ARTICLE IN PRESS

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Pleasecitethisarticleinpressas:S.Lamnini,etal.,Theroleoftheattritionmillingonthegrainsizeanddistributionofthecarbonnanotubes w w w . e l s e v i e r . e s / b s e c v

The role of the attrition milling on the grain size and distribution of the carbon nanotubes

in YSZ powders

Soukaina Lamnini

^a^,^b

, Zsolt Fogarassy

^b

, Zsolt Endre Horváth

^b

, Sára Tóth

^c

, Katalin Balázsi

^b^,∗

, Csaba Balázsi

^b

aDoctoralSchoolofMaterialScienceandTechnologies,ÓbudaUniversity,Bécsistr.96/B,1034Budapest,Hungary

bInstituteforTechnicalPhysicsandMaterialsScience,CentreforEnergyResearch,HungarianAcademyofSciences,Konkoly–ThegeM.

str.29-33,1121Budapest,Hungary

cWignerResearchCentreforPhysics,HungarianAcademyofSciences,Konkoly–ThegeM.str.29-33,1121Budapest,Hungary

a r t i c l e i n f o

Articlehistory:

Received14May2018 Accepted17October2018 Availableonlinexxx

Keywords:

MWCNT Milling

Yttria-stabilizedzirconia

a bs t r a c t

Theaimofpresentworkwastheexaminationoftheroleofthehighefﬁcientattrition millingonthestructureandgrainsizeofZrO2–8mol.%Y2O3(YSZ)andonthedistribution ofmultiwallcarbonnanotubes(MWCNTs)inthesepowdermixtures.Themicrostructureof YSZ/MWCNTspowdermixtureswith1,5,10wt.%MWCNTswasinvestigated.Detailedstudy conﬁrmedtheYSZgrainsizedecreaseandsimultaneouslythenon-destructivehomoge- nizationofMWCNTsintheceramicpowdermixtures.Thebesthomogenizationdegreewas achievedinthecaseofYSZ/1wt.%MWCNTpowder.Ramanscatteringmeasurementswere performedtoinvestigatetheatomicbondingandstructuralintegrityofcarbonnanotubes.

ThepresenceoftheGandDbandsineachsamplesat∼1590and∼1355cm⁻¹conﬁrmedthe unaffectedstructuralintegrityofMWCNTsafterthemillingprocess.InadditiontoRaman measurementsthehighresolutiontransmissionelectronmicroscopy(HRTEM)studieshave shownthatthestructureofMWCNTsremainsintactaftermilling.

Elroldelamoliendaporatricióneneltama ˜nodegranoyladistribución denanotubosdecarbonoenlospolvosYSZ

Palabrasclave:

MWCNT Fresado

Zirconiaestabilizadaconitria

r e su m e n

El propósitodeeste trabajo fuela evaluacióndelroldelamolienda poratriciónenla estructuraytama ñodegranodeZrO2–8mol.%Y2O3(YSZ)yladistribucióndetubosdecar- bonomultipared(MWCNTs)enlasmencionadasmesclasmixtaspulverizadas.Seinvestigó lamicroestructuradelospolvosYSZ/MWCNT.Elestudiodetalladoconfirmóladisminu- cióndeltama ñodegranoYSZy,simultáneamente,lahomogeneizaciónnodestructivade

∗ Correspondingauthor.

E-mailaddress:balazsi.katalin@energia.mta.hu(K.Balázsi).

https://doi.org/10.1016/j.bsecv.2018.10.001

creativecommons.org/licenses/by-nc-nd/4.0/).

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Pleasecitethisarticleinpressas:S.Lamnini,etal.,Theroleoftheattritionmillingonthegrainsizeanddistributionofthecarbonnanotubes MWCNTenlasmezclascerámicaspulverizadas.Elmejorgradodehomogenizaciónfue alcanzadoenelcasodeYSZal1%enpesodeMWCNT.Serealizaronmedicionesdedisper- siónRamanparainvestigarlauniónatómicaylaintegridadestructuraldelosnanotubos decarbono.LapresenciadelasbandasGyDencadamuestraa∼1590cm-1y∼1355cm-1, conﬁrmóquelaintegridadestructuraldelosMWCNT,nofueafectadadespuésdelproceso demolienda.AdemásdelasmedicionesdeRaman,losestudiosdemicroscopíaelectrónica detransmisióndealtaresolución(HRTEM)demostraronquelaestructuradelosMWCNT permaneceintactadespuésdelamolienda.

