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AdvancedCeramicsProcessingLecturenotesLouisWinnubstUT-code:373701UniversityofTwenteFacultyofScienceandTechnologyP.O.Box2177500AEEnschedeTheNetherlandse-mail:a.j.a.winnubst@utwente.nlAugust2008 3PrefaceTheselecturenotesareintendedforacourseonAdvancedCeramicsProcessing.Thisisstilladraftversion.Somepartsarenotcomplete(especiallymorequantificationisnecessary)Thegeneralaimofthiscourseistoobtaininsightsinprocesses,whichplayaroleinthefabricationofinorganic(orceramic)materialsandceramiccoatings.Thefabricationprocessistreatedandtheimportanceisemphasizedforunderstandingtheeffectsofprocessingvariablesontheevolutionofmicrostructuralparameters.Ifonehassufficientinsightintheseveralprocessstepsitispossibletomakeareproduciblematerialwithregardtopropertiesandmicrostructure.Theobjectiveinmaterialsprocessengineeringistofindrelationsbetween(desired)materialspropertiesandrelevantmicrostructuralparametersononesideandtounderstandwhichprocessparameterchangesacertainmicrostructuralparameterontheotherhand.Themicrostructurein-cludescharacteristicslikecrystalstructure,chemicalcomposition,crystallitesizeandaggregateoragglomeratemorphology,poresizeandporemorphology.Theintegralceramicfabricationprocessisdividedintothefollowingprocessstepsorbasicproc-esses:·Powderpreparation·Powdertreatmentslikemillingandmixing·Formingintoagreenshape·Coatingtechniques·SinteringEachstephasitsspecificinfluenceonthemicrostructure.Thereforecontrolandknowledgeofthewholeprocessisimportant.Optimalpropertiesrequireoptimisationandcontrolofeachofthesestepsinacoherentway.Thefollowingbasicphenomenawillbetreatedinthecourseforobtainingabetterunderstandingofthebasicprocesses:·Particlesizeandmorphology·Interactionbetweenparticles(agglomeration,suspensionstability,rheology,etc.)·Nucleationandcrystallisation·Reactionsbetweensolidstateparticles 4Literature.Thefollowingbookscoverthebroadfieldofceramicprocessengineering:§J.S.Reed“PrinciplesofCeramicProcessing”2nded.,JohnWilley&Sons(NewYork)1995(ISBN0-471-59721-X)§W.D.Kingery,H.K.Bowen,D.R.Uhlmann,“IntroductiontoCeramics”2ndEdition,JohnWiley&Sons(NewYork)1976.§D.W.Richerson,“ModernCeramicEngineering”2ndEdition,MarcelDekker(NewYork)1992.§T.ARing,“Fundamentalsofceramicpowderprocessingandsynthesis”AcademicPress(SanDiego)1996(ISBN0-12-588930-5)§D.Segal,“Chemicalsynthesisofadvancedceramicmaterials”CambridgeUniversitypress(Cambridge)1989§D.H.Everett“Basicprinciplesofcolloidscience”RoyalSocietyofChemistry(Cambridge)1988(ISBN0-85186-443-0)§Y.M.Chiang,D.BernieIII,W.D.Kingery,“Physicalceramics;Principlesforceramicscienceandengineering”,JohnWiley&Sons,Inc.(NewYork)1997(ISBN0-471-59873-9)§R.A.Terpstra,P.P.A.C.PexandA.H.deVries“CeramicProcessing”Chapman&Hall(Lon-don)1995(ISBN0-412-59830-2)§P.J.vanderPut“Theinorganicchemistryofmaterials:Howtomakethingsoutofelements”Plenumpress(NewYork,London)1998§H.Yanagida,K.KoumotoandM.Miyayama,“Thechemistryofceramics”JohnWilley&Sons(Chichester)1996(ISBN0-471-95627-9) 5ContentsPreface3Contents51Introduction71.1Theprocess–microstructureandhomogeneity71.2Preparationofbulkceramics91.3Phasediagramsinceramics112Characteristicsofpowdersandcompacts152.1Definitionsofceramicpowders152.2Particlemorphology182.3Particlesizedistribution192.4Chemicalandphysicalcharacterisationmethods202.4.1Flowbehaviourofpowders202.5Compactcharacteristics213Interactionsbetweenpowderparticles233.1Adhesiveforcesinagglomerates233.2Particlesinasolvent;theDLVOtheory253.2.1Electrostaticforcesaroundasingleparticle;theelectricdoublelayer253.2.2VanderWaalsattraction293.2.3TheDLVOpotential303.3Rheology313.3.1Newtonianfluid313.3.2Non-Newtonianbehaviour323.3.3Shearrate-dependentbehaviour323.3.4Time-dependentbehaviour353.3.5Weissenbergeffects383.4Solidstatemechanics394Preparationofceramicpowdersandcoatings414.1Solidstatepreparationofinorganic(multicomponent)powders414.1.1Anintroductiontosolidstatediffusion414.1.2Examplesofsolidstatepreparation414.2Nucleation,crystallisationandcrystalgrowth434.2.1Theory434.2.2Examples434.3WetChemicalpreparation454.3.1Dispersionmethods464.3.2Co-Precipitation484.3.3Complexation494.3.4Sol-Gel494.4Preparationfromthegasphase504.5Preparationfromthe(partly)meltedphase514.5.1Plasmaspraying515Treatmentsofpowders53 65.1Millingorcomminution535.2Mixing535.3Granulation(controlledagglomeration)535.3.1Layeringgranulation535.3.2Pressgranulation535.3.3Spraydrying546Processesforcompaction556.1Drypressing556.1.1Powderproperties576.1.2Fillingthedieormould576.1.3Microstructuredevelopmentduringdrypressing.576.1.4Die-wallfrictionduringuniaxialcompaction606.1.5Drypressinginpractice626.1.6Specialtechniquesindrypressing636.2Suspensionprocessing636.2.1Slipcasting636.2.2Colloidalfiltration646.2.3Centrifugalcasting646.2.4Tapecasting646.2.5Othersuspension/slurrytechniques656.2.6Coatingsfromsuspensions666.3Pasteprocessing666.3.1Extrusion666.3.2Injectionmoulding677Thermalprocessingofgreencompacts697.1Drying697.2Binderburn-out698Sintering738.1Drivingforceinsolidstatesintering748.2The(solidstate)sinteringprocess758.2.1Thecoordinationmodel778.2.2Graingrowthanddensification808.3Liquidphasesintering808.4TheMasterSinteringCurve(MSC)818.5Reactivesintering818.6Specialsinteringtechniques818.6.1Pressuresintering818.6.2MicrowaveandRFsintering828.6.3Fastfiring828.6.4SparkPlasmaSintering838.7Practicalaspectsofsintering838.7.1Furnaces838.7.2Temperaturecontrol848.7.3Atmosphereandadditives84 71IntroductionInthiscourseon“CeramicProcessingandMicrostructure”themainsubjectwillbethefabricationofproductsfrompowders.Skillswillbehandedforunderstandingprocessesthatplayanimportantroleinthepreparationandprocessingofinorganicmaterials,especiallyceramicmaterials.Inmate-rialtechnologyinsightisrequiredinbasicphenomenaaswellasinbasicprocesses.Inthischapterfirsta“definition”ofceramicsisgivenfollowedbysomegeneralaspectsonceramicprocessing.Ceramicsareinorganic(non-metallic)materials,abletowithstandelevatedtemperatures(inexcessof500°C).Theyareavailableasbulkmaterialsorasacoatingonothermaterials(metals,poly-mersorceramics).Oneoftheprocessstepsinthefabricationofbulkceramics(andsometimesalsoforceramiccoatings)isafiringorsinteringstep.Sinteringmeansthatitobtainsatemperaturetreatmentsuchthatmeltingdoesnotoccuroronalimitedscaleonly.Aceramicobtainsitsfinalcharacteristicpropertiesafterthesinteringstep.Theresultingmaterialhasacertainmicrostructure;ceramicsmaybesingle-phaseormixedphaseandthereisgenerallyporosity,rangingfromalmost100%tonear0%.Ceramicsfindnowadaysawidespreaduseinabroadrangeofapplications.Generallyceramicmaterialsaredividedintwomaingroups:Tradi-tionalceramicsandadvanced(ortechnical)ce-ramics(seeFigure1-1).Examplesarebricks,householdporcelain,high-temperaturefurnacerefractorymaterials,theinsulatingpartofsparkplugs,ferritemagneticmaterials,BaTiO3capaci-tormaterials,etc.Themainattentioninthiscoursewillbefocus-sedonbulkceramics.However,alsosomeex-Figure1-1:Subdivisionofceramicmaterialsintwomainamplesofceramiccoatingswillbetreated.groups1.1Theprocess–microstructureandhomogeneitythUptoaboutthefirsthalfofthe20centurymostnewproductswereseenasinventionsratherthantheplannedoutcomeofresearchanddevelopment.Beforethedevelopmentofscientificinsightsof(ceramic)materialprocessing,thepropertiesoftheproductwereoftencorrelatedwithchangesinaprocessingoperation.Theprobabilitythatadjustmentsbasedontheseempiricalcorrelationswillproducesignificantadvancesissmall,becausethepotentialnumberofunsuccessfulcombinations1ofvariablesisalwaysrelativelylarge.Theobjectivesofthescienceofceramicprocessingaretoidentify(chemicalandphysical)charac-teristicsofthematerialsystemandunderstandtheeffectsofprocessingvariablesontheevolutionofthesecharacteristics.Thesecharacteristicsormicrostructuralparametersdeterminetheproper-tiesofthematerial.1“Principlesofceramicprocessing”ed.byJamesS.Reed,JohnWiley&Sons(NewYork)1995 8AdvancedCeramicsProcessingTheobjectiveinmaterialsprocessengineeringisthereforetofindrelationsbetween(desired)ma-terialspropertiesandrelevantmicrostructurepa-rametersononesideandtounderstandwhichprocessparameterchangeacertainmicrostruc-turalparameterontheotherhand.Themicro-structureincludescharacteristicslikecrystalstructure,chemicalcomposition,crystallitesizeandaggregateoragglomeratemorphology,poreFigure1-2:Relationshipsinmaterialsprocessingsizeandporemorphology.Anotherimportantaspectwithin(inorganic)materialstechnologyisthehomogeneity.Agoodho-mogeneitymeansa/oanarrowsizedistributionofbothparticlesandporesinthecompactincom-binationwithmorphologicalandcrystallographichomogeneityonascale,whichisintheorderoftheparticlesize.Anexampleofadensehomogeneous(green)compactofTiO2powderanditsmi-crostructureaftersinteringareshowninFigure1-3.Forcomparisonthegreencompactofanag-glomeratedTiO2powderanditsfinalmicrostructureaftersinteringareshowninFigure1-4.Figure1-3:ScanningelectronmicrcoscopepicturesofadenseuniformcompactofTiO2powder(A)andthemicrostruc-turesinteredto>99%ofthetheoreticaldeinsityin90minutesat1050ºCFigure1-4:Scanningelectronmicrographsof(A)aporouscompactofagglomeratedTiO2and(B)thefinalmicrostruc-turein90minutesat1000ºC 1Introduction91.2PreparationofbulkceramicsTheproductionprocessforceramicmaterialsisschematicallygiveninFigure1-5.Theintegralfabricationprocessisdividedintofoursubsequentsteps:1.powderpreparation,2.compactionintoagreenshape,3.sinteringand4.posttreatment.Eachstepcanhaveitsspecificinfluenceonthemicrostructureandthereforecontrolandknowledgeofthewholeprocessisimportant.Optimalpropertiesrequireoptimisationandcontrolofeachofthesestepsinacoherentway.Powderpreparationisthefirststepinceramicfabrication.Powderscanbemadebysolidstatere-actions,recrystallisation,precipitationorpolymerisation.Powdercompactsareformedbybringingthepowderparticlesclosetoeachother.Thisprocessingsteptransformsthepowderfeedmaterialintoagreenproductwithaspecificsize,shape,strengthandmicrostructure.Theformedcompactshouldhavesufficientstrengthforhandling.Sinteringthiscompactbyheatingittohightemperaturesgivestheproductitsfinalstrengthandmicrostructure.Inordertoobtainproductsthatrequireclosetolerances,post-treatments,likemachiningwithmoreorlessstan-dardworkshoptools,arecarriedoutafterthesin-teringstep.PowderpreparationInclassicalceramicsminerals,e.g.naturalclays,areusedastherawmaterial.Theseclaysareinfactpaste-likesuspensionsofsilicate-basedparti-clesinwater.Withthedevelopmentofceramicsandmorespecific,technicalceramics,inthepastcenturyrequirementsoncomposition,purity,par-ticlemorphology,sizeanddegreeofagglomera-tionincreasedenormously.Consequentlythemin-eral-typerawmaterialsweresubjectedtovariousrefiningstepssuchasparticleandagglomeratereduction,purificationandsizeclassification.Forceramicswithhighaddedvalue(asisthecaseforbioceramics)thepreparationofpurelysyntheticpowderscameintoscope.Figure1-5:DifferentstepsinthefabricationprocessSomeexamplesofsyntheticceramicpowderofbulkceramics.preparationare:·RecrystallisationprocessessuchastheBayerprocessforthepreparationofaluminapowders·Precipitationisoneofthewet-chemicalpowderpreparationmethods.Hereaprecipitateisformedfromasolution.Acalcinationprocesstoformthefinalcrystalphaseoftenfollowsthisprocess.·So-gel.Withthistechniqueparticlesaregrownfromasolutioninaverycontrolledwayinwhatonemightcallan“inorganicpolymerisationprocess”.·Solid-statereactionsinwhichthecompoundispreparedbyreactionoftwoormoresolidcomponents.Preparationbysolid-statereactionmayhavebeenprecededbyaprecipitationprocessinwhichforexamplehydroxides,nitrates,sulphates,acetates,citratesandcarbonatesareformedfirstandafinaloxidephaseisformedbyacalcinations(solid-statereaction)step.·Methodsinvolvingvapourasreactants.(e.g.CVD=ChemicalVapourDeposition)Thepurityandmorphologyofpowdersandthedegreeofagglomerationshowastrongcorrelationwiththemethodofpreparation.Foradvanced,highqualityceramiccomponentspowdersareoften 10AdvancedCeramicsProcessingpreparedbycontrolledprecipitationtechniques.Advancedpowderscanalsobepreparedfromgasphasecomponents(CVD)inwhichhomogeneousnucleationandsimultaneousgrowthisoftenas-sistedbylaserbeamirradiation.Thesolid-statereactiontechniqueisoftenappliedtopreparepowdersofcomplexmulti-componentsolids.Arathercoherent,sintered,massisgenerallyobtainedwiththismethod.Aggregationcan,inprinciple,beexpectedforsolid-statereactionpowderswhilehomogeneitywillneedattention.Powderpreparationtechnologyincludesafter-treatmentstoimprovetheparticlemorphology.Theprimarygoalofthesetreatmentsisgenerallydeagglomerationordeaggregationsothatintheidealcaseanassemblyof“loose”primaryparticlesisobtainedwithanarrowsizedistribution.Well-knownexamplesofsuchtreatmentsarewetattritionmilling,vibrationmilling,dryyetmillingandforlabscaleapplicationsultrasonicdeagglomerationtreatments.Alsocontrolledgranulationbye.g.spraydryingisamethodtoimprovepowdermorphology(seechapter5).CompactionProcessingTable1-1givesasurveyofthemainprocessingtechniquesforthefabricationofa“green”ceramiccompact.Ascanbeseenfromthistable,thefeedmaterialformostoftheprocessingtechniquescontainsamixtureofpowderandadditives.Additivescanbedividedinsolvents,deflocculants,binders,lubricantsandplasticisers.Itmustbementionedthatalltheseadditivesmustbeevaporatedorburned-outfromthecompactpriortothepore-closurestepinthesinteringprocess.Solventsareusedtowetthepowderparticlesandtoprovidefluidity.Theextentofdispersionisinfluencedbydeflocculants,whichaffecttheelectrostaticinteractionsorsterichindrancebetweenparticles.Themainobjectiveofdeflocculantsisthede-agglomerationofpowdersintouniformpar-2ticlesandthepreventionofflocculation.Bindersareaddedtoprovideenoughadhesivestrength,toincreasetheviscosityofthesuspensionorthepasteand,thereby,influencetherheologyofthesystem.Inaddition,bindersareusedtoprovidestrengthtothegreenbody.Plasticisersaremostlyusedincombinationwithabindertosoftenthebinderinthedrystate,therebyincreasingtheflexi-bilityofthegreencompact.Lubricantsareusedinmanycompactiontechniquestoreducethemu-tualfrictionbetweentheparticlesandthefrictionbetweentheparticlesandthedie.Foreachoftheprocessingtechniques,whicharediscussedinthiscourse,theuseofadditiveswillbeaddressedincombinationwiththeirpurposeforthatspecifictechnique.Afterthisgreenformingsteptheproductmusthavesufficientstrengthforfurtherhandling.Themostcommoncompactiontechniqueforceramicspowdersistherelativecheappressingtechnique.Table1-1;Feedmaterialsandshapesofthegreencompactforthecommonformingmethods.FormingmethodFeedmaterialShapeofgreenproductdryorsemidryprocessinguniaxialpressingpowderorfree-flowinggranulesSmallsimpleshapesisostaticpressingpowderorfragilegranulesLarger,morecomplexshapesSuspension/colloidalprocessingslipcasting,colloidalfiltrationsuspensionwithlowconcentrationofadditivesThincomplexshapestapecastingsuspensionwithrelativelyhighbindercontentThinsheetsPasteprocessingextrusionhighviscousmixtureofpowderandbinderUniformcrosssectioninjectionmouldingmixtureofpowderandthermoplasticbindersSmallcomplexshapes2Theworddeflocculantisderivedfromtheprocessofconvertingaflockedsystemorabatchofpowderintoasystemofwell-dispersedparticles.Inthesamewaytheworddispersantisused,derivedfromtheprocessofdispersingpowdersinaliquid.Astabiliserisaddedtomaintainasysteminawell-dispersedstate.Iftheseagentsaredescribedinchemicaltermsonespeaksofsurfactantsincaseofsmallinterface-activemoleculeswhilelargermoleculesaresimplyindicatedaspolymers.Inadditionitiswell-knownthatsimpleioniccompoundssuchasHNO3canhaveasignificantcolloidalactiv-itytoothroughsurfacechargingeffects. 1Introduction11SinteringSinteringisthecrucialtechnologicalstepforobtainingstrengthtopowdercompacts.Itcanbegener-allydefinedasahigh-temperatureprocessofmatterredistributionstimulatedbythefreeenergyasso-ciatedwiththelargefreesurfaceenergyoffineparticlesinpowdercompacts.Decreaseoftheinternalfreesurfaceduringsinteringleads,asarule,todensificationandstrengtheningofthecompact.Gen-erallynoliquidphase(solidstatesintering)orjustalimitedamountofliquidphase(liquidphasesin-tering)ispresentduringsinteringoftechnicalceramics.Sothedrivingforceforsinteringisthereductionoftheinterfaceenergy.Ifnosecondphasesarepresentequation(1.1)holds:DG=DG+DG=gDA+gDA(1.1)sbssbbWhereDGisthechangeinGibbsenergyofthecompact,gsandgbthesurfaceenergy,andDAsandDAbthechangeinsurfaceareaofthesolid-vapourandthesolid-solid(grainboundary)inter-face,respectively.SinteringoccursifDG<0.EarlyintheprocessthedecreaseinGsisthemostimportantfactor.LaterintheprocessDGsissmallerandaccordinglytheincreaseinDGbmustslowdowntokeepDGnegative.Thiscanbereal-isedbygraingrowth.Solidstatesinteringiscommonlydividedintothree,notclearlydistinguishable,stages(seealsoFigure1-6):·Initialstage;thepowderparticlesgrowto-getherandnecksareformedattheinterfaces.Someparticlerearrangementmayoccuraswell.Porosityisintherangeof40-60%.Alargeinfluenceoftheparticlesizeontherateisobserved.Figure1-6:Schematicrepresentationofthemicrostruc-·Intermediatestage;theporesandparticlesturedevelopmentduringtheseveralstagesofsintering:formanintersectingnetwork.Thedensity(A)“green”compact,(B)neckformation,(C)porechan-increasesto85or90%whereporechannelsnels,(D)closedporesandgraingrowth.breakupandformdiscretepores.Conse-quently,theporositytypechangesfromopentoclosed,andgraingrowthoccurs.·Finalstage;discreteporesarepresent,whichcanonlyberemovedbyfurthergraingrowth.1.3PhasediagramsinceramicsInmaterialssciencephasediagramsareimportanttools.Aphasediagramisagraphicrepresenta-tionofthephasesaspresentinequilibriumstateofmaterialswithacertaincompositionandasfunctionoftemperature.Inthisparagraphthezirconia-yttriasystemwillbetreatedasanexample.Alsoinotherpartsofthiscoursezirconia-yttriasystemswillbeusedasexamplesforthedifferentprocessingstepsAtambientpressure(undoped)ZrO2canoccurinthreecrystallographicdistinctpolymorphs:cubic,tetragonalandmonoclinic.Atroomtemperature,purezirconiahasamonoclinic(m)structurethatisstableuptoabout1170ºC,atwhichtemperatureittransformsintothetetragonal(t)structure.Uponfurtherheatingatransformationtothecubicphasetakesplaceat2370ºCandthisphaseisstableuptothemeltingpointofabout2680ºC.ByadditionofstabilisingoxideslikeMgO,CaO,Y2O3andCeO2thecubicphasecanbestabilisedtoroomtemperature.InFigure1-7aphasedia-gramisgivenforthezirconiarichpartoftheZrO2-Y2O3system. 12AdvancedCeramicsProcessingItisevidentfromthephasediagramthatthetetragonalphaseforpurezirconiaisunstableatroomtempera-ture.Retainingthehightemperaturephasebyquench-ing(rapidcooling)isnotpossibleduetothediffu-sionless,martensitic,characterofthetetragonal-to-monoclinicphasetransformation.Thistransformationisaccompaniedbyadilatationalstrain(volumeexpan-sion)of0.067andashearstrainof0.080.Sincethistransformationprovidesthemainsourceoftoughen-ing,itisofutmostimportancetoretainthetetragonalphaseinametastablestateatroomtemperature.Stabi-lisationofthetetragonalphaseatroomtemperatureispossiblewhencertainmicrostructuralconditionsarefulfilled.Thiscan(qualitatively)beexplainedbyre-gardingthefreeenergychangeofthetetragonaltotàmmonoclinic(tàm)phasetransformation(DG).tàmTransformationwillnotoccurwhenDG>0.Ac-4cordingtoM.Yoshimurathetàmtransformationisnotonlythermodynamicallydrivenbutalsokineti-cally.Attemperaturesbelow~1200°CthisphaseFigure1-7:Phasediagramofthezirconia-richpart3transformationismorekineticallydeterminedthanofthezirconia-yttriasystem;afterthermodynamically.Thiskineticterm(anenergybarrier)increaseswithdecreasinggrainsize,meaningthatametastabletetragonalzirconiaphasecanbeobtainedatroomtemperaturewhenasufficientlylargemolarsurfaceareaandsubsequentlyasmallgrainispresent.Forundopedzirco-niathecriticalgrainsizebelowwhichnotàmtransformationoccursatroomtemperature(=dc)is30nm.Thiscriticalgrainsizeincreaseswithyttriacontent;dcvaluesof350and500mmarementionedforzirconiaceramicsdopedwith4and6mol%YO1.5respectively.Ifzirconiaparticlesarepresentasasecondphaseinaceramicmatrixofe.g.Al2O3thekinetictermforphasetransformationcanincreasemorebyaconstraintofthematrixactingonthezirconiaparticles.Thismatrixconstrainta/odependsontheYoung’smodulus(E)ofthematrix.AlargerE-modulusofthematrixresultisamorestabletetragonalstructure.TheEmodulusofaluminaismuchhigherthanthatofzirconia(seechap-ter4).Soitispossibletostabilisethetetragonalstruc-turetoroomtemperatureforundopedzirconiawithgrainsofabout0.5mmifthiszirconiaisincorporatedinanaluminamatrix.Inthiswayitispossibletomakeso-calledZirconiaToughenedAlumina(ZTA)where15weight%undopedtetragonalzirconiaishomoge-neouslydispersedinanaluminamatrix.AnexampleofsuchamicrostructureisgiveninFigure1-8.Agingofzirconia-basedceramicsTetragonalzirconiaceramicsarepronetoaginginthepresenceofwater.Thismeansthataslowttomphasetransformationoccursinthepresenceofwaterorwa-Figure1-8:SEMpictureofzirconia-toughenedtervaporattemperaturesbetween150and200°C.Ag-alumina(ZTA):ingstartsinisolatedgrainsonthesurfacebyastress15wt%ZrO2(whitegrains:200nm)-corrosiontypemechanism.Studiesshowedthatan85wt%Al2O3(darkgrains:500nm)3M.G.Scott,J.Mater.Sci.,10(1975)15274M.Yoshimura,SolidStateIonics,86-88(1969)1131-1149 1Introduction13increaseininternalstress,causedbydiffusionofwaterradicalsintotheZrO2lattice,triggerstheinitiationofthettomtransformation.Tensilestressesgeneratedbythisphasetransformationcaninducemicrocracksatthegrainboundaries,makingiteasierforwatertodiffuseinsidethebulkofthematerial.Inordertoreduceagingeffect,thedensity,grainsize,homogeneityofthephasedis-tributionandtheresidualstressstateofthesurfaceofthematerialneedtobecontrolled. 152CharacteristicsofpowdersandcompactsInthischapterseveralmicrostructuralcharacteristicsofpowdersandcompactswillbediscussedThepropertiesofaceramicproductdependlargelyontherawmaterialsfromwhichtheyareformed.Itisthereforeessentialtoknowandtocontrolpowderproperties.Notonlythechemicalcomposition,purityandcrystalstructurecontrolthefinalpropertiesoftheceramicbutalsonumer-ousotherpowderpropertiessuchassizeandsizedistribution,packingdensity,flowandcompac-tioncharacteristics.Themostimportantpropertiesandtheircharacteristicscanbedividedasfol-lows:·Propertiesdependentonthephysicalnatureofthepowder,àparticlesizeandparticlesizedistributionàparticleform(formfactor)àsurfaceareaàporosityàporesizeandporesizedistributionàpackingdensityandpackingstructureàdynamicproperties(e.g.flow)·Propertiesdependentonthechemicalnatureofthepowder,àchemicalcompositionàpurity/impurityàphasecompositionàsurfaceenergyàsurfacereactivityàsurfacecomposition·Propertiesdependentonbothphysicalandchemicalnature,àdegreeofaggregation/agglomeration.Thepropertiesofagreencompactandasinteredproductstronglydependonthecharacteristicsofthestartingmaterial.So,characterisationofpowdersisanimportantstepinalmosteveryceramicprocess.InPartIIIofReed[1]importantcharacteristicsofceramicpowdersaregiven.Thepurityandmorphologyofpowdersandthedegreeofagglomerationofpowderparticlesshowastrongcorrelationwiththemethodofpreparation.Thegeneralidealmorphologyofpowdersformodernceramicprocessingshould,generallyspoken,meetthefollowingrequirementsinordertobeabletopreparehomogeneouscompacts:·Narrowparticlesizedistribution,·single-phase,·lowdegreeofagglomerationoraggregation,·lowcontentofunwantedimpurities,·nodestructivephase-transformationsduringfurtherprocessing2.1DefinitionsofceramicpowdersInordertodeterminethepropertiesof(ceramic)powdersitisnecessarytomakesomeagreementsconcerningnomenclaturethatwillbeusedforthepresentationandevaluationofpowders.Notonlythedimensionsofthesolidparticlesbutalsotheirshapeandmutualattractiveandrepulsiveforceshastobetakenintoaccount.InthiscourseweusethedefinitionsthatareacceptedforASTMterminology[2].Theparticleisacommonworkingunitusedtodescribeparticulatematter.Itisthatstateofsubdi-visionofmatterwhoseshapedependsontheprocessbywhichitwasformedandonthein-tramolecularadhesiveforces.Suchadefinitiondescribesallparticulateentitiessuchasasingle 16AdvancedCeramicsProcessingcrystal,amultiphasesystem,anamorphousmaterial,aggregates,agglomerates,etc.Theseworkingunitsdonotchangeinsizeandotherphysicalpropertiesduringthemeasuringprocedure.Particlescanbedividedinultimateparticles,aggregatesandagglomerates.UltimateparticleAnultimateparticleofasubstanceisthesmalleststateofsubdivisionwhichretainsallthephysicalandchemicalpropertiesofthatsubstance.Thesepropertiesarealsohomogeneousonthatscale.Someexamplesofultimateparticlesare:·Crystallites.Acrystalliteisacrystallographicorderedassemblyofunitcells,·amorphousparticles.Thisisthesmallestgroupofmoleculeswithoutastrictlyorderedarrangement(glasses,polymers),·unitsofliquids(ingasoremulsion).Theseunitscanreachthedimensionofasinglemolecule,·gasunitsinaliquidorsolidstate;e.g.,pores.Figure2-1:Schematicrepresentationofcrystallites,ag-Onallthesetypesofultimateparticlesthesamegregates,andagglomeratesmathematicalcompilationscanbeapplied(size,sizedistribution,etc.).Inthiscoursetheultimateparticleisinmostcasesthecrystallite.Ifonlythistypeofultimateparticleisregarded,anultimateparticleisoftencalledaprimaryparticleorcrystallite.AggregateAnaggregateisanassemblyofsolidparticlesheldtogetherbystronginter-orintramolecularoratomic(adhesive)forces.Theseforceshaveachemicalcharacter.Incrystallinematerialsaggregatescanbetheproductofsolidstatereactionsandcalcination(sin-tering)treatments.Duringthesetemperaturetreatmentssufficientdiffusionofmatteroccursintotheneckregionsbetweenindividualparticleswhichcreatesstronginterparticlebonds.Aggregationorsolidbridgingdoesnotonlyoccurbetweentwosolidparticles,butasolidparticlecanalsoreactwiththecarryingfluid(liquidorgas;e.g.oxidation)orwithabinder.Crystallisationofdissolvedmaterialatthepointofparticlecontact(Oswaltripening)mustalsobementionedinthisfield.Fi-nallypartialmeltingandsubsequentcoolingcanbeasourceforaggregation.Aggregatesarestabletonormalhandlingandordinarydispersiontechniquessuchashigh-speedmixingandultrasonictreatments.Extremegrindingdecreasesitsdimensions.Aggregatesarestrongenoughtoretaintheiridentityduringgreenformingandcanthereforeaffectthedevelop-mentofthemicrostructureduringsintering(densification,graingrowth)andcaninfluencethefinalproperties(e.g.densityandgrainsize).Non-uniformaggregatescangiverisetoanon-uniformcompact.Inordertoobtainaceramicmaterialwithawell-definedmicrostructureandwell-definedpropertiesitisimportantthataggregatesaresmallandespeciallyuniforminsize,therebyprevent-ingtheformationofinhomogeneousproductscontainingcracksandfaults.AgglomerateInagglomeratessolidultimateparticles(crystalline/amorphous)oraggregatesareheldtogetherbyrelativelyweakadhesiveforces.Inmanycasestheseforcesareduetoanelectrostaticsurfacecharge,whichcanbegeneratedduringhandlingandprocessingoperationslikesievingordrying.Thesmallertheparticledimensions,thelargerthespecificsurfacechargedensity,hencethemoresevereagglomerationoccurs.Besidestheseelectrostaticforces,liquidbridgesandVanderWaalsforcesalsocauseadhesiveforcesinagglomerates(seesection3.1).Inthecaseofsinteractivefine-grainedceramicpowders,agglomeratesarealwayspresent.Controlofagglomerationisthereforeimportant.Thismeansthattheagglomeratesmustbeuniforminsize,regularinstructureandweakenoughsothattheyarefracturedduringcompaction,resultingina 2Characteristicsofpowdersandcompacts17uniformstackoftheindividualcrystallitesoraggregatesinthegreencompact.Agglomeratescanbereducedinsizebymilling,ultrasonificationordispersiontreatments,likefluidisation.Agglomeratescantransformintoaggregatesdur-ingtemperaturetreatmentslikesolid-statereactionandcalcination.Inthiswayirregularaggregatestructurescanarisewhichareevenlessfavourableforobtainingawell-definedceramicmaterial.AschematicdrawingcontainingthesethreeparticleworkingunitsisgiveninFigure2-1.Inliteraturethepowderisnotoftenaspecialsub-jectforanalysis.Inmanycasesitisnottreatedseparatelybutusedasapartof,e.g.wet-chemicalpowderpreparationorthestudyofsinteringbe-haviour.Inthesecasessometimesotherdefinitionsforparticlesareused.Forexample:·Grain.Thiscanbeusedforalltypesofparti-Figure2-2:Transmissionmicrographofazirconiapow-dercles(e.g.crystallitesize,agglomeratesize,crystallitesizeinasinteredcompact),·anagglomerateisoftendenotedasasoftag-glomerate,whileinthatcaseanaggregateisahardagglomerate,·flocculateorcoagulate:anagglomerateinaliquidmedium,·granule:granulesareagglomeratesaccordingtothedefinitionsmentionedabove.Thewordgranuleisoftenusedwhenagglomerationoc-cursinacontrolledway.Thisgranulationprocesscanbeveryimportantforformationofthegreencompactsandwillbediscussedinchapter6.Figure2-3ScanningelectronmicrographofazirconiapowderafterdyingandcalcinationInFigure2-2-Figure2-4severalmorphologiesofa(zirconia)ceramicpowderareshown.TheTEMpictureinFigure2-2showsanultimateparticlesizeorcrystallitesizeofapproximately10nm.Thispowderwaspreparedbymeansofawet-chemicalmethod(gel-precipitationtechnique),whichafterdryingandcalcinationresultsinir-regularagglomerates(seeFigure2-3).Figure2-4showsascanningelectronmicroscopepictureofsphericalgranules(agglomerates)madebyspraydrying.Figure2-4:Scanningelectronmicrographofsphericalgranulesmadebyspraydrying(TOSOHInc.) 18AdvancedCeramicsProcessing2.2ParticlemorphologyInonlyarelativelyfewtypesofpowderstheClassificationshapeparticleshapeissufficientlyregulartoprovideasingledefinitivedimensionforthe“size”.Foraacicularneedleshapesphericalparticlethisisthediameter,whileforacubethelengthoftheedgeisauniqueparame-angularroughlypolyhedralshapewithsharpterfortheparticlesize.Ingeneral,particlesizeedgesdataarerelatedtothediameterofasphere,whichisequivalenttotheparticleregarded.Thisdendriticbranchedcrystallineshapeapparentspheresizewillbedependentuponthefibrous(ir)regularlythread-likeshapespecificmeasurementtechniqueemployed.TheISOstandard(ISO,1995)recommendstheflakyplate-likeshapesymbolxtobeusedfortheparticlesizeandagranularirregularbutapproximatelyequidi-subscriptdenotingtheequivalentdiameterused.mensionalExamplesoftypesofparticlesizesandthetech-niquesusedforanalysisare:irregularlackinganysymmetry·xv:equivalentvolumediameter®gasadsorptionnodularroundirregularshape·xs:equivalentsurfacediameter®sphericalnominallysphericalshapemicroscopy·xw:equivalentsettlingdiameter®Table2.1:Qualitativeparticle-shapeclassificationssedimentationaccordingtoISOStandard3252.·xn:equivalentmeshdiameter®sieving.Inthediscussiongivenaboveeachparticleischaracterisedbyonlyonescalarnumber,whichdoesnotcontainanyinformationabouttheshapeoftheparticle.Ameasureoftheshapeofaparticlecanbeobtainedfromthesphericityindexy,whichisdefinedastheratioofthesur-faceareaofaspherewiththesamevolumeasthetestparticletotheactualsurfaceareaoftheparticle.Thesphericityofthepowderalsohasaninfluenceonthepackingdensity.Awaytoderivethesphericityofaparticleisfromtheratioofanytwoequivalentdiameters.ThemostcommonexampleistheWadellsphericityfactor:2æöxy=ç÷v(2.1)wxèøsFigure2-5:Particlemorphologiesofpowders:a)acicular,Theshapeoftheindividualparticleswithina(b)angular,(c)dendritic,(d)fibrous,(e)flaky,(f)granu-powdersignificantlyinfluencesitsbulkproper-lar,(g)irregular,(h)nodular,(i)spherical.ties,forexample,packing(apparentdensity),flowabilityandcompressibility.Thequalitativeparticle-shapeclassifications,definedbytheInter-nationalOrganizationforStandardizationinISOStandard3252arerepresentedintable2.1.ThesegeneralpowdermorphologiesareshowninFigure2-5 2Characteristicsofpowdersandcompacts192.3ParticlesizedistributionWhenthesizeofanindividualparticleisdefinedanddetermined,thenthenextproblemishowtoexpressthedistributionofparticlesizeswithinapowdersample.Asetofparticlesizescanbeplot-tedasafrequencydistribution.Inordertocharac-terisesuchadistributionthefollowingthreestatis-ticalquantitiesareveryoftenused(seeFigure2-6):·Modeisthevaluewherethefrequencydistri-butionreachesitsmaximumvalue,·Mediandividesthefrequencydistributioncurveintwopartswithequalsurfacearea,·Meanisthe“mainpoint”ofthefrequencydis-Figure2-6:Graphicrepresentationofthemode,mediantribution.andmeanvaluesofaparticlesizedistributionThesethreevaluesgivesomeinformationontheparticlesizedistribution;however,thesedataarenotsufficientforacompletedefinition.Forex-amplethe“broadness”(standarddeviation)ofthedistributionisnotknowninthisway.Theparticlesizedistributionisusuallyexpressedbyadistributionfunctionf.Awell-knowndistri-butionfunctionisthelog-normaldistributionfunction(seeFigure2-7)andthefrequencybywhichaparticlewithsizexiispresent,isthendefinedby:_21(lnxx-ln)igf(xi)=-exp[]2(2.2)sp(2)2sInthisequationsisthestandarddeviationofthedistributionandcanbecalculatedformacumu-_lativedistributionaswillbeshownlaterinthissection.xisthegeometricalmeanparticlesize:giI=ånxiilog_i=1logx=(2.3)gNInequation(2.3)niexpressesthenumberofparti-clesintheintervalIwithmeansizexi.Thesum-mationisoverI,whereIisthenumberofintervalsinwhichtheparticlesizedistributionisdivided.ThetotalnumberofparticlesanalysedisgivenbyN.Alog-normalparticlesizedistributionissymmet-ricandthereforemode,medianandmeanareiden-ticalandlieexactlyat50%ofthedistribution.InFigure2-7:Alog-normaldistributionplottedasarela-Figure2-7andFigure2-8,respectively,afractionaltivepercentagedistribution,usingalogarithmicscaleforandacumulativelog-normalparticlesizedistribu-theparticlesizetionaregiven[6].Acumulativedistributioncurveismostcommonlyusedbecausethesecurvescanbemoreeasilyinterpolatedandnormalised.Thecumulativecurvesmaybechoseneitherundersizeoroversize.Foralog-normaldistributionastraightlineinthedistributioncurveisobtainedifthelogarithmoftheparticlesizeisplottedagainstthecumulativepercentageoversizeorundersizeonaprobabilityscale.Fromthecumula-tivedistributionthemeanparticlesizeandthestandarddeviationcaneasilybedetermined.Themeanparticlesizeisat50%.Thestandarddeviation(s)isdefinedinsuchawaythatthesizeof68%oftheparticlesinthedistributioniswithintheinterval:__.Thegeometrical[xx-+ss,]ggstandarddeviation(sg)canbecalculatedfrom: 20AdvancedCeramicsProcessing84505016logs=logx-=-logxlogxxlog(2.4)g84wherexmeansthesizeat84%(40mminFigure2-8)etc.Thusmeanandstandarddeviationaresufficientparameterstocompletetheidentificationofthewholelog-normalparticlesizedistribution.Adetaileddescriptionofparticlesizemeasure-mentsandothermathematicalfunctionsusedtodescribeparticlesizedataaregiveninthebookofAllen[6].Figure2-8:Alog-normaldistributionplottedonlog-probabilitypaper2.4Chemicalandphysicalcharacterisationmethods·Particlesize.Severalmethodsfordeterminingtheparticlesizedistributionareavailable,e.g.:àSievingàLaserdiffractionàCrystallitesizedeterminationbyX-raylinebroadeningà(Electron)microscopy·Specificsurfacearea:Thisisdefinedastheaccessibleareaofsolidsurfaceperunitmassof2material(m/g)·Thermalanalysis(DTA,DSC,TGA)·Chemicalanalysis(X-rayfluorescence,…)2.4.1FlowbehaviourofpowdersTheflowabilityofapowderisdeterminedbyus-ingtheHallflowmeterasillustratedinFigure2-9.Thetimerequiredfor50gofthepowdertoflowthroughtheorificeof2.5mmisveryoftenre-portedastheflowabilityof(metal)powders.An-othermeasureforpowderstackinganduniformdiefillingistheHausnerindex.Twotypesofden-sitiesaredefinedforcalculatingtheHausnerin-dex,i.e.:·Theapparentdensity(ra)isthedensityafterfillingthedie.ThismassperunitvolumeofaloosepowdercanbemeasuredbyusingaHallflowmeter.Inthiscasethetestinvolvesfilling3a25cmcupwithpowderfallingthroughtheorificefromafixedheight(25mm).Thefilledlevelledcupisweighedandtheapparentden-3sityisdetermineding/cm.Theapparentden-Figure2-9:Hallflowmeter.sityofanon-free-flowingpowderisdeter-minedinasimilarmannerexceptthata5.1mmfunnelorificeisused(Carneyfunnel).Theap-parentdensityisequaltothepowderbulkdensity, 2Characteristicsofpowdersandcompacts21·thetapdensity(rt)isthedensitymeasuredaftervibratingacupwithpowderuntilthepowdervolumeremainsconstant.Theratiort/raiscalledtheHausnerindexandisameasureforfrictionalforcesinamovingmassofpowder.AlowvalueoftheHausnerindex(rt/ra®1)resultsinthemostuniformfillingofthedie.ForsmallparticlesoragglomerateslowHausnerindicescannotoftenbeobtainedbecauseofahighparticle-to-particleinteractionandbecauseoftheenhancedpossibilityofentrapmentofair(largevoids).2.5CompactcharacteristicsDensity/porosityManysolidscontainacertainvolumeofvoidsandemptyspaces.Thesecanbedistributedintheformofpores,cavitiesandcracksofvariousshapesandsizes.Thetotalsumofthisvoidsvolumeiscalledporosity.Porosity(e)isdefinedastheratioofthevoidvolumeVvtothetotalvolumeVtotofthespecimen.VvVtotVeryoftenthefollowingtypesofporosityaredefined:·Openoreffectiveporosity.Thesearecontinuouschannels,whichconnecttheinteriorofthespecimenwiththesurroundinggasorliquid.Openporescanbefurtherdividedindead-endandinterconnectedpores.·Closedporosity:Pores,whicharecompletelyisolatedfromtheexternalsurface.·Totalporosity.ThisthesumofopenandclosedporosityThedensityrofamaterialisdefinedastheratioofmasstovolume(r=m/V)Sometimesdistinctionismadebetweentrue,apparentandtotaldensity·Ingeneraltheopenporesizedistributioninapowder,powdercompactoraporousceramicbodyismeasuredby:àMercuryintrusionporosimetry,àGasadsorption-desorptionMercuryporosimetryAusefultechniquethatprovidesinformationaboutdistributionandamountofopenporosityismercuryintrusionporosimetry[1,10].Thismethodisbasedonthepropertythatmercuryisinter-actingwithmostsolidsasanon-wettingliquid(contactangle>130°)andthereforepressurehastobeappliedtoforceitinanevacuatedsurfacepore.Thesampleisplacedinanimpermeablecontainer.Thecontainerisevacuatedandback-filledwithameasuredvolumeofmercury.Pressureappliedtothemercurycausespenetrationofporeswithaparticularsize.Theintrudedradius(R)atanyappliedpressureiscalculatedfromtheWashburnequationforacylindricalpore:2LVcosRPwhereP=pressure,q=contactangleandg=surfacetensionofHgattesttemperature.Thecontactangleofmercuryofmostoxidesisabout130-140°.Pressuresupto400MPa(orevenmore)areavailable,whichenablespenetrationofporesrangingfromabout200µmto2nmAnintermezzoonwettingbehaviourandcontactangleTheequilibriumsituationthatexistsafteraliquiddropisbroughtintocontactwithasubstratedependsonthebalancebe-tweenadhesionandcohesion.Thestaticcontactangle(q)isameasureforthe 22AdvancedCeramicsProcessingwettingbehaviourandisgivenbytheYoung(-Gibbs)equation:g-gsvslCosq=glvHereglvistheliquid/vapoursurfacetensionandgslandgsvarerespectivelythesolid-liquidandsolid-vapourinterfaceenergies.Avalueofq=90ºisoftendefinedastheboundarybetweenwettingandnon-wetting.Avalueofq=0ºmeanscompletewet-ting;090ºweareinthenon-wettingre-gime.Afreeenergyofde-wetting(Ws)(alsocalledspreadingparameter)canbedefined:W=g-g-g=g(Cosq-1)SsvlvsllvIfWs<0cohesionoftheliquidoccursresultingintheformationofadroplet.Forma-terialswithahighsurfacefreeenergy(gsv)theformationofasolid-liquidsurfaceisfavourable.Thesetypesofsolidsurfacesshowhydrophilicbehaviour.Nonwettingsurfacesshowhydrophobicbehaviour.References1.Reed,J.S.Principlesofceramicsprocessing,2ndEd.,J.Wiley&Sons,NewYork(1995).2.Irani,R.R.,Calis,C.F.Particlesize:Measurement,interpretationandapplication,J.Wiley&Sons,NewYork-London(1963).3.Rumpf,H.,Schubert,H.AdhesionforcesinagglomerationprocessesinCeramicProcessingbeforefiring,Eds.Onoda,G.Y.,Hench,L.L.,J.Wiley&Sons,NewYork(1978)357.4.Everett,D.H.Basicprinciplesofcolloidscience,Royalsocietyofchemistry,Cambridge(1994).5.Sheppard,L.M.Am.Ceram.Soc.Bull.71[5](1992)715.6.Allen,T.Particlesizemeasurement,4thEd.,ChapmannandHall,(1990).7.Lukasiewicz,S.J.J.Am.Ceram.Soc.72[4](1989)617.8.Stamoto,H.J.Am.Ceram.Soc.Bull.70(1991)1651.9.Scarlett,B.Mater.Sci.Technol.17A(1996)100.10.D.W.Richerson“ModernCeramicEngineering”2nd.ed.MarcelDekker,Inc.(1992) 233InteractionsbetweenpowderparticlesInthischapterwedescribeinteractionforcesbetweenparticlesinthe“dry”state(agglomeration)andtheforces,whichtakeplacebetweenparticlesinasolvent,asisthecaseforsuspensions.Alsoattentionwillbepaidtoparticlesundershearinasuspensionorpaste(rheology).3.1AdhesiveforcesinagglomeratesWhenaparticleisinthe“dry”statethefollowingadhesionforcescanoccurbetweenthepowderparticles:·VanderWaalsforces,·Electrostaticforces,and·Liquidbridges.Forparticleslargerthan1cm,gravityforcesactingonparticlesaremuchlargerthanthenaturaladhesionforcesbetweenparticles.Iftheparticlesizeisreduced,thegravityforcedecreasesveryrapidly(bythethirdpoweroftheparticlediameter),whilenaturaladhesiondecreasesapproxi-1matelybythefirstorsecondpowerofthediameter.Fromaroughcalculation(asgiveninReed)itcanbeconcludedthatforceramicpowdersagglomerationoccurswhentheparticlesaresmallerthanapproximately40mm.Incaseof1mmparticlesthevanderWaalsforcesareaboutsixordersofmagnitudelargerthangravity.Thisistheworldoffineparticles:theirbehaviourdependsmuch2moreonsurfacephenomenathanonvolumeforces.Theseveraladhesionforceswillnowbetreatedseparately.VanderWaalsforcesTheVanderWaalsforcesarewell-knownbondingforcesbetweenmoleculesandatoms.Itconsistsofthreedifferentdipoleinducedforces,theKeesominteraction,theDebyeinteractionandtheLondoninteraction.·Keesominteractionoccurswhenapermanentmoleculardipolecreatesanelectricfield,whichorientsotherpermanentdipolesinsuchawaythattheywillattracteachother.·Debyeinteractionoccurswhenapermanentdipoleinducesadipoleinapolarisableatomormolecule.Theinduceddipoleisorientedinsuchawaythatattractionoccurs.·Londoninteractionoccursbyfluctuationsintheelectronsinatomsormoleculesinsuchawaythatinstantaneousdipolesareformed.Thiseffectleadstoattractionbetweenthetwoinduceddipoles.IneverysubstanceLondon-vanderWaalsforcesoccurwhicharetheresultoffluctuatingdipoles.Theforcesthatoccurbetweenindividualmoleculesandatomsdependonthedistance(r)between7thecentresofthemoleculestothepower-7(FvdW~1/r).ThedistancedependenceofthevanderWaalsforcesbetweentwomacroscopicsolidbodies(parti-cles)canbedescribedaccordingtotheHamakeranddeBoertheory.AccordingtothistheorythevanderWaalsmolecularforcesinthesolidbodycanbeintegratedoverthewholebodyandde-pendontheirsurfaceseparationa.FortwoparticleswithradiusRthevanderWaalsforcesareex-3pressedby:22ARplqR0F=-=-(3.1)vdW2212aa121J.S.Reed,“Principlesofceramicsprocessing”(1995)Example2.6page312H.Rumpf,H.Schubert,“Adhesionforcesinagglomerationprocesses”in“Ceramicprocessingbeforefiring”Ed.byG.Y.OnodaandL.L.Hench,J.Wiley&Sons,NewYork(1978)3573D.H.Everett,“Basicprinciplesofcolloidscience”Royalsocietyofchemistry(Cambridge)1994 24AdvancedCeramicsProcessing22WhereAistheHamakerconstant(=pq0l),q0isthenumberofatomsperunitvolumeandlisthevanderWaalsconstant.32l=hna(3.2)4-34Inequation(3.2)ishPlanck’sconstant(6.63x10Js),athepolarisabiltyoftheatomormoleculeandnacharacteristicfrequencycorrespondingtothefirstionisationpotential;nliesconsequentlyintheultravioletregionoftheelectromagneticspectrum.ThusvanderWaalsadhesionforcesarenotonlyshort-rangechemicalbondsbetweenthesurfaceofmoleculesbutarestillmeasurableuptoseparationsaof50-100nm.Theseforcesdependonthewholesolidcontinuumnearthesurfacebutalsoonthepropertiesofafluidphasebetweentwosolids.Inafirstapproximationanaqueouslayermaybeconsideredaspartofthesolid.Theforego-ingimpliesthatsmallamountsofmoisturehaveastronginfluenceontheagglomerationtendencyoffine-grainedmaterials.ElectrostaticforcesWhentwodifferentparticlescomeintocontact,electronstendtoflowfromoneparticletotheotherbecauseofdifferencesintheelectronicworkfunctionsatbothsurfaces.Theseworkfunctionsdependonlocalimpuritiesandareoftenunknown.Thecontactpotentialdifference(DU)rangesfrom0-0.5V.Oppositechargesbetweenparticlescanariseduringparticlecontact,asaresultofelectrontransferfromoneparticletoanother.Anexcesschargecanbecausedforexamplebyfriction.Thelargestsurfacechargedensitiesarerealisedifparticlesarecontactedandseparatedmanytimes,whichcanbeperformedbyforinstanceimpactgrinding(milling).Fracturealsoproducessurpluschargesonthenewsurfaces.Thesechargingphenomenaareimportantfeatures,whichcausesagglomerationduringmilling.Sphericalparticlesofoppositesurpluschargedensitiess1ands2attracteachotheraccordingtoCoulombslaw.2pssx12F=(3.3)el24e[1+(ax/)]0Inequation(3.3)isxthediameterofthespheres,atheseparationdistancebetweenthespheresande0theabsolutedielectricconstantofvacuum.Foridealinsulators,thisformulagivestheadhesionforcesincaseofcontact.Theadhesionforcesbetweennon-conductorsaresmallerthanbetweenconductors.Innon-conductorstheaccumulatedchargesmayextenduptoadepthofabout1mm,whileconductorsmayhavechargesconcentratedinalayer,oflessthan1nmthickness,atthesur-face.LiquidbridgesSmallamountsofliquidcanformthinliquidbridgesbetweenparticlesduetocapillarycondensa-tion,liquidfilmmovementorevaporation.Suchaliquidbridgeisstablewhentheliquidvapourpressureisequaltothepartialpressureofthesurroundingenvironment.Theliquidbridgeadhesionforceisequaltothecapillaryforcewhentheparticlesareincontact(sothesurfacedistanceaisequaltozero).Therelationofthecapillaryforce(Fcap)betweentwosphericalparticleswithradiusR,asurfacetensiongattheliquid/airinterfaceandaliquidcontactangleqisexpressedby:FR=2pgqcos(3.4)capThesurfacetensionofwaterisnearly3timeshigherthanthatofethanolandthecapillarypressureforawatersystemisthus3timesgreaterthanthatforanethanolsystem.Iftheparticlesareseparatedbyadistanceathentheliquidbridgeadhesionforcebecomesafunc-tionoftheparticle-distance/particle-radiusratio(=a/R).Furthermorethisadhesionforceisafunc-tionrationbetweenliquidandsolidvolume(VL/VS).Wethencomethefollowinggeneralexpres-sionfortheliquidbridgeadhesionforce(Fl): 3Interactionsbetweenpowderparticles25æöaVLF=Ff*ç÷(3.5)ll,maxRVèøSWhereFl,max=2pRg.Thisgeneralrelationisillus-tratedinFigure3-1whereadimensionlessadhesionforce(Fl,max/2Rg)isplottedagainstthea/2RratioatseveralvaluesofVL/VS.Atsmallamountsofliquid-2(10³VL/VS)andata=0thisdimensionlessforcehasavalueofp.Thismeansthatitcanbeassumedthatthecontactangleq=0atlowVL/VSorinotherwords:theliquidbridgeforceincreasesasthevol-umeofthebridgedecreases.Figure3-1alsoshowsthatatsufficienthighliquid/solidratios(VL/VS>-210)theliquidbridgeforceata=0islowerthanFigure3-1:Dependenceofthenormalisedadhesionthemaximumforce,butagglomerationoccursatforceonseparationdistanceasafunctionofvolumelargerparticledistancea.ratiosliquid/solid.3.2Particlesinasolvent;theDLVOtheoryWhenapowderparticleissuspendedinasolventseveralforcesbetweentheparticlesoccur.Thesecanbedividedin:·Electrostaticinteractionforces.A(net)surfacepotentialatthesurfaceofaparticleina(polar)solutionresultsinadiffuselayeraroundthisparticlewithahigherconcentrationofionswithoppositecharge(counterions).·VanderWaalsattractionforcesTheDLVO-theory(Derjaguin,Landau,VerweyandOverbeek)simplyisthesumoftheelectro-staticinteractionandVanderWaalsattractionforcesbetweenparticlesina(diluted)suspension.InthisparagraphtheseveralforceswillbetreatedresultinginaDLVO-equation.Alsoattentionwillbepaidtotheinfluenceofadditives(surfactants)onsuspensionstability.Floccula-tion/deflocculationwillbetreatedaswell.Insightinthesephenomenaisespeciallyimportantforsuspension-formingtechniqueslikeslipcastingandtapecasting(seechapter6)3.2.1Electrostaticforcesaroundasingleparticle;theelectricdoublelayerSurfacechargeTheoriginoftheelectrostaticforceisthesurfacechargethatsolidparticlesacquirewhentheyareimmersedinaliquidthatcontainsasufficientamountofions.Possiblechargingmechanismsareiondissolution,ionisationandionadsorption.Ionisationandionadsorptionaretheimportantchargingmechanismsforsystemsasdescribedinthiscourse.DependentofionconcentrationorpHthereisacertainstatewheretheoverallsurfacechargeiszero.Thisiscalledthepointofzerochargeortheisoelectricpoint(IEP).IondissolutionIonicsubstancescanacquireasurfacechargebyunequaldissolutionoftheionsofwhichtheyarecomposed.Anexampleissilveriodideparticlesinanaqueoussuspension,whichareinequilibrium-withasaturatedsolution.WithexcessI-ions,thesilveriodideparticlesarenegativelycharged.+WithanexcessofAg-ions,theparticleswillbepositivelycharged.Ionadsorption 26AdvancedCeramicsProcessingIngeneralitcanbestatedthatifa(powder)materialisincontactwithapolarsolutionelectrostaticforcesbetweensurfaceandsolutionarise.Anetsurfacechargecanamongotherthingsbeachievedbyadsorptionofionsfromthesolution.Theseionsdeterminesignandsizeofthesurfacepotentialandarethereforecalledpotentialdeterminingions.Thechargeofthesurfaceinthesolutionde-+pendsonthepH.AtlowpHthesurfacechargeispositive(anetHadsorption),whileatahighpHtheoverallsurfacechargeisnegative.HereagainapHisfoundatwhichthesurfacehasanoverallchargeofzero(IEP).IonisationColloidalmetal-oxideparticles,withhydroxylgroupsattheirsurface,mayundergoprotonassocia-tionordissociationdependingonthepHofthesolution.AtlowpH,ametal-oxideparticlewillbechargedpositivelyandathighpHnegatively.ThepH,atwhichthenetchargeiszero,iscalledtheisoelectricpoint.CounterionsTherearealsoionspresentinthesolutionthatdonotdirectlyinfluencethesurfacechargearecalledindifferentions.Howevertheseionsdoinfluencethepotentialprofilesaroundthesur-face.Inthesolutionthesurfacechargeiscompen-satedbyoppositecharges(counterions),whicharediffuselydistributedinanelectricdoublelayer(seeFigure3-2).Co-ionshavethesamesignasthesurfaceandtheirconcentrationinthedoublelayerislowerthaninthebulk.Thecon-centrationdistributionsofco-andcounter-ionsaretheresultofthecompetitionbetweenBrownianmotionandelectricforcesontheions.Figure3-2:Sketchoftheelectricdoublelayer.ThepotentialenergyVi(x)ofioniatadistancexofthesurfacewhereapotentialdifferencey(x)existswithrespecttothebulk,isexpressedby:Vxibg=zeiybxg(3.6)2+-Whereeisthechargeofaprotonandzithevalenceoftheionincludingitssign.(ThusforCa:zi=+2).TheforceKontheionisgivenby:dVxibgdyK=-=-ze(3.7)idxdxEquation(3.8)showstheforcebalancefortheionswithaconcentrationci(x).dPdyi+zecxbg=0(3.8)iidxdxVan’tHoff’slawforioniisgivenby:P=cbxgkT(3.9)iiSolvingequation(3.8)resultsintheBoltzmanndistribution:LzeiybxgOcxibg=ci,¥expM-P(3.10)NkTQ 3Interactionsbetweenpowderparticles27-3Hereci,¥isthebulkconcentrationofionI(expressedasthenumberofionsm).Inordertocalcu-lateci(x)thecurveofthepotentialy(x)hastobeknown;thiscurvewillbederivedforaflatplate.Equation(3.11)representsthePoissonequation:2dyr=-(3.11)2dxee0Hereisrthenetspacechargedensity,etherelativepermittivity(the“dielectricconstant”)ofthesolutionande0thepermittivityofvacuum.ThechargedensityrisasumofthedensityofallchargesandcanbedescribedbytheBoltzmannequation:zeiY-r=åcze=å(czee)kT(3.12)iii,¥iiiCombinationofequations(3.11)and(3.12)leadstothePoisson-Boltzmannequation:2zeiYdY1-=-å(czee)kT(3.13)2i,¥idxee0ixThisequationcanbesimplifiedbylinearisingtheexponent(e~1+x)whichispermittedforzey/kT<1.Thisso-calledDebye-Hückelapproximation,whichisalsousedforthetheoryofstrongelectrolytes,leadsto:2dy2=ky(3.14)2dxHerekisdefinedby:1Fe2cz2I2åi,¥ik=GGiJJ(3.15)eekTH0KkistheDebyeconstant.Integrationofequation(3.14)finallyyieldstheDebye-Hückelpotential:y=yexp-kx(3.16)0WhereY0isthesurfacepotential.Aplotofyversusxillustratesthephysicalmeaningoftheparameterk:thereciprocalvalue-1kisthedistanceatwhichthepotentialhasde-creasedtoy0/e(seeFigure3-3).ThisdistanceisalsocalledtheDebyelength(=theinverseDe-byeconstant).Accordingtoequation(3.15)add-ingsaltcanreducethethicknessofthedoublelayer.Consequently,thepotentialenergyandtheionconcentrationsquicklydecreasetotheirFigure3-3:Curveofthepotentialinthedoublelayeratabulkvaluesatahighsaltconcentration(=ahigh1)highand2)lowsaltconcentrationionconcentrationinthesolution).TheionicstrengthIcombinestheconcentrationoftheionsinsolutionandthevalenceoftheions.Theionicstrengthcanbedefinedby:12I=cz(3.17)åi,¥i2i 28AdvancedCeramicsProcessingandthereforetheionicstrengthisagoodmeasureforthedoublelayerthickness.AqualitativemodelfortheelectricdoublelayerUptothispointatheoreticalmodelisgivendescribingthedistributionofcounterionsintheelec-tricdoublelayer.Inmostofthe(morequalitative)modelsthedou-blelayerisdividedinalayer“adhered”tothesurfaceandanouterlayerofmore“free-movingcounterions”.TheSternmodelgivesthebestrepresentationofthearrangementofcounterionsanddipolesinthedoublelayerandisschemati-callyrepresentedinFigure3-4.Thisisacombi-nationHelmholtzmodelandtheGouy-Chapmanmodel.IntheSternmodelthereisafairlyimmobilelayerofionsthatadheretightlytothesurfaceofthematerial.Theseionsarecon-strainedintoarigidHelmholtzlayer,whichtakestheformofamolecularcapacitor.Theremainingcounterionsaredispersedina“diffuselayer”.TheplanebetweentheHelmholtzlayerandthediffuselayeriscalledtheshearplane.ThepotentialatthisshearplaneisanimportantFigure3-4:SchematicrepresentationoftheSternmodelshowingthepotentialdropasfunctionofdistance.parameterunderdynamicalconditions,whered=InnerHelmholtzPlane;D=OuterHelmholtzPlaneelectrolytesolutionandchargedparticledmoverelativelytoeachother.Thispotentialcanbedeterminedfrommeasurementsofthevelocityofpar-ticlesinanelectricfield.Theelectricpotentialattheshearplanerelativetoitsvalueinthebulkmediumiscalledthez-potential(zetapotential).Thisz-potentialcanberegardedasthepotentialataninterfaceas“experienced”bytheenviron-ment.InFigure3-5andFigure3-6severalz-potentialdataaregivenFigure3-5theIEPofthein-vestigatedtitaniaisshownbutalsotheinfluenceofionicstrengthonz-potentialisvisible.ThedatainFigure3-6notonlyshowtheinfluenceonz-potentialoncounterionvalencebutalsoonthera-diusofthecounterion.Figure3-6:ZetapotentialasfunctionoftheradiusandvalenceofthecounterionsFigure3-5:Zeta-potentialasfunctionofpHatseveralsaltconcentrations(powder:TiO2anatase)ItmustbenotedthattheshearplaneisnotexactlyattheOHPbutliessomewhatfurtherfromthepowderparticle.Thishoweverisnotofgreatimportancefortheinterpretationofthez-potentialdataofsuspensions. 3Interactionsbetweenpowderparticles29Insummary:theSternmodelstatesthatthesurfacechargeiscompensatedbycounterionsrigidlybondtothesurface,whiletherestofthecounterionsaresituatedinadiffuselayer.Athigherionicstrength(highersaltconcentration)morecounterionsaresituatedintherigidlayer,whichmeansthatthephysiologicalenvironment“experiences”alower“surface”(=shearplane)charge.DoublelayeroverlapUptothispointonlyoneparticleisregarded.Nowwewilldescribetheelectrostaticinteractionbetweentwoparticlesinasolution.Iftwochargedcolloidparticles(heremodelledastwoflatplates)approacheachotherasaresultofBrownianmotionoraflowfield,theionatmospheresaroundtheparticleswillpartiallyoverlap.Intheareaofoverlaptheionconcentrationishigherthanthe(constant)bulkconcentrationn¥.Onecanimaginethatbythecompressionofsuchan“iongas”theapproachingplateswillexperienceacounteractingosmoticpressure.SupposethatwekeepthetwoplatesatafixeddistanceHbymeansofanexternalforce(seeFigure3-7).Theiondistributionbetweentheplatesisatequi-librium.Thekeyobservationhereisthatforallionsoftypeiexactlythesameforcebalanceisoperativeasforonefreedoublelayer[equa-tion(3.8)],whereyisnolongertheDebye-HückelpotentialbutfollowsacurveasdepictedinFigure3-7ThepotentialyH/2inthecentralplaneisunknownbutagoodapproximationispossibleiftheover-lapofthedoublelayerissmall.Inthatcaseequa-tion(3.18)isvalid.yy»2(3.18)HH/2/2,singledoublelayerUsingtheDebye-Hückelpotentialofequa-tion(3.16)foryleadsto:Figure3-7:Potentialcurvebetweentwoflatdoublelay-ersatadistanceH.yH/20»-2ykexp[H/2](3.19)TheDebye-Hückelapproximationisrestrictedtolowpotentials,therefore,atasmalldoublelayeroverlap(kH>>1)ThechangeinfreeenergyDGrep,whichistheworknecessarytoputtheplatesatdistanceH,canthenbeexpressedby:HDG=-PdHrepò¥¥22=-2ee00kykòexp[H]dH(3.20)H2=-2ee00kykexp[H]Notethatthisisarepulsiveenergyperunitsurfacearea.3.2.2VanderWaalsattractionIfonlyanetrepulsionwouldexistbetweenparticles,theywouldnotcoagulate.Thereforealsoanadditionalattractionforcehastobepresentbetweentheparticles.ThishasnothingtodowiththeelectricdoublelayerbutwithVanderWaalsattractionwhichistheresultofinteractionbetween(permanent)dipolesofmoleculesasisdiscussedinsection3.1.ThevanderWaalsattractionbe-tweentwocolloidsisthesumoftheeffectsofallattractionsbetweenseparatemoleculesofboth 30AdvancedCeramicsProcessingcolloids.TwoflatplatesatadistanceHaretakenasamodelforthecolloids.ThevanderWaalsattractionenergyperunitareaofsurfaceisforthisspecialcasegivenby:ADG=-(3.21)attr212pHHereAistheHamakerconstant.NotethattheVanderWaalsattractionenergyasdescribedinsec-tion3.1isderivedfortwoparticleswitharadiusRwhileinthissectionadescriptionisgivenoftwoflatplatesatacertaindistance(H).3.2.3TheDLVOpotentialThetotalinteractionenergyDGtotbetweenthetwoplatesconsistsoftheattractionenergyasgiveninequation(3.21)andtherepulsiveenergygivenbyequation(3.20)andisthusexpressedby:2ADGHtot=2ee00kykexp[--]2(3.22)12pHNoteagainthatequation(3.22)isvalidunderthesamerestrictionsasholdforequation(3.20),whichare·Thesurfacepotentialy0issmall(Debye-Hückelapproximation),·thedoublelayeroverlapissmall(kH>>1),and·thecolloidisaflatplate.InmanycasestheserestrictionswillnotbefulfilledandamorecomplicatedexpressionforDGtotisnecessary,however,thefollowingexaminationremainsvalidinaqualitativesense.Aninteractionpotentialasinequation(3.22)withadoublelayerrepulsionofthetypeexp[-kH]andanattractionwhichdecreasesal-gebraicallyissometimescalledtheDLVOpo-tential.Essentialforthispotentialisthatatverylargeandverysmalldistances(H)attractiondominateswhileinthecentralregiontherepul-sioncanbestronger(seeFigure3-8).Inthiscasethereisanenergybarrierthatpreventstheparticlestoapproachsufficientlyclosetoaggre-gate.TheheightofthisbarrierDGmaxdecreaseswithdecreasingrepulsion,forexamplewhenelectrolyteisadded.IfDGmax1.BinghamandplasticbehaviourMaterialswithayieldvaluebehavelikeanelastic(solid)matteratapressurebelowthisyieldvalue,whiletheystarttoflowabovethisyieldvalue.Incaseofalinearcorrelationbetweenshearstressandshearrateabovetheyieldvalue,thebehaviourofthematerialisdescribedasBingham[seecurves4)inFigure3-11].TheBinghambehaviourcanbedescribedby:·t=+tgk(3.33)0Wheret0istheyieldvalueandkaconstant(sometimescalledplasticviscosity).Amaterialshowinganon-linearcorrelationbetweenshearstressandshearrateabovetheyieldvalueisdescribedbythetermsplastic,plasticityorviscoplastic.Thecorrelationbetweenstressandvelocityisrepresentedbycurves5)inFigure3-11.Theplasticbehaviourisexpressedbyequa-tion(3.34),whichincludesapowerlawdependence(Herschel-Bulkly).·n(3.34)t=+tgk0Althoughthereareprobablynomaterialswhichbehaveexactlyaccordingtoequation(3.33)or(3.34)therearemanymaterialsshowingayieldvalue.Examplesofmaterialswithayieldvalueare:·Rinsingliquidsfordrillings(drillingmuds).Theundisturbedsystemhastobeastablesuspen-sionthatcontainstheboredustandatacertainpressureitmustbepossibletopumpoffthesystemagain,·greases,creams,·toothpasteandsoap,·paints.Theundisturbedsystemneedsayieldvalueinordertoprevent“runningdrops”butthe2processingofpaintshouldbeeasy.Theaverageyieldvalueforpaintis1-10N/m,·manyfoodstuffsshouldhaveacertain“firmness”atrestbutneverthelesstheirconsumptionshouldbenottoodifficult.Examplesareicecreamandmargarine.Inliquidsystemssuchasfruitjuicesandchocolatemilkanartificialnetworkisprovidedthatpreventsthedispersedpar-ticles(fruitoildroplets,cocoaparticles)fromseparation.Inallthesesystemsanetworkisformedbymo-lecularbondsorbyinteractionsbetweenparticles(vanderWaals,electrostatic).Whentheexter-nallyappliedstressislargerthanthebondstrengththenetworkwillbreakdownandthematerialstartstoflow(seeFigure3-15).3.3.4Time-dependentbehaviourFigure3-15:DispersionnetworkIftheviscosityinastationaryshearexperiment(seeFigure3-10)doesnotonlydependontheshearratebutalsochangesasafunctionoftime,thematerialcanbehavethixotropicallyorrheopectically.Asamatteroffact,thematerialconstantvis-cositycaninprinciplenotbeused.Therefore,theviscosityusedincaseoftime-dependentflowbehaviouristhepresentshearstressdividedbytheshearrate. 36AdvancedCeramicsProcessingThixotropyAmaterialiscalledthixotropicifitsviscositydecreaseswithtimeinastationaryshearexperi-ment.Atresttheviscosityincreasesagainuntilitreachesitsoriginalvalue.Whentheexperimentisrepeatedaftersometime,thesamecurveisobserved(seeFigure3-16).Figure3-16:ViscosityasafunctionoftimeforthixotropicbehaviourIftheshearrateisraisedfromzerotoacertainvalueandreducedtozeroagaininashearex-periment,therespectivecurveswillnotcoin-cide(seefigureFigure3-17a).Theareabetweenthebackandforthrunningcurve(hatched)isameasureforthethixotropy.Itsunitisforcepervolume.·éùNNm1[]tg·==êú23ëûmssmFigure3-17;a)Shearstressandb)thecorrespondingviscosityasafunctionofshearrateforthixotropicbehav-bshowstheviscosityasafunctionofthesheariour.rate.Itisobviousthattheviscosityisdifferentforthedifferentdirectionsofthecurve.Thereasonforthethixotropicbehaviouristhenetworkstructureofthematerial(seeFigure3-18).Ataconstantshearmoreandmoreofthenetwork“junctions”breakasafunctionoftime(therebydecreasingtheviscosity)untilallconnec-tionsarebrokenoruntilanequilibriumbetweenformationandbreakdownofconnectionsisreached.Atrestthenetworkisrestoredagain.Figure3-18:ExamplesofnetworkstructuresInsomematerialslikeyoghurttheoriginallypre-sentnetworkisnotobvious.Thesematerialsshowafalsethixotropicbehaviour,seeFigure3-19.Theviscosityremainsconstantatalowvalueaftertheshearratereturnstozero.RheopectyMaterialsarecalledrheopecticiftheirviscosityincreaseswithtimeinastationaryshearexperi-ment(seeFigure3-20).Thisbehaviourisalsocallednegativethixotropy.Anexampleofamate-Figure3-19:Viscosityasafunctionoftimeforfalserialwithsuchabehaviourispertubanlatex.thixotropicbehaviour 3Interactionsbetweenpowderparticles37ViscoelasticityTheviscosityofthixotropicandrheopecticmateri-alsisdependentonthehistoryoftheshear.Vis-coelasticmaterialshavea“memory”forthehis-toryoftheirdeformation.Theyshowfeatureslikestressrelaxationandelasticrecoil.Letuslookbackontheshearexperimentwherethesampleisshearedbetweentwoflatplanesofwhichtheloweroneiskeptfixedandthetheup-peroneismovedwithconstantvelocityV(Figure3-10).Nowtheupperplaneissuddenlystoppedandkeptfixed.IfthesampleconsistsofaNewto-Figure3-20:Viscosityasafunctionoftimeforrheopec-ticbehaviournianfluidoranyothernon-viscoelasticmaterial,thestressinthematerialwillinstantaneouslybedisappeared.Ifthesampleconsistsofaviscoelas-ticmaterial,thestresswillnotimmediatelybereducedtozerobutwilldisappearintime.Thecourseofstressasafunctionoftimeissche-maticallyshowninFigure3-21.Ifthisrelaxationfunctioncanbedescribedbyanexponentialfunc-tion,therelaxationtimeisdefinedasthetimeafterwhichthestresshasreached1/eofitsoriginalvalue.However,oftenaspectrumofrelaxationtimescanbeobserved.Inthiscaseanaveragere-laxationtimeisused.Figure3-21:Stressrelaxationinaviscoelasticmaterial.Inafollowingexperimentthemovingplaneissuddenlystoppedoncethestationaryshearisreached(thedrivingforceisremoved)andtheplaneisnotkeptfixed.Ifthesampleisinelastictheplanewillstandstill.Aviscoelasticsamplewillmaketheplanerecoil(seeFigure3-22).Thisrecoildoesnotoccurinstantaneouslybutrequiressometimeduetotheviscousmufflingofthemate-rial.Thetimeduringwhichthesheardecreaseswithafactorof1/eiscalledtheretardationtime.DeborahnumberFigure3-22:RecoilinaviscoelasticmaterialInprincipleallmaterialsshowviscoelasticbehav-iour.InordertoindicatewhetheramaterialbehavesmainlyviscousorelastictheDeborahnumbercanbeused.relaxationtimeDebora=(3.35)observationtimeIfDeborah<<1,thematerialbehavesasa(quasi-)fluid,whereasDeborah>>1expressesa(quasi-)solidandthematerialisviscoelasticasDeborah»1.(DeborahwasaprophetessdescribedintheOldTestament.ItiswritteninJudges5,verse5:“ThemountainsmeltedawayinthefaceoftheLord...”.Fortheshort-livinghumanbeingsmountainsconsistofasolid,elasticmaterialbutfortheLordwithhisinfinitelifeevenmountainsflow.) 38AdvancedCeramicsProcessing3.3.5WeissenbergeffectsEspeciallymoltenpolymersandpolymersolu-tionsshowconspicuouseffectswhicharecharac-teristicforan“elastic”fluid.RodclimbingWhenanelasticfluid,e.g.apolyoxsolution,isstirredinabeaker,thepolymerwill“climbup”thestirrer(Figure3-23a),whereasinaNewtonianfluidtheliquidwillbepressedagainstthewallsofthebeakerbythecentrifugalforce(Figure3-23b).Figure3-23:Stirringinabeakercontaininga)anelasticThe“rodclimbing”effectcanbeexplainedus-andb)aNewtonianfluidingFigure3-24showingthecrosssectionofthebeaker.Thearrowsindicatethedirectionofflow.Thepolymermoleculesareelongatedinthesedirectionsbuttendtoreturntoarandomlyorientedshapeinwhichtheendsofthechainsareincloserproximitytoeachothercomparedtotheelongatedstate.Thisresultsinaneffectiveforceonthechainspointinginward.Conse-quently,theliquidiscompressedtowardthestir-rer.Thisforceisprovokedbyentropy,i.e.theFigure3-24:Rodclimbingeffect.preferenceforchaos.Thisexperimentclearlyindicatesthatincaseofshear,normalstressesoccur.A“kitchen”exampleofthiseffectistheris-ingofdoughagainstamixer.DyeswellThepressingofapolymermeltorsolutionthroughaconstrictioncanleadtopolymerswellingafterpass-ingtheejectionorifice(seeFigure3-25).Thisphe-nomenonoccursinasimilarwayasrodclimbing.Thelongpolymermoleculesarefirstelongatedintheflowdirectionduetotheconstrictedflowarea.Atthemomentthepolymerleavestheejectionori-fice,orientationandelongationareeliminatedresult-inginaswelledejectedflow.Thiseffectiswellknowninextrusionprocessesusingpolymermelts.ReferencesonrheologyFigure3-25:DyeSwelling1.Blom,C.,Jongschaap,R.J.J.,Mellema,J.Inleidingindethreologie,3Ed.,KluwerTechnischeBoekenB.V.,Deventer,TheNetherlands(1991).2.Sherman,P.RheologyofemulsionsinEmulsionscience,Ed.Sherman,P.,AcademicPress,London-NewYork(1968)217.3.Brenner,H.SuspensionrheologyinProgressinheatandmasstransfer,Vol.5,Ed.Showalter,W.R.,PergamonPress,Oxford(1972).4.Brenner,H.DynamicsofneutrallybuoyantparticlesinlowReynoldsnumberflowsinProgressinheatandmasstransfer,Eds.Sideman,S.,Hartnett,J.P.,Pergamon,NewYork(1972).5.Jiuescu,V.V.Int.Chem.Eng.14(1974)397.6.Mewis,J.,Spaull,A.J.B.Adv.Coll.Int.Sci.6(1976)173.7.Jeffrey,D.J.,Acrivos,A.AiChEJ.22(1976),417.8.Mewis,J.J.Non-NewtonianFluidMech.6(1979)1.9.Ball,R.C.,Richmond,P.Phys.Chem.Liq.9(1980)99.10.Russel,W.B.J.Rheol.24(1980)287.11.Tempel,M.v.d.RheologyinthefoodindustriesinRheometry:industrialapplications,Ed.Walters,K.,Researchstudiespress,Chichester(1980). 3Interactionsbetweenpowderparticles3912.Overbeek,J.Th.G.Colloids,afascinatingsubject:introductorylectureinColloiddispersions,Ed.Goodwin,J.W.,RoyalSocietyofChemistry,London(1982)1.13.Goodwin,J.W.SomeusesofrheologyincolloidscienceinColloiddispersions,Ed.Goodwin,J.W.,RoyalSocietyofChemistryBritain,London(1982)165.14.Doi,M.,Edwards,S.F.Thetheoryofpolymerdynamics,ClarendonPress,Oxford(1986).15.Russel,W.B.EffectsofinteractionbetweenparticlesontherheologyofdispersionsinTheoryofdispersedmulti-phaseflow,Ed.Meyer,R.E.,AcademicPress,London-NewYork(1983).16.Russel,W.B.,Saville,D.A.,Schowalter,W.R.Colloidaldispersions,CambridgeUniversityPress,Cambridge(1989)/17.Macosko,C.W.Rheologyprinciples,measurementsandapplicationsVCHPublishers,Inc.,USA.ExerciseWhattypeofstabilizerisPMAA?Explainthebehaviourasgiveninthisfigure·ViscosityasfunctionofpH·Viscosityasfunctionofbindersolidconcen-tration3.4Solidstatemechanics 414PreparationofceramicpowdersandcoatingsThetechnicalpropertiesofceramicmaterialsarenotonlydeterminedbyitschemicalcompositionbutalsobyotherceramicmicrostructuralparameterslikecrystalstructure,crystallitesize,aggre-gate/agglomeratestructureandthe(chemical)homogeneitywithintheceramicpowder.Thewholeceramicfabricationprocess(fromstartingpowdertofinalceramic)determinesthismicrostructure.Soduringtheceramicpowderpreparationprocessseveralpropertiesofthefinalceramicareal-readyfixed.Heterogeneouspowders(incompositionaswellasinaggregate/agglomeratestructure)cangiveproblemsduringthesinteringprocesscausedbylocaldifferencesinreactivity,shrinkageandgraingrowth.Abroadgrainsizedistributionorthepresenceofaggregates(strongagglomer-ates)causesinhomogeneousparticlepackingduringgreenforming.Thiscanresultinincompletedensificationand(local)anomalousgraingrowthduringsintering.Duringtheceramicfabricationprocessmuchattentionispaidtothepreparationofceramicpow-ders.Thereisanincreasinginterestinalternative(synthetic)powderpreparationroutesforseveralreasons.Notonlythesinteringbehaviourisinfluencedbythepowdermorphology.Chemicalim-puritiescanalsohavedeleteriouseffectonhigh-temperaturemechanicalbehaviourofengineeringceramics(presenceofglassyorviscousphase).Theelectricalpropertiesofelectroceramicsareingeneralimprovedwhenthepurityishigher.Hencearequirementarisesforpowderswithphysicalcharacteristicsthatallowreliablefabrication.Inthispartofthecourseseveralpreparationtechniquesfor(oxide)ceramicpowdersarediscussed.4.1Solidstatepreparationofinorganic(multicomponent)powders4.1.1AnintroductiontosolidstatediffusionDrivingforcesinthesolidstate.(seee.g.booksofJ.S.ReedandT.A.Ring)4.1.2ExamplesofsolidstatepreparationInmostcasesonephaseofaceramicmaterialscontainsmorethanonemetalion:forexampleperovskites(ABO3)oradditionsforstabilisationofahightemperaturephase(e.g.Y2O3isaddedtoZrO2forstabilisingthehightemperaturecubiczirconiaphasetoroomtemperature)Thetimenecessaryforcompletereactionbe-tweentwosolidparticlesisstronglydependentontheinterdiffusionofthereactantsthroughtheintermediatereactionproduct(Figure4-1).Thethicknessofthatlayercanbeseveralmicrons.Amathematicdescriptionwillnowbegivenofasolidstatereactionwherewesimplifythesystemtoasphericalparticlewheretheproductforma-tion(growthrate)isdiffusionlimited(soapara-Figure4-1:Schematicrepresentationofaproductlayerwiththicknessyonparticlewithradiusrbolicformationoftheproductlayer).Thetime(t)necessaryforthereactionofavolumefractionxofreactantAwithradiusrtotransferintoreactionproductsisgivenbytheCarterrelation:2/32/32[1+(z-1)x]+(z-1)(1-x)=z+-2(1z)(Ktr/)(4.1)zistheratioofvolumeformedtovolumeconsumed.Kistheapparentrateconstant.AnArrheniusrelationcommonlyexpressestheeffectoftemperature: 42AdvancedCeramicsProcessingK=-Kexp(Q/)RT(4.2)0K0isarateconstantthatdependsonthediffu-sionpathlengthandQistheapparentactivationenergyfordiffusion.Thereactionisafunctionoftimeandtemperature,buttemperaturehasagrea-terinfluenceontherate.InFigure4-1thesolidsatereactionbetweenMgOandAl2O3fortheformationoftheMgAl2O4isgiven.Forthereactionofpowderswithparticlesizeinthemicrometerrangeatemperatureofmorethan1100°Cisnecessaryfortheformationofthespinel.Equation(4.1)isonlyvalidiftheinterfacereac-tionistherate-limitingstepduringsolid-statereaction.Thisise.g.thecasiftheproductlayerisnotclosed(dense).Aporousreactioninterfaceisformedbye.g.gasevaluationduringthereactionorbyvolumedifferencesbetweenreactantsandproducts.Iftheratelimitingstepfortheconver-sionofreactantstoproductsisnotthechemical(interface)reactionitselfthetimeexponentinequation(4.1)isnotequalto1butcanhaveotherFigure4-2:Theformationofspinel(MgAl2O4)fromthevalues.Ifadense,closedproductlayerisformedsolid-statereactionofMgOanda-Al2O3asafunctionofduringreaction-diffusionofreactantsthroughthethereactiontemperatureforaconstantreactiontimeof8hours.solidphaseoftenistheratelimitingstepandre-1/2actionratedecreaseswithtime(tdependence).Thissituationcanbedesiredfortheformationofoxidelayersonmetals(diffusionbarriers).Solid-solidreactioninapowder,resultinginasolidreactionproductcanberegardedasalargenumberofreactionsatcontactpointsbetweengrains.Itisthereforeimportantthatthereareasmuchaspossiblecontactpointsperunitofvolume.Thisnumberofcontactpointsisinversepropor-3tionalwithr.2Thetimenecessaryforthereactionisinverseproportionaltor(seeequation(4.1)).Alargereac-tivitythereforedemands:·Smallparticles·Goodmixing·Densestacking·HighconcentrationgradientsSomixingandmillingareimportantstepsinthepreparationofpowdersina"dry"preparationway.Thehightemperaturesnecessaryforthecompleteconversiontoreactionproductsandthedensestackingoftheparticlesduringreactionoftenresultsinlargeprimarycrystallites,whicharesinteredtogetherinlargeaggregates.Anotherexampleofasolid-statereactionistheformationofbariumtitanate:BaCO3+TiO2-->BaTiO3+CO2Thereactiontemperatureis500-700°C.Theformationoftheperovskiteinvolvesthefollowingreactionsteps:2BaCO3+TiO2===2BaCO3.TiO22BaCO3.TiO2===Ba2TiO4+2CO2Ba2TiO4+TiO2===2BaTiO3Thesecondstepistheratelimitingone.ThepartialpressureofCO2intheporesoftheproductin-fluencesreactionkinetics.Ba2TiO4formedinthesecondstepisundesirableinthecalcinedprod- 4Preparationofceramicpowdersandcoatings43uct.Agoodparticlecontact(bymixing)isveryimportant.Forthatpurposecalcinationandmixinginonestep(inafurnace)mayproduceamoreuniformproduct.Ingeneralonecanconcludethatthedrypreparationmethod(solidstatereaction)isarelativecheaptechniqueforthepreparationofceramicpowders.Ithoweverresultsinlargecrystallites/aggregatesandarelativenon-uniformpowder.4.2Nucleation,crystallisationandcrystalgrowth4.2.1TheorySection6.3–6.4(pp.183–210)ofthebookofT.A.Ring“Fundamentalsofceramicpowderproc-essingandsynthesis”givestheoreticalbackgroundsonthissubject.4.2.2ExamplesAl2O3Aluminaisthemostwidelyusedinorganicchemicalforceramicsinporousaswellasdenseform.Theapplicationoftheporousstructureisinthefieldofcatalystsandceramicmembranes/filters.Densealuminaisusedinseveralfields:abrasives,electricalinsulation(sparkplugs),semi-conductorindustry,transparent(lamp)envelopes,thermalinsulationandseveralstructural(me-chanical)applications.AluminapowdersareproducedworldwideintonnagequantitiesusingtheBayerprocess.Theprin-cipaloperationsintheBayerprocessarethechemicaltreatmentsofbauxite,inthepresenceofcausticsodaatanelevatedtemperature,clarification,precipitationandcalcination,followedbycrushing,millingandsizing.Duringthechemicaltreatment,mostofthehydratedaluminagoesintosolutionassodiumaluminate.Thechemicalreactionsforextractingaluminafrombauxiteare:+-Al(OH)3+NaOHàNa+Al(OH)4+-AlOOH+NaOH+H2OàNa+Al(OH)4Thedissolutionreactionsareendothermicand,therefore,anincreaseintemperatureincreasesthesolubilitySettlingandfiltrationremoveinsolublecompoundsofiron,siliconandtitanium.Aftercooling,thefilteredsodiumaluminatesolutionisseededwithveryfinegibbsiteAl(OH)3,andatlowtemperaturethealuminiumhydroxideisformedasthestablephase.Theagitationtimeandtemperaturearecarefullycontrolledtoobtainaconsistentgibbsiteprecipitate.Thegibbsiteiscon-tinuouslywashforreductionofthesodiumcontentandclassified.Thepowderiscalcinedat1100-1200°Ccrushedandgroundtoobtainarangeofsizes.Figure4-3:Transformationsequencesinalumina 44AdvancedCeramicsProcessingCalcinationisperformedatthistemperatureinordertoobtainthestableaphase.Severaltransfor-mationsequencesofaluminaareshowninFigure4-3.Fromthisfigureitcanbeseenthatattemperatures<1000°Cseveraltypesofaluminacanbepresent:gibbsite,boehmiteorbay-erite.Thetypeofaluminiumhydroxideformedisdependentofthepreparationtechnique.MgOMgOispreparedbyprecipitatingmagnesiumhydroxideinabasicmixtureoftreateddolomite(MgCO3.CaCO3)andusingnaturalbrinesorseawatercontainingMgCl2andMgSO4,followedbywashing,filtering,dryingandcalcination.Thisso-calledseawaterprocesswillnowbedescribedintomoredetail:Thedolomitemineralispyrolysedathightemperatures,whichresultsintheoxidesMgCO3.CaCO3àMgO.CaO+CO2Reactionofthisoxideinwater(strongreactionofCaOwithwaterandalmostnoreactionofMgO):MgO.CaO+H2OàMgO.Ca(OH)2Thishydratedmaterialisdissolvedinseawater(whichcontainsabout1.3g/lMgO)2+2+MgO.Ca(OH)2+MgàMgO.Mg(OH)2+Ca(calciumhydroxidesdissolves200timesbetterinwaterthanmagnesiumhydroxide).TheamountofCO2mustbeaslowaspossiblebecauseotherwiseCa-compoundsprecipitatetogetherwith2+Mg(OH)2.ThisresultsinCacontaminationinMgOandconsequentlyworseproperties.ZrO2Zirconiumoxidehasmanyspecificapplicationsastoughceramics,aselectrolyteinsensorsorsolidoxidefuelcells(SOFC)andasaceramic(thermalbarrier)coating.Forthesepurposes2-10mol%ofY2O3,CeO2orCaOisaddedasastabilisingagent.Inmanycasesverypurestartingma-terialsarenecessary.Thepreparationofdopedzirconiasystemsisgiveninthechapteronwet-chemicalpreparationmethods.Themineralsourceforzirconiaiszircon(ZrSiO4),whichoftencontainsmanyimpurities.Inoneprocessmilledzircontogetherwithcokearechlorinatedinafluidizedbedusingchlorineasthefluidizingmedium:ZrSiO4+4C+4Cl2àZrCl4+SiCl4+4COZrCl4evaporatesat150-180°CandSiCl4at10°C.Sothesecomponentscanbeseparatedbydistillation.Mostoftheotherimpuritiesaresolidatthesetemperatures.Veryhighpuritiescanbeobtainedinthisway.FurtherprocessingofZrCl4tozirconiacanbedoneinseveralwayse.g.bygelroutes.Bymeansofthesemethodsextremelyfinepowderswithprimaryparticle(=crystallite)sizesrangingfrom10-50nm(seesectiononwet-chemicalpreparation).AnotherstartingmaterialforthepreparationoftechnicalzirconiaceramicsisZrOCl2.FortheproductionofZrOCl2finelygroundzirconisfusedwithNaOHtoobtainsodiumsilicate,so-diumzirconateandsomesilicozirconate.MostFigure4-4:FlowschemeforthepreparationofZrOCl2ofthesodiumsilicatedissolvesinwater,leavingfromzirconsand 