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Densification and polymorphic transition of multiphase
Y2O3 nanoparticles during spark plasma sintering
Rachel Marder, Rachman Chaim, Geoffroy Chevallier, Claude Estournès
To cite this version:
Rachel Marder, Rachman Chaim, Geoffroy Chevallier, Claude Estournès. Densification and
polymor-phic transition of multiphase Y2O3 nanoparticles during spark plasma sintering. Materials Science and
Engineering: A, Elsevier, 2011, vol.528, pp. 7200-7206. �10.1016/j.msea.2011.06.044�. �hal-00720725�
To link to this article: DOI: 10.1016/j.msea.2011.06.044
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ttp://dx.doi.org/10.1016/j.msea.2011.06.044
This is an author-deposited version published in:
http://oatao.univ-toulouse.fr/
Eprints ID: 5668
To cite this version:
Marder, Rachel and Chaim , Rachman and Chevallier, Geoffroy and
Estournès, Claude Densification and polymorphic transition of multiphase
Y2O3 nanoparticles during spark plasma sintering. (2011) Materials
Science and Engineering A, vol.528 (n° 24). pp. 7200-7206. ISSN
0921-5093
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Densification
and
polymorphic
transition
of
multiphase
Y
2
O
3
nanoparticles
during
spark
plasma
sintering
R.
Marder
a,
R.
Chaim
a,∗,
G.
Chevallier
b,
C.
Estournes
baDepartmentofMaterialsEngineering,Technion–IsraelInstituteofTechnology,Haifa32000Israel bCNRS,InstitutCarnotCirimat,F-31602ToulouseCedex9,France
Keywords:
Sparkplasmasintering Phasetransformation Densification Y2O3
a
b
s
t
r
a
c
t
Multiphase(MP)monoclinicandcubicY2O3nanoparticles,40nmindiameter,weredensifiedbyspark
plasmasinteringfor5–15minand100MPaat1000◦C,1100◦C,and1500◦C.Densificationstartedwith
pressureincreaseatroomtemperature.Densificationstagnatedduringheatingcomparedtothehigh shrinkagerateincubicsingle-phasereferencenanopowder.ThelimiteddensificationoftheMP nanopow-deroriginatedfromthevermicularstructure(skeleton)formedduringtheheating.Interfacecontrolled monoclinictocubicpolymorphictransformationabove980◦Cledtotheformationoflargespherical
cubicgrainswithinthevermicularmatrix.Thisresultedinthelossofthenanocrystallinecharacterand lowfinaldensity.
1. Introduction
Rapidsinteringanddensificationofceramicpowderstofull den-sityare nowadaysa routineprocedure, usingthespark plasma sintering (SPS) method.As wasnoted in many hot-press stud-iesincludingSPS,themaindensificationshrinkageofthepowder aggregatetakesplaceduringtheheatingbyparticlesliding,tothe close-packedarrangement[1–4].Furtherdensificationofthe pow-dercompactmaybeaccomplishedeitherbyplasticdeformation orbydiffusionalprocesses,attheparticlenecks.Nevertheless, dif-fusionalprocessesareinevitableduringthefinalstagesintering, whenisolatedporesformandcanbeeliminatedviabulkorgrain boundarydiffusion[5,6].Consequently,microstructureevolution duringtheSPSprocess,duetothechangeintheprocess param-eters,such astemperature,pressure,time,atmosphere, vacuum level, etc. mayaffectthe densificationmechanism. Many types ofphasetransformationsandtransitions associatedwithcrystal symmetrychangesinvolvechangesinthemicrostructureand mor-phology[7].Therefore,effectsofthephasetransformationsduring thedensificationbySPSmaybeofprimeimportancetothe densifi-cationprocess,aswellastothefinalphaseassemblageinthedense compact.Thephasecontentandassemblageoftenhavea consid-erableimpactonthefinalpropertiesofthesinteredceramic[8]. Inthisrespect,Takeuchietal.[9]investigatedthedensificationof
∗ Correspondingauthor.Tel.:+97248294589;fax:+97248295677. E-mailaddress:rchaim@technion.ac.il(R.Chaim).
submicrometersizetetragonalBaTiO3 powdersandfoundSPSto beeffectiveforpreservationofthesubmicrometersizemetastable cubicBaTiO3atroomtemperature.Thepreservednanometricgrain sizebySPSwasalsofoundtobeacauseforthecubicphase stabi-lizationatlowertemperatures[10,11].