Introduction

Investigationofadvancedceramics,namelyzirconiaandtheir compositesbecameanimportanttaskfortremendousscien- tificresearchdealingwithenergyproduction,conversionand storage.Theincorporationofanadditivephaseintozirconia matrixappearsasaverypromisingapproach,especiallyfor enhancingthethermo-mechanicalandtheelectricalproper- tiesofthecomposites[1].Therefore,advancedceramicsare widelyexploredforenergyapplicationsanddifferentceramic componentsareappliedtosolidoxidefuelcells,supercapac- itors,hydrogenstoragesystemsandphotovoltaicsolarcells [2]. Fuel cells are classified according totwo different cat- egories. The first one depends on the nature of the fuels usedforpowergeneration[3]suchashydrogen[4],methanol [5],fossilfuels[4]orbiomass-derivedmaterials[6].Thesec- ond category includes the composition of the catalysts to speeduptheelectrochemicalprocessesregardingthediffer- ent possibleworking temperature ofthe cell [7]. Thesolid oxidefuelcells (SOFC)operating athightemperature from 800^◦Cto1000^◦Careoneofthebestefficientpowergeneration devices devoted toproduce clean energywithout environ- mentaldamages [8].Yttria-stabilized zirconia(YSZ)isused tobethe most frequentlyelectrolytematerial forSOFC[9]

toconvertthechemicalenergyofthereactantsdirectlyinto electricity and heat. The output of the reaction is water.

Thehigh temperature can cause many problemsin terms ofanode-electrolyte–cathodedegradationandlifetimeofthe cells [10]. The main challenge is the decrease of the high workingtemperature of fuelcells. Liuet al.identiﬁed and analyzedthemainissuesresponsibleforcathodedegradation inSOFC[11].Themicrostructuralchangesattheinterfaceof LSM((LaxSr_1−x)yMnO3)/YSZcathodeandYSZelectrolyteintro- ducedareductionoftheLSMcratersandtheformationofnew phasesoftheinsulatingzirconate.Thelackofoxygencon- tentintheairintroducedtothecathodegaswasattributed asamajorfactor responsibleofLSMcraters reduction.Ni- YSZcermetanodesarethemostcommonelectrodeforSOFC operatingathightemperature.ThemetallicNiparticlesare responsibleofelectronconductionproducedfromhydrogen oxidationaccordingtothefollowingreaction[12]:

2H₂+2O²⁻→2H₂O+4e⁻ (1)

ThehighporositydensityisrecommendedforNi-YSZcer- metanodestoensuretherequiredperformanceofthecells.

Indeed,TriplePhase Boundary(TPB)isthepoint whereNi, YSZ, and the poremeet and participateinelectrochemical reaction[13].Theproducedwatermoleculeistransferredout- sideoftheanodethroughthepores.Ni-YSZanodesworkas acatalystusedtoreformthemethane.Therefore,impurities suchascarbondepositionnamedalso“cooking”or“sulphur deposition”remainthemajorissuesresponsibleforthefast anodedeterioration.Thepresenceofsulphurandintensive sulphurdepositioncontributedtothedelaminationofanode layerandblockthenickelgrains.Thisfactcausedthelimi- tationofthehydrogenatom’smovementandthusleadstoa significantdecreaseofthecellefficiency.Chengetal.found thattheformationofsulphide(Ni₃S₂,NiS)canbelimitedbya highcoolingrateabout70^◦C/min[14].Whilethecarbondepo- sitionontheporousanodecanoccurindifferentformssuch as fibres,whiskers orgraphitic carbon causing micro mor- phologicalchangesoftheanoderesultingindeactivationor breakdownofthecatalysts[15].Manyrecentstudiesproved thatthetemperatureofsinteringisanimportantparameter inobtainingasmallgrainedstructureforabetterperformance ofSOFCelectrolyte[16].