4Preparationofceramicpowdersandcoatings45asodiumzirconatewhichissolubleinmostmineralacids.Ifitisdissolvedinhydrochloricacid,theZrOCl2.8H2Oisrecoveredfromsolutionbycrystallization.AcommercialprocessusedbyTo-sohCorptomanufactureZrO2involvesthehydrolysisofzirconiumoxychloridetoproduceparti-clesofzirconiumhydroxides,whichisthermallyconvertedtoZrO2TiO2Titaniaisproducedbythesulphateorbythechlorideprocess.Inthesulphateprocess,ilmeniteFeTiO3istreatedwithsulphuricacidat150-180°CtoformthesolubletitanylsulphateTiOSO4:FeTiO3+2H2SO4+5H2O--->FeSO4.7H2O+TiOSO4Afterremovingundissolvedsolidsandtheironsulphateprecipitate,thetitanylsulphateishydro-lysedat90°CtoprecipitatethehydroxideTiO(OH)2:TiOSO4+2H2O--->TiO(OH)2+H2SO4Thetitanylhydroxideiscalcinedatabout1000°Ctoprocesstitania.Inthechlorideprocess,ahigh-gradetitaniaoreischlorinatedinthepresenceofcarbonat900-1000°C,andthechlorideTiCl4formedissubsequentlyoxidizedtoTiO2.Thechloridecanalsobede-composedinaplasma(archeating).4.3WetChemicalpreparationWetchemicalpreparationmethodshaveanadvantageifcomparedwithsolidstatereactionbe-cause:1.Theintermediateproductspriortocalcination(crystallisation/oxideformation)areextremelyfine(5-100nm).Sothereaction(calcination)stepisatalowertemperature(200-400°Clower).Thereforelesssintering(aggregateformation)occursduringcalcination.2.Thehomogeneityisbetterbecauseofshorterdiffusionpaths.3.Millingprocedurescanbeminimizedresultinginhigherpurityofthepowder.4.Chemicalandphysicalpropertiescanbebettercontrolled.Ingeneralwetchemicalpreparationstartswithasolutionofmetalsaltsormetal-organiccom-pounds.Inthisso-calledprecursorsolutionthereactantsaremixedonatomicscale.Duringfurthertreatmentstoasolidpowderthishomogeneitymustbepreserved(counteractsegregation)Mostwet-chemicalmethodscanbesubdividedinfourgroups,whichdifferinthewayhowthesolidstateisseparatedfromtheprecursorsolution:·Dispersion·Co-Precipitation(dispersionpluschemicalreaction)·Complexation·GelationInFigure4-5theseveralwaysofimmobilisationaregiven.Thiswillbediscussedindetailinthefollowingsections.Theorderisfromrighttoleftsowestartwithdispersionandend-upwithgela-tion.Inthisorder(fromdispersiontogelation)thetechniquesbecomemorecomplex,butthere-sultingpowdersgenerallyhaveabetterperformance“fromrighttoleft”(e.g.inhomogeneity,pu-rity,agglomerationbehaviour) 46AdvancedCeramicsProcessingFigure4-5:Schemeoftheseveralwaysofwet-chemicalpreparationof(ceramic/inorganic)powders4.3.1DispersionmethodsThistechniqueisbasedonthedispersionofaprecursorsolutioninsmalldroplets("atomization")priortodrying(inairorinanonmixableliquid).Inthiswaythedistanceofphaseseparation(seg-regation)duringdryingandcalciningislimitedbythedimensionsofthedroplets.Theliquidhastoberemovedfastsothatsegregationwithinthedropletsisminimized.Afterdryingthesedropletsthepowderconsistofindividualgranularparticles.AgglomerationbetweenthisparticlesislimitedTheagglomeratestrengthwithintheseparticlescanhoweverbeveryhigh.SpraydryingTheprocessstartswithaconcentratedsolution,whichistransformedindropletsbyapplyingpres-suresoftheorderof50-300kPa.Dropletsofquiteuniformsize(100-500mm)areformedanddriedinair.Ingeneralhollow,sphericalagglomeratesareformed.Inthecaseofmulti-componentsystemsradialconcentrationgradientcanbeformedduetoadifferenceinsolubilityofthereac-tants.LiquidaerosolthermolysisThisprocessisamoreadvancedwayofthespraydryingprocess.Herethedropletsizecanbecon-trolledinabetterway.Thismethodhasattractedmuchattentionrecently,becauseofitscapacitytoproducehomogeneousreactivepowdersinarelativelysimpleequipmentinonesinglestep.Liquidaerosolthermolysis(alsocalledaerosoldecomposition,spraypyrolysisorsprayroasting)involvestheatomisationbyanultrasonictreat-Figure4-6:Schematicpictureofaliquidaerosolequip-mentmentofthestarting(precursor)solution.Themistisdriedandsubsequentlydecomposedinahotfurnace.Eachdropletsactsasa"container"andreactionscanbeconductedwithinthedropletstoyieldsphericalparticles.Theequipmentissche-maticallygiveninFigure4-6Thedropletsinthe"mist"oftheprecursorsolution,generatedbytheultrasonicatomizer,haveaverynarrowsizedistribution.Themeandropletsize(Dg)isdeterminedbytheequation: 4Preparationofceramicpowdersandcoatings47gpDg=3(4.3)24rFgisthesurfacetensionofthesolution,rthedensityofthesolutionandFtheultrasonicfrequencyofthepiezoceramictransducer.Foraqueoussolutionadropletsizeof5µmisobtainedatafre-quencyof750kHz,whileatafrequencyof2.5MHzthedropletsizeamounts2µm.Byusingasolutionwithalowersurfacetensionthanwater(alcohol)thedropletsizeissmaller.Alcoholshowevergenerallyevaporatetoofast.Theliquidlevelabovethepiezoceramictransducermustbeofaconstantheightforstableoperat-ingconditions.Themistyielddependsontheexcitationpoweroftheapparatusandisintheorder3of300cm/h.Afteratomisation(ornebulization)thedropletsareintroducedinthehotzone.Duringthispyroly-sisprocessthesolventevaporatesandthesaltdecomposestotheoxides.Thesizeofadenseparti-clecanbecalculatedfrom:M0CDp=Dg3(4.4)Mir0whereM0isthemolecularweightoftheoxideobtained,Miisthemolecularweightoftheinor--3-3ganicsalt,r0(g.cm)isthedensityoftheoxideformed,C(g.cm)istheconcentrationofthestart-ingsolutionandDdisthedropletsizeinµm.Inthefollowingpaperstheexperimentalprocedureandresultsaredescribedforthepreparationofseveralsystems(e.g.:yttriadopedtetragonalzirconiaandthehighTcsuperconductingmaterialYt-triumBariumCopperoxide).àT.Ogihara,T.Ookura,T.Yanagawa,N.OgataandK.Yoshida,"Preparationofsubmicro-metresphericaloxidepowdersandfibresbythermalspraydecompositionusinganultra-sonicmistatomizer"J.Mater.Chem.1[5](1991)789-794àB.Dubois,D.RuffierandP.Odier"Preparationoffine,sphericalyttria-stabilizedzirconiabythespray-pyrolysismethod"J.Am.Ceram.Soc.72[4](1989)713-715àS.Z.ZhangandG.L.Messing,"Synthesisofsolid,sphericalzirconiaparticlesbyspraypy-rolysis"J.Am.Ceram.Soc.73[1](1990)61-67Themorphologyofthefinalparticleafterpyrolysisnotonlydependsonthedropletsizeafternebu-lisation.Otherparameterwhichdeterminesthestructure(e.g.hollowspheres)aredryingrate,resi-dencetimeinthefurnace,gasflow,temperatureprofileinthefurnace.Theinfluenceofthesepa-rametersaretheoreticallydescribedin:S.W.Lyons,J.OrtegaL.M.WangandT.T.Kodas,"Multi-componentceramicpowdergenerationbyspraypyrolysis"pp.907-917in"BetterceramicsthroughchemistryV",MaterialsResearchSocietyproceedingsvolume271,Ed.by:M.J.Hampden-Smith,W.G.KlempererandC.J.Brinker,MRS,(Pittsburgh,Pennsylvania),(1992).InthispaperalsothepreparationoftheperovskitesBaTiO3andSrTiO3aredescribed.FreezedryingFreezedryingconsistsofrapidlyfreezingasaltsolutioncontainingthedesiredmetalions.Forthatpurposethesaltsolutionissprayedinacoldsolution(e.g.hexaneat-60°C)understirring.Thesaltparticlesarefrozeninthisway.Demixingofthesaltcomponentsduringsolidificationcanbebettercounteractediffreezingtakesplacefaster.Thelowtemperatureinthisdispersiontechniqueisanadvantageintermstohomogeneitycomparedtospraydrying.Thefrozensaltspheresarethendriedundervacuum(atabout-30°C)andheatedslowlywhileavoidingmelting.Afterremovingwaterandothersolventsthepowderiscalcinedresultinginagranularproduct. 48AdvancedCeramicsProcessingHotliquiddryingThistechniqueusesawarmliquidasdryingmediuminsteadofagas(inspraydrying).Kerosineisaliquidoftenusedforthismethod("hotkerosinemethod").Theadvantageofthistechniqueifcomparedwithspraydryingisthatitcanbeperformedeasilyinsmallquantities.Anaqueoussaltsolutionisaddeddropwisetoahotliquid(160-170°C)understirring.Thewaterisevaporatedimmediately.Thesmallsaltparticlesarefilteredfromtheliquid(kerosine)phase.4.3.2Co-PrecipitationThisprocedureisingeneralbasedonthecontrolledhydrolysisofanaqueoussolutionofmetalsaltsoranalcoholsolutionofmetalorganiccompounds.Theprecursorsolutionisstronglydiluted,whileprecipitationtakesplaceinanexcessofbasicorneutralmedium.Inthiswayitispossiblethattheprecursorsolutionprecipitatesinstantaneouslyathighnucleationrate.This"nucleationburst"isimportantfortheformationofmanynuclei.Duetothehighdegreeofdilutiontheconcen-trationofnucleiperunitofvolumeissmallandconsequentlythegrowthoftheparticlesisfurtherminimizedresultinginaparticlesizenotlargerthanitsstablenucleussize(about5–8nanometre).Priortodryingandcalcinationthereactionby-products(alcohols,chloridesornitrates)arewashedoutbymeansofwater.Thefreewateralsohastoberemovedbyaliquidwithalowersurfaceten-sion(veryoftenanalcohol)inordertopreventtheformationofstronglargeagglomeratesaftercalcination.Thereplacementofwaterbyanalcoholsignificantlylowersthecapillaryforcesactingonthegelduringdrying.Thisalcohol-washingstepcanalsobedonebyazeotropicdistillationofawater/alcoholmixture.Afterthefiltrationanddryingstepsaverylooselypackedpowderisobtainedresultinginalmostno(orasmallamountsof)sinternecksduringcalcination.Theresultingpowdersareweaklyagglom-erated.Duringformingpriortosintering(e.gpressing)theseagglomeratesareeasilyfracturedandregularlystackedgreenmicrostructureisobtained.Inthiswayforzirconiaagreenmicrostructurewasobtainedconsistingofhomogeneouslystackedaggregateswithsizeofabout15nmenclosingporesintherangeof8nmindiameter.Severalcompositionsaremadebymeansofaprecipitationtechnique.AcommercialprocessusedbytheTosohCorp.tomanufactureyttriadopedzirconiainvolvesthehydrolysisinammoniaofasolutionofZrOCl2andYCl3.Crystallizationoftheamorphousprecipitatesisgenerallydonebymeansoftemperaturetreatmentsatatmosphericpressures.Inrecentliteraturealsoattentionispaidtothehydrothermaltreatmentofprecipitates.Thistechniquehastheadvantagethatlowertemperaturesforcrystallizationcanbeusedresultinginsmallerprimaryparticlessizeandweakeragglomerates.Seee.g.thebook:àS.Somiyae.a."Hydrothermalreactionformaterialsscienceandengineering"ElsevierAp-pliedSicencePublishers(1989)Problemswiththeprecipitationtechniqueare:·Deposition(precipitation)rateforeachcomponentisnotthesame.Eachalkoxidehydrolysesatadifferentrate.Duringthisheterogeneousprecipitation,theconcentrationofanioninthepre-cipitatediffersfromthatinthesolution,andthecompositionoftheprecipitatemaychangeasprecipitationprogressesresultinginsegregationwithinthepowderbatch.Thisphenomenoncanbeminimizedbychoosingthose(alkoxide)precursormaterialsthathaveanalmostidenti-calhydrolysisrateand/orusingaverydiluteprecursorsolution.Anotherapproachistheuseofmetal-organiccomplexescontainingallmetalionsinonecomplex(complexa-tion/precipitation).ForexampleSrTi-carboxyl-alkoxideswerepreparedandhydrolysedgivingsphericalSrTiO3particles.Anexampleofasingle-compoundoxalateprecursorisBaTi(C2O4).4H2OwhichprecipitatestotheidealstoichiometricBaTiO3andismixedonatomicscale.·Washingcanselectivelyremoveaprecipitatedcomponent;changeinchemicalcompositionandchemicalhomogeneitycanoccurinthisway. 4Preparationofceramicpowdersandcoatings494.3.3ComplexationInthisprocesstheprecursorsolutionisimmobilizedbytheformationofahighviscous(gelati-nous)matrixinwhichtheprecursorisdispersedorbymakingapolymericnetwork.IngeneraltheviscosityofthemediumhastoincreasedrasticallyTheprecursorcanbeimmobilizedbycomplexationwithorganiccompounds.Theimportantstepforpreparationaviscousamorphouscomplexisthefastdehydrationatlowtemperatureandpres-surestartingfromasolutionofcationsandcomplexingagent.Thisviscoussolutionisthermallytreated(pyrolysed)resultinginthefinalpowder.Pyrolysishastobeperformedinsuchawaythatthenon-desiredcomponents(e.g.organics,ammoniumsalts)decomposeingaseouscomponents.Anexampleistheso-calledcitrateacidmethod.Thismethodstartswiththepreparationofanitricacidsolutionofthecationsinthedesiredcomposition(stoichiometry).Thesecationsarethencom-plexedbymeansofcitrateacid,whichresultsinafixationofthestoichiometryonatomicscale.ForthiscomplexationtherightpHhastobechosen.EachindividualcationhasaspecialpHrangeforcomplexation.SoforcomplexationofallcationsinvolvedtheremustbeacommonpHrange.SomeammoniaisaddedtothisprecursornotonlyforthefixationofpHbutalsofortheformationofammonium-nitrate(ammoniaplusthenitricacidsolvent).Duringthesubsequenttemperaturetreatmentdecompositionoftheammonium-nitrateoccursjustpriortothedecompositionoftheorganics.Thegassesevolvedbythedecompositionofammonium-nitrateinflatetheviscousfluidinsuchawaythataverylowpackedsystemarises.Thevolumeofthesystem(ammonium-nitrategaspluspolymericprecursor)canincreaseseveralhundredspercentsifcomparedwiththestartingcomplexsolution.Duetothislargevolumetheaccessofoxygentotheprecursorsolutionisim-provedresultinginahomogeneouscombustion(pyrolysis)oftheorganics.Anotheradvantageofthisinflationpriortodecompositionisthefewamountsofcontactpointsbetweentheparticles,resultinginlimitationofparticlegrowthduringthisstageofthermaltreatment.Theadvantageofthismethodistheversatilityinthechoiceofmaterials.Thismethodcanbeeasilyperformedonlabscale.Theresultingpowdersshowagoodhomogeneityandsinter-reactivity.Ex-amplesofpowderspreparedbythiscitratemethodareseveralperovskites,yttriaorgaddoliniadopedzirconiaandthesuperconductingyttriumbariumcopperoxide.4.3.4Sol-GelSol-gelprocessinghasattractedmuchinterestforthepreparationofspecialpowdersbutalsoforthepreparationofceramicmembranesandforformingthincoatingsorextrudedshapes.Insol-gelprocessing,colloidalparticles,dispersedinasuspension(asol)undergofurtherreactions,whichcausesthecolloidalparticlestojointogetherintoacontinuousnetwork,calledagel.Thegelisdried,calcined,andinthecaseofapowdermilled.Inmostcasesthestarting(precursor)materialforsolpreparationisametalalkoxide.Theprepara-tionofsolsandsubsequentgelscanbedoneinseveralways.Someexamplesfortheso-called“col-loidalroute”willnowbedescribedHydrolysis/peptisationmethodApuremetal-alkoxideisdropwiseaddedintowaterwhilestirring.Inthiscasethehydrousmetaloxidesareprecipitatedforvaluesof[OH]/[M]>z(M:metalwithvalencez).Thisprecipitateisconvertedtoasolbypeptisation(=introducingasurfacecharge).Forthatpurposeoftennitricacidisadded.Anexampleisthepreparationofaboehmite(g-ALOOH).Aluminiumsecondarybutoxideisaddedtowaterthatwasheatedtoatemperatureabove80°Candstirredathighspeedduringthealkoxideaddition.Thesolutionmustbekeptatthistemperaturetoavoidtheformationofbayerite(seeFigure4-3).Thegelwaspeptisedwith0.07molHNO3.Afterremovingthebutanol(inthiscasebyevaporation)thesolwaskeptatabout100°Cduringabout16hoursunderrefluxafterwhichaclearsolwithuniformparticlesizewasobtained. 50AdvancedCeramicsProcessingAnotherexampleisthedropwiseadditionofanalcoholsolutionofametalalkoxidetoamixtureofwaterandalcoholwhilestirring(H2O:alkoxideratio=20:1).Theprecipitateisfilteredandwashedwithalargeamountofwatertoremovethealcohol.Theprecipitateisredispersedintoawater/nitricacidmixtureandtheparticlesarepeptisedatroomtemperatureorduringrefluxingatabout80°C.Thismethodisusedforthepreparationofnanocrystallinetitaniapowderandisfur-therdescribedin:àK.P.Kumar,K.Keizer,A.J.Burggraaf,T.Okubo,H.NagamotoandS.Morooka,"Densifi-cationofnanostructuredtitaniabyaphasetransformation"Nature358(1992)48-50Controlledhydrolysis/condensationInthiscasethemetalwaterrationislessthanthestoichiometricratio.Forthepreparationofnanocrystallinetitaniaparticles(so-calledQ-particles)water(+HNO3pH=2)ismixedinanalco-holanddropwiseaddedtoametal-alkoxide/alcoholmixture.TheH2O:Timolarratiowasfixedto2.5:1.Theformationofthenanosizedsolparticlescanbecontrolledbythewater/metalratio,thepHandvaryingthealkoxidegroups.Large,branchedalkoxidegroupsstericallyhindertheconden-sationstepduringhydrolysis.DialysisThemetalalkoxideisaddedtoawater/HNO3mixture(pH<0).AtthesepHvaluesthemetalhy-droxideformedisdissolved.Partofthecounterionsareremovedbydialysisagainstpurewater(thepHisincreased).GelationFromthesolsgelscanbeformedbygradualremovalofthedispersionmedium(inmostcaseswa-ter)orbycolloidaldestabilisation.Destabilisationcanbedonebyremovalofthecounterionsorbyadditionofdestabilisingionse.g.bypHchanges.Thewayofgelationhasastronginfluenceontheresultinggelstructureandgelmorphology.Gradualremovalofthedispersionliquidoftenresultsinamicrostructurebasedonaregularstackofultrasmallcrystallites.Duringliquidextractiontheparticlesareforcedtoapproachwhiletheconcentrationincreases.Duringthisprocessthecolloidalparticleskeeptheirsurfacecharge.Theserepulsionforcesincombinationwithincreasingconcen-trationfinallyresultinaratherdenseandregularlystackeddryproduct.InthepaperofK.P.Kumaretal.(Nature358(1992)48-50)theinfluenceofparticlecharge(controlledbyHNO3addition)onthestructureofthedriedproductisdescribed.Ifonthisdriedgelnotemperaturetreatmentwillbegivenattemperaturesabove150°Cthesystemiscalleda"xerogel"(=water-freegel).Theindividualparticlesstillhavetheirsurfacechargeandcanberedispersedasasolbyagainaddingwater.Suchoxidesarecommercialavailableforseveralelements("dispersable"alumina,silica,titania)usedasasemimanufacturedarticlefortheprepara-tionofcatalystsandcarriermaterials.4.4PreparationfromthegasphaseCVDChemicalvapourdeposition(CVD)isatechnique,whichisveryoftenusedtoapplyacoatingonasubstrate.CVDcanbedefinedasachemicalpreparationtechniqueofsolidmaterialsstartingfromreactantsinthegasphase.Severalbookshaveappearedonthissubject.Themethodise.g.de-1scribedinSputteringSputteringasanexampleofaphysicalvapourdeposition(PVD)technique,meaningthatina“physical”wayavapourisproduced.1pp.207-234andpp.302-315in:Paul.J.vanderPut“TheInorganicChemistryofMaterials,HowtoMakeThingsoutofElements”PlenumPress(NewYork,London)1998 4Preparationofceramicpowdersandcoatings514.5Preparationfromthe(partly)meltedphase4.5.1PlasmasprayingPlasmasprayingisaoftenusedtechniqueforapplyinge.g.hydroxyapatitecoatingstomedicalimplantdevices.Itisoneoftheseveralthermal-sprayingtechniques,whichentailthe(partial)meltingofpowderedmaterialsanditsdeposition(spraying)onanappropriatepreparedsurface.Duringplasmasprayingthethermalenergysup-pliedbyanionisedgas,orplasma,(partially)meltstheceramicparticle.Thecarriergas(usu-allyargon)isionisedasitpasseswithinthehightemperaturedischargezonelocatedatthecurrentFigure4-7:schematicrepresentationofplasmasprayingarcacrossthegapbetweentheanodeandcath-withpowderfedintotheplasmastreaminternally.ode.Figure4-7showsaschematicdiagramofa2plasma-sprayingset-up.Whentheparticleshitthesubstrate,itdeformsintowhatisknownas“splats”(seeFigure4-8).Thelayerformedhasaresidualporosityofabout2%andcancontainamorphousaswellascrystal-linephases.Postheattreatmentcanincreasesomeofthemechanicalandbondingpropertiesofthelayer.Figure4-8:SchematicrepresentationofacharacteristicmorphologyasmadebyplasmasprayingTheparticlesizedistributionofthestartingpow-dershouldnotbetoobroad,whileotherwisetheparticlesdonotmeltuniformly;somewillbeoverheatedorvaporised.Ifforhydroxyapatiteaparticlesize<45mmisused,mostlyacoatingisobtainedwithanalmosttotallyamorphousstructureduetocompletemeltingoftheverysmallpar-3ticles.Itmustbementionedthatthiscriticalparticlesize,belowwhichcompletemeltingoccurs,ofcoursealsodependsonexperimentalconditions(e.g.flametemperature,residencetimeofpow-derinflame).Theplasmasprayingprocessrequiresrougheningofthe(metallic)substratesurfacee.g.bygritblasting,inordertoobtainbondingofthecoatingtothesubstrate.Theadhesionofthesprayedpar-ticlesisbasedonamechanicalclampingoftherapidlyhardeningofthepowderparticlesontheroughenedpeaks.2W.R.Lacefield“Hydroxylapatitecoatings”pp.223–238in“Anintroductiontobioceramics”ed.L.L.HenchandJ.Wilson,WorldScientific(Singapore)1993.3C.P.A.T.Klein,J.G.C.WolkeandK.deGroot“Stabilityofcalciumphosphateceramicsandplasmasprayedcoating”pp.199–221in“Anintroductiontobioceramics”ed.byL.L.HenchandJ.Wilson,WorldScientific(Singapore)1993 