Effect of amount of doping agent on sintering, microstructure and optical properties of Zr- and La-doped alumina sintered by SPS

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Effect of amount of doping agent on sintering,

microstructure and optical properties of Zr- and

La-doped alumina sintered by SPS

Lucile Lallemant, Nicolas Roussel, Gilbert Fantozzi, Vincent Garnier,

Guillaume Bonnefont, Thierry Douillard, Bernard Durand, Sophie

Guillemet-Fritsch, Jean-Yves Chane-Ching, Domingo Garcia-Gutierez, et al.

To cite this version:

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To cite this version : Lallemant, Lucile and Roussel, Nicolas and

Fantozzi, Gilbert and Garnier, Vincent and Bonnefont, Guillaume and

Douillard, Thierry and Durand, Bernard and Guillemet-Fritsch, Sophie

and Chane-Ching, Jean-Yves and Garcia-Gutierez, Domingo and

Aguilar-Garib, Juan Effect of amount of doping agent on sintering,

microstructure and optical properties of Zr- and La-doped alumina

sintered by SPS. (2014) Journal of the European Ceramic Society, vol.

34 (n° 5). pp. 1279-1288. ISSN 0955-2219

Open Archive TOULOUSE Archive Ouverte (OATAO)

OATAO is an open access repository that collects the work of Toulouse researchers and

makes it freely available over the web where possible.

This is an author-deposited version published in :

http://oatao.univ-toulouse.fr/

Eprints ID : 13979

To link to this article : doi:

10.1016/j.jeurceramsoc.2013.11.015

URL :

http://dx.doi.org/10.1016/j.jeurceramsoc.2013.11.015

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Effect

of

amount

of

doping

agent

on

sintering,

microstructure

and

optical

properties

of

Zr-

and

La-doped

alumina

sintered

by

SPS

Lucile

Lallemant

a,b

,

Nicolas

Roussel

c

,

Gilbert

Fantozzi

a,b

,

Vincent

Garnier

a,b,∗

,

Guillaume

Bonnefont

a,b

,

Thierry

Douillard

a,b

,

Bernard

Durand

c

,

Sophie

Guillemet-Fritsch

c

,

Jean-Yves

Chane-Ching

c

,

Domingo

Garcia-Gutierez

d

,

Juan

Aguilar-Garib

d

a_Université_de_Lyon,_CNRS,_France

b_Insa-Lyon,_MATEIS_UMR5510,_F-69621_{Villeurbanne,}_France

c_Université_de_Toulouse,_{CNRS-UPS-INPT,}_CIRIMAT,₁₁₈_route_de_Narbonne,₃₁₀₆₂_Toulouse,_France

d_Centro_de_Innovacion,_{Investigacion}_y_Desarrollo_en_Ingeniera_y_Tecnologica,_UANL,_Carretera_al_Aeropuerto,_Apodaca,_Nuevo_Leon,_Mexico Received30January2013;receivedinrevisedform5November2013;accepted12November2013

Availableonline2December2013

Abstract

SPS-produceda-aluminasamplesarepreparedfrompowdersdopedwithdifferentamountsofZr4+_and_La3+_cations._Zr4+_cations_segregate_at

grainboundaries.m-ZrO2particlesareformedat570butnotat280catppm.Ab-aluminaLaAl11O18structureisfoundat310catppmwhen

thelanthanumgrainboundarysolubilitylimitisexceeded(∼200catppm).100catppmLaissufficienttoblockthediffusionpathacrossgrain

boundariesandinhibitgraingrowth.Bothdopingcationsdisturbthegrainboundarydiffusionwhatevertheiramount.Theydelaythedensification

athighertemperatureswhilelimitinggraingrowth.Therealin-linetransmittance(RIT)ofa-aluminaisimprovedduetothereducedgrainsize.

Nevertheless,increasingthecationamountleadstoanincreaseinporosityoreventheformationofsecondaryphaseparticles,bothdetrimental

foropticalproperties.Finally,optimisedamountsofcationof200and150catppmarefoundforLa-andZr-dopedalumina,respectively.

Keywords:Al2O3;Doping;Sparkplasmasintering;Grainsizes;Opticalproperties

1. Introduction

Properties of ceramic materials are governed by their microstructure. Therefore, the best microstructure suitable for a given application should be obtained after sintering. Grain boundariesareknown to playasignificant role onthe microstructureformationduringdensificationandgraingrowth. Inthecaseofalumina,dopingcationswithalargerionicsizethan Al3+(forexampleY3+,La3+,Mg2+)maybeaddedtothepowder beforesintering.Theyshowastrongtendencytoeitherforma solidsolutionwiththealuminaorsegregateatgrainboundaries. Masstransportinthegrainboundaryplaneandgrainboundary mobilityisthereforemodified.1–5Asaresult,thedensification caneither be improvedor hindered dueto ahigher or lower

∗_{Corresponding}_author_at:_Insa-Lyon,_MATEIS_UMR5510,_F-69621 Villeur-banne,France.Tel.:+33472438498.