Oneoftheadvancedapplicationofceramicsassolarcells resulted inlowthermalemittance and increasedcorrosion resistanceatelevatedtemperaturesbyapplyingoftheaddi- tional layersofzirconium thinﬁlm between substrate and absorber[17].Supercapacitorsaredevelopedonthebasisof zirconiadopedCNTorgraphenecharacterizedbyenhanced mechanical andelectrochemicalproperties. Amiranteetal.

concluded that the capacitance of supercapacitor materi- als isdepending strongly on the carbon nanotubes (CNTs) orientation [18]. Alves et al. studied the new concept ofa ternarycompositeconsistingofZrO2nanoparticles,reduced grapheneoxide(rGO) andinsitupolymerizedPyrrole (PPy).

It was proved that the addition of zirconiainfluenced the morphology and increased the porosity of the electrodes resultinginremarkableenhancementinthecapacitanceof the supercapacitor [19]. On the other hand, a significant improvementin bothcompressive and torsionalresistance hasbeenreported.Differentalloysofmetalhydridesmate- rials have been studied in the literature namely: cerium, lanthanum and nickel (Ce-La-Ni) or magnesium hydride combined with zirconium oxide and single-walled carbon nanotubes. However, this modern method of storage has somelimitationsashighthermalstability,difficultytodecom- pose the hydride system into metal and hydrogen gas or apoorkineticsrateofhydrogenationanddehydrogenation.

El-Eskandaranyetaletal.studiedthedopingeffectofMgH2

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Pleasecitethisarticleinpressas:S.Lamnini,etal.,Theroleoftheattritionmillingonthegrainsizeanddistributionofthecarbonnanotubes nano-powderswith10wt.% ofnanocrystalliteZrNi5 onthe

hydrogenation/dehydrogenationbehaviour[20].Theseresults showed improved properties with a short absorption time of1 and 10min torelease 5.3wt.% ofH2. This study con- ﬁrmedthatthezirconiumtetrachloride(ZrCl₄)asacatalyst intoMgH2exhibitedanotablereductioninthedehydrogena- tion and rehydrogenation temperature. More importantly, ZrCl4 prevented thegrain growth of the hydridefor better cyclicperformance[21].

The aim of our study was the development of YSZ/MWCNTs as nickel free SOFC material. This work presents the mainresults ofﬁrst preparationstep; milling andhomogenization.Thedetailedstructuraldesignofmilled YSZ/MWCNT powder mixtures with various addition of MWCNTsfrom 1to10wt.% was performed. Thestructural andmorphologicalpropertiesofpowdermixtureswereinves- tigatedbyscanningelectronmicroscopy(SEM),transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and Ramanspectroscopy.

Experimental

Preparationofpowdermixtures

ZrO2 –8mol.%Y2O3 (yttria-stabilizedzirconia (YSZ),Sulzer MetcoAMDRY6643) with anaveragegrain size40␮mwas usedasbasematerialforpowdermixturepreparation(Table1, Fig.1a).Commerciallyavailablemultiwallcarbonnanotubes (MWCNT,typeNC3100^TM,Nanocyl)withanaveragediameter 9.5nmandanaveragelength1.5␮mwereaddedtozirconia

Table1–Compositionofreferencesample andYSZ/MWCNTspowders.

Sample YSZ(wt.%) MWCNT(wt.%)

YSZ-100 100 0

YSZ-99 99 1

YSZ-95 95 5

YSZ-90 90 10

powderatdifferentconcentrations(1,5and10wt.%)(Table1, Fig.1b,c).Highefﬁciencyattritormilling(UnionProcess,type 01-HD/HDDM) wasused(4000rpm, 5h)forpowdermixture preparation.Eachbatchwith80gﬁnalproductwasmixedin zirconiatank(750cm³)by130gethanoland280mLzirconia balls(eachof1mmdiameter).Theobtainedpowdermixtures were driedat172^◦C for25minutes, then sievedby100␮m mesh(Fig.1d).

Characterizationmethods

Thephase analysiswas performedbyX-raydiffractometry (XRD)withCