Effect of polyethylene glycol on the surface exfoliation of(2)

TherheologicalbehaviorsoftheSiCslurriesweremeasuredbyarotationalrheometer(CVO100,BohlinIn-strumentGmbh,Germany) withgapsetupof50 mataconstanttemperatureof25C.ThezetapotentialofSiCpowderwascalculatedfromthemeasured

electrophoretic

Fig.1.Theprocess owchartofsiliconcarbidegelcasting.

mobilityusingelectrophoreticlightscattering(ELS)with5mWlasersource.ThebulkdensitymeasurementsofgreensamplesweredeterminedwiththeHgintrusionmethodbasedonArchimedes’principle.The exuralstrengthofgreenbodywasexaminedbyatri-axialshearinstrument.Three-pointbendingwasemployedforthetest,withaspanof30mm.Thebarsofgreenbodieswerenormally5mm×6mm×40mminsize.Fracturesurfacesofgreensampleswereexaminedwithscanningelectronmicroscope(SEM,PHLIPS515,Holland).

3.Resultsanddiscussion

3.1.EffectofdispersantandPEGonthecolloidalbehaviorsofSiCsuspension

Thestudyoftheelectrokineticbehaviorsthroughmea-surementofzetapotentialisveryimportantforunderstand-ingthedispersabilityofceramicparticlesinaliquidmedium.Thehigherthezetapotential,thehighertherepulsiveen-ergyandthemorestabletheslurries.Fig.2showsthezetapotentialsofSiCpowderwithandwithoutdispersantand/or

F.Lietal./MaterialsScienceandEngineeringA368(2004)255–259

257

20

100

V

m /-10laitn-20teop-30 taZe-40-50-60

2

4

6

8

10

12

pH

Fig.2.Zetapotentialvs.pHvalueofsiliconcarbidesuspensionindeinoizedwater.

PEGindeionizedwater.Itisfoundthattheisoelectricpoint(IEP)ofSiCindeionizedwaterwithoutdispersantwas3.1.AtstronglybasicpHvalues,theabsolutevaluesofzetapo-tentialswererelativelyhigh.Withtheadditionof0.5wt.%TMAHsolution,theIEPofSiCshiftedto2.3;withtheaddi-tionof0.5wt.%TMAHsolutionand2.0wt.%PEG(basedonSiCweight),theIEPofSiCshiftedto2.0.Incompari-sonwiththeabsolutevaluesofthereceivedSiC,thatoftheSiCwithdispersantand/orPEGbecamemuchhigherinthebasicpHrange.

Itiswellknownthatthedispersant,TMAH,isanorganicalkali,whichcanbeionizedinwateraccordingtothefol-lowingreaction:

N(CH3)4OH N(CH3)4++OH

(1)

TheSiCparticlesarecoatedwithanoxygen-richlayeronitssurfaceduringthesynthesisprocess.Theoxygen-richlayercanbehydratedwhenSiCparticleisexposedtoam-bientconditionsorisinwatertoformsilanol(Si-OH)onthesurfaceofSiCparticles.AtlowpHrange,silanolreactswithH+tobecomeSi-OH2+,sothezetapotentialofSiCispositivewhenthepHvalueisbelow3.1.AfterTMAHisaddedtothesuspension,the+IEPofSiCshiftstolowerpHvaluebecausetheN(CH3)4group,whichcomesfromreaction(1),canalsobeabsorbedonthesurfaceofSiCpar-ticlesandtheamountofpositivechargesonSiCparticlesincrease.AthighpHrange,silanolreactswithOH tobe-comeSi-O ,sothezetalpotentialofSiCisnegative.WhenTMAHisaddedtothesuspension,moreOH anionsthatcomefromthedispersantabsorbonthesurfaceofSiCpar-ticles,thusthezetapotentialincreasesandahigherrepul-siveforcebetweentheSiCparticlesisobtained.ItshouldbenotedherethattheadditionofpolyethyleneglycoltoSiCsuspensionhasnoobviouseffectonzetapotentialofSiCparticles.Thismightbebecausethatthehydroxylgroupinthemacromolecularchainofpolyethyleneglycolisnotas

10

10

.s

Pa / tyiosiscV11

Shear Rate / s

-1

Fig.3.Viscosityofsiliconcarbidesuspensionwithdispersantvs.shearrate.

activeasthatinTMAHmoleculeandcannotbeeasilyion-izedinwater.SoitcannotaffectachangeinSiCsurfacecharges.

3.2.RheologicalpropertiesofSiCsuspensionwithPEGFromthemeasuredvaluesofzetapotentialforSiCsus-pension,itcanbeseenthatTMAHhasanobviouseffectonimprovingtherepulsiveenergybetweenSiCparticlesintheaqueoussystem.However,excessdispersantmaydamagethe uidpropertyofceramicsuspension.Thus,theamountofdispersantinSiCsuspensionwithasolidload-ingof50vol.%wasoptimized.Fig.3givesthelog-linearplotsofapparentviscosityversusappliedshearrateofSiCsuspensionswithoutandwithTMAHaftermilling12h.ItcanbeseenfromFig.3thatallthesuspensionsexhibitedshear-thinningbehavior.Thisisbecauseofaperturbationofthesuspensionstructurebyshear.Atlowshearrates,thesuspensionstructurewasclosetoequilibriumbecausether-malmotiondominatedovertheviscousforces.Athighershearrates,theviscousforcesaffectedthesuspensionstruc-tureandshear-thinningoccurred.Atveryhighshearrates,theviscousforcesdominatedandtheviscosityplateaumeasuredtheresistanceto owofasuspensionwithacom-pletelyhydrodynamicallycontrolledstructure.Fig.3showsthattheSiCsuspensionwithoutTMAHexhibitedhighestviscosityatvariousshearrates.AfteraddingTMAH,theviscosityofSiCsuspensiondecreasedandtheoptimumamountofTMAHinSiCsuspensionwas1.0wt.%(basedonSiCweightinthesuspension).

WiththeadditionofpolymerssuchasPEGintomonomersolution,thesystembecomesamixtureoforganicmonomerandpolymer.TheabsorptionofPEGonthesurfaceofSiCparticleshadanobviouseffectonthesteadyviscosityofSiCsuspension.Fig.4showsthelinear–linearplotsofapparent

258

876

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7654321

100

200

300

-1

Viscosity / Pa.s

543210

400

0500

Fig.5.Surfaceofsiliconcarbidegreenbodiespreparedfromthesuspen-sionswithout(a),andwith1.5wt.%(b)and3.0wt.%PEG(c).

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