535Treatmentsofpowders5.1Millingorcomminution5.2Mixing5.3Granulation(controlledagglomeration)Granulationisveryoftenusedinpowdertechnologyasacontrolledagglomerationtechnique.Granulesobtainedinthiswaycanbeeitheranend-product(e.g.catalystsorpharmaceuticals)orasemi-manufacturedproduct(inceramictechnology).Muchattentionispaidtothisprocessinpow-dertechnology,becauseitisofimportanceforseveralpowderproperties.Withthisprocessitispossibletoobtainauniform,homogeneous,non-dustingandfree-flowingpowderresultingingoodstackingandhomogeneousdiefillingpriortocompaction.Theflowabilityofapowderisdeter-minedbyusingtheHallflowmeterasisdiscussedinchapter2.Ingeneral,uniformgranulesarepreparedbyagglomeration(sizeenlargement)andsometimesbycomminution.Granulationtechniquesbycontrolledagglomerationcanbedividedinlayeringgranulation,pressgranulation,andspraydrying.Thesetechniqueswillbedescribedinthissection.1InPartVIofReedcomminution,mixingandothertechniquesforimprovingthepowderpropertiespriortogreenformingaredescribed.5.3.1LayeringgranulationLayeringgranulationisawidelyusedtechniqueforthepreparationofgranuleswithasizeof0.1-1mmstartingfrompowderparticleswithasizeofabout10mm.Thismethodstartswiththeadditionofsmallamountsofliquidtoapowder.Thefinelydispersedliquidresultsinstrongadhe-sionforcesbymeansofliquidbridges(capillaryforces).Thismaterialisplacedinaninclinedro-tatingpan,arotatingdrumoramixerandsubjectedtoarollingmotion.Therandommotionresultsinsphericalagglomerates.Inagranulationpantheparticlenucleiandsmallparticlesmovenearthebottomofthepanandaretransportedupwardsbecausetheinitiallyirregularparticlesexhibitmorefriction.Thelarger(spherical)granulesrollmoreeasilyoverthesmallergranulesandarefinallydischargedwithawell-definedsize.Thegranulesizeofthematerial,whichisdischargedthroughoverflow,issouni-formthatasubsequentclassificationisoftennotnecessary.Inagranulationdrumtheaxialinclinationistoolowtoresultinclassification.Itservesrathertotransportthematerialthroughthecylinder.Accordingly,thesizedistributionofthegranulesthatleavethedrumissubstantiallybroaderthanofthoseleavingagranulationpan.Inmixerstheproductisagitatedeitherbymovingpaddlesorbyrotationofthewall.Ifpowderandliquidareintroduced,agglomeratescanbebuiltupinthesamewayasinagranulatingpanordrum.High-speedcutterheadsensurethatagglomeratesthathavebecometoolargearecommin-uted.5.3.2PressgranulationThepressgranulationtechniqueismainlyusedforthepreparationofendproducts(e.g.catalysts,pharmaceuticals).Duringpressgranulationofdrypowdersthematerialissohighlycompactedthatthismethodresultsinagglomeratesofadequatestrength.Thecompactionprocessincreasesboththenumberofcontactpointsbetweentheparticlesandtheadhesionoftheparticlesbydeforming1ndJ.S.Reed,“Principlesofceramicsprocessing”2Edition(1995) 54AdvancedCeramicsProcessingthemintheircontactzone.Compactionofdrypowders,whichissometimescarriedoutwithbind-ers,isperformedbypressing(tabletting)orbyrollpressingwithsmoothandbriqettingrolls.Moistmassessuchasfiltercakesandpastesaregranulatedindiepresses,wherethematerialisforcedthroughholesandisthuscompactedtoextrudates.5.3.3SpraydryingGranulescanalsobeproducedbydryingawet,loosemassofmaterial,suchasasuspensionorasolution.Equipmentsusedforthismethodare,e.g.spraydryersandfluidisedbeddryers.Inthespraydryingprocessasuspension(mostlywater-based)ispumpedtoanatomiserandsprayedintoahotdryingmedium.Whenthespraydryingprocessisperformedproperlytheresultinggranuleswillbesphericalanduniforminsizeanddensity.Thesegranulatedpowdersshowabetterflowabil-itythanirregularlyshapedpowdersand,consequently,abetterfillingof,e.g.apressingmould.21Oneofthemainproblemsofspraydryingistheformationofhollowspheresandchapter20inDeflocculants/surfactantsandbindersareaddedtosuspensionusedforspraydryingtoobtainsta-ble(aqueous)powdersuspensionswithalowviscosityandenoughadhesivequalityandstrengthforstabilisationofthesphericalparticles,whichareproducedbyatomisation.Theseparticlesmustremainsphericalduringthemechanicalactionsperformedonthedropletsinthespraydryerandinthespraydriedstate.Lubricantsmaybeintroducedbeforespraydryingorareaddedafterspraydryinginordertocoatthegranulesnexttodecreasewearandfrictionalongthediewall.Lubricantsenhancetheflowbe-haviourofthepowder,improvedensityuniformityaftercompactionandreducethepressureneces-saryforejectionfromthedieaftercompaction.Severalparametersinfluencethepropertiesofthefinalspray-driedgranule.Notonlythedropletsizeaftersprayingdeterminesthefinalparticlesizebutalsothesolutionviscosityandsolidscon-tentofthesolutionorslurry.Duringdryingthedropletslosemoistureandthecompactwillshrink.Ifthesolidmaterialcannotadjusttothedropletshrinkageorifitformsaskin,theresultingparti-clescanbehollow.Thesehollowparticlesoftencollapseonthemselvesresultinginanirregularagglomeratestructure.Fastdryingconditionsandlowsolidcontentinthespraycontributetothistypeofparticleformation.Amoresophisticatedwayofspraydryingismakinguseofanultrasonicnozzleforgeneratinga3spray.Herethepowdersuspensionispumpedintothedryingvesselbymeansofanultrasonicnozzle.Thenozzlefrequency,whichdeterminesthedropletsize,isvariedfrom25-100kHz.Inthiswayitwaspossibletovarytheagglomeratesizefrom30-100mm.Bythisultrasonictech-niquethedropletsizedistributionisverynarrowrelativetootherspraytechniques.Therefore,theresultinggranulesizedistributioncanalsobeverynarrow.2S.J.Lukasiewicz,J.Am.Ceram.Soc.72[4](1989)6173H.J.Stamato,"Passive-FlowSprayDrying-ANewTechniquefortheLaboratory",BulletinoftheAmericanCeramicSoci-ety,70[10](1991)1651-57 556ProcessesforcompactionAfterpowderpreparationandeventuallypowderconsolidation(millingetc.)compactionisthesec-ondstepintheceramicfabricationprocess.Thegoalofthisstepistoformthepowderinacertainshapewithsufficientstrengthandalsosufficientcontactpointsbetweenthepowderparticles,nec-essaryforthesinteringprocess.Contrarytowhathasbeenthoughtforalongtime(andisstilloftenheard)maximumdensityisnottheprimarygoal.Theprimarygoalistoobtainacompactashomo-geneousaspossible.Thismeanshomogeneityinpackingoftheparticles(coordinationnumber),poresizedistributionanddensity.Ofcourse,higherdensitiesarehelpfulinafurtherdensification(sintering)processbutthebenefitsofhigherdensitiesareonlyfullyrealisedinhomogeneouscom-pacts.Manymethodsareusedtoconsolidatepowdersandsomeofthemwillbediscussedinthischapter.6.1DrypressingDrypressingisoneofthemostpopularshapeformingprocesses.Itinvolvesarelativelysimpletechnology,whilehighproductionratesarepos-sible.Drypressingmaybedefinedasthesimul-taneous(uniaxialorisostatic)compactionandshapingofa(granulated)powder.Inmostcasessmallamounts(few%)ofwaterand/ororganicsareusedaslubricants.Theextensivepracticeofdrypressingstemsfromtheinherentabilitytoformrapidlyawidevarietyofshapeswithclosetolerancesusinghighlyautomatedequipments.Generallyonecandividethepressingprocessinuniaxialandisostaticpressing.Foruniaxialpressing,apressureisappliedinonedirectionbyapunchonapowder,embeddedinadie(seeFigure6-1).Thedie,usuallymadofhardsteel,isfilledwitheitheradrypowderorapowdercon-taininguptoseveralweightpercentofabinder.Insomecasessimplefew%ofH2Oisaddedasabinder.(Cold)uniaxialpressingisused,forin-stance,intheproductionofceramictransformerFigure6-1:Pressandejectionmotionusingafloatingdiecores(ferrites).(uniaxialpressing).Inisostaticpressing,pressureistransferredviaaliquidmediumwhiletheproductisencapsulatedinsideanelastomeric“bag”.In“wet-bag”isostaticpressing(seeFigure6-2),the“bag”isplacedinapressurevessel,containingoilorwater.Inthe“dry-bag”pressingmethod,however,the“bag”ispartofthepressurevessel(seeFigure6-3).The“dry-bag”methodisthepreferredmethodtoproducelargerseriesofcompacts.Inbothcasesthepressureisapplieduniformlyovertheoutsideofthepowdercompact.Pressingcanalsobeperformedatelevatedtemperaturesinsuchawaythatpressingandsinteringarecombined.Inthatcaseonespeaksofhotuniaxialpressing(HUP)andhotisostaticpressing(HIP).Thelattertechniquesgenerallyresultinhigh-qualityproductsbuthavenotyetfoundwide-spreadusebecauseoftheirlargecosts.Contrarily,thecoldpressingtechniqueshavemanyapplica-tionsinmass-productionofceramicproducts.Notonlyceramicpartsaremadebycompactionburalsovariousmetalpartsaremadebycompac-tion.Fromamesoscopicpointofview,thedeformationmechanismtoformacompactistherear- 56AdvancedCeramicsProcessingrangementofparticles,whereasincaseofametalpowdertheparticlesthemselvescanshowplasticdeformation.Figure6-2:“Wet-bag”isostaticpressing.Coldisostaticpressing(CIP)canbeusedtomakerathercomplexshapesonamassscale,forexam-pletheceramicpartofsparkplugs.Apropercon-ditioningofthepowderbeforepressingisofex-tremeimportancetoobtainhomogeneousgreenstatecompacts.However,twooppositerequire-Figure6-3:“Dry-bag”iosostaticpressingmentsconcerningtheparticlesizeshouldbemet:·Ontheonehand,toobtainagoodsinteractiv-ityofthegreenshapetheaverageparticlesizeshouldgenerallybeintherangeof0.1-10mm,·ontheotherhand,thepowdershouldhavegoodflowproperties,whichcanbeachievedbylargerparticlesizes,i.e.largerthan20mm.Incaseofgoodpowderflow,thedieisreproduci-blyfilled,thepressureishomogeneouslytransmit-tedthroughthecompactduringpressingandthepressingratecanberapid.Powdersthatarepackedefficientlywithinthedieresultincompactswithsmallvariationsinpackingdensity.Thesmallerthesedensityvariations,themorehomogeneouswillbetheshrinkagewithinthesinteringstage,resultinginaproductwithoutdistortion.Thisemphasisestheimportanceofareproducible,homogeneousparticlestackingwithanarrowpore-sizedistributioninthecompactwithoutanyspecialorderingphenomena(spatialcorrelationsofphases).Figure6-4:ScanningelectronmicrographofagreenTherequirementofhomogeneityinthegreenstatecompactofarelativelystronglyagglomeratedzirconiaisverygeneralandofextremeimportanceincom-powderinwhichresidualagglomeratesarepresent.pactionprocessing.Toencounteralldemandscon-cerningthefavourableparticlesizeforsinteringandflow,thepowdermicrostructureisoftenadaptedandthepowderparticlesaregranulatedintosoftagglomerates.Thecompleteprocess,however,mustbedesignedinsuchawaythatpreferablynotracesofpowderagglomerationareleftinthegreenstateafterpressing. 6Processesforcompaction57Figure6-4showsaSEMpictureofagreenzirconiacompact,inwhichtheagglomeratesstillarevisible,althoughtheyarebrokendowntosomeextent.6.1.1PowderpropertiesThepowderusedforcompactionshouldusuallybefree-flowing,resultinginauniformfillingofthepressingmould.Thismeanse.g.that,afterfilling,porosityandporesizedistributionareuni-formandthatnolaminatingtendencyofthepowderoccursduringfilling.Otherrequirementsofpressingpowdersarethattheyarenon-dusting,whileduringdiecompactiontheydonotadheretothepunchandcauseaminimumindiewearanddiefriction.Forthatpurposegranulatedindustrialpressingpowdersareused.Thegranulesmustbestableunderambientconditions.Thismeansthatthegranulesmustnotfractureordeformduringstorageorduringfillingofthemould.Thegranulesshould,however,fractureduringcompactioninordertoobtainauniformstackingoftheabsoluteparticles(oraggregates)aswillbediscussedinsection6.1.3.6.1.2FillingthedieormouldThefillingstepduringpressingiscritical.Inhomogeneitiescreatedinthisstageremainwithinthecompactduringtheremainderoftheceramicfabricationprocess(compaction/sintering)orcanevencausefractureduringsintering.TheHausnerindex(seechapter2)expressestheextenttowhatthedieisfilleduniformly.Priortocompaction(soafter“tapping”)notonlyuniformfillingisimportantbutalsoasufficientlyhighdensity(say±20%relativedensity).Manyfinepowdershaveafillingdensityoflessthan10%whichcanbeimprovedonlyslightlybytapping(upto15%relativedensity).Asaresultoftheselowdensitiesthecompactionratiowhichistheratiogreencompactdensitytofilldensitycanbeupto10.Suchhighcompactionratiosareimpracticalformostcommercialdrypressingopera-tionsandalsoleadtonon-uniformgreencompacts.Sinter-activepowdersusedforthepreparationofadvancedceramics,oftenhaveahighHausnerindexandahighcompactionratioand,therefore,showbadcompactionbehaviour.Atechniqueforimprovingthecompactionbehaviouroffineceramicpowdersisthegranulationoftheparticlestolargersphericalagglomerates(granules).Atechnique,whichisoftenusedforthepreparationof“pressgranules”,isspraydrying(seechapter5)Ingeneralonestrivesforthefollowingcharacteristicsforpowders:·Sphericallyshapedagglomeratesthateasilyflowovereachothertouniformlyfilladie(lowHausnerindex,lowinternalparticle-particlefriction),·controlled,uniformporosityandporesizedistributionwithinthegranules,·highfilldensitypriortocompaction(lowcompactionratio),and·uniformcompositionwithinandbetweenagglomerates.6.1.3Microstructuredevelopmentduringdrypressing.Generally,onecansaythatcompactionofceramicpowdersinvolvestherearrangementandfrag-mentationofessentiallynon-deformableparticlesresultinginadecreaseinporesizeandabetterparticlecontact.So,theinternalstructureofthecompactchangesduringthecourseofthepressingprocess.Intermsofmicrostructureparametersthismainlyconcernschangesinagglomerateandporemorphology.Techniqueslikegasadsorption/desorptionandmercuryintrusionporosimetryareusedforanalysingtheporesizeandporesizedistribution.Ashortdescriptionofbothmethodsise.g.giveninthebookofJ.Reed. 58AdvancedCeramicsProcessingThecompactionprocesscanbefollowedbymeasuringcompactioncurves,wheredensityisplottedagainstthepressureapplied.InFigure6-5anexampleofaweaklyagglomeratedzirco-niapowderisshown(thispowderwasprepared1byanalkoxideprecipitationtechnique).Tworegions,alow-pressureandahigh-pressurere-gioncanbedistinguished.ThisbehaviourisalsoreflectedbyanalysingtheporesizedistributionduringcompactionasgiveninFigure6-6.Theporemorphologyisanalysedaftercompactionat4,8,95and400MPa.Aftercompactionat4MPaclearlyabimodalporesizedistributionisFigure6-5:Compactioncurveofazirconiapowder,visibleforbothpowders.AftercompactionatshowingatransitionpressurePj.qindicatepressuresat8MPathebimodalityisremoved,althoughawhichporesizedistributionisdetermined1.longtailintheporesizedistributionremains.Poreswithradii<20nmshownochangeinsizeandvolumeifcompactionat4and8MPaarecompared.Forpressuresabovethe“transitionpressure”Pj(95and400MPa)thesesmallerporeschangeinsizeandvolume.Severalaspectsaboutthemicrostructuredevelopmentduringcompac-tioncanbederivedfromtypicalfigureslikeFigure6-5andFigure6-6·Atlowpressuretheagglomeratesrearrange,settleanddeformslightly.Theinternalag-glomeratestructureremainsintactatthesepressures.Thiscanbeverifiedinthefollow-ingways.Thedensityoftheagglomeratescanbedeterminedbymeansofgasadsorp-tion/desorptionanalysisofthefreepowder(4nm100nm)havecompletelydisappearedbeforetheagglomeratesfractureanddensify·Thesmallporeswithradii<3nmdidnotchangeinsizeandvolume.Theseporesareascribed1totheporeswithintheaggregates.AdifferentmicrostructuredevelopmentduringcompactionispossibleasisshowninFigure6-7andFigure6-7.Aftercompactionabovetheyieldpointintergranularporesarestillpresent.Theseporesaredenotedasthe“persistentintergranularpores”inthepressedpiece(seeFigure6-8)Sointhiscase,abimodalporesizedistributionispresentinthegreencompactpriortosintering.Thismaybecausedbythepresenceofrelativelyductileagglomerates(granules).Thesegranulescandeformtoalargeextentwithoutfracturingandconsequentlylargeforceswithinthecompactare1M.A.C.G.vandeGraaf,J.H.H.terMaatandA.J.Burggraaf,J.Mater.Sci.,20(1985)1407 6Processesforcompaction59transmittedinthecompactbyplasticdeformationofthegranules.Especiallywhenthoseductilegranuleshaveahighdensity(relativedensityabout50%)theywillnotfractureeasilyand(partof)theagglomeratestructuremayremainaftercompactionresultinginaninhomogeneousstackingoftheparticles.Theanisotropyinthisgreencompactwillresultintodifferentialshrinkage.Figure6-7:Cumulativeporesizedistributionsindicatingintergranularandintragranularporosityandporesizesonfillingdieandchangeswhenpressed(Hg-porosimetry).Figure6-8:Changeinshapeandbimodalporesizedis-tributionduringcompactionsInordertobesurethatno“persistentintergranu-lar(orinteragglomerate)pores”arepresentafter(isostatic)compactionadensificationbehaviourisnecessaryasgiveninFigure6-9.Thecurvecanbedividedintothreelinearportions.InthepressurerangelowerthanPy(=46MPa),therela-tivedensityincreasesslowly,whichcorrespondstotherearrangementoftheagglomeratesasdis-cussedbefore.InthepressurerangefromPy(=46MPa)toPa(=73MPa),therelativedensityincreasesrapidly,whichisascribedtotherear-rangementintheinternalstructureofdeformedordisruptedagglomerates.ThepressurePy,atwhichagglomeratesdisruptcanagainbecharac-Figure6-9:Densificationofveryweaklyagglom-terizedastheagglomeratestrength.TheloweratedanduniformpowdervalueofPy(=46MPa)isanindicationforweakagglomeratesinthepowders.ThepressurePaiscalledthe“joiningpressure”asdiscussedbyMa-2tsumoto.AtpressuresabovePaonlyarearrangementofprimarycrystallitesor(inourcase)ag-gregatesisassumed.Pahasavalueof73MPa,whilethecompacthasarelativedensityof56%atthatstage.Atthefinalpressureusedinthisexperiment(300MPa),therelativedensityofthegreencompactreachesabout63%.Suchahighdensityofthegreencompactandauniformstackingoftheparticlesmayallowsinteringofthecompacttoadenseceramicatarelativelylowtemperature2R.L.K.Matusumoto,“Generationofpowdercompactionresponsediagrams,”J.Am.Ceram.Soc.,69(10),C264-7(1986) 60AdvancedCeramicsProcessing6.1.4Die-wallfrictionduringuniaxialcompactionForanumericaltreatmentofpowdercompactiontheeffectofwallfrictionhasalsotobetakenintoaccount.Datacanbegatheredbyawallfrictiontester(seeFigure6-10)whichmeasuresthefric-tionbetweenacertainwallandapowdercompact.Fromtheplateauintheexperimentalcurvethevalueofthefrictioncoefficientiscalculated.Theresultsforavarietyofpowdersaregiveninfigurefromwhichitisclearthattherearetwoplateau’sdependentonparticlesize/wallroughnessratioaswellastheparticlehardness/wallhardnessratio.Simplemodelscanbeusedtorationalisetheseresults(seefigure).Overawiderangeofveloci-ties(typically0.01-1mm/s)thefrictioncoeffi-Figure6-10:Powderwallfrictionapparatus.cientappearstobeindependentofthevelocity.Thefrictioncoefficientisrelatedtothefollowingcharacteristicsofthepowderandthewall:·Hardnessofthepowderparticle(Hp),·hardnessofthediewall(Hw),·surfaceroughnessofthediewall(Rw),and·particlesize(dp).InFigure6-11theeffectoftheseparametersonthedynamicfrictioncoefficientfdynareshown.TheresultsinFigure6-11arederivedfrommeasurementsonferricoxideasdiscussedby34StrijbosandWelzen.Ifdp/Rw<1thefineparti-clesformastationarylayerofpowderonthewall.Themovingpowderparticlesinthecom-pacthavenodirectcontactwiththediewallbutwithathinlayerofsimilar,non-moving,parti-cles.Theobservedvalueoffdyn(seeFigure6-11a,fdyn»0.6)isassumedtobethepowder-powderfrictioncoefficient.Thisphenomenonofthenon-movingpowderlayeratthewallandthemovingpowderinthemouldresultsinadiffer-enceinstresseswithinthecompact(closetothewallversusthecentreofthemould).Conse-quently,adifferenceispresentindensitieswithinthecompact.Ifdp/Rw>1thepropertiesofthewallaswellasthatofthepowderinfluencefdyn.Softparticlescompressedinaharddie(Hp/Hw<1)showacontinuousdecreaseoffdynwithincreasingratioofdp/Rw.Adisintegrationoftheparticlesatthediewallcanoccurduetothefactthatthepow-Figure6-11:Dynamicfrictioncoefficientfdynforavarietyofpowders(particlesizedp)andwallrough-derislesshardthanthewall.Withincreasingnessvalues(Rw)aswellasvariousratiosofparticledp/Rwthenumberofcontactpointsofaparticletowallhardness(Hp/Hw).(perunitarea)withthediewallincreases.3S.Strijbos,ScienceofCeramics8(1976)4154J.T.A.M.Welzen,J.MaterialsEducation8(1985)187 6Processesforcompaction61Thereforestressesattheparticlesdecreasewithincreasingdp/Rwresultinginadecreaseinparticledisintegration.Consequentlythefrictionathighdp/Rwratioisapurelypowder-wallfriction.Soverylowfrictionsarepossibleinthiscase.Iflarge,hardparticlesarepressed(dp/Rw>1andHp/Hw³1)thereisasharpdecreaseinfdynwithincreasingdp/Rwratioreachingaconstantvalueatdp/Rw>10(seeFigure6-11bandc).Ingen-eralonecanstatethatinthiscaseataboutdp/Rw=1arapidtransitiontakesplacefrompowder/powderfrictiontowallscratchingbythepowder.Oneofthemajorproblemsofthisdiewallscratchingbythepowderisthecontamina-tionofthepowdercompactbythediewallma-terial.Adiewalllubricant(forexamplestearicacid,dissolvedindiethylether)isanotherpossibilityforfrictiondecrease(seefigure).ThecoefficientoffrictionbetweenfinepowdersandlubricatedFigure6-12:Simplemodelfortheexplanationofwalls(dp/Rw<1)decreasesgraduallywithin-thetwolevelsoffriction.Atsmallparticlesizesthecreasinglubricantfilmthickness.Forcoarsepowderfillsthewallroughnessesandfrictionises-powders(dp/Rw>1)thecoefficientoffrictionsentiallypowder-powderfriction.Thisresultsinhighfrictioncoefficientsindependentofthethewallpa-slightlydecreasesbytheadditionoflubricantsrameters.Atlargeparticlesizesonlyasmallnumberandisnearlyindependentofthethicknessoftheofcontactpointsexistsbetweenthepowderandthelubricantlayer.Duringpressinglubricantsarewallandthefrictiondependsonthewallparameters.notonlyintroducedtoreducediewearandejec-tionpressure.Lubricantsarealsointroducedbeforegranulation(spraydrying)oraddedasacoat-ingonthegranulesinordertoimprovethepow-derflowability(duringfillingofthemould)andtoreleasepowder-powderstressdistributionsduringcompaction.Inordertominimisetheshearingstressesatthediewallalowpowder-wallfrictioncoefficientisnecessary.Fromthediscussionaboveitcanbeconcludedthatforuniaxialcompactionofpow-dersthebestresultscanbeobtainedwhendp/Rw»1andHp/Hw<1.Thismeansalowfric-Figure6-13Typicalresultsshowingthesheartioncoefficientandnocontaminationofthecom-forceversuswalldisplacement.Fromtheconstantpactduetopowder-wallscratching.Sorelativelyvalueatlargerdisplacementthedynamicfrictionlargepowderparticlescanbewellcompactedinacoefficientiscalculated.diemadefromahardmaterialwithasmoothsur-face.Inpracticethesurfaceroughnessofthediewallaftergrindingisabout0.2mm.Smootherwallsdemandanextra,expensivesurfacefinishing.Thepowder-wallfrictioncanbereducedbygrindingthediewallparalleltotheslidingdirection.FromFigure6-11aitcanbededucedthatforthecompactionofferricoxideinadiewithawallroughnessof0.2mmandadesiredfdynof0.23theparticlesizemustbeabout20mm.Whenusingpowderswithsmallerparticlesizesitisnecessarytogranulatethepowderstothedesiredparticlesize.Compactionofthosegranulatedpowderscanfulfiltherequirementsmentionedabove.IfhoweverduringcompactionthegranulesarefracturedatapressurePytheparticleswhicharerear-rangedatP>Pycanagainbesmallerthanthecriticalsizerequiredforalowpowder-wallfriction.Anotherconclusion,whichcanbedrawnfromthisdie-wallshearingstresstheoryisthatitisal-mostimpossibletouniaxiallycompressfine-grainedhardpowders(forexamplesinter-reactivezir-conia). 62AdvancedCeramicsProcessingAnotherproblem,whichcanariseduringuniaxialcompaction,isanon-uniformtransmissionoftheaxialpressurethroughthecompact.Ingeneralitcanbestatedthatpressuretransmissionfromtoptobottomofacompactislargerforcompactsofsmallerthickness/diameterratioand/orwhenthediewallislubricated,consequentlyresultinginagreencompactwithmorehomogeneousdensity.6.1.5DrypressinginpracticeDiepressingcanbepractisedusingadryora“wet”powder.Indrypressingpowderisusedeitherreallydryorwiththeadditionofupto5vol%watertoimprovetheadhesionofthepowder.Ifthisisnotsufficient(whichisoftenthecase),bindersareused.Hereachoicecanbemadebetween‘hard’or‘soft’binders.Thefirstclassyieldsratherhardagglomeratesinthepowderwhichmakesthepowderfree-flowingbutnotself-lubricating.Thelatterclassyieldssoftagglomerateswhichresultinflowproblemsbutlesslubricationproblems.ExamplesofhardbindersaredextrineoracrylatewhilewaxandArabgumareexamplesofsoftbinders.Acompromiseisachievedby‘intermediate’binderslikepolyvinylalcohol(PVA)ormethylcellu-lose.Typicallyafewpercentsofbinderareused.Sometimesafewtenthsofapercentofalubri-cant,suchasparaffinoilorastearinesolution,isaddedtoimprovethelubrication.Apressure-1rangeof20-100MPaisgenerallyused,whereasthepressingraterangesfrom0.01-5sforsin--1gleactionpresses,whileforhighspeedrotarypressesratesupto100scanbereached.Thetoler-anceinmassthatcanbeachievedindrypressingistypicallyabout1%.Insizeatoleranceof0.02mminthicknessandabouttwicethatinplan-parallelismcanbeachieved.Inwetpressingmuchmorewaterisaddedtothepowder,typically10-15vol%.Forwetpressingtheuseofbindersisquitenormal(about2%).Problemsthatcanarisewiththedie-pressingarewearofthedies(contaminationandlossofsizetolerance),cracksinthecompactanddensityvaria-tions.Inpressingdensityvariationsalmostalwaysoccuroverthecompact.Thesevariationsareduetoinhomogeneousfillingofthedieandthepressingprocessitself.Amorehomogeneousfillingcanbereachedbyusingapowderwithincreasedflowability.Theflowabilityofapowderisoftenin-creasedbydeliberateagglomeration,e.g.byspraydrying.Furtherimprovementmaybereachedsometimesbyso-calledtap-filling:after(orduring)fillingofthedie,tappingisappliedbeforethepunchesareloweredtoconsolidate.Asimpleimprovementinthepressingprocessitself,toavoidthesedensityvariationstosomeextent,istheapplicationofthecompactionforcefromtwosides.Arelativelynewimprovementistheuseofultrasonicenergy.Byapplyingultrasonicwaves(typically20kHz)duringcompaction,particularlyintheearlystages,asignificantimprovementinhomoge-neitycanbeachieved.Inparticular,largervoidsareremoved.Figure6-14showsseveralkindsofdefectsand/orfracturemodeswhichcanoccurafterincorrectisostaticcompaction.Figure6-14Defectsinisostaticallypressedcompacts,a)neckingduetounderfilloronevenfillpossiblyorigi-natingfrompoor-flowingpowder,b)irregularsurfaceduetounevenpowderfillorunsupportedbag,c)‘ele-phantsfoot’duetorigidclosuresinwetbagtoolingortoahighlycompactablepowder,d)‘banana’duetoun-supportedbaginwetbagtooling,e)compressioncrackduetoaxialspringback,typicalforhardpowders,f)laminationduetocompressioncrackingoriginatingfromunsuitableortoothickbagmaterialorweakcompacts,g)irregularsurfaceduetounsuitableortoothickbagmaterial,weakcompactsorsmallcornerradii,h)axialcracksduetoinsufficientelasticspringback(intubes). 6Processesforcompaction63Insummaryitcanbesaidthatpowderpressingisarelativesimplemethodbutacomplexoperationdependentonseveralparameters.Notonlythepowderpropertiesareincludedbutalsothoseofthedieandtheprocessingoperationitself.Thus,forhighqualitycompactsagreatdealofattentionhastobegiventothepressingprocedure.Modellingofthepressingprocessispossiblebutonlywhenmanydataareavailable.Consequently,experiencewithpressingremainsahighlydesirableassetforqualityproducts.6.1.6Specialtechniquesindrypressing·Explosivecompaction·Magneticpulsecompaction6.2SuspensionprocessingSuspensionprocessingincludesslipcastingandtapecasting.Thefeedmaterialfortheseprocessesisasuspension,i.e.acolloidalsolution,containingasolvent,adeagglomeratedstabilisedpowderandcertainadditives.6.2.1SlipcastingSlipcasting(seeFigure6-15)isoneofthemostwellknownceramicformingtechniquesandisusedinthefabricationofmosthouseholdandsanitaryware.Inslipcastingaslipispreparedbydispersingfineparticlesinaliquid(inmostcaseswater)usingadispersionaid(deflocculant).Theactualshapingprocessinslipcastingoccursbypouringtheslipintoaporousmould,generally.madeofplasterofParis(CaSO4½H2O).Theliquidphaseoftheslippenetratesintotheporesofthemouldundercapillaryforcesandtheparticlesstarttosettleatthewallofthemould.Oneoftherequirementsforslipcastingisthatthefineparticlesuspensionissufficientlystabletoremainin-variantunderprocessingconditions.Thecolloidalstabilityoftheslipisgenerallyaccomplishedbyaddingastabilisingaidmakingtheceramicparticlesinthesliprepeleachotherduetoelectrostaticrepulsionorsterichindrance.Itisevidentthatinterfaceandcolloidchemistryisanimportantdisci-plineinceramicsuspensionprocessing.Therefore,aseparatechapterinthiscourseisdedicatedtocolloidchemistry.Figure6-15:Slipcastingprocess.Duringparticlesettlinginthemoulditisofextremeimportancethatthestackingoccurshomoge-neously,whichinmostcasesmustresultinahighstackingdensity.Therefore,oftenmetastablesuspensionsareusedinwhichtheinteractionsbetweenparticlesonlybecomeattractivewhentheyareveryclose,likeathighsolidconcentrationduringsolventremoval.Finally,dryingofthecon-solidatedproductmustalsobeperformedashomogeneouslyaspossibleinordertopreventcrackformationduringthisprocess. 64AdvancedCeramicsProcessing6.2.2Colloidalfiltration6.2.3CentrifugalcastingIncentrifugalcastingofceramictubularcompacts,awell-dispersedsuspensionispreparedofparti-cleswith,preferably,anarrowparticlesizedistribution.Thesuspensionisbroughtintoatubularmouldthatissubsequentlyrotatedrapidlyarounditsaxissothatatubular-shapedsedimentisformedattheinnertubewallbytheactionofcentrifugalforcesontheparticles.Mouldreleaseandhandlingstrengthofthedepositedsedimentisgenerallyproblematic.Centrifugalcastingisappliedinseveralareas,e.g.theproductionofopticaltelecommunication56,789fibres,steeltubes,polyesterandpolyvinylpipes,homogeneousdenseceramicpartsorporous10ceramicsupportsformembraneapplications.Structuresmadebycentrifugalcastinghavedistinctpropertieswhencomparedtomoretraditionaltechniquesasextrusionandisostaticpressing.Themainadvantageistheuniformityinparticlepackingwhichresultsinaveryhomogeneousproductwithahigherstrengthandcorrosionresistanceincaseofthesteeltubes.Fortheproductionofdenseceramics,centrifugalcastingwillresultinahighersintereddensityoranequaldensityatalowersinteringtemperature.Acentrifugedtube,e.g.usedfortheproductionofporousceramicmembranesupports,showsanextremelygoodroundnessandaverysmoothinsidesurfacewhencomparedtoanextrudedtube.Thisisofimportancewhentubesaresealedintoreactormodulesandforthequalityofmembranes10thataredepositedonthissurfaceinalaterstage.Thestabilityofthedispersionhasalargeinfluenceonthecastingproperties.Ifthedispersionistoostablethesedimentwillremainfluid-likesothatanactualcompactisnotformedandredispersionoccursassoonasrotationhasstopped).Amethodtodestabiliseacolloidalfluidinsituisdirect11coagulationcasting(DCC).Ifthedispersionislessstabletheparticleswillstartattractingeachotheratagivenparticledensityintheliquid/sedimentinterfaceareasothatflock-likestructuresareformedwiththepossibleconsequenceofinferiorsinteringpropertiesifthehomogeneityofthegreencompactisseriouslyaffected.Formationofflocksinthesediment,ontheotherhand,maybeadvantageousformouldreleasebecausethepowderpackingdensitywillnotbeattherandomdensepackingmaximumandsomeplasticcompressibilityisleft(comparedrypressingofgranules).Centrifugalcastingisprobablymostusefultopreparetubularcompactsonalaboratoryscale;anadditionaladvantagemaybethatratherhomogeneouscompactsareobtainedwithneartheoreticaldensity,possiblyresultinginfa-vourablesinteringproperties.Anicedescriptionofthepreparationofvitreoussilicatubesbycen-12trifugaldepositionwithmanyrelevantexperimentaldetailsisgivenbyBachmann.6.2.4TapecastingInthetapecastingprocess(seeFigure6-16),whichisthemostoftenusedfilmcastingprocess,aparticlesuspensionisspreadona(continuous)beltthroughaslit-likeorifice(doctor-blading).Theresultingthinsuspensionlayerissubsequentlydriedandthedriedtapeisreleasedfromthebelt.Thedriedtapeshouldbesufficientlyflexiblesothatitcanbehandledandstoredforsubsequentprocessingwithoutdamagetotheparticlestacking.Intapecastingaparticlesuspensionispreparedlikeinslipcasting,althoughthesuspensionformulationfortapecastingdeviatessystematically5thMorissette,S.,Lewis,J.A.Abstractbook99annualmeeting&expositionAm.Ceram.Soc.(1997)103.6Northcott,L.,Dickin,V.J.Inst.Metals,70(1944)3017Royer,A.J.Mater.Shaping.Tech.5(1988)1978Jones,F.R.CentrifugalCastinginHandbookofPolymer-FibreComposites,Ed.,Jones,F.R.PolymerScienceandTechnologySeries,LongmanScientific&Technical(1994)1449a)Steinlage,G.A.,Roeder,R.K.,Trumble,K.P.,Bowman,K.J.,Li,S.,McElfresh,M.J.Mater.Res,9[4](1994),833;b)ibid.Am.Ceram.Soc.Bull.75[5](1996)9210Nijmeijer,A.,Huiskes,C.,Sibelt,N.G.M.,Kruidhof,H.,Verweij,H.Am.Ceram.Soc.Bull.(1997)11Graule,T.J.;Si,W.;Baader,F.H.;Gauckler,L.J.“Directcoagulationcasting(DCC):Fundamentalsofanewformingprocessforceramics”CeramicTransactions,51(1995)457-46112"P.K.Bachmann,P.Geittner,E.Krafczyk,H.Lydtin,G.Romanowski"ShapeFormingofSyntheticSilicaTubesbyLayerwiseCentrifugalParticleDeposition”,Amer.Ceram.Soc.Bull.,68[10](1989)1826-31 6Processesforcompaction65fromthegeneralrecipeforslipcastingslurries.Oneofthemostimportantdifferencesisthatthetapecastingmixshouldcontainabinderandplasticisertoensuremechanicalcoherencyandflexi-bilityofthedriedtape.Furthermore,intapecastingmostusuallyanon-aqueoussolventisusedbecausedryingoccursfasterthanincaseofwaterasasolvent.However,becauseofhealthanden-vironmentalregulations,aqueoustapecastinggainsgrowinginterest.Burnoutofthebinderoutofthegreencompactinsubsequentthermalprocessingcanbeaverytediousandtime-consumingop-eration.Casting-beltreleasemaybeanimportantpracticalproblemaswell.Tapecastingisusedmostlytopreparecomponentsforelectronicindustrywithasheet-likemor-phology.Examplesareinsulatorsubstratesforadvancedelectroniccircuits(thickfilmtechnology)andelementsofelectronic(multi-layer)componentsandhybridcircuits.A“simple”multi-layercomponent,manufacturedwithsheetsmadebytapecastingistheceramicmulti-layercapacitorthatconsistsofalternatelayersofdielectric(BaTiO3)andelectrodematerial(Pd).Tapecastingisconsideredmoreandmoreasasuitablemethodtopreparecriticalceramicpartsofmoltencarbon-ateandsolidoxidefuelcells(MCFCandSOFC)forhighlyefficientenergyconversion.Likeforslipcastingtechnology,ascientificdisciplineofmajorimportanceintapecastingtechnol-ogyiscolloidandinterfacechemistry.Awell-dispersedandstablesuspensionisofutmostimpor-tance.Sinceinnon-aqueoussolutionstheelectrostaticinteractionsaregenerallynegligible,thesuspensionsfortapecastingareusuallystabilisedbysterichindrance.Bystartingwithawell-dispersedsuspensionahomogeneousparticlestackinginthegreentapecanbeobtained.Thisshowstheimportanceofknowledgeofparticlestackingincompactionprocessingaswell.Figure6-16:Tapecastingequipment6.2.5Othersuspension/slurrytechniquesGelcastingDirectcoagulationcasting(DCC)Freezecasting.Seee.g.F.DoganandS.W.Sofie,“Microstructuralcontrolofcomplex-shapedceramicsprocessedbyfreezecasting[383] 66AdvancedCeramicsProcessing6.2.6CoatingsfromsuspensionsDipcoatingSpincoating6.3PasteprocessingPasteprocessingisbasedonthecontrolofandthechangesintheviscosityduringtheformationprocess.Viscosity(rheologyistreatedinchap-ter2).Figure6-17summarizesdifferentrheologi-calbehaviours.Thecompactsareshapedfromamixturethatisdeformableunderanappliedpres-sure,containingaceramicpowderandvariousadditives.Theplasticityisusuallycausedbytheadditionoforganicmaterials.Themostimportantpasteprocessingtechniquesareextrusionandinjectionmoulding.ScreenprintingisaspecialtechniquefortheprocessingofpastesFigure6-17:Shearrate-stresscurvesfordifferenttypesofrheologicalbehaviourofsuspensionandpastes6.3.1ExtrusionExtrusionisashapingtechniqueinwhichaplas-ticpasteobtainsacertainshapebyforcingitthroughanozzle.Extrusioncanbeusedtopro-ducecylindricalshapessuchastubes,rodsand(multi-bore)capillariesor,moregeneral,prod-uctswithauniformcross-section.Anexampleofaverycomplexshapethatcanbemadewithex-trusionarehoneycombstructuresforautomotiveexhaustcatalystcarriers.Translucentaluminatubesforhighvapourpressuredischargelampsarenowadaysmadealmostexclusivelyusingex-trusionasshapingmethod.Typically,extrusionisamethodformassproduction,likedrypressing.ExamplesofextrusionformsareshowninFigureFigure6-18:Extrusionforms.6-18.Theextrusionpasteconsistsofamixtureoffineparticles,liquidphaseandorganicbinder.Theviscosityofextrusionpastesisveryhigh,resultingingreenceramicextrusionshapeswithasolid-likeappearance.Therheologicalpropertiesofextru-sionpastesarenon-Newtonianandgenerallyshow“Bingham-type”behaviourwhichmeansthatthepasterequiresacertainyieldstresstoinitiateflow.Inextrusion,thecompactcanobtainitsshapebecauseofchangesinviscosityduetoshearratesduringtheformationprocess.Themostfavourableflowbehaviourisshear-thinning,whichmeansthattheviscositydecreaseswithincreas-ingshearrate.Pasterheology,die-wallfrictionormoregenerally“extrusionmechanics”areimportantareasofinvestigation.Theextrusionpropertiesofpastesdependverycriticallyoncompositionandevenonmixingandageingtime.Avariationof1%intheliquidphasecontentmayresultinachangeintheapparentviscosityofseveralordersofmagnitude.Ageingandgellingofbindersmayresultintime-dependentbehaviour.Muchattentionispaidtothedryingstepofextrudedshapes.Dryingcanbeverytime-consumingifnospecialprecautionsaretakeninthefullsequenceofthermalprocessingofthegreenshapeintoafinalproduct. 6Processesforcompaction67Figure6-19:Extrusionunit6.3.2InjectionmouldingInjectionmoulding,wellknownfromorganicpolymertechnology,isarelativelynewtechniqueinceramicsprocessing.Awell-dispersedmixtureoffineparticlesandanorganicbinderphaseisin-jectedatelevatedpressureandtemperatureintoamould.Verycomplexshapescanbemadeinthisway.However,investmentandtotalprocessingcostsareveryhighand,therefore,theactualappli-cationofinjectionmouldingtoproduceceramicpartsonacommercialscaleisnotverywide-spread.Typicalexamplesofinjectionmouldedproductsarecomplex,relativelysmallceramicpartsmadefromnew,high-strengthtechnicalceramicssuchasautomotiveturbo-chargersandcombustionchambers.Theviscosityatinjectionconditions,soatelevatedtemperatures,mustbeverylowandliquid-likeinordertoensurearapidandcompletefillingofthemould.Alsointhiscase,thecom-pactisformedduetodifferencesinviscosityatdifferentshearratesbutalsoatdifferenttempera-turesduringtheprocess.Consequently,therheologicalpropertiesofthegreeninjectionmouldingmixtureareimportantandgenerallyspoken,thecontentoforganicbinder(orwax)isratherhigh,about50%ormore.Oneofthemostdifficultpartsoftheinjectionmouldingtechnologyofceramicpartsisthebinderremoval(ordewaxing)ofthegreenshapebythermaltreatmentorburn-out.Therefore,thisbinderremovalofinjectionmouldedcompactsisanimportantareaofattentioninthermalprocessingofceramics.Forpastesusedforbothextrusionandinjectionmouldingahomogeneousdistributionofthepow-derparticlesinthebinderphaseisofutmostimportance.Contrarilytothecaseofpowdersfordrypressing,particleagglomerationmustbeavoidedasmuchaspossible.Again,muchattentionmustbepaidtohomogeneityinthegreenstate.Inadditiontothemethodsofextrusionandinjectionmouldingtherearevariousexamplesofshap-ingtechniquesinwhichamoreorlessplasticpasteisformedintoashape.Verywell-knownexam-plesaretheclassicalmanufacturingofpotterywareand“wet-pressing”ofbrickpre-forms.Theunderlyingtechnologicalprinciplesinthesecasesarenotverydifferentfromthoseofextrusionandinjectionmouldingwhilethespecificationsoftheproductsproducedareoftenlesscritical. 