E-mailaddress:vincent.garnier@insa-lyon.fr(V.Garnier).

diffusivityatgrainboundaries.3_Moreover,_the_grain_size_is

gen-erallyfineraftersinteringwithsuchdopingagentsduetolower grainboundarymobility.3,6,7Thosedopingelementshavebeen successfullyusedtoobtaintransparentpolycrystallinealumina (PCA).7–12Indeed,becauseofthebirefringentnatureofPCA,a finemicrostructureisrequiredaftersintering13asdemonstrated bythemodeldevelopedbyApetzetal.(Eqs.(1)and(2)).

RIT = I2 I1 =(1−RS·exp(−γtot·D)) (1) γtot=γG+γp= 3π2r∆n2 λ2₀ + p (4/3)π·r3 p ·Csca,p (2)

whereRITistherealin-linetransmittance,I₁ andI₂the light beam intensitiesbefore andafter travellingthrough a sample having athickness D; Rs the totalnormal surfacereflectance

(=0.14forPCA);γtotthetotalscatteringcoefficient;γGthelight

scatteringcoefficientbygrainboundaries;γpthelight

scatter-ingcoefficientduetoporosity;rtheaveragegrainradius;1n

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Fig.1.Schematicdrawingof(a)solidsolution,(b)grainboundarysegregation, and(c)secondaryphase(blue)formation.(Forinterpretationofthereferences tocolorinthisfigurelegend,thereaderisreferredtothewebversionofthe article.)

theaveragerefractiveindexchangebetweentwoadjacentgrains (=0.005forPCA),λ0thewavelengthoftheincidentlightbeam

invacuum;pthetotalporosity,rptheaverageporeradiusand

Csca,pthescatteringcross-sectionofonesphericalpore.Stuer

et al.14 recently showed that even ifa highdegreeof poros-ity is present in the sample, there is little effect on the RIT belowacriticalporesize ofabout50nm.Theporosityeffect could thus be overestimated. Concerningthe grainboundary effect,withexperimentsthemodelhasshowngoodcorrelation forgrainsizebetween300nmand3mmforλ=640nm.15

Nev-ertheless, thismodel does notconsidera possiblecorrelation betweengrainboundaryscatteringandgrainorientation.16 How-ever,Choetal.17 haveobservednosignificantgrainboundary misorientationonaluminasamplesdopedwith100catppmZr and500catppmLacomparedwithpuresamples.Ithas there-forebeendecidedtocompareourresultswiththeApetzmodel. In theliterature,the roleof oneparticular dopingelement on densificationandgraingrowthofPCAisgenerallyobservedfor onesinglequantityofthiselement.Nevertheless,thelocationof dopingcationsmaychangedependingontheiramount,resulting indifferentsinteringbehaviourandsodifferentmicrostructures andassociatedproperties:

(A) When the amount of doping agent is below the bulk solubilitylimit,dopingcationsareinsolidsolution homo-geneouslydistributedwithinthegrain(Fig.1(a)).However, thiscaseusuallydoesnothappenasbulksolubilitylimits areverylow(afewhundredppm4)dependingonthedoping agentionicradiiorcharges.

(B) When the amount of doping agent is higher than the bulk solubility limit and lower than the grain boundary solubility limit, the additional cations segregate within a few nanometres18 at the grain boundaries (Fig. 1(b)). The atomicstructural environmentaroundthesegregated dopingcations is similarto thatit would haveinknown secondaryphases.4,19 _Moreover,_it _should_be_highlighted

that,ontheonehand,thegrainboundarysolubilityis sur-facedependenti.e.itdependsonthegrainsizewherefiner grainswillaccommodatealargeramountofdopingagent duetotheirlargersurfacearea.Ontheotherhand,thegrain boundarysolubilityisalsodependentonthegrainboundary planeorientation,4resultinginaheterogeneousdistribution ofthesegregatingareasalongthegrainboundaries. (C) Whenthedopingagentamountexceedsthegrainboundary

solubility limit,secondaryphasesappearatgrain bound-ariesandattriplepointsalongwithalatticediscontinuity betweenthealuminaandthesecondaryphases(Fig.1(c)).

Dependingonitscharacteristics(ionicradius,charge,etc.), thedopingelementmaybeinsolidsolution,segregateatgrain boundariesorformsecondaryphases.Thesephasesareoneof thepossible light scattering sourcesinPCA.Their scattering coefficientcanbedescribedbyEq.(3)9andisaddedtothetotal scatteringcoefficientγtotinEq.(2).

γdop=

hppmwi_dop·ρAl2O3

4/3·Π·r_dop3 ·ρdop

·Csca,dop (3)

whereγdopisthelightscatteringcoefficientbysecondaryphase

particles;hppmwidoptheamountofdopingagentinweightppm;

ρAl2O3thetheoreticaldensityofa-Al2O3;rdoptheaverageradius

ofsecondaryphaseparticles;ρdopthetheoreticaldensityof

sec-ondaryphaseparticlesandCsca,dopthescatteringcross-section

ofonesphericalsecondaryphaseparticle.Thus,theformationof secondaryphaseparticlesshouldbeavoidedinordertoimprove opticalproperties, unless theyarehomogeneously distributed alongthegrainboundarieswithalowsizeandarefractiveindex very closetothat of PCA.In the present study, the effectof dopingagentamounton themicrostructure andoptical prop-ertiesof Zr-andLa-doped aluminasinteredbySparkPlasma Sintering(SPS)willbedetermined.BothZr4+andLa3+cations are largerthan Al3+ cations: their ionic radii are 0.72 ˚A and 1.03 ˚A,respectivelyfor6-foldcoordinationcomparedto0.53 ˚A forAl3+forthesamecoordinationnumber.20Zr4+wasfoundto beinsolubleinana-aluminalattice21_and_the_bulk_solubility_of

La3+_is_really_low_(∼80_cat_ppm22₎_or_even_lower.4_Those_doping

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L.Lallemantetal./JournaloftheEuropeanCeramicSociety34(2014)1279–1288 1281

Table1

ICPmeasurementsondopedaluminapowdersafterfreeze-drying.AP:pure alumina,AZ:aluminaandzirconium,AL:aluminaandlanthanum.

Sample Introduced dopingagent amount (catppm) Measured dopingagent amountafter freeze-drying (catppm) Ratio (mea-sured/introduced) dopingagent amount AP 0 – – Zr-dopedalumina AZ150 250 150 0.6 AZ280 500 280 0.6 AZ570 1000 570 0.6 La-dopedalumina AL40 60 40 0.7 AL100 150 100 0.7 AL200 300 200 0.7 AL310 500 310 0.6 AL670 1000 670 0.7

2. Materialsandmethods

The starting material was a commercial (BA15psh,

Baïkowski) high purity a-Al2O3 aqueous slurry (solid

con-tent 73.5wt%) with an average particle size Dv₅₀ of 160nm (measured by laser diffraction). The total impurity was less than0.01wt%(14ppmNa,60ppmK,7.1ppmFe,13ppmSi, 4ppmCa)as quotedbythe manufacturer.The dopingagents wereintroducedbyweighingouttherequiredamountofhigh purity(>99.99%)water-solublechloridesalts(Sigma–Aldrich, Germany)ofzirconium(ZrOCl2,8H2O)andlanthanum(LaCl3,

7H2O),addingthemtothealuminaslurryandmixingfor24hby

rotationofthecontainer.Theslurrywasfrozeninliquid nitro-genthenfreeze-driedforapproximately48h at−40◦Cunder 0.1mbar (Freeze-dryer: Christ Alpha 2–4) to obtain a pow-der.Thiswasthensievedat500mm. Theamountsof doping agent (cationic ratio [doping elementX+]/[Al3+]) introduced were250,500, 1000catppmfor zirconiumand60,150,300, 500,1000catppmforlanthanum.InductivelyCoupledPlasma (ICP) measurements (Horiba Jobain Yvon Activa) were per-formed to characterise the real amount of dopingagent still present in the powder after freeze-drying and sieving (see

Table1).

As can be seen from Table 1, a constant ratio of doping cationsiseliminated duringthedryingstep.Thissuggestsan equilibriumbetweenthe dopingcationspresent atthe surface of alumina particles and thosepresent within the slurry.All amounts of doping agent given hereafter will refer to those measuredbyICPafterfreeze-drying.

Thefreeze-driedpowdersweresinteredbySPS(HPD25/1, FCTSystem,Rauenstein,Germany)withoutanythermal pre-treatment. Indeed, this avoids large agglomerates within the powder beforedensification.11 The sinteringcycle was previ-ouslyoptimisedtoobtainpuretransparentPCAsamples.11,24It includesauni-axialpressureof80MPaappliedthroughoutthe cycle,rapidheatingto800◦_C,_a_heating_rate_of₁₀◦_C/min_from

800◦_C_to₁₁₀₀◦_C_followed_by_slower_heating₍₁◦_C/min)_up_to

Fig.2.RITs640nmofZr-dopedaluminasamplessinteredatTf=1280◦Cand 1300◦_C_for_different_amounts_of_doping_agent.

thefinalsinteringtemperature(Tf)inordertoremovethe

resid-ualporosity.11,24–26Rapidcoolingendsthecycle,interruptedby a10-minperiodat1000◦_C_to_release_the_residual_stresses.25,26

Thefinalsinteringtemperaturewasoptimisedforeachkindof dopedsampleintherangeof1230–1330◦_C_in_order_to_obtain_the

bestopticalproperties.Thesinteringtemperaturewasmeasured 3mmfromthesamplewithanopticalpyrometerfocusedonthe non-throughhole(3mmdiameter)ofagraphitedie.Then,the 20mmdiametersampleswerecarefullymirror-polishedonboth sidesusingdiamondslurries(downto1mm).Thetransparency inthecentreofthepellet(spotsize=5mm×2mm)was evalu-atedbyaRITmeasurement(JascoV-670),asitonlytakesinto account theunscatteredlight throughthe sample(i.e.the real transmittedlight).To ensurethat onlytheRITwasmeasured, ashieldwithanappropriateaperturewas placedbetweenthe sampleandthedetectorinordertohidelightscatteredbymore than0.5◦_._For_comparison_purposes,_all_the_RIT_values_given_in

thispaperareevaluatedforλ0=640nmandEq.(4)allowsthe

RITtobeobtainedforthesamethicknesst2=0.88mm:

RIT(t2)=(1−RS)

RIT(t1)

1−RS

t2/t1

(4) whereRSisthetotalnormalsurfacereflectance(=0.14forPCA)

andRIT(ti)istheRITforasamplethicknessti.

An SEM ZEISS Supra55 was used to investigate the

microstructure of thesamples. Grainsizes wereevaluatedon fracture surfaces and a factor of 1.22 was applied to obtain a revised grain size.10,13 _The _presence _of _secondary _phases

was determinedusing X-raydiffractometry(XRDBruker D8 Advance)andtheirlocationwascharacterisedbytransmission electron microscopy(TEM)observations (TEMTitan G2

60-300andTEMJEOL2010F).

3. Resultsanddiscussion

3.1. Zr-dopedalumina

3.1.1. Opticalproperties

Zr-dopedpowdersweresinteredatdifferentTf:1230,1250,

1280,1300and1330◦_C._The_highest_RITs

640nmwereobtained

atTf=1280and1300◦CandtheseresultsarepresentedinFig.2.

DopingwithZr4+cationsenablesustoincreasetheRIT640nm

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1282 L.Lallemantetal./JournaloftheEuropeanCeramicSociety34(2014)1279–1288

Fig. 3.Average grain sizes of Zr-doped alumina samples sintered at

Tf=1280◦Cand1300◦Cfordifferentamountsofdopingagent.

For both temperatures, the best RITs640nm (45±1% and

48±1%forTf=1280and1300◦C,respectively)areobtained

for thelowestZr4+dopingamount(AZ150).However,Fig.3

indicatesthatthe lowertheamountofZr4+ doping,thelarger thegrainsize.Forexample,at1300◦C,theRIT640nmdecreases

byabout5%betweenAZ150andAZ570whereasthegrainsize decreases from0.59±0.13mmfor AZ150 to0.44±0.13mm forAZ570.

Inthesameway,thehighestRIT640nmisalwaysobtainedat

Tf=1300◦Cfor agivenamountofdopingagent whereasthe

grainsizeislargeratthistemperature.Infact,themodel(Eqs.

(1)–(3))indicatesthattheRITshouldbehigherforlowergrain sizeunless porosity or secondaryphaseparticles arepresent. TEMobservationsarethusperformedtoexplainthehigherRIT atlargergrainsize.

3.1.2. TEManalysis

Inatransmissionelectronmicroscope,HAADF(HighAngle AnnularDarkField)imagesarehighlysensitivetovariationsin theatomicnumberofatomsinthesample(Z-contrastimages). As the atomic number of Zr (Z=40) is higher than that of Al(Z=13),brightcontrasts inHAADF images mayindicate thepresenceofZr4+ cationswithinthealuminamatrix.Thus, HAADFobservationsandEDXspectroscopywereperformed onAZ570samplessinteredatTf=1300◦C(Fig.4(a)).Theresult

indicatesthat noZr4+ cations are locatedwithin the alumina grains.Thewhite graincorresponds toatopologicalcontrast. Onthecontrary,Zr4+_cations_are_segregated_along_grain

bound-ariesasindicatedbythebrightcontrastandEDXspectroscopyin

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Fig.5.XRDmeasurementsofAZ280(bottomleft)andAZ570(topleft)samples sinteredatTf=1280◦CandAL670(right)samplesinteredatTf=1330◦C (m-ZrO2:ICDD37-1484;LaAl11O18:ICDD33-0699,Al2O3:ICDD46-1212).

Fig.4(b).Finally,secondaryphaseparticlesarealsosometimes foundattriplepoints(seeFig.4(c)).

Thesesecondaryphaseparticles(Fig.4(c))areresponsible forthereflectionofmonocliniczirconia(m-ZrO2)inXRD

mea-surements(Fig.5).Asm-ZrO2reflectionappearsonlyonAZ570

samples(Fig.5)andnotforloweramountsofdopingagent,this suggeststhatsecondaryphaseparticlesofm-ZrO2areformed

atasufficientlyhighZrconcentration.Forloweramounts,Zr4+ cationsareonlysegregatedalonggrainboundaries.

TheseresultsareingoodagreementwithastudybyWang et al.19 who reported that for a doping agent amount of 100catppmZr,theZr4+cationssegregatedalonggrain bound-arieswithoutsecondaryphaseformation.Itiswellknownthat Zr4+cationsareinsolubleinana-aluminalattice.21However,it isinterestingtopointoutthatthegrainboundarysolubilitylimit ofZr4+cationsbeforetheformationofsecondaryphase parti-clesisquitehigh.Forgrainsizesaround450nm,thisamountis thusbetween280and570catppmZr.

SegregationofZr4+_cations_along_grain_boundaries_explains

whythe grainsize issmaller for agreater amountof doping agent (Fig. 3). Indeed, grain growthmechanisms induce dif-fusionacrossthegrainboundaries.Suchdiffusionishindered bythepresenceofimpurities,suchassegregatedZr4+cations. Moreover,impuritiesleadtoasolutedragphenomenonthat dis-turbsthegrainboundary mobility, allthemoreas thedoping agentamountincreases.27Thismeansthatgraingrowthwillbe moresignificantatsmallamountsofdopingagent.ForAZ570 samples,thepresenceofsecondaryphaseparticleswillinducea pinningeffectthatwillbeaddedtothesolutedragphenomenon topreventgraingrowth.

Concerningtheinfluenceofmicrostructureonoptical prop-erties, two behaviours have to be considered depending on whethersecondaryphaseparticlesarepresentornot.ForAZ150 and AZ280, XRD measurements indicate that no secondary phaseparticlesareformed. Thissuggeststhat lightscattering onlydepends on grain size andporosity (Eqs. (1)–(3)). This assumptionimpliesthat thesegregationof Zr4+ cationsalong grainboundaries does not contribute tolight scattering. This hasbeendemonstratedbyBernard-Grangeretal.9,10who suc-ceededinobtainingtransparentPCAdopedwithMg2+,Ti4+and Ca2+/Ti4+.Intheirstudies,thedopingcationsonlysegregated

Fig.6.ComparisonofRITs640nm ofAZ150andAZ280samplessinteredat

Tf=1280◦Cand1300◦Cwiththeoreticalmodels(λ0=640nm,D=0.88mm, φp=100nm).

alonggrainboundarieswithoutsecondaryphaseparticle forma-tion andtheRITmeasurementsinthevisiblerangepresented agoodcorrelationwiththetheoreticalcurvesobtainedwiththe Apetz model (Eqs. (1) and(2)).That is whyfor AZ150 and AZ280,onlyscatteringduetoporosityandfromgrain bound-ariesisconsidered.

Transparencyrequiresnoporosityorsuchalimitedamount thatitcouldnotbemeasuredbytheclassicalArchimedemethod. However,asmeasuredgrainsizesarebetween300nmand3mm and no secondary phase particlesare formed on AZ150 and AZ280 samples,itispossibletocompare ourresultswiththe theoreticalmodeldevelopedbyApetz(Eqs.(1)and(2)). Theo-reticalcurves(Fig.6)werecalculatedfora100nmporesizeas itisthelargestporesizewhichcanbeobservedbyTEM(Fig.4). Moreover,thiscorrespondstothecriticalporesize.14

Asthegrainsizeislargerat1300◦_C_than₁₂₈₀◦_C₍_Fig.₃_),

thismeansthatthelowertransparencyobtainedat1280◦_C

com-paredto1300◦_C_can_be_explained_by_greater_porosity₍_Fig.₆_).

Thus, at 1280◦C, the densification is not yet complete and somecriticalporosity(>50nm14)isstillpresent,affectingthe RITs640nm.HigherRITs640nmarethusobtainedat1300◦Cfor

allamountsofdopingagent(Fig.2).Atthistemperature,the densificationissufficienttolimitthepresenceofporeswitha limitedgraingrowth.Lightscatteringbybothporesandgrain boundariesisthenlimited.11Moreover,foragiventemperature, theRITs640nmofZr-dopedsamplesdecreasewiththeincrease

in amount of dopingagent (Fig. 2). This decrease is dueto theincreaseinporosity,asshowninFig.6,indicatingdifferent densificationbehaviours.

ForAZ570,m-ZrO2particlesarefoundwithinthealumina

matrix (Figs. 4(c) and 5) which will also contribute to light scattering(Eq.(3)).

3.1.3. Sintering

Asdiscussedabove,samplesdopedwithdifferentamountsof Zr4+cationsmayhavedifferentsinteringbehaviours.The den-sificationratesofthedifferentpowderssinteredatTf=1280◦C

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Fig.7.Densificationrateoffreeze-driedaluminapowderspureordopedwith differentamountsofZr4+_cations.

AscanbeseenfromFig.7,dopingaluminapowderswith Zr4+ cations delays the densification at higher temperatures whatever the amount of doping agent. Moreover, increasing thedopingagentamountalsotendstodelaythedensification at higher temperatures evenif thiseffect isless pronounced. Indeed,thedensificationofAPstartsat850◦_C,_compared_to_930,

960and1000◦CforAZ150,AZ280,andAZ570,respectively. Thehigher densificationratesareobtained at1050,1060and 1070◦_C_for_AZ150,_AZ280,_and_AZ570,_{respectively,}_compared

to1000◦_C_for_pure_alumina_samples_(AP).

SegregatedZr4+ _cations_along _grain_boundaries_favour _the

presence of aluminiumvacanciesattheir vicinitytopreserve electrical neutrality. Consequently, diffusion mechanisms of Al3+cationsalongthesegrainboundariesshouldbefavoured. Some authors28–31 have proved that the densification of a-aluminabySPSiscontrolledbygrainboundary diffusionbut the rate controlling species are not known for certain. Cur-rently,nostudieshavebeenabletoascertainwhetheraluminium grainboundarydiffusionisgreaterorsmallerthanoxygengrain boundary diffusion.32 Thismeansthat the promotionof Al3+ cations diffusionmechanisms maynot improvethe densifica-tionofa-aluminaiftheratecontrollingspeciesareO2−anions. Inourstudy,segregatedZr4+cationsatgrainboundariesdelay thedensificationathighertemperatures.AccordingtoYoshida,2

thesubstitutionofanAl3+_cation_by_a_Zr4+_cation_increases_the

ionicbondingstrengthbetweenAl3+_cations _and_O2−_anions.

It thus slows downthe grainboundary and latticediffusions ofbothspecies(grainboundariesaretheoriginofdiffusionin bothcases).Asaresult,thedensification isdelayed athigher temperatures.

Finally,asthedensificationofsamplesdopedwithagreater amountofZr4+cationsismoredifficult(slightdelayathigher temperatures),thiscouldexplainwhythesesamplesshowmore porosityaftersinteringandthusreducetransparency.

3.2. La-dopedalumina

3.2.1. Opticalproperties

La-dopedpowdersweresinteredatdifferentTf:1230,1250,

1280,1300and1330◦_C._The_highest_RITs

640nmwereobtained

atTf=1250,1280and1300◦Candtheseresultsarepresented

inFig.8.

Fig. 8.RITs640nm of La-doped alumina samplessintered at Tf=1250◦C, 1280◦_C_and₁₃₀₀◦_C_for_different_amounts_of_doping_agent.

Fig. 9.Average grain sizes of La-doped alumina samples sintered at

Tf=1250◦Cand1280◦Cfordifferentamountsofdopingagent.

DopingwithLa3+cationsenablesanincreaseintheRIT640nm

ofaluminasamplesduetoareducedgrainsize(Fig.9). More-over,RITs640nm of samplessintered at1250and1280◦Care

verysimilar.ThisisingoodagreementwithworkbyRoussel etal.33 whofoundthatdopingPCAwithLa3+cationsenables hightransparencytobeobtainedoveralargerangeofsintering temperatures.

Interestingly,addingLa3+cationsabove100catppmhasno additionaleffect onthe grain size whichremains constantat around0.35mmand0.45mm,at1250◦_C_and₁₂₈₀◦_C,

respec-tively.AsγG,thelightscatteringcoefficientbygrainboundaries,

dependsonlyongrainsize(Eq.(2)),thismeansthatthe differ-encesbetweentheRITs640nmof samplessinteredatthe same

temperatureareduetoeitherporosity(γP)orsecondaryphase

particles(γdop)(Eqs.(1)–(3)).TEMobservationscouldhelpus

clarifythispoint. 3.2.2. TEManalysis

TheatomicnumberofLa(Z=57)ishigherthanthatofAl (Z=13).HAADFobservationscanthusbeperformedto indi-cate the presence of La3+ _cations _within _an _alumina _matrix.

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Fig.10.HAADFobservationsandEDXspectroscopyof(a)AL200,(b)AL310,and(c)AL670samples.ArrowsindicatetheEDXanalysispoint.

Table2

Atomicratiosoftheelementspresentwithinalanthanum secondaryphase particle.

Element Atomicratio(%)

Carbon 1.29±0.06

Oxygen 56.36±0.33

Aluminium 37.53±0.25

Chlorine 2.83±0.07

Lanthanum 1.97±0.18

solubilityisbetween200and310catppmLafora400nmAl2O3

grainsize.

EDXspectroscopywithinasecondaryphaseparticlereveals the presence of lanthanum, aluminium,oxygen andchlorine. Chlorineisduetothelanthanumchloridesaltusedasthe pre-cursorfordoping.Theatomicratiosofthesedifferentelements aregiveninTable2.Forthisestimation,thepeakswerefirst iden-tifiedbeforesubtractingthebackgroundofthespectrum.Then, theCliff–Lorimermethodwasusedtocorrelatetheintensityof thesignalwithelementconcentration.Finally,itwasassumed thatthesummationofallcalculatedconcentrationsisequalto1. Basedontheseresults,thesecondaryphaseissupposedtobea

b-aluminaLaAl11O18structureastheatomicratiosarecloseto

thoseinsuchastructure(3.4%La,36.6%Aland60%O).Itis alsoinagreementwithXRDresultsobtainedonAL670(Fig.5). As for Zr4+ cations, the presence of La3+ cations atgrain boundaries hinders diffusion across these boundaries. More-over,thesolutedrageffectpreventstheirmobility.27From0to 100catppm,thosephenomenareducethegrainboundary mobil-ityallthemoreastheamountofdopingagentincreases.Above 100catppm,grainsizeisconstant(Fig.9).Thissuggeststhat thediffusionacrossgrainboundariesisblockedandthatneither anadditionofsegregatedcationsnortheformationofsecondary phaseparticleswillfurtherinhibitgraingrowth.

Concerningtheinfluenceofmicrostructureonoptical prop-erties when no secondaryphaseparticlesare formed (for the samples AL40,AL100 and AL200),onlythe degree of light scatteringduetograinsizeandporosityhastobeconsidered. Theresultsarethuscomparedwiththetheoreticalmodel devel-opedbyApetz(Eqs.(1)and(2)).Theoreticalcurves(Fig.11) werethereforecalculatedfora100nmporesizeasitisthelargest poresizeobservedbyTEM(Fig.10).

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Fig.11.ComparisonofRITs640nmofAL40,AL100andAL200samplessintered atTf=1250◦Cand1280◦Cwiththeoreticalmodels(λ0=640nm,D=0.88mm, φp=100nm).

Fig. 12.Theoretical curves of RIT640nmversus thediameter of LaAl11O18 particlesfordifferent amountsofdoping agent(λ0=640nm, D=0.88mm, φG=450nm,φp=100nm,p=0.01%).

beobtainedbecausetheporosityisgreater(0.03%).Thestudy of the sintering behaviour of La-doped samples will thus be discussedinthenextsection.

For samples dopedwith a larger amount of La3+ cations (AL310 and AL670), b-alumina LaAl11O18 particles are

observed(Fig.10(bandc)).Duetoitsdifferentrefractiveindex comparedtoalumina(about1.7834and1.76forLaAl11O18and

a-alumina,respectively),theLaAl11O18phasecaninducelight

scattering(Eq.(3)).Forexample,at1280◦_C,_the_RIT

640nmvalue

decreasesfrom50±1%to43±1%betweenAL310andAL670 samples.Thisdecreaseoccurswithoutgraingrowth.The con-tribution of the secondary phase particles to thisdecrease is evaluatedinFig.12.Theoreticalcurveswerecalculatedfora grainsizeof450nmasitcorrespondstothatobtainedfor sam-plessinteredatTf=1280◦C.Theporesizewasfixedat100nm

as itisthelargestporesizeobservedbyTEM.Moreover,the largestsecondaryphaseparticlesobservedbyTEMarearound 100nm forbothsamples. Accordingtothe Apetzmodel, the residualporosityshouldbearound0.01%,whichislowerthan theonefoundonlessdopedsamples(0.03%).Alvarez-Clemares etal.35haveshownthatceriumoxidesecondaryphaseparticles are locatedatgrainboundariesandtriplepointswithinPCA, closingtheporesusuallylocatedatthesepositions.Wecanthus assumeanequivalentbehaviour ofb-alumina particleswhich

Fig.13.Densificationrateoffreeze-driedaluminapowders,pureordopedwith differentamountsofLa3+_cations.

werealsolocated attriplepoints. That iswhytheporosityis lowerforsampleshavingsecondaryphaseparticles.

3.2.3. Sintering

Freeze-driedpowdersdopedwithdifferentamountsofLa3+ cationsweresinteredbySPSatTf=1280◦C.Thedensification

ratesofthedifferentpowdersareshowninFig.13.

