Control of the γ-alumina to α-alumina phase transformation for an optimized alumina densification

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w w w . e l s e v i e r . e s / b s e c v

Control

of

the

␥-alumina

to

␣-alumina

phase

transformation

for

an

optimized

alumina

densiﬁcation

Samir

Lamouri

a,∗

_,

_Mohamed

_Hamidouche

b

_,

_Noureddine

_Bouaouadja

a

_,

Houcine

Belhouchet

a,c

_,

_Vincent

_Garnier

d

_,

_Gilbert

_Fantozzi

d

_,

_Jean

_Franc¸ois

_Trelkat

e

a_Non-Metallic_Materials_Laboratory,_Optics_and_Precision_Mechanics_Institute,_Ferhat_Abbas_University_Setif_1,_Setif,_Algeria b_Emerging_Materials_Research_Unit,_Ferhat_Abbas_University_Setif_1,_Setif,_Algeria

c_Physics_Department,_Faculty_of_Sciences,_University_Mohamed_Boudiaf_of_M’sila,_M’sila,_Algeria

d_Laboratory_MATEIS_(UMR_CNRS_5510),_Bat._Blaise_PASCAL,_INSA_{Villeurbanne,}_{Villeurbanne,}_Lyon,_France e_Valenciennes_University,_LMCPA_Boulevard_Général_de_Gaulle,_Maubeuge,_France

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received26May2016 Accepted17October2016 Availableonlinexxx Keywords: Transitionalumina Phasetransformation Densiﬁcation Microstructure

a

b

s

t

r

a

c

t

Inthiswork,westudiedtheaptitudetosinteringgreenbodiesusing␥-Al2O3transition

alu-minaasrawpowder.Wefocusedontheinﬂuenceoftheheatingrateondensiﬁcationand microstructuralevolution.Phasetransformationsfromtransitionalumina␥→␦→␪→ ␣-Al2O3werestudiedbyinsituX-raysdiffractionfromtheambientto1200◦C.XRDpatterns

revealedcoexistenceofvariousphasetransformationsduringtheheatingcycle.DTAand dilatometry resultsshowedthatlowheatingrateleadstoasigniﬁcantreductionofthe temperatureofthe␣-Al2O3aluminaformation.Around1190,1217and1240◦Cwerefound

when using5,10 and20◦C/minofheatingrate,respectively.Theactivationenergyfor ␪-Al2O3→␣-Al2O3transformationcalculatedbyKissingerandJMAequationsusing

dilatom-etrymethodwere464.29and488.79kJ/mol,respectivelyandbyDTAmethodwere450.72and 475.49kJ/mol,respectively.Inaddition,thesinteringofthegreenbodieswithlowheating ratepromotestherearrangementofthegrainsduring␪-Al2O3→␣-Al2O3transformation,

enhancing therelativedensityto95%andpreventingthedevelopmentofavermicular structure.

Control

de

la

transformación

de

fase

gamma-alúmina

a

␣-alúmina

para

una

densiﬁcación

óptima

Palabrasclave: Alúminadetransición Transformacióndefase Densiﬁcación Microestructura

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s

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n

Enestetrabajo,sehaestudiadolacapacidaddesinterizacióndemuestrasenverdeapartir de␥-Al2O3detransiciónenformadepolvo.Eltrabajosehafocalizadoenlainﬂuenciade

lavelocidaddecalentamientosobreladensiﬁcaciónylaevoluciónmicroestructural.Las transformacionesdefasedealúminas detransición␥→␦→␪→␣-Al2O3 sehanestudiado

insitumedianteDifraccióndeRayosX(DRX)desdetemperaturaambientehasta1.200◦C.

∗ _{Corresponding}_author.

