintothe colloidalsuspensionfor5minandthenwithdrawnfromthesilica suspension.Fig.5showsFEG-SEMcrosssectionalviewofthetop andthebottomoftheanodicfilmafterthedipping.These micro-graphsshowthatjustafewparticlesarepresentonthetopofthe filmbutnoparticlesareinsidethepores.

Duringdrying,solventevaporationinducesaconvectivemotion of the particles toward the surface of the anodic film where adsorptionoccurs.Simultaneouslyanincreaseoftheparticle con-centration occursat thesolvent–filminterface. Above a critical concentration,aggregationoccursandtheparticlesdepositonto thefilmexplainingthepresenceofparticlesatthetopofthepores. Thisexperimentrevealsthatsimpleimmersiondoesnotpermit fillingthepores.

3.3.2. Influenceoftheelectricfield

Several electric fields (33Vcm−1_{, 66Vcm}−1_{, 133Vcm}−1_,

200Vcm−1_{)wereappliedfor5mininordertostudyitsinfluence}

on the electrophoretic impregnation (suspension conductivity: 74mScm−1).FEG-SEMcrosssectionalviewsrevealsthatelectric fieldimprovesinsertionofparticlesinsidepores(Fig.6).Whenan electricfieldof33Vcm−1_{isapplied,particlesdepositonlyatthe}

topofthepores,whereasparticlesarepresentinsideporeswithan electricfieldof200Vcm−1_{.Thechangesoftheaverage}

impregna-tiondepthversustheappliedvoltagearesummarizedinFig.7.No particlesaredepositedatthebottomofporeshowever.

Fig.7. Impregnationdepthversustheappliedvoltage(suspensionconductivity: 75mScm−1,duration:5min).

3.3.3. Influenceofthesuspensionconductivity

Theinfluenceofthesuspensionconductivitywasalsostudied usinganelectricfieldof200Vcm−1_{for5min.Tochangethe}

con-ductivitydifferentvolumesofanI2–acetonemixture(6gL−1)were

addedtothesuspension.FEG-SEMcrosssectionalviewsshowthat lowsuspensionconductivity(40mScm−1)leadstotheformation ofadepositatthesurfaceofthepores(Fig.8aandc)whereasan increaseofthesuspensionconductivityallowscompletefillingof thepores(Fig.11bandd).Thechangesofthepenetrationdepth versusthesuspensionconductivityaresummarizedinFig.9.The saturationoftheimpregnationdepthforconductivitieshigherthan 120mScm−1correspondstothecompletefillingoftheanodicfilm.

Fig.8.FEG-SEMcrosssectionalviewsoftheporousanodicfilmaftertheelectrophoresisprocessusing200Vcm−1_{during5minwithtwodifferentsuspensionconductivity:}

Fig.9.Impregnationdepthversusthesuspensionconductivity(200Vcm−1_,

dura-tion:5min).

3.3.4. Studyoftheelectrophoreticporefilling

Tofill entirelythepores,anelectric fieldof 200Vcm−1 _was

appliedfor5minwithasuspensionconductivityof120mScm−1. Theconductivitywasmeasuredbeforeandafterelectrophoresis. Initiallyitwasequalto120mScm−1anddecreasesto95mScm−1 after theimpregnation. The current was also measuredduring electrophoresis(Fig.10).Atthebeginningoftheexperimentthe currentdensitydecreasesthenincreasesalittlebit(orstabilizes) beforeendingupdecreasingagain.Sincetheconductivityofthe suspensiondoesnotchangemorethan20%,thefirstdecreaseof the current (between 0sand 50s) probably corresponds toan increase ofthe resistivityofthefilm due totheaggregationof

Fig.10.Currentdensityversusdepositiontimefordepositionofsilicaparticlesat 200Vcm−1_{during5min(suspensionconductivity:120mScm}−1_).

particlesatthesurfaceandinsidethepores.Itisreportedinthe literaturethatthisresistancedepositiscausedbytheformation ofaniondepletionlayerinthedeposit[28]andmasstransport limitations through the growing deposit layer [29]. The small increase(orstabilization)between50sand100scanbeduetothe dielectricbreakdownoftheinitialinsulatinganodicfilm.

