电泳溶胶凝胶沉积法.docx
- 文档编号:18344057
- 上传时间:2023-08-15
- 格式:DOCX
- 页数:29
- 大小:1.96MB
电泳溶胶凝胶沉积法.docx
《电泳溶胶凝胶沉积法.docx》由会员分享,可在线阅读,更多相关《电泳溶胶凝胶沉积法.docx(29页珍藏版)》请在冰点文库上搜索。
电泳溶胶凝胶沉积法
ElectrophoreticSol–GelDeposition
INTRODUCTION
ElectrophoreticdepositionisaversatileprocessforformationofthinandthickfilmsonsubstrateswherebyelectricallychargedparticlesaredepositedontothesubstratesasanelectrodefromastabilizedsuspensionunderaDCelectricfield(Sarker,1996).Thisprocesshaswidelybeenappliedtovariousfieldssuchaspreparationofphosphorsfordisplays,solidelectrolytesforelectrochromicdisplays,cathodesforlithiumsecondarybatteries,superconductors,ferroelectricmaterialsandbiomaterials(Mizuguchi,1983;Kuwabara,1991;Nojima,1991;Nagai,1993;Koura,1995;Yamashita,1998).Themainadvantageoftheelectrophoreticdepositionisthefactthatthickfilmscanbepreparedonvarioussubstrateswithacomplicatedshapeinmuchshortertimecomparedwiththeothercoatingtechniques.Thefilmthicknesscanbecontrolledbyvaryingthepreparationconditionssuchasappliedvoltage,depositiontimeandconcentrationofthesuspensions.Inpractice,however,thepreparationofstablesuspensionswithfineanduniformparticles,whichisessentialtoobtainexcellentthickfilms,isnotsoeasy.Forexample,whenrawmaterialsaregroundusingaconventionalballmill,theshapeoftheparticlesisnotuniformandthesizedistributionisbroad.Thesol–gelmethodpermitsthepreparationofuniform,sphericalandfineparticles.So-called“electrophoreticsol–geldeposition”,whichiscombinedsol–gelmethodforparticlepreparationandelectrophoreticdepositionofthesol–gelderivedparticles,hasbeenproposedasapromisingproceduretopreparethickfilms(Kishida,1994).Inthischapter,theprincipleoftheelectrophoreticsol–geldepositiontechniqueisdescribedandtheprogressinresearchandtechniqueisreviewed.
PREPARATIONPROCESSES
Theelectrophoreticsol–geldepositiontechniqueisbasedonthepreparationofparticlesfrommetalalkoxidesbysol–gelmethodandthedepositionofthesol–gelderivedparticlesbyelectrophoresis.
PreparationofParticlesbySol–GelMethod
Theformationofsolidparticlesfromsolutionsproceedsthroughnucleationandgrowthofhydrolyzedandcondensedmetalalkoxides.VariouskindsofparticlessuchasSiO2,TiO2,ZrO2,Al2O3etc.canbepreparedfromcorrespondingmetalalkoxides(Pierre,1998).Amongthesemetaloxides,monodispersedSiO2particlesarewellknowntobeobtainedbyStöberprocessbasedonthegrowthoftheprimaryparticlesbyOstwaldripeningduetotheirlargesolubilityofandgreatsize-dependentsolubilityunderbasicconditions(Stöber,1968;Iler,1979).Recently,organosilsesquioxane(RSiO3/2)microparticles,whichareakindofinorganic–organichybridwithaSi—Ccovalentbond,havebeenalsopreparedfromorganotrialkoxysilanes(Katagiri,1998).Theparticlesformedinthesolutionaredirectlyusedforelectophoreticdeposition.However,centrifugationoftheparticlesfollowedbyredispersioninadequàtesolventsispreferredtoobtainelectophoreticallydepositedfilmsdependingoncircumstances.
DepositionofParticlesbyElectrophoresis
Electrophoreticdepositionprocessbasicallyconsistsofthreesteps:
(i)electricalchargingofparticlesinasuspension(electrification),(ii)movingofthechargedparticlestowardtheoppositelychargedelectrodeunderaDCelectricfield(electrophoresis),and(iii)coagulationoftheparticlesontheelectrodetoformafilm(deposition).Thesurfacechargeontheparticlesinthesuspensionischaracterizedbythechargedensityandsurfacepotential.Themechanismbfchargingisexplainedeitherbythedissociationandionizationofthesurfacefunctionalgroupsontheparticlesorbytheadsorptionofpotential-determiningionsontheparticles.Thechargedparticlesattractoppositelychargedionstodeterminethezetapotential(Stern,1924).
Schematicdrawingoftheelectrophoreticsol–geldepositioncellisshowninFigure14-1.Whenanexternalelectricfieldisappliedtothechargedparticles,theparticlesmoveinthesuspensionatanelectrophoreticvelocitydeterminedbythebalancebetweentheelectricfieldandtheviscousforceswhichactontheparticles(O’Brien,1978).Withrespecttothedepositionprocess,severaltheoreticalapproacheshavebeendonesofar.HamakerandVerwey(1940)proposedthatdepositionoccursviaaccumulationofparticlesattheelectrode.Theysuggestedthatthephenomenonofelectrophoreticdepositionisidenticallythesameasthatofsedimentation.Theaccumulatedparticlesclosetotheelectrodearetobedepositedbecauseofthepressurethatovercomestheinterparticlepotentialbarrier.
Figure14-1.Schematicdrawingoftheelectrophoreticsol–geldepositioncellwhichillustratestheprocess.