E-mailaddress:samirlamouri@univ-setif.dz(S.Lamouri).

http://dx.doi.org/10.1016/j.bsecv.2016.10.001

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LosdiagramasdeXRDhanreveladolacoexistenciadediversastransformacionesdefase duranteelciclodecalentamiento.LosAnálisisTérmicosDiferenciales(ATD)realizadosy losdatosdedilatometríahanmostradoquevelocidadesdecalentamientobajasconducen a unareducciónsigniﬁcativadelatemperaturadeformaciónde␣-Al2O3.Detectándose

alrededorde1.190,1.217y1.240◦Ccuandoseutilizan5,10y20◦C/mincomovelocidad de calentamiento,respectivamente. LaEnergíadeActivación parala transformación ␪-Al2O3→␣-Al2O3 calculadamediantelasecuacionesdeKissingery JMAusandométodos

dilatometricoshansido464,29y488,79kJ/mol,respectivamente,ymedianteATD450,72 y475,49kJ/mol,respectivamente.Además,lasinterizaciónconvelocidaddecalentamiento bajapromuevelareorganizacióndelosgranosdurantelatransformación␪-Al2O3→␣-Al2O3,

elaumentodeladensidadrelativaal95%ylaprevencióndeldesarrollodeunaestructura vermicular.

Introduction

Duetoitsinterestingphysico-chemicalandmechanical prop-erties,aluminaisoneofthemostusedtechnicalceramicsas structureparts,appliedinmedicaluse(orthopaedicimplants) and electronics[1–5]. To obtain densealumina products, a perfectprocessingcontrolfrompowdersynthesistosintering

[6,7]isrequired.Gibbsite(Al(OH)3)andboehmite(AlOOH)are themostusedprecursorsforthepreparationof␣-Al2O3. Dur-ingtheheattreatment,aluminiumhydroxidestransforminto transitionaluminaintheformofmetastablestructurebefore endingas␣-Al2O3thermodynamicstablephase(Fig.1)[8,9]. Boehmitetransformsinto␥-Al2O3transitionaluminaundera temperaturerangeof500–550◦Cwithadepartureofstructural water[10].Thetransformationof␥-Al2O3monoclinicphase (d=3.56gcm−3)to␣-Al2O3hexagonalphase(d=3.98gcm−3) isaccompaniedbyavolumereductionofabout10%causinga largedensityincrease[11–14].Thetransformationof␪-Al2O3 transitionaluminato␣-Al2O3takesplaceinthetemperature range 1050–1200◦C. This transformation occurs through a nucleationandgrainsgrowthmechanism[15–18]andis influ-encedbyseveralparameterssuchasgrainsize[19],chemical composition[20]andheatingrate[21].The␪-Al2O3→␣-Al2O3 transformation isvery significant for the sintering process and consequently for the control of microstructure [22]. Accordingtothesinteringconditions,theformationof crys-talline␣-Al2O3canproduceavermicularmicrostructureform consistingofawidepores network,whichdevelopsduring the␣-Al2O3transformation[23,24].Inaddition,itisdifficult to study with precision the transformation of transition aluminaphases duetotheir instability and highreactivity

[21]. Different studies showed that a high energy milling influencesthephasetransformationfromtransitionalumina to␣-Al2O3 stablephase [25–29]. Specific diagrams defining the pathway of phase transformations were established accordingtotheheatingtemperatureandparticlegrainsize

[10,30].Somestudieswereconductedusingcoarseandﬁne grained gibbsite in order to examine the transformation pathsusinginsituX-raysdiffraction[19].Othersconcernthe gibbsiteandboehmiteto␣-Al2O3phasetransformationcases. Theynoticedthatsmall␣-Al2O3crystallitesofsphericalform are obtained by boehmite transformation and larger ones in plate form from gibbsite [31]. The raw powder and the

millingmodearethereforefundamentallyimportantforthe sinteringprocessandthedensiﬁcation.Manytechniquesare usedtoimprovethedensiﬁcationprocesslike␣-Al2O3seeds

[32,33]andthealuminadopingagents[34].

Inthis work weused␥-Al2O3transitionaluminaasraw powdertostudythedensiﬁcationprocessandthe microstruc-ture of sintered samples using various heating rates. The ␥-Al2O3to␣-Al2O3transformationwascharacterizedinsitu byXRD,andtheactivationenergyaccompanyingthe␪-Al2O3 to␣-Al2O3transformationwasevaluated.

Experimental

procedures

Theusedrawpowderis␥-Al2O3transitionaluminaobtained byboehmitecalcinationsat600◦C(boehmitebeingobtained by partial dehydration of gibbsite at 450◦C). The chem-ical composition expressed in weight % is given as follows: Al2O3(88.34%), SiO2(3.86%), F(0.89%), Na2O(0.34%), CaO2(0.18%), Fe2O3(0.096%) and structural water(6%). The powderwasattritionmilledduring3husingaluminaballsof 2mmdiameter.A0.25%massratioofDARVANCasa disper-santand1%ofduramaxasabonderwereadded.Duringthe millingprocess,thepHwasadjustedat10.4[35].Aftermilling, thesuspensionswerecompletelydriedat110◦C.Thepowder wassievedtoobtainaggregatessmallerthan45␮m.Inorder tohighlightthephasetransformations,thethermalbehaviour ofthepowderwasstudiedusingananalyserSETERAMTGA92 allowingadifferentialthermalanalysis(DTA).Thermalcycles wererecordedfromambientupto1600◦Cwiththreeheating rates(5,10and20◦C/min).Apowdermassofnearly40mgwas usedinallcases.TheanalysisbyX-raysdiffractionwas car-riedoutusingBrukerD8Advancediffractometerworkingat hightemperatures.Greensamplesthermalshrinkingwasalso studiedbetweenroomtemperatureand1300◦C.Dilatometry testswerecarriedoutusingaNETZSCHDIL402Cdilatometer. Thegreencompactswereinitiallyshapedbyuniaxialpressing thenbycoldisostaticpressingat300MPa.Thesampleswere sinteredat1700◦Cfor2h.Theﬁnaldensitiesweredetermined byArchimedes’smethodusingdistilledwater.TheSEM micro-graphswereobtainedusingaJOEL840Ascanningelectronic microscope.A20kVvoltageandaworkingdistancebetween 10and15mmwereused.Thegoldmetalizedpolishedsurfaces wereobservedafterathermaltreatmentat1600◦Cduring1h.

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100

Gibbsite Chi Kappa

Theta Theta 1400 K 1200 1000 800 600 400 Delta Gamma Boehmite Boehmite Bayerite RHO Diaspore Eta Gel. Alpha Alpha Alpha Alpha 300 500 700 900 1100 ºC

Fig.1– Transformationsequencesofgibbsite(Al(OH)3)to␣-A12O3[10].

Fig.2–SEMmicrographsofboehmitepowderbeforemilling.

Results

and

discussions

Rawpowder

Themorphologyoftheusedrawboehmiteisillustrated in

Fig.2.Itconsistsofagglomeratesofaround75␮m.Theraw powderwasattritionmilledreducingitsmeandiameter(D50) downto0.29␮m(Fig.3).Theabsolutedensityofthemilled powdermeasuredwithheliumpycnometer was3.08g/cm3.

0.4 0.6 0.8 1.0 0.2 Size (μm) 0.0 –2 0 2 4 6 Volume, % 8 10 12 14

Fig.3–Particlesizedistributionforboehmiteafter3h

attritionmilling.

Toobtain the ␥-Al2O3 transitionalumina phase,the milled boehmitewasheatedat600◦Cinordertocompletely elim-inatethestructuralwateraccordingtoestablishedmethods

[30].

Differentialthermalanalysis

Thethermalbehaviour(DTA) ofthe␥-Al2O3 transition alu-minapowder,fordifferentheatingrates,isillustratedinFig.4.

0 200 400 7 6 5 4 3 2 1 Heat flov ( μ V) 0 –1 –2 600 800 Temperature (ºC) 5ºC/min D A B C E 10ºC/min 20ºC/min 1000 1200 1400 1600 1800

Fig.4–DTAcurvesofthe␥-Al2O3rawpowderupto1600◦C

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Duringtheheatingcyclebetweenambientand1600◦C,the curvesrevealedthat:

- For a 5◦C/min heatingrate,the pointsA, Band C,three exothermic peaks of transition alumina transformation phases are observed. The ﬁrst peak at around 800◦C is relatedtothetransformationof␥-Al2O3tothe␦-Al2O3,the second ataround1080◦Cisduetothe␦-Al2O3→␪-Al2O3 transformationandﬁnallythethirdpeakat1150◦Ctothe ␪-Al2O3→␣-Al2O3phasetransformation.

- For 10and 20◦C/minheatingrates,onlythe␪-Al2O3→ ␣-Al2O3 phase transformation appeared. They respectively startat1217◦C(PointE)and1240◦C(pointF).Ourresults correspondtoWefferandMisradiagram[10].Ashiftinthe ␪-Al2O3→␣-Al2O3transformationpeakforthethree heat-ingratesisalsoobserved.Thelowestheatingratereduces thetemperatureof␣-Al2O3formation.Thetransformation seemsmoreprogressive,withalargerandlessintensepeak thanforhigherheatingrates.

Dilatometry

Theshrinkagecurvesandtheirderivativesofthepressed sam-plesareshownon(Fig.5).Theyindicatetwonotablevariations alongtheheatingcycle.Thesamplesshrinkagewasdisrupted byasmallexpansionindicatingthe␪-Al2O3→␣-Al2O3phase transformationat1056,1082and1140◦Cfortheheatingrate of 5, 10 and 20◦C/min respectively. The lowest heating ratepromotesamaximumtransformationrateat1150◦C.This typeofexpansionwasobservedbydifferentauthorsunder theinﬂuenceofalphaaluminadopingseeds,sample com-paction pressure rates and dopingagents [8,34]. From the crystallographicpointofview,the ␪-Al2O3→␣-Al2O3 phase transformation occursby crystallographicstructurechange (oxygen anion moving positions) from cubic face centred towardsacompacthexagonalstructure.

Thistransformationtakesplacebynucleationandgrowth mechanisms.Itoccurs withhighactivationenergy depend-ingontheheatingrate[16–36].Insuchtransformation,the activationenergyismostlyused inthenucleationprocess. This induces the temperatures shifts at the beginning of the ␪-Al2O3→␣-Al2O3 phase transformation caused bythe crystallographicreorganizationsmechanisms[37].Inorderto completeandevaluatethesinteringprocess,adilatometric cycle of ␥-Al2O3 transition aluminaof pressed green sam-plefromambientto1700◦Cwitha5◦C/minheatingratewas made(Fig.6).Thistestenablesustomakeashrinkageprocess study.Novolumevariationwasobservedatthebeginningof sinteringprocess.Thiscanbeexplainedbythefactthatthe transitionaluminaphasetransformationsarecharacterized bystructural changes without shrinkage. The␪-Al2O3→ ␣-Al2O3 phasetransformationinitiatesat1050◦Candreaches itsmaximumat1192◦C.

DRXinsitu

Fig. 7 shows the XRD patterns carried in situ from ambi-entto1200◦Conboehmiteattritionmilledrawpowder.The choiceofthismonohydratecomesfromthefactthat␥-Al2O3 transitionaluminaisproducedfromthedehydrationofﬁne

1 2 0 200 600 800 Temperature (ºC)

a

b

Temperature (ºC) Derivatives (Δ L/Lo)/% 5ºC 10ºC 20ºC 400 1000 1200 1400 –1 –2 –3 –4 –5 –0.1 0.00 –0.01 –0.02 –0.03 –0.04 0 200 400 600 800 1000 1200 1400

Fig.5–Dilatometrycurves(a)andtheirderivatives(b)

ofthe␥-Al2O3transitionaluminasamplesheatedup

to1300◦Cfordifferentheatingrates.

boehmite[10].TheXRDdatawerecollectedonaBrukerD8 Advancediffractometerwithangularsweepingfrom2=42◦ until2=50◦at0.1◦/min.Thephaseswereidentiﬁedby com-parisonwiththeJCPDStables(␥-Al2O3:ICDD50-741,␦-Al2O3: ICDD16-394,␪-Al2O3:ICDD23-1009,␣-Al2O3:ICDD46-1212). Fromtheambientto450◦C,thediffractogramsshowsthatthe

–2.5 –2 –1.5 –1 –0.5 0 0 200 400 600 800 1000 Temperature (ºC) Δ L/L o (mm) 1200 1400 1600 1800

Fig.6–Dilatometrycurveofthe␥-Al2O3transitionalumina

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2θ(º)

B

3000 2000 1000 1200 1000 800 600 400 200 0

B

42 43 44 45 46 47 48 49 50 Coups (u.a) Temperatur e (ºC )

γ

θ

δ

α

δ

_δ

Fig.7–XRDinsitupatternsoftherawboehmiteattritionmilledshowingtransformationfromambientto1200◦C.

observedpeakscorrespondtocrystallineboehmite.Boehmite iscompletelytransformedto␥-Al2O3inthetemperaturerange 550to850◦C.The␥-Al2O3and␦-Al2O3phasescoexistupto 950◦C.Beyondthistemperature, the␪-Al2O3starts toform progressively. Thespectraof those “pseudo-amorphous” ␦-Al2O3and␪-Al2O3phasesdecreasegraduallywithformation ofthe␣-Al2O3stablephase.Beyond1200◦C,onlythe␣-Al2O3 isobserved.

Activationenergy

DilatometryandDTAtestswereundertakentoevaluatethe activation energyof␪-Al2O3→␣-Al2O3 transformation.The activation energy (Ea)values were obtained from transfor-mation temperature peak measurements [20,38] using the threeheatingrates(Fig.8).Theactivationenergyvaluesfor ␪-Al2O3→␣-Al2O3transformationwereestimatedbyKissinger and(JMA)methods[39]usingthe followingEqs.(1)and(2), accordingtothepeaktemperatureTpatthedifferentheating rates: ln

T2 p

=− Ea RTp+Cst (1) log[−ln(1−x)]=−mlog−₂_.3031 nEa RTp+Cst (2) where‘’istheheatingrate(K/s),Tpisthetemperature corre-spondingtothemaximumof␪→␣-Al2O3transformationpeak (K),Eaistheactivationenergyofthe␪→␣-Al2O3 transforma-tion(kJ/mol)andRistheidealgasconstant(8.314J/molK).

Therelationcharacterizingthevariablex:

x=A_AT 0,

Table1–Avramiparameter(n)valuesatdifferent

heatingrates.

Heatingrate(◦C/min) n t0.75/t0.25

5 1.83 1.1.

10 2.25 1.08

20 2.73 1.05

whereATistheareaoftheexothermicpeakintheDTAcurve

attemperatureTandA0istheareaunderthepeak.

The plots obtainedin Figs. 9 and 10, (ln(/Tp2) vs 1/Tp) and(ln()vs1/Tp)respectively,arecharacterizedbystraight lineswithdifferentslopesEa atdifferentheatingrates.The obtainedactivation energyvalue bydilatometrywas about (464kJ/mol). The oneobtained byDTA isabout 450kJ/mol. TheseclosevaluesremainlowerthanthosereportedbyMa. KrellandWang(517kJ/mol)[38,39].Accordingtotheseresults, the ␥-Al2O3 transitionalumina rawpowderattrition milled contributestoloweringactivationenergyfortheformationof thestable␣aluminaphase.Ontheotherhand,thevalueofthe exhibitorAvramiparameter[40,41],whichreﬂectsthe trans-formationratebygerminationandgrowth,isdeﬁnedby(Eq.

(3))fromtheendothermicshapeofcrystallizationobtainedby DTA.Themorphologyofthecrystalgrowthcanbeobtainedat thetransformationratios(75%and25%)[42].

n= 2.5 T

T2 p

Ea/R (3)

whereTisthewidthofthecrystallizationpeakathalf max-imumvalue.ThevaluesoftheAvramiconstantsarelistedin

Table1.Theyincreasefrom1.83to2.73withincreasingheating rate.Theaveragevaluesoft0.75/t0.25foreachheatingrateare alsolistedinTable1.Thesevaluesareveryclosesuggesting

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Shr inkage speed r ate Heat flo w (mV) Tp =145 327

a

b

Tp =15 189 149 763 146 579 14 832

5ºC/min 10ºC/min 20ºC/min

150 415 1073 1433 1473 1513 1573 1273 Temperature (K) Temperature (ºK) 1473 1673

Fig.8–␪to␣-Al2O3maximumtransformationpeak

ofdilatometryderivativecurves(a)andDTAcurves(b)

atvariousheatingrates.

thatthegrowthmorphologyisthesameatthedifferent heat-ingrates.