697Thermalprocessingofgreencompacts7.1DryingInsoliddryingprocessesaliquidisseparatedfromasolidbymeansofevaporation.Inthemajorityofcasestheliquidtoberemovediswater.Thefinalproductofadryingprocessisasolidmaterial(e.g.aceramicpowder,ashapedpieceofceramics),whichstillmaycontainsomeresidualmois-ture.Asharpdistinctionshouldbemadebetweenmechanicaldewateringanddrying.Inamechanicaldewateringprocess(filtration,centrifugation)thesolidandliquidareseparatedbymechanicalforces.Onecanimaginethate.g.waterbeingkeptinsidetheporesofthematerialandanytypeof‘bound’watercannotberemovedcompletelyinthisway.Thereforeinmostcasesadryingprocesswillfollowamechanicaldewateringprocess.Atypicalenergyconsumptioninconvectivedryingprocessesisabout1.5-2.5timestherequiredevaporationenergyoftheliquid.Theenergyneededmaybesuppliedbyhotdryinggas(convectiondrying),directcontactwithheatedwalls(conduc-tiondrying),infraredradiation(radiativedrying)orhighfrequency(microwave,di-electricheat-ing).Energyuseinmechanicaldewateringprocessesisconsiderablylower,sothereis-fromthispointofview-agoodeconomicalreasontoremovetheliquidasmuchaspossibleinthemechani-caldewateringstep.Agreatdiversityindryingequipmentexistsdueto:·Largedifferencesinrequireddryingtimes,e.g.tensofsecondsinspraydryingofceramicsus-pensions,orseveraldaysinchamberdryingofclayformsinbrickindustry,·astronginfluenceofdryingconditionsonthefinalproductquality,and·theconditionofthefeed,e.g.claysuspensioninspraydrying,shapedpieceofclayinchamberortunneldryers,ceramicfilminbeltdryer.Importantqualityaspectsinceramicsdryingarerelatedtocrackformation,deformationandshrinkageoftheshapedforms.Evennowadayssafedryingpathsforshapedpiecesarestillestab-lishedmainlybyempiricalmeans.Thereexistsalargeneedfordevelopingmorescientificap-proachesfordesignandoptimisationofdryingprocessesandforabetterunderstandingofthecomplicatedrelationbetweenmaterialproperties,dryingconditionsandtheobtainedqualityofthedryproduct.7.2Binderburn-outInmanycasesagreenceramiccompactisformedwithaconsiderableamountoforganicbinder.Thisbinderhastoberemovedpriortosinteringandseveraltechniquesareusedorhavebeenproposedtoaccomplishthis:·Normaloxidativeburn-outinwhichthebinderisconvertedintosimplegasessuchasCO2andH2O.Duringthisprocessoxygenandreactionproductshavetodiffuseintoandoutofthecompact,respectively.Oxidativeburn-outmaybeaccompaniedbyotherprocessessuchasde-compositionofthebinderintosimpleorganicfragmentsormeltingofthepolymerandcapil-larytransportthroughthecompact.Oxidativeburn-outisgenerallyexothermic;thereactionratemustbekeptwithinpredefinedlimits,dependingongeometrytoavoidbloatingandcrack-ingduetoexcessiveinternalpressurebuild-up,·removalofthermoplasticbindersbymeltingandcapillaryextraction,andembeddingthecom-pactinafinepowder.Thismethodisusedespeciallyforcompacts,containingseveralwt%ofthermoplasticbinderasobtainedininjectionmouldingprocesses.Binderremovalisamajorprobleminthemanufactureofceramiccomponentsbyinjectionmoulding(seechapter7), 70AdvancedCeramicsProcessing·lowtemperaturebinderdegradationbyUV-irradiation;thismethodworksonlytotreatthenearsurfaceareaofcompactsbutcanbeusefultocreateopenporosityatthesurfacetoavoidinter-nalpressurebuild-upinsubsequentprocessing.Systematicstudiesonthematerialsaspectsofbinderburn-outarescarceandageneraltreatmentcanhardlybegivenduetothewidevarietyofbindersystemsandtheextremelycomplexnatureofbinderburn-outreactions.Thedescriptionofburn-outreactionsingeneralcanbegivenintermsoftwoextremepictures:·Homogeneousbinderburn-outinwhichthecompacthasanopenporosityandtherearenomass-transferlimitationsofgaseouscomponentswithintheporouscompact,and·shrinkingcoreburn-outinwhichthereisacleardistinctionbetweenabinder-freeouterzoneinthecompactandabinder“core”.Theinterfacebetweenthecoreandthebinder-freezonecanbequitedistinctonamacroscopicscale.Ifburn-outoccursintwoormoreseparatestages,multiplecoreburn-outmayoccur.Attemptstodescribebinderburn-outofceramiccompactsgenerallyrefertotheshrinkingcoremechanism.Apioneeringtreatmentofburn-outofa(smallamountof)carbonaceousresidueinacylindricalporouscompactof30mmdiameter,50%porosityandanaverageporediameterof150mm(!)isdescribedbyStrijbos.Itisfoundthatunderisothermalconditionstheburn-outproc-essisdiffusion-limitedattemperaturesinexcessof660°Candhenceoftheshrinkingcoretype.Below480°Ctheprocessisreactionrate-limitedandconsequentlyofthehomogeneoustype.Atintermediatetemperaturesamixedsituationoccurs.Aconditionisthatsufficientoxygenissup-pliedattheoutsideofthecompactwhichiseasilyfulfilledundernormalcircumstances.2CalvertandCimagaveamorerecenttreatmentonbinderburn-out,focusedonthermaldepoly-merisationofPMMAintomonomers.Thereactionislimitedbythetemperature(ceilingtempera-ture)becauseoftheconditionthattheequilibriumpressureofmonomershouldnotexceed1atm.atanyplaceinthecompact.Thiswouldresultinboiling,accompaniedbydestructivebubbleformation.Itisfoundthattheeffectivedebindingratesarenotaffectedsignificantlybytheformationofinternalporesorthetypeofburn-outmechanismassumedasthedecompositionreactionisinthiscaserate-limiting.Theonlyimportantdifferenceisthatforthehomogeneousmodel(parallel)theceilingtem-peraturecanbeconsiderablyhighersothatthepredictedmaximumslabthicknessthatcanbeburnt-outwithinreasonabletimeincreasessignificantly(<10mm).Externalmasstransferlimitationsaregenerallynotexpectedtobeimportantandporediffusionisinmostcasessaidtobenonrate-limiting.Thissituationmaybedifferentforoxidativeburn-outandhomogeneouscompactswithporesizesinthenmrange.Detailedinsightintonatureofbinderburn-outmechanismsisimportantforsubsequentprocessing:·Incompleteburn-outoforganicbindermayresultin“trapped”carbonaceousresiduesthatareverydifficulttoremovefromadensifyingordensifiedcompact,duringoraftersintering,·capillarytransportofliquidbindermayresultinmigrationofsmallparticleswithpossiblyse-vereconsequencesforsubsequentprocessingandproperties,·theoccurrenceof“self-propagation”duringoxidativeburn-outmayresultinseverewarpingandcracking;boilingmaybethereasonforinternalblisterformation.Themeasuresthatcanbetakentomakethatthermalburn-outoccurswithoutproblemsoftenfocusontheselectionofapropertemperatureprogramincombinationwithaspecificchoicefortheburn-outatmosphere.Examplesare(seealsofigure7.9):·Theuseofhighambientoxygenactivities(plasmas,elevatedoxygenpressures,ozone)incombinationwithlowburn-outtemperaturestoobtainanopenporosityrightatthesurfaceandsubsequentshrinkingcoreburn-outinsuchawaythatnobindermeltingorcarbonificationoc-cursandself-propagatingreactionsareavoided,·theuseoflowambientoxygenactivities(bylettingthecompactburn-outforinstanceinitsownatmosphere)incombinationwithhightemperaturestoobtainahomogeneousburn-out1S.Strijbos,Chem.Eng.Sci.28[1](1973)2052P.CalvertandM.Cima,J.Am.Ceram.Soc.73[2](1990)575 7Thermalprocessingofgreencompacts71situationwithoutexcessivereactionheats.Inanumberofcasesoxidativeburn-outcannotbeappliedduetounwantedoxidationreactionsoftheceramicparticles.·activecontroloftheburn-outprocess.Thiscanbeachieved,forinstance,inoxidativeburn-outbylettingthetemperatureincreaselinearlyinsuchawaythatthemaximumtemperatureisat-tainedwithinanacceptabletime.Inthesametimetheoxygenconsumptionismeasuredandcontrolledtoaconstantlevelbyadjustingtheoxygeninletsothatthereactioncanbeexpectedtoproceedgraduallyandwithoutexcessiveheatformation.Thisapproachreliesontheoxida-tiveaspectsofburn-out,neglectingotherpossiblephenomenasuchassimpledecomposition.3Adifferentapproachisbasedonthecontrolofproductweight.Inthiscasetheoxygenpressurecouldbekeptfixedwhilethesampleweightlossduringdecompositionwascontrolledtoafixedratebyadjustingthetemperature.Thisapproachhastheadvantagethattheprogressofburn-outiscontrolleddirectlyattheproductbutoxidativeaspectsarenotdirectlyincluded.Specificweight-timecurves,obtainedfrommodelconsiderationsonbinderburn-outforspecificshapes,maybeusedintheactualprocessprogramminginsteadofaconstantrateofweightloss(seefig-ure7.9).Activecontrolmethodshaveonlyrecentlyreceivedindustrialinterestandmanyprob-lems,connectedwithpropersensoringandscale-upwillhavetobesolvedbeforelargerscaleap-plicationscomeintoscope.Fastremovalofthewaxesandoilsmaybeimprovedbyplacingthecompactsinawickingpowderbedoronawickingsubstrate.Thewickingpowdermustbefiner-sizedthanthecompactpowderinordertodrawoutthewaxbycapillaryforces.Advantagesofthismethodarethefasterdebinding,alsoofthicksections,andtheshapestability(thepowderbedsupportsoverhangingcompactparts).Disadvantagesarelackofelegance(workingwithdustyfinepowders)andcontaminationofthecompactswithdifficulttoremovewickingpowder.Figure7.9:Overalltemperature-time,weight-time,andweight-temperaturecurvesforconventionalbinderburn-outandsinteringofceramicmultilayercapacitorsina)airandb)O2.Anotherkeytomorecontrolleddebindingistheuseofamultiplecomponentbindersystemthatdebindsprogressivelyoveranumberofsteps,sothatremovingafirstbinderconstituentleavesbehindasufficientquantityofasecondconstituenttoholdtheparticlesinplace.Typicallylowmeltingwaxesandoilsareusedasthefirstbinderconstituent.Polymersholdtheparticlesinplaceduringtheearlyportionofdebinding.Thecomponentthatisremovedfirstshouldconstitute30%ormoreofthebinder.Solventextractionofbindercomponentsisanothermethodtoacceleratethedebindingprocess.Solventdebindingrequiresthebinderbecomposedofatleasttwocomponents,onethatisex-tractedbythesolventandtheotherwhichholdstheparticlesinplaceduringandafterextraction.Thebindercomponentthatisextractedbythesolventmustbeconnectedtothecompactsurface.Sufficientinterconnectivityofthesolublephasetypicallyrequiresthatitconstitutesatleast30%of3H.VerweijandW.H.M.Bruggink,J.Am.Ceram.Soc.73[2](1990)226 72AdvancedCeramicsProcessingthebinder.Swellingmayoccurduringimmersionduetosolventuptakebythesecondbindercom-ponent.Anoptimaltemperatureexistsforsolventdebind-ingasillustratedinfigure7.10.Thisfigureplotstheweightofbinderextractedinfourhoursfromceramicinjectionmoulded(CIM)cementedcar-bidecompactsinheptaneatvarioustemperatures.Thewax,binderandheptanearesolubleinoneanother.Asaconsequence,highertemperaturesincreasethedebindingrate,becauseofahigherdiffusivity.However,thereisatemperaturerangeoverwhichthecompactretainsitsintegrity.Crackingoccursatlowtemperaturesbecausethesolventdiffusesintothewax,butthewaxdif-fusestooslowlyintothesolvent,causingswell-ingandinternalstressesinthecompact.Alterna-tively,ifthetemperatureistoohighthecompactslumpsduetobindersoftening.Solventextractionisinterestingwhenthicksec-Figure7.10:TemperatureeffectonsolventdebindingofCIMcementedcarbidecompactsintionedcompactshavetobedebinded.Whenor-heptane.Atlowtemperaturethecom-ganicsolventsareused,precautionswithrespectpactswillcrack,whileathightempera-totheenvironmenthavetobetaken.Develop-ture,thecompactslumpsinspiteofamentswithwater-solublebindersareinterestingrapiddebindingrate.inthisrespect. 738SinteringTheprincipalstepsintheceramicprocessingroutearedensification,accomplishedbythesinteringprocess.Sinteringistheprocessofbringingthepowderatelevatedtemperaturesothat,duetothehigh(er)mobility,thecompactdensifies,therebyreleasingthesurfaceenergyofthepowderparti-cles.Generallynoliquidphase(solidstatesintering)oronlyalimitedamountofliquidphase(liq-uidphasesintering)ispresentduringsinteringofadvanced(ortechnical)ceramics.Intraditionalceramicsupto30vol%liquidphasecanbepresentinwhichcasethedensificationissometimescalledvitrification.Itisalsopossibletoapplypressureduringsin-tering(pressuresinteringorhot-pressing)ortocarryoutthesinteringprocesssimultaneouslywiththereactionofprecursorstotheproperchemicalcomposition(reactivesintering).Inspiteofthefactthatthetheoryofsinteringhasbeenstudiedforalongtime,realprogresshasbeenmadeslowly.Practically,however,anum-berofaspectsofthetheoryofsinteringhavehadanenormousinfluenceandhaveledtoasignifi-cantimprovementinhomogeneityofmicro-structuresandthusinthepropertiesofceramicmaterials.Solidstatesinteringiscommonlydividedintothree,notclearlydistinguishable,stages(seealsoFigure8-1):·Initialstage;thepowderparticlesgrowto-getherandnecksareformedattheinter-faces.Someparticlerearrangementmayoc-curaswell.Porositiesareintherangeof40-60%.Figure8-1:Stagesinsintering;a)greencompact;b)neck·intermediatestage;theporesandparticlesformation;c)neckareaformationandcontinuous,openformanintersectingnetwork.Thedensityperchannels;d)finalstagewithclosedporosity.increasesto90or95%whereporechannelsbreakupandformdiscretepores.Consequently,theporositytypechangesfromopentoclosed,andgraingrowthoccurs(Figure8-2andFigure8-3),·finalstage;discreteporesarepresent,whichcanonlyberemovedbyfurthergraingrowth. 74AdvancedCeramicsProcessingFigure8-3:Variationofopenandclosedporosityversustotalporosityforsinteringuraniaat1400ºC.Figure8-2:DensityandgrainsizeofTiO2versussinteringtemperature.8.1DrivingforceinsolidstatesinteringThedrivingforceforsinteringisthereductionofthesurfaceorinterfacefreeenergy.Inotherwords:Theresultofsinteringisadecreaseinsurfaceareabyeliminationofsolid-vapourorsolid-solidinterfaces.Ifnosecondphasesarepresentequation(8.1)holds.DG=DG+DG=ggDAA+D(8.1)sbssbbWhereDGisthechangeinGibbsenergyofthecompact,gsandgbtheinterface(surface)energy,andDAsandDAbthechangeinsurfaceareaofthesolid-vapour(s)andthesolid-solidinterface(b),respectively.SinteringoccursifDG<0.EarlyintheprocessthedecreaseinDGsisthemostimpor-tantfactor.LaterintheprocessthedecreaseinDGsislessandaccordinglytheincreaseinDGbmustslowdowntokeepDGnegative.Thiscanberealisedbygraingrowth.Mattercanbetransportedbyanumberofmechanisms.Itisnowclearthatdiffusionisbyfarthemostimportantmechanisminsintering.Wewillconsideronlythevacancymechanisminsimpletermsandomitallcomplicationsduetotheionicnatureofmostceramicmaterials.Inabsenceofanexternalstressandachemicalreaction,thesurfacecurvatureprovidesthemaindrivingforceforsintering.Heretheinterface(surface)energyisreducedwhenconcavere-gionsarefilledinandthusconcaveregionsareasourceforvacancies.Similarly,theconvexre-gionsaresinksforvacancies.Possiblesourcesandsinksofvacanciesarethesurfaces(thepore-solidinterface)andtheinterfaces(thegrainboundariesanddislocations).GrainboundariesFigure8-4:Movementofsurfaceatomsduringfirststageareperfectsinksforvacanciesandforcrystallineofsinteringmaterialthepresenceofagrainboundarybe-tweenthegrainshasbeenshowntobeessentialforshrinkage.Theeffectivenessofthegrainboundaryasasinkis,however,alsodependentonthemobilityofthegrainboundarydislocations.Soluteatomsexertingadragcanreducethisdislocationmobility.Alsograinboundaryparticlescaninhibitgrainboundarymotionbydislocationpinning. 8Sintering758.2The(solidstate)sinteringprocessIntheinitialstagetheprimaryprocessisthefor-mationofnecksbetweentheparticles(seeFigure8-5).Ofthevariousmechanismspossible,onlythosethatremovematerialfromthespacebe-tweenthegraincentres(thegrainboundaryarea)totheneckscontributetodensification.Wecon-siderthesimplecaseoftwosphericalparticlesofthesamesize,sinteringtoeachotherbya(va-Figure8-5:Modelforinitialsintering;cancy)latticediffusionmechanism.NeckgrowthLeft:nocentreapproach;Right:centreapproachandshrinkageoccursimultaneously.DuringearlystagesinteringtheGibbsenergychangedGperunitvolume,isrepresentedby:dGgdAssdG@gsdAor==(8.2)ssdVdVWheregsisthesolid-vapourinterfaceanddAs/dVisthechangeinsurfaceperunitvolumeduetonecking.Thechangeinfreeenergycanbeinterpretedasaneffectivecompressivestresss.Ifweassumethatallnecksgrowatthesamerate,thendAs/dV=ndA’wherenisthenumberofnecksperunitvolumeanddA’theareachangeperneck.Thetotaldrivingforceisthusproportionaltothenumberofnecksperunitvolumeandsisexpressedby:sg=ndA'(8.3)sTwoparticlereactivityThemostsimplewaytodescribethesinteringprocessisbymeansofthetwoparticlereactivity.Inthiscaseonlyparticlesareconsidered,whoarenotinfluencedbyanyotherparticle.Thissimplemodelgivesaquantitativedescriptionoftheinitialstagebutcanalsobeusedforaqualitativeinter-pretationofthesinteractivityintheintermediateandfinalstage.InFigure8-6anoverviewoftheseveralpossibletransportpathsaregiven.Themechanismsbelong-ingtoeachpathsandwhetheritcontributestodensificationisindicatedintable8.1.Itisobviousthatredistributionofmaterialoverthesurfacebysurfaceand/orvolumediffusionwillnotresultinshrinkage,butonlyresultsinanin-creasedstrengthbyenlargingparticlecontactareasandreducingthenotcheffectofsharpporecon-tours.Figure8-6:Possibletransportpathsduringsintering 76AdvancedCeramicsProcessingMechanismTransportpathMatterfluxContributiontoSourceSinkdensification?1vapourtransportgasphasegrainsurfaceneckno2viscousflowgrainboundarygrainboundaryneckyes3surfacediffusiongrainsurfacegrainsurfaceneckno4grainboundarydiffusiongrainboundarygrainboundaryneckyes5volumediffusiongrainbulkgrainboundaryneckyes6volumediffusiongrainbulkgrainsurfaceneckno7bulkdiffusiongrainbulkbulk(dislocations)neckyesTable8-1:Mechanismsandpathsformattertransportduringsintering;thenumbersrefertoFigure8-6FinalstageAsstatedbeforethefinalstageofsinteringischaracterizedbythepresenceofclosedpores.Furtherdensificationisonlypossiblebydiffusionofvacanciesfromtheporesalongtheboundaries1(seeFigure8-7)orthroughthebulk.However,latticediffusionisslowcomparedtograinboundarydiffusionandinpracticeporescanonlyFigure8-7:Densificationmechanismduringthefinalstagea)forporesattachedtoagrainboundaryandb)fordisappearaslongastheystayonthegrainporeswhicharedetachedfromagrainboundaryboundaries.ThereforetoreachtheoreticaldensityweneedtopreservethemicrostructureassketchedinFigure8-7aandFigure8-8a.Ifaporeislooseningfromamovinggrainboundary(e.g.byabnormalgraingrowth)diffusionpathsincreaseinlengthandgrain-boundarydiffusionfordensificationisnotpossibleanymore(seeFigure8-7b).Manysmallporesinsteadofafewlargeporesatthegrainboundaryreducetransportdistancesandthedensificationprocessrunsfaster.Graingrowthisanintegralpartofthefinalstage.Inordertodescribethisphenomenonmoreparticleshavetoberegarded.Thiswillbediscussedlaterinthischapterafterwehavefirsttreatedthe“coordinationmodel”.Figure8-8:Twopossibleporegrainboundarymicrostructuresinthefinalstageofsintering;a)poresongrainboundariesandb)poresisolatedinthegrains.1R.J.Brook“Controlledgriangrowth”in:“Ceramicfabricationprocesses”ed.byF.F.Y.Wang,“Treat.Mater.Sci.Tech.Vol.9’Acad.Press,NewYork(1976)331 8Sintering778.2.1ThecoordinationmodelOneapproachforaquantitativeexpressionofshrinkageisthecoordinationmodel.InthismodelacentralroleisplayedbytheconceptofporecoordinationnumberasexploredbyLange2,3,4andKellet.Acoordinationnumberisattrib-utedtoeachpore.Thisisthenumberoftouchingparticlesthatformthesurfaceoftheentityinthevoidphaseofapowdercompactcalledapore(seeFigure8-9).Theoriginoftheideaiseluci-Figure8-9:Schematicviewofporecoordination.Surfacecurvaturefor2poreswithapproximatelythesamevol-datedandsomeconsequencesarediscussed.umeanddihedralangle,butwithdifferentnumberofThedihedralangleq(orcontactangle)isdeter-coordinatinggrains.Ina)thecoordinationnumberislar-gerandinb)smallerthanthecriticalcoordinationnum-minedby:ber.qgbcos=22gsWheregbandgsarethegrainboundaryandsurfaceenergy,respectively.Assumeforsimplicitythatwehaveatriangularporeformedbysinteringofthreecylinderstogether(seeFigure8-10).Forporeswithstraightsidesthedihedralanglewillbeq=60°.Thisresultsin:q3cos==or3ggbs22Figure8-10:Poreatathree-grainjunction,a)whengb/gs=3theporeisinequilibriumandwillneithergrownorshrink,b)whengb/gs<3theboundaryisconvexandtheporewillgrow,andc)whengb/gs>3theboundaryiscon-caveandtheporewillshrink.SupposethattheporeshrinksfromABCtoA’B’C’.Eachsideoftheporeisshortenedby2XandeachgrainboundaryislengthenedbyC-C’,whichisequalto2XÖ3.TheGibbsenergychangeisequalto0.DG=-6XXgg+=6/30(8.4)sbThereisthusnonetdrivingforceforporeshrinkageorsintering(Figure8-10a).Ifgb<Ö3gs,theporeisconvexandwillgrow(Figure8-10b)whileifgb>Ö3gs,theporeisconcaveandwillshrink(seeFigure8-10c).Thegeneralconclusionisthatporeshavingconcavesides(viewedfrominsidethegrain)willshrink,whileporeswithconvexsideswillgrow.Forthetriangularporejustdis-cussedtheporeisconvexwhengb/gs>Ö3,andconcavewhengb/gs<Ö3.Forothergeometriesothercriticalvalueshold.Toputitdifferently,forthisexamplewithgb/gs=Ö3(Figure8-10a),thecritical2F.F.Lange,J.Am.Ceram.Soc.,67(1984)833B.C.KelletandF.F.Lange,J.Am.Ceram.Soc.,72(1984)7254B.C.KelletandF.F.Lange,J.Am.Ceram.Soc.,72(1984)735 78AdvancedCeramicsProcessingporecoordinationnumberNc=3.Iftheporecoordinationnumber,N>Nc=3,porescannotdiffuseawaysincethelocalsituationisthermodynamicallystable.IfN