AscanbeseenfromFig.13,thedensificationbehaviourof slightlydopedsamples(below100catppm)issimilartothatof undopedsamples.Wecanthusinitiallyassumethattheamount ofLa3+cationsislowerthantheirbulksolubilitylimit(no seg-regatedcationsalongthegrainboundaries)sincedensification iscontrolledbygrainboundarydiffusionmechanisms. Never-theless,Fig.9 showsthatthe grainsizeisalreadydecreasing fordopingamountsas lowas 40catppm, suggestingthat the dopingagentisalreadysegregatedatthegrainboundariesand isactiveinlimitinggraingrowth.Therefore,itisasthoughthe lanthanumbulksolubilitylimit isbelow40catppm,as calcu-latedbyGalmarini,4evenifbelow100catppmsegregatedLa3+ cationshavenodetectableeffectsonthedensificationbehaviour of alumina samples. Above 100catppm, the densification of La-dopedsamplesisdelayedathighertemperatures.The densi-ficationofAPstartsat850◦_C,_compared_to_940,₉₉₀_and₁₀₁₀◦_C

forAL200,AL310,andAL670,respectively.Thehigher densi-ficationratesareobtainedat1050,1070and1090◦_C_for_AL200,

AL310andAL670comparedto1000◦Cforpurealumina sam-ples (AP). This is due to the larger amount of La3+ cations alonggrain boundariesor attriplepointsthat hinder the dif-fusionmechanismsresponsiblefordensification.However,too manysegregatedLa3+_cations_may_hinder_porosity_reduction_at

theendofsinteringthusexplainingwhyAL40andAL100have asimilardensificationrateprofilebuttheporosityaftersintering isfoundtobegreaterforAL100samples(Fig.11).

3.3. OptimisedZrandLadopingconditions

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inthegrainsizewithoutsecondaryphaseformation.Moreover, samplesdopedwithdifferentagentsofdifferentnatureand/or amount couldlead tosimilar RIT640nm valuesproviding that

thesinteringcycleisadapted.Forexample,AZ280andAL40 bothleadtoRIT640nm∼44±1%buttheir sintering

tempera-tures are different (Tf=1300◦C and 1280◦Cfor AZ280 and

AL40,respectively).

Finally,the optimisedcationamountsfor transparencyare foundto be150 and200catppmfor Zr-and La-doped sam-ples,respectively.ThehighestRIT640nmof53±1%obtainedin

thisstudyforAL200samplessinteredatTf=1250◦Cis

equiva-lenttotheoneofStuerfor225catppmLa-dopedalumina7also obtained with freeze-dried powders. Nevertheless, this value also correspondsto theone obtained inourprevious study24 onthegreenbodyprocessingoptimisationforpurea-alumina beforesinteringbySPS.Thus,combiningthebeneficialeffects ofthegreenbodyparticlepackingoptimisationandtheaddition ofthegoodamountofgraingrowthinhibitorinafineralumina powderwillagainimprovetheopticalpropertiesofa-alumina sinteredbySPSupto71% RIT640nmasdemonstratedinRef.

33.

4. Conclusions

Theeffectofamountofdopingagentonmicrostructureand opticalpropertiesofZr-andLa-dopedaluminahasbeen inves-tigated.BothZr4+andLa3+aregraingrowthinhibitorsandthus enabletheopticalpropertiesofsparkplasmasinteredalumina tobeincreased.Moreover,bothdopingcationsareinsoluble(or nearly) inthe a-alumina latticeand disturbthe grain bound-arydiffusion.Densificationisthusdelayedandoccursathigher temperatures.

Zr4+ cations segregateat grainboundaries for amountsof dopingagentashighas280catppmwithagrainsizeof450nm. At 570catppm, a few particles of monoclinic zirconia are formed.TheoptimisedamountofZr4+ cationstoobtain trans-parentPCAisaround150catppmevenifthegrainsizeisnot thefinest,becausenosecondaryphaseparticlesareformedand thedensificationiseasierthanforhigherdopingamounts(slight delaytowardshighertemperatures).AmaximumRIT640nmof

48±1%isfoundforsinteringatTf=1300◦C.

La3+cationsfirstsegregateatgrainboundariesbefore form-ingb-aluminaLaAl11O18particlesat310catppmLaforagrain

sizeof 450nm.Nevertheless, 100catppmofsegregated La3+

cations are sufficient toblock thediffusion pathacross grain boundaries.Asaresult,thegrainsizeofLa-dopedsamplesis constantforhigherthan100catppmLa3+.

Forbothdopingagents,thereduction inopticalproperties (transparency)isattributedtoanincreaseintheporositywhen nosecondaryphaseparticlesareformed.Aftertheformationof asecondaryphase,thoseparticlescontributetolightscattering; themoreparticlestherearethemorescatteringisobserved.

Finally,thehighestRIT640nmof53±1%isobtainedwhen

dopingwith200catppmofLa,thoughitcanstillbeimproved byoptimisingthegreenbodyprocessing.33

Acknowledgements

ThisworkispartoftheCeraTRANSproject.Wewouldliketo thanktheANR(FrenchNationalResearchAgency)forits finan-cialsupport.WewouldalsoliketothankSandrineTrombertand LionelBonneaufromBaïkowskifortheirhelpinsample char-acterisation.OurfinalthanksgototheCLYM(CentreLyonnais deMicroscopie:www.clym.fr)–supportedbytheCNRS,the “GrandLyon”andtheRhône-AlpesRegion–foruseoftheJEOL 2010Fmicroscope.

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