Microstructure

TheSEMmicrographsofgreen bodies’sintered samplesat 1700◦CareshowninFig.10.The␥-Al2O3samplesheatedwith aheatingrate20◦C/minhadaporousstructurewith85%of relativedensityand1.6␮mgrainmeansize(Fig.10a).Forthe sampleﬁredwitha10◦C/minheatingrate,amicrostructure

y=–5.587x+21.36

a

b

R²_=0.986 Ea=464.29 Kj/mol –17.2 –17 –16.8 –16.6 –16.4 –16.2 –16 –15.8 –15.6 –17.2 –17 –16.8 –16.6 –16.4 –16.2 –16 –15.8 –15.6 6.6 6.65 6.7 6.75 6.8 6.85 6.9 Ln( Φ /Tp 2) 1/Tp (10–4_{) [°K}–1_] y=–5.423x+19.9 R²_{= 0}_,983 Ea=450.72 Kj/mol 6.55 6.6 6.65 6.7 6.75 6.8 6.85 Ln( Φ /Tp 2) 1/Tp (10–4_{) [°K}–1_]

Fig.9–Plotsofln(/Tp2)versus1/Tp(a)bydilatometry

curves,(b)byDTAcurves.

y=–5,882x+37.95

a

b

R²=0.987 Ea=488.79 Kj/mol –3 –2.5 –2 –1.5 –1 –0.5 0 6.6 6.65 6.7 6.75 6.8 6.85 6.9 1/Tp (10–4) [K] Ln( Φ ) y=–5.722x + 36.51 R²_{= 0}_,984 Ea=475,49 Kj/mol –3 –2.5 –2 –1.5 –1 –0.5 0 6.55 6.6 6.65 6.7 6.75 6.8 6.85 1/Tp (10–4_)[K] Ln (Φ )

Fig.10–Plotsoflnversus1/T_p(a)bydilatometrycurves,

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improvementwas noticed. The grain boundaries are quite apparentwithimportantintergranularporosity.Therelative densityis90%andthemeangrainssizeis1.54␮m(Fig.10b). Ontheotherhand,thesamplesheatedat5◦C/minhadadense andhomogeneousmicrostructurewithlowresidualporosity and95%relativedensityand3.2␮mmeangrainsize(Fig.10c). Theobservedhighdensityandhomogeneouscrystalstructure withclearlyapparentgrainboundariesareacausedbyawell establishedparticlesrearrangementduringthe␪-Al2O3→ ␣-Al2O3transformationprocess(Fig.11).Theresidualporosityin intra-granularpartswasdifﬁculttoeliminate[19].Otherwise,

Fig.11–SEMimagesof␥-Al2O3greenbodiesﬁredat

1700◦Cwithvariousheatingrates:20◦C/min(a),10◦C/min

(b),and5◦C/min(c).

Fig.12–SEMimagesshowinggrainsboundariesof␥-Al2O3

samplessinteredat1700◦Cwith5◦C/minheatingrate.

the presenceofimpurities inthe startingalumina powder couldbeattheoriginofliquidsilicatesformation.Generally, this glassy phase islocated ingrain boundariesand triple points.Theformationoftheamorphousphasefacilitates alu-minadensiﬁcationduringsintering(Fig.12).

Conclusion

Theinﬂuence ofthe heatingrateon the densiﬁcation and microstructuresofgreenbodiesusing␥-Al2O3rawpowderwas studiedduringsintering.Withalowheatingrate(5◦C/min), theDTAcurvesshoweda␥-Al2O3→␣-Al2O3phase transfor-mations. For higher heating rates (10 and 20◦C/min), only the ␪-Al2O3→␣-Al2O3 phase transformation was detected. Thedilatometrycurvesshowedthatthetemperatureatthe beginning ofthe ␪ to ␣ alumina transformation decreases with decreasing heating rates. The insitu XRD resultsfor the boehmiteraw powderusedshowed coexistenceof dif-ferenttransitionaluminaduringthephasetransformations. Thediffractionpatternsinthetemperaturerange(550–850◦C) correspondtothe␥-Al2O3phase.At950◦C,thisphaseisstill present and coexists with the ␦-Al2O3 phase. Beyond this temperature,weobservedthe␪-Al2O3→␣-Al2O3phase trans-formation.Theactivationenergyforthistransformationwas evaluatedusingKissingerequationfromtheresultsobtained by both DTAand dilatometrytechniques. TheSEM micro-graphsshowedthatsamplesheatedata5◦C/minratehada highrelativedensity(95%)andhomogeneousmicrostructure withlowresidualporosity.

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