Shear Rate / s
Fig.4.Viscosityofsiliconcarbidesuspensionwithpolyethyleneglycolvs.shearrate.
viscosityversusappliedshearrateofSiCsuspensionswithPEG.ItcanbeseenfromFig.4thatallthesuspensionsex-hibitedshear-thinningbehaviorandtheviscosityincreasedwiththeamountofPEGinSiCsuspensions.PEGhashy-droxylgroup,soitiseasilydissolvedinwater.However,PEGcannotbeadsorbedonthesurfaceofSiCinalargeamountandimprovethestabilityofSiCsuspension.ThelargemolecularchainsofPEGdistributeinthesuspension,whichpartlyinhibitsthe uidityandcausestheincreaseofthesuspensionviscosity.
3.3.PreparationandpropertiesofSiCgreenbodiesAccordingtoFig.1,thestablesuspensionsofSiCwith50vol.%solidloadingwerepreparedbydispersingSiCpowdersintheorganicmonomersolutionswithoutandwithPEG.Aftermixingwith1vol.%(basedonthemonomersolution)initiatorsolution(ammoniumpersulphate,5wt.%aqueoussolution)and0.1vol.%(basedonthemonomersolution)catalyst(TEMED),thesuspensionswerecastintopolymermoulds.Theconsolidationtimeofthesuspen-sions,thedegreeofsurfaceexfoliation,bendingstrengthandrelativedensitiesoftheSiCgreenbodiesareshownin
Table1.ItcanbeseenfromTable1that,withtheincreaseofPEGamountinthesuspensions,theconsolidationtimewasextended.Thisisprobablyduetothefactthatthecontactamongmonomers(AM)wasinterferedwithandhinderedbyPEGinthesuspensionduringtheprocedureofin-situpolymerizationofmonomericsystems.Duetotherheologicalbehaviorandconsolidationtime,aminimumPEGconcentrationinSiCsuspensionisrequiredtoinhibitthesurfaceexfoliationphenomenon.Fromtheexperimentsitcanbeconcludedthatatleast2.5wt.%ofPEGinthesuspensionisrequiredtoinhibitsurfaceexfoliation.ThebendingstrengthandrelativedensityofthedriedgreenbodieswithPEGwerelowerthanthatwithoutPEG;butthereducedstrengthishighenoughtoallowinexpensivegreenmachiningofthegreenbodiesintomorecomplexshapes.Fig.5showsthephotosofthegreenbodiesgelcastfromSiCsuspensionswithoutandwith1.5and3.0wt.%PEG.ItcanbeseenthatthegreenbodywithoutPEGhadanobvi-ousexfoliationlayerthatwaseasilybroken.Theboundarybetweenthelayerandrigidpartwasveryclear.Thegreenbodywith1.5wt.%paratively,thegreenbodywith3.0wt.%hadarigidsurfacewithoutexfoliationlayer.ThemechanismofPEGinhibitingthesurfaceexfoliationmightbethatthepolymerpreventsthereactionbetweenoxygenandmonomerfreeradical, R–CH2 duringthegellingpro-cedureinair.Fig.6(a)and(b)showthemicrostructures
Table1
Propertiesandgelationofthesuspensionwith50vol.%siliconcarbideatvariousconditionsNo.12345678
ab
AmountofPEG(wt.%)00.250.51.01.52.02.53.0
Consolidationtime(min)a2030354244505460
SurfaceofsamplesObviousexfoliationObviousexfoliationObviousexfoliationObviousexfoliationPartialexfoliationPartialexfoliationNoexfoliationNoexfoliation
Bendingstrength(MPa)11.798.404.574.344.152.073.374.02
Relativegreendensity(%)b57.9654.4551.2349.8250.2247.8851.4553.78
Consolidationtimereferstothetimeofchange
fromsuspensiontowetgreenbody.Thetheoreticaldensityofsiliconcarbideis3.127g/cm3.
F.Lietal./MaterialsScienceandEngineeringA368(2004)255–259
259
Fig.6.MicrostructuresofsiliconcarbidegreenbodiespreparedfromsuspensionswithoutPEG(a)andwith3.0wt.%PEG(b)(Thearrowsinthepicturesindicatethepolymernetworkingreenbodies).
obtainedbythegelformedbypolymerizationofmonomerwithoutandwithPEGmixedSiCpowder.Maetal.[12]re-portedthattherewasnopolymernetworkinaluminagreenbodypreparedfromaluminasuspensionwithPEG.ButfromFig.6(a)and(b),itcanbeobservedthatboththesampleshadobviouspolymernetwork,whichindicatesthatPEGonlypartlyinhibitedthecontactofmonomersAM,butnotcom-pletelypreventedthecross-linkingofthemonomers.ThusarigidgreenbodycanbeobtainedfromtheSiCsuspensionwithPEG.
Acknowledgements
TheauthorsthanktheNationalKeyBasicResearchDe-velopmentProgramofChina(973Program,GrantNo.G1999064900-4)andPostdoctoralFundofChina(GrantNo.2003033295).TheauthorsalsothankMr.M.J.Dongforthemeasurementofzetapotentialoftheceramicpowder,Dr.Q.WuforthesupporttorheologicaltestsandMr.E.K.Mengfortheassistancetomechanicaltests.References
4.Conclusions
Anewgelcastingsystembasedonaqueoussuspensionofsiliconcarbide/polyethyleneglycol/acrylamidecross-linkedwithacouplingagentN,N -methylenebisacrylamidehasbeendeveloped.Theroleofpolyethyleneglycolinthissystemistoeliminatethesurfaceexfoliationofsiliconcarbidegreenbodiesgelledinairandmakethegel-castingasimpleprocess.Theadditionofpolyethyleneglycolhadnoobviouseffectonthecolloidalbehaviorofsiliconcarbidepowderinaqueoussuspension,butin-creasedtheviscosityoftheslurriesanddecreasedthemechanicalstrengthofthegreenbodies.Astheamountofpolyethyleneglycolincreased,thesurfacesofsiliconcarbidegreenbodiesbecamemoreandmorerigidandthesurfaceexfoliationwascompletelyeliminatedwhenthepolyethyleneglycolinthegreenbodieswasover2.5wt.%(basedonsiliconcarbideweight).Themechanismofpolyethyleneglycolinhibitingthesurfaceexfoliationofsiliconcarbidegreenbodiesisprobablybecausethepoly-merpreventsthereactionbetweenoxygenandmonomerfreeradical, R–CH2 duringthegellingprocedureinair.
[1]K.Komeya,M.Matsui,in:R.W.Cahn,P.Haasen,E.J.Kramer(Eds.),
MaterialsScienceandTechnology:AComprehensiveTreatment;in:M.V.Swain(Ed.),StructureandPropertiesofCeramics,vol.11,VCH,Weinheim,1994,p.517.