Constant-VoltageandConstant-CurrentDeposition.Constant-voltageandconstant-currentelectrophoreticdepositiontechniqueshavebeenemployedsofar(Zhang,1992).
Intheconstant-voltageelectrophoreticdeposition,thepotentialbetweentheelectrodesismaintainedconstant.Thedepositionoftheaccumulatedparticlesrequiresasteeperpotentialthanthattheelectrophoresisofthedispersedparticlesneeds.Therefore,electricalresistancelargelyincreasesduringthedeposition.Thecurrentresultingfromconstant-voltageelectrophoresisdecreasesasafunctionofthedepositiontime.Thecurrentdecreasesbecausethegrowingfilmcontinuallyincreasesthetotalcellresistance.Thevoltagedropinthefilmisequaltotheproductofthecurrentandthefilmresistance.Therefore,thecurrentreflectsthechangesinresistanceduetophysicalandchemicalpropertiesofthefilms.Thefilmwillceasetogrowwhenthevoltagedropofthefilmbecomesequaltotheconstantappliedvoltage.
Underconstantcurrentelectrophoreticdeposition,thepotentialinducingelectrophoresisismaintainedconstantbyincreasingthetotalpotentialdropbetweentheelectrodes.Asaresult,inhomogeneityindensity,microstructureandmorphologycanbeavoided.Constantcurrentelectrophoreticdepositionprovidesahighdepositionrate.Therefore,constantcurrentelectrophoreticdepositioncanavoidthelimiteddepositionanddeposition-rateproblemsintheconstant-voltageprocess(Sarker,1996).
SolventandElectrification.Aqueousandnon-aqueoussolventshavebeenusedforthesuspensionsfortheelectrophoreticdeposition.Althoughaqueoussuspensionsareharmlessandcosteffective,theformationofhydrogengasatthecathodeandoxygengasattheanodethroughelectrolysisofwaterresultsinthedegradationoftheresultantfilms(Ryan,1979).Ingeneral,non-aqueoussuspensionsarepreferentialforthedepositionofmetaloxidefilms.Severalkindsoforganicsolventssuchasalcohol,acetone,acethylacetone,cycrohexanonehavebeenreportedtobeusedforthepreparationofthickfilmssofar(Koura,1995;Ishihara,1996;Yamashita,1997).TheadditionofsmallamountsofI2andwatertoketonegeneratesH+ionsbyaketo–enoltautomerism.TheadsorptionofH+ionstothemetaloxideparticlesdeterminestheirzeta-potentialandpermitstheelectrophoreticdepositionoftheparticlesatthecathode.
PRACTICALAPPLICATIONS
SilicaThickFilms
Silicathickfilmshavebeenpreparedonastainlesssteelsheetbytheelectrophoreticsol–geldepositioninthepresenceofsodiumdodecylsulfate(SDS)asdispersantinasolcontainingsilicaparticles(Nishimori,1995,1997).Silicaparticleswerepreparedbyhydrolysisoftetraethoxysilane(TEOS)underbasicconditions.SDSwasdissolvedin0.5moldm–3NH3aqueoussolution,andTEOSwasdilutedseparatelywiththesameamountofethanol(EtOH).Thesetwosolutionsweremixedandstirredat25°C.ThemolarratioofTEOS/H2O/EtOHwasfixedtobe0.2/10/10.ThecontentofSDSwaschangedfrom0to0.3mass%againstthetotalweightofthesols.
Thesizeofsilicaparticlesandthegeometricalstandarddeviation,σg,asafunctionofthecontentofSDSisshowninFigure14-2.Thegrowthoftheparticlesiscompletedin12hofthereactiontime.Monodispersedsilicaparticleswithsmallσg(<1.1)areobtainedwhenthecontentofSDSisupto0.25mass%.ThevalueofσgisdecreasedwithanincreaseofSDS.Theparticlediameterisincreasedfromca.0.09toca.1.2μmwithanincreaseofSDS(0–0.25mass%).WhenthecontentofSDSislargerthan0.3mass%,theparticlesaggregateandmonodispersedparticlesarenotobtained.Inthisway,monodispersedsilicaparticleswithvariousparticlesizesupto1.2μmindiameterareeasilypreparedbychangingtheamountofSDS.
Figure14-2.Sizeofsilicaparticlesandthegeometricalstandarddeviation,σg,asafunctionofthecontentofSDS(Nishimori,1997).
Figure14-3.SEMphotographsofthesilicaparticlesextractedwithdifferentreactionperiods:
(a)fortheparticleswithreactiontimefor30minand(b)for12h(Nishimori,1997).
SEMphotographsoftheparticlesextractedwithdifferentreactionperiodsareshowninFigure14-3.ThecontentofSDSwas0.2mass%.Doublesphericalparticlesbycoalescencewithanorderofsub-micrometerinparticlesizeareobservedwhenreactiontimeis30min(Fig.14-3(a)).Thecoalescenceisnotobservedandsphericallargerparticlesarefinallyobtainedbystirringfor12h(Fig.14-3(b)).Thisindicatesthatthecoalescenceoftheparticlesoccurinearlierreactionperiodandlargersphericalparticlesarefinallyobtained.
Theweightofsilicafilmspreparedbytheelectrophoreticsol–geldepositionusingSDS-containingsilicasolisshowninFigure14-4asafunctionofappliedvoltage.ThecontentsofSDSaddedwerechangedfrom0to0.2mass%.IntheabsenceofSDS,thedepositedfilmscollapsein10minafterwithdrawing.Ontheotherhand,thesolscontainingSDSyieldthickfilmsonthestainlesssteel
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 电泳 溶胶 凝胶 沉积