Densification and polymorphic transition of multiphase Y2O3 nanoparticles during spark plasma sintering

To link to this article: DOI: 10.1016/j.msea.2011.06.044

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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

_,

_R.

_Chaim

,

G.

Chevallier

,

C.

Estournes

a_{DepartmentofMaterialsEngineering,Technion–IsraelInstituteofTechnology,Haifa32000Israel} b_{CNRS,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 was10−2_mms−1_.

Thisindicated thehigh capillaryforces in theSPto drive den-sification,incontrasttotheMPpowder,wherenodensification was recorded during the heating up. A higher densification rate(1.5×10−2_mms−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(1.5×10−2_mms−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}

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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