Kumaretal.[12]tookadvantageofthehigheratomic mobil-ityneartheAnatasetoRutilephasetransformationtemperatureto enhancethedensificationoftheTiO2 nanoparticles.SPSof mul-tiphaseTiO2 (70%Anatase and30% Rutile)with20-nmparticle sizeat 62MPafor 5minand600◦C resultedincomplete phase transformationtoRutile[13].Forcomparison,onlyannealingof the same precursor powder for 5min at 600◦C preserved the multiphasecharacter ofthe powder.This exhibitstheeffect of theappliedpressureandpossiblytheelectricfieldonthephase transformationduringtheSPS.Fundamentalinvestigationofthe currenteffectonsolid-statereactivityduringSPSwasperformed onstackedMo–Si–Molayers[14].Nochangeinthereaction mecha-nismwasobserved,albeittheenhancedgrowthrateofthereaction productlayer(MoSi2),whichwasrelatedtotheenhancedmobility orthechangeinthedefectconcentration.
The present paperfocuses on theeffect ofthe polymorphic phasetransitiononthemicrostructureanddensificationbehavior ofmultiphaseY2O3nanoparticlesduringtheSPS.
2. Experimental
Commercial pure (99%) multiphase (MP) Y2O3 nanopowder (NeomatCo.,Riga,Latvia)withaverageparticlediameterof40nm
Fig.1.X-raydiffractionspectrafromtheY2O3nanoparticles.(a)Singlephase(SP)
cubic.(b)Multiphase(MP)cubic+monoclinic.
wasused.Asecondhighlypure(99.99%)nc-Y2O3powder(Cathay AdvancedMaterials,China)with100%cubicphase,designatedas singlephase(SP),wasalsousedasareferencespecimen[15]. Con-stantamountofthepowdersamplewaspouredintothegraphite dieusinggraphitefoils(Grafoil)toseparatebetweenthepowder, thediewalls and theplungersurfaces.Thepowders were sin-tered(Dr.Sinter,SPS2080)atdifferentconditionsfor5–15minand 100MPaat1000◦C,1100◦C,and1500◦C.Thestartingtemperature wasroomtemperatureforthe1000◦Ctreatment(designated‘cold compaction’),but600◦Cforthe1100◦C and1500◦Ctreatments (designated‘hotcompaction’).Theuniaxialpressurewasapplied eitherafewsecondaftertheprocessstartedorwhentheSPS tem-peraturewasreached.Inbothcases,thepressurewasincreased linearlywithtime,andheldconstantduringtheisothermal treat-mentattheSPStemperature.Theprocessdurationreferstothe isothermalSPStreatment.Aheatingrateof100◦C/minand vac-uumlevelof3Pahasbeenused;thepulsedurationwas3.3ms.The SPSparameterswererecordedduringtheprocess.The tempera-turewascontrolledbyathermocouplefor‘coldcompaction’,while anopticalpyrometerwasusedabove600◦Cfor‘hotcompaction’ andathighertemperatures.Thefinalspecimendimensionswere 8mmindiameterand1.7–2.5mmthick.Theramdisplacements wereexpressedintermsofthelinearshrinkageandthetemporary relativedensity,followingthespecimenthicknessversustime, tak-ingintoaccountthethermalexpansion/shrinkagebehaviorofthe specimenandthegraphiteplungers[15].
Thephasecontentoftheas-receivednano-powdersandthe sin-teredspecimenswerecharacterizedbyX-raydiffractionusinga diffractometer(PhilipsPW3710)withmonochromaticCuKa radi-ation (XRD),operatedat 40kVand 30mA.A scanningspeed of 0.5◦/minhasbeenused.Themicrostructureswerecharacterized usingtransmission(TEM,FEITecnaiG2T20,operatedat200kV) andscanning(FEIE-SEMQuanta200,operatedat20kV)electron microscopes.Thespecimensfortheelectronmicroscopy observa-tionswereprepared bytheconventionalmethods.Thethermal stabilityofthenanopowderswascharacterizedusingdifferential scanningcalorimetry(Labsys1600,Setaram)upto1400◦CinArgon atmosphere,ataheatingrateof5◦C/min.Thefinaldensityofthe specimenswasdeterminedbytheArchimedesmethodfollowing ASTMstandardC20-92(±0.5%accuracy).
3. Results
X-raydiffractionspectrafromtheas-receivednanopowdersare showninFig.1.Thesinglephase(SP)Y2O3nanopowderwithcubic
symmetry(JCPDS41-1105)wascharacterizedindetailelsewhere [16](Fig.1a),whereasthemultiphase(MP)nanopowderrevealed polymorphswithcubicandmonoclinic(JCPDS39-1063) symme-tries(Fig.1b).Quantitative analysis ofthe spectrum inFig. 1b, assumingapowdermixture[17],resultedin30%cubicand70% monoclinicphaseinthenanopowder.Theseresultswerein agree-mentwiththe25:75cubictomonoclinicphaseratiodetermined byothers[18].
TEMobservationofthetwopowders(notshownhere)exhibited sphericalmorphologyforthemultiphasepowder,comparedtothe equiaxedpolyhedralshapeforthecubic,singlephasepowder[16]. Bothpowdersexhibitedlog-normalgrainsizedistributions,with averagegrainsizeof18±8nmand41±22nmforthesinglephase andthemultiphasepowders,respectively.
XRDspectrafromtheMPnanopowdercompacts,subjectedto SPSfor5minat100MPaandatdifferenttemperatures(Fig.2a), showedthe transformation of themetastable monoclinic poly-morphtothestable cubic phase tooccurat1100◦C. However, further XRD characterization of the MP nanopowders sintered fordifferentdurationsat1000◦Cand100MPa(Fig.2b)revealed the continuous nature of the phase transformation kinetics alreadyatthistemperature.Themonoclinictocubicpolymorphic phasetransformationwasnotcompleted,evenafter15minSPS duration.
Following the above phase transformation in the sintered specimens,thethermalstabilityofbothnanopowderswere char-acterizedbyDSC(Fig.3).Atthestagewherethefiner,cubicSP nanopowderwasrelativelystable,thecorrespondingcurvefrom
Fig.2.XRDspectraoftheas-receivedMPY2O3nanoparticles,aftersintering(a)for
Fig.3.DifferentialscanningcalorimetryfromnanocrystallineY2O3.(a)Singlephase
(SP)cubic.(b)Multiphase(MP)cubic+monoclinic.
theMPnanopowderexhibitedastrongexothermicpeakstarting at∼980◦C,withthemaximumaround1200◦C.Thisisin agree-mentwiththeexpectedmonoclinictocubicphasetransformation reportedat∼950◦C[19],andinagreementwiththeXRDresults. Therefore,specialattentionwaspaidtothisphasetransformation duringthedensificationbySPS.
Followingthedensificationbehaviorofthetwonano-powders, severalimportantfeatureswereobserved.First,averyrapid com-pactionoftheMPpowderwasdirectlyassociatedwiththepressure whenappliedatroomtemperature(Fig.4a);thisroom tempera-turecompactionwiththepressureincreasepersistedforallSPS temperaturesinvestigated.However,furtherdensificationat ele-vatedtemperaturesdependedonthefinalSPStemperature.Inthe 1000◦C‘coldcompaction’treatment(Fig.4a),thedensityincreased
Fig.4.Relativedensity–time–temperature–pressuredependenciesduringSPSof (a)MPand(b)SPY2O3nanoparticlesat1000◦Cfor5minand100MPa.
simultaneouslywiththepressureincreasefromroomtemperature (i.e.SPSstartingtime),andleveledat67%within2min,whenthe pressurereacheditsmaximumvalue(100MPa).Sincethe temper-aturewasincreasedonlyafter5minfromtheSPSstart(Fig.4a), theobserveddensificationatroomtemperaturecanberelatedto particlesliding withnegligiblecontributionfromthediffusional processes.However,negativeramdisplacementwasrecordedat ∼600◦Cduringheatingataconstantpressure.Thisdisplacement couldbeassociatedwithacertaindecreaseindensity,butasitis ceasedwhentheSPStemperatureof1000◦Cwasreached,itcan berelatedtothethermalexpansionmismatchesbetweenthatof thegraphiteplungersandtheclosepackednano-particlenetwork. Apparently,theclosepackednano-particlesundergosurface dif-fusionduringtheheatingtoformarigidskeleton.Insuchacase, therigidskeletonmayopposefurther shrinkageunderthe con-stantpressure;ifthethermalexpansionofthisskeletonishigher thanthatofthegraphiteplunger,negativeramdisplacementmay occur. (Negativedisplacement due tothermal expansionofthe graphitemold,plungersandspacersisusuallyobservedwiththe increasingtemperatureusingblankspecimens.)Thisaspectwillbe discussedlaterindetail.Thedensitydidnotchangesignificantly duringtheSPSisotherm.Inaddition,increaseintheSPSduration to10 and15minresultedinhigherdensitiesof74.6and76.0%, respectively.
DensificationoftheSPreferenceY2O3nanopowderwasrecently investigatedindetailundersimilarSPSconditions[20].Someof theresultswillbeusedhereforcomparison.ThedensityoftheSP referencespecimenat1000◦Ctreatmentincreasedwiththe pres-sureincreaseatroomtemperature(Fig.4b),althoughitleveledoff atamuchlowerdensityof42%.However,incontrasttotheMP nano-powder,furtherincreaseinthedensitywasobservedduring theheatingprocess.Densificationwasacceleratedaround∼680◦C andreacheditsmaximumvalueof93%around∼880◦C,beforethe finalSPStemperatureof1000◦Cwasreached.
TheheatingratetotheSPStemperatureof1100◦Cwasof‘hot compaction’anddifferedfromthatat1000◦C.Inthisrespect,a heatingpulsewasusedtoreach600◦Cwithin3minunderthe holdingpressureof 2MPa(Fig.5a).Thiswasfollowed by heat-ingto1100◦Cforadditional5min.Thenthepressurewaslinearly increasedto100MPa,resultingin simultaneousincrease inthe density to its final value of 66%; nofurther densification was observedattheSPSisotherm.Similardensities(i.e.63.4and62.8%) were reached with further increase of the SPS duration to 10 and15min,respectively.Comparisonbetweenthedensities mea-suredattwoSPSregimes,i.e.at1000◦Cand1100◦C,revealedthat theapplicationofpressure atthebeginning (‘coldcompaction’) resultedinhigherdensities.
ThecorrespondingSPspecimenexhibitedsignificantincrease in the density during theheating process between800◦C and ∼1050◦C, underthe2MPa holdingpressure only(Fig.5b)[20]. The maximum displacement/(shrinkage)rate was10−2mms−1. Thisindicated thehigh capillaryforces in theSPto drive den-sification,incontrasttotheMPpowder,wherenodensification was recorded during the heating up. A higher densification rate(1.5×10−2mms−1)wasmeasuredwhen the pressure was increasedtoitsmaximumvalue.Afinaldensityof93%,whichis higherthanthatoftheMPspecimen,wasreached.
Thedensificationbehavior intheMPspecimenat1500◦C is shown in Fig.6, as in the other experiments,a very fast den-sification rate(1.5×10−2mms−1)wasrecordedsimultaneously with the pressure increase. However, in this ‘hot compaction’ experiment(i.e.,aheatingpulseto600◦Cwasappliedbeforethe pressurewasincreased),densificationceasedwhenthemaximum pressurewasreached.Densitywasstagnatedduringfurther heat-ing upto 1350◦C, where anadditional rapid densificationrate (5×10−3mms−1) upto thefinal density wasrecorded. During
Fig.5.Relativedensity–time–temperature–pressuredependenciesduringSPSof (a)MPand(b)SPY2O3nanoparticlesat1100◦Cfor5minand100MPa.
theperiodofdensity stagnation(i.e.,between200and650sin Fig.6), thepressureexperiencedtwodisturbancesexpressedby adecreaseof8–10MPaintherecordedpressure.Thesedecreases inpressureoccurredaround1030◦Cand1280◦C,andwere recov-eredat 1130◦C and 1350◦C, respectively. The second pressure recovery(increase)wasresponsiblefortherepeatedincreasein densificationat1350◦C,aswasmentionedabove.Thesepressure disturbancesareassociatedwithincreaseintheresistanceofthe nano-particlestoundergosliding,whetherduetotheformation ofarigidskeletonorjammingoftheagglomeratednano-particles. Ineithercase,thermalexpansionsofboththegraphiteplungers andthespecimentogetherwiththelackofplasticityinthe speci-men,introduceinternalcompressivestresses.Sincetheexternal pressureappliedinthesystemisregulatedtoremainconstant, itsactualvalueshoulddecreaseinordertobalancetheinternal
Fig.6.Relativedensity–time–temperature–pressuredependenciesduringSPSof MPcubicY2O3nanoparticlesat1500◦Cfor5minand100MPa.
Fig.7.SEMimageshowingthehomogeneousdistributionofthesphericalgrains throughoutthespecimensinteredat1100◦Cfor5minand100MPausing
multi-phaseY2O3nanoparticles.
thermalpressuresformed.Thismayleadtothedisturbancesand decreasesobservedinthepresentSPSexperiments.
SEMimagesfromthespecimenssinteredat1100◦Cfor differ-entdurations(i.e.5min,Fig.7)showedhomogeneousdistribution ofmanysphericalshapegrainsthroughoutthematrixoftheMP Y2O3 nanoparticles.Thelargestdiameterofthesphericalgrains was∼15mmafter5min,and∼60mmafter10mindurations,with verywidegrainsizedistributions.Thesesphericalgrainsexhibited alowvolumefractionofthespecimensevenafter15minof sinter-ingat1100◦C.Thesesphericalgrainswereabsentaftersintering at1500◦C;aregularpolyhedralshape grainmicrostructurewas observed.
TEMimagesfromthespecimenssinteredatdifferent temper-aturesclearly revealedthe microstructureevolution duringthe SPS. First, at 1000◦C and 1100◦C, closeto the phase transfor-mationtemperature,many polycrystallinesphericalparticles of submicrometerandmicrometer-sizeindiameterwereobserved withintheporousnanoparticlematrix(Fig.8a).Higher magnifica-tionprovedtheinternalstructureofthelargeparticles(Fig.8band c)tobecomposedofsub-grainsseparatedbydislocationnetworks; insomeoccasions,internalsubmicrometer-sizeporeswerealso observed.Thematrixwascomprisedofpartiallysintered nanopar-ticleswhichformedaporousnetworkresemblingthevermicular structure(theupperpartinFig.8b).Detailedexaminationofthe interfacebetweenthesphericalparticlesandthevermicular struc-turerevealedthattheformergrowontheaccountoftheporous nanoparticleskeleton(Fig.8c).Selectedareadiffractionpatterns confirmedthecubiccrystal symmetryof thesphericalparticles (Fig.8d).Therefore,itseemsthatthesphericalparticlespresent nucleationandinterface-controlledgrowthofthecubicgrainson theaccountofthemonoclinicnanoparticles.
Finally, TEM images from the 1500◦C treated specimen, shown in Fig. 9, exhibit the micrometer size polyhedralshape grains with nanometric closed pores. Many grains were com-prisedofsub-grainsseparatedbydislocationnetworks(Fig.9b). A few nanometer size grains and closed pores were still visible along the sub-grain boundaries and at their corners (arrowed in Fig. 9b).This microstructure can beconsidered as analmostfullytransformedversionofthevermicular structure to thecubic grains. The microstructure evolution in the refer-ence single-phase cubic nanoparticle compactsduring the SPS were described in detail elsewhere [15,16,20]. At SPS temper-atures below 1100◦C, the nanometric character of the dense
Fig.8.TEMimagesshowingthe(a)nucleationandgrowthofthesphericalcubicY2O3grainsonaccountofthemultiphasenanocrystallinematrixat1000◦C.(b)Higher
magnificationof(a)showingthevermicularstructureofthematrixnanoparticles.(c)Theinterface-controlledgrowthofthesphericalcubicgrainsat1100◦C.(d)Selected
areadiffractionpatternconfirmedthecubicsymmetryofthesphericalY2O3grains.
compacts waspreserved. However, significant grain growthto micrometer-sizegrainswasobservedathighertemperatures[16]. The microstructural evolution associated withthe polymorphic phase transformationisthebasisfor theobserveddensification behavior ofthemultiphasenanoparticlesand willbediscussed below.
4. Discussion
Theobserveddensificationbehaviorofthetwonanopowders canbeexplainedbythemetastablenatureofthemonoclinic poly-morph.ThemonoclinicphaseisahighpressureversionofY2O3, but can be retained in a metastable state at the atmospheric conditionswheninthenanoparticleform.Theseaspectsof poly-morphism,especiallyintheY2O3nano-particles,werediscussed indetailelsewhere[19,21,22].Althoughbothpowdersexhibiteda closetonanocrystallineparticlesize(18-nmvs.41-nm),the sur-faceenthalpyofthemonoclinicpolymorphissignificantlyhigher (2.78Jm−2)thanthatofthecubicphase(1.66Jm−2)[19]. There-fore,thehighlyactivesurfacesofthemonoclinicnano-particles act as an efficient driving force for low-temperature sintering andneckformation.Thistypeofhighsinterabilityiswell char-acterized in transition alumina (g-alumina) and lead to rapid formationofa rigidporousskeletonbysurfacediffusionatlow
temperatures [23]; the resultant vermicular structure leads to low-densitysintered compacts.In this respect,consolidation of aluminananoparticlesbySPSshowedenhanceddensificationof the a-alumina compared to that of transition g-alumina [24]. Surprisingly, dense a-alumina was obtained at a considerably lowerSPStemperature,albeitoriginallywithalargerparticlesize. Thisbehaviorwasrelatedtog→aphasetransformationduring the SPS, which in turnled to the formation of the vermicular structure.
Thetheoreticaldensitiesofthecubic(c)andmonoclinic(m) Y2O3polymorphsare5.030gcm−3and5.468gcm−3,respectively
[19]. Consequently, the polymorphic m→c phase transforma-tion is associated with∼8% volume increase. Nevertheless,the DSC results indicated the polymorphic transformation temper-ature to be around1000◦C, where a rigid skeleton of mY
2O3 hasbeendevelopedfairlywell.Ononehand,thekineticsofthis interface-controlled polymorphic phase transformation is more sluggish compared tothe SPSheating time scale, as evidenced byXRD analysis.Ontheotherhand, thepresenceof the nano-poreswithinthesphericalparticlesisanevidenceforaveryrapid interface-controlledprocess(i.e.nano-poreswerenotannealedby diffusion).Consequentlythechangeinthecompactvolume dur-ingtheheating,duetothephasetransformation,maybemarginal. SimilareffectswerereportedduringdensificationofSi3N4 with
Fig.9.TEMimagesshowingthe(a)micrometer-sizecubicgrainsformedat1500◦C.
Afewporesarevisiblealongthegrainboundaries.(b)Manysub-grainboundaries decoratedwithdislocationnetworksandnano-grainswerepresent(arrowed).
metallicsinteringadditivebySPS[25],wheresignificant shrink-ageoccurredbyparticlerearrangementandwhilealiquidphase wasformed.However,minorshrinkagewasobservedwhenphase transformationandgraingrowthviasolution-reprecipitationtook place.
Based on the microstructure developed in the multiphase nanoparticlecompacts,thefollowingprocessesmaybeconsidered. First, theapplication ofexternal pressure at roomtemperature enablesparticlesliding andrearrangement.Theearlyand rapid densificationstageceaseswhenreachingthemaximal pressure applied.Second,highlyreactivesurfacesof themnanoparticles enableenhancedneckformationandgrowthbysurfacediffusion duringtheheating.The partiallysinteredmnanoparticlesform a rigidand porous skeleton witha vermicular structure which opposesfurtherdensificationbyparticlesliding.Atandabovethe polymorphicm→cphasetransformationtemperature,nucleation ofthestablecphasetakesplace,homogeneouslythroughoutthe porousskeleton(thehomogeneous/heterogeneousnatureofthe nucleationeventwasnotinvestigatedhere).Furthergrowthofthe cubicnucleibythisinterfaced-controlledtransformationresults insphericalpolycrystallineparticles.Apparently,the8%volume
increaseaccompaniedtothetransformation,is notsufficientto overcomethevolumeconstraintsimposedonthegrowing spher-icalparticles withintherigid porousmatrix. Consequently,the elasticconstraintsmaybereducedbytheformationofdislocation networksandsub-grains,which,inturn,growontheaccountof them+cnanoparticles,attheirgrowingfront.Whentherapidly growingfrontofthesphericalparticlefacesalargecavityofthe vermicularstructure,itmaysurpassit,duetoinsufficienttimefor diffusion,resultinginoccludednano-pores.Athighertemperature, whenthephasetransformationisaccomplished,particlesliding, dislocationcreep,andgraingrowthmayberesponsibleforthelater stagedensificationclosetofulldensity.
Finally,theeffectofthevolumechangeduringthe polymor-phicphasetransformationtodensificationofthemultiphaseY2O3 nanoparticleswasnegligible.Nevertheless,themetastablenature andthehighsurface activityofthis polymorphwerethemajor causefortheformationofthevermicularstructure,whichinturn, inhibitedthedensificationduringtheheatingbySPS.The interface-controlledcharacterof thetransformation ledtotheformation ofverylargecubicgrains,responsibleforthelossofnanometric characterofthecompactsubjectedtodensification.
5. Summary
Sparkplasmasinteringofthemultiphasemonoclinicandcubic Y2O3 nanoparticles at 1000◦C exhibited limited densification comparedtotherapiddensificationofthecubicsingle-phase coun-terpart.XRDofthemultiphasesinteredcompactsrevealedthatthe polymorphicmonoclinictocubicphasetransformationoccurred duringtheSPS around1000◦C, andwascompletedbyreaching 1100◦C.Themetastablemonoclinicphaseledtorapidneck forma-tionduringtheheatingwhichresultedinavermicularnanometric matrixwithopenporositynetwork;thislimitedfurther densifi-cationofthemultiphasenanopowderandendedinverylowfinal densities.AtSPStemperaturesabovethepolymorphic transforma-tiontemperature,homogeneousnucleationofthesphericalcubic Y2O3grainswasobservedwithinthevermicularmatrix.This inter-facecontrolledmonoclinictocubicphasetransformationresulted inthelossofthenanocrystallinecharacterofthecompact.SPSofthe multiphasenanoparticlesat1500◦Cshowedsimilardensification behaviorasthepurecubicY2O3,resultinginadensemicrostructure andcoarsemicrometer-sizepolyhedralshapedgrains.
Acknowledgments
ThefinancialsupportoftheIsrael MinistryofScienceunder contract#3-3429isgratefullyacknowledged.WethankDr.Ori YeheskelfromNRC-NegevforsupplyingtheMPnanopowder. References
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