Inordertoknowthechronologyofthedeposit,FEG-SEM micro-graphies were made at different electrophoresis times. Fig. 11 revealsthatparticlesarepresentonlyatthebottomofthepores after15swhereas poresarecompletelyfilled byparticlesafter 1min.Atthebeginningoftheelectrophoresis,particlesgotothe bottomoftheporesandthenprogressivelyfilltheporesentirely.

Fig.11.FEG-SEMcrosssectionalviewsoftheporousanodicfilmaftertheelectrophoresisprocessusing200Vcm−1_{during15s(top(a)andbottom(c)ofthepores)and60s}

Duringelectrophoresisthecurrentlinespassthroughthe bar-rierlayerandnotthroughthetooresistiveporewalls.Thereforethe particlesgotothebottomoftheporesandfilltheporesasthe elec-trophoresisprocesscontinues.Oncetheporesarefilled,particles depositonthesurface.

As shown in Figs. 6 and 7 an increase of the electric field improvestheimpregnationdepthofparticles.Theseobservations revealthatahighelectricfieldislikelyneededduring electrophore-sistoavoidaggregationofparticlesatthetopofpores.Animportant electricfieldisrequiredherebecauseofthepresenceoftheresistive barrierlayeratthemetal-porousanodicfilminterface.The migra-tionspeedoftheparticlesisdescribedbythefollowingequation:

v

with

v

(Vm−1_{). According to Eq. (6) an increase of the electric field}

enhancesthedrivingforcesofparticlesandsofacilitatestheir inser-tioninsidepores.

Anincrease oftheconductivityimprovestheelectrophoretic impregnationtoo(Figs.8and9).Theseresultscanbeexplainedby theestablishmentofahigherpotentialgradientthroughtheanodic film[30].Theprincipaldriving forceofparticles forEPD isthe appliedelectricfield.Sohighpotentialgradientacrosstheanodic filmisrequiredtoallowtheinsertionofparticlesinsidepores.If thesuspensionhasalow conductivity,thepotentialgradientis mainlylocatedintotheresistivesuspensionandparticlescannot enterinsideporesduetotheweakpotentialgradientintheanodic filmregion.Soanincreaseofthesuspensionconductivitytransfers thepotentialgradientfromthebulksuspensiontothebarrierlayer regionbyreducingthepotentialdropinthesuspension.Therefore anincreaseoftheconductivityconcentratestheelectricfield in theanodicfilmmakinginsertionoftheparticleinsidepores eas-ier.Moreoveranincreaseofthesuspensionconductivitypromotes thethinningoftheelectricaldoublelayeratporewallsenhancing masstransportthroughthepore[31].Thereforeitappearsthrough theseresultsthattheconductivityislikelytobeoneofthemost importantparametertofillporousanodicfilmbyEPD.

4. Conclusion

Electrophoreticimpregnationofporousanodicfilmformedin phosphoricacidbasedelectrolytewasperformedinanorganicSiO2

colloidalsuspension.Indeedalmostallporeshavebeencompletely filledbygraftedAPTMS-SiO2.Theelectricfieldandthesuspension

conductivityaretwokeyfactorsthatcontroltheimpregnation.At thebeginningofEPD,particlesgotothebottomofporeswhich continuetofillprogressively.TheseresultsattestthatEPDcanbe usedtofillsuccessfullyporousanodicfilmdespitethepresenceof theresistivebarrierlayeratthemetal–oxideinterface.

Acknowledgements

ThisworkwasfinanciallysupportedbyMECAPROTEC Indus-tries. We thank B. Daffos and P. Lenormand for the FEG-SEM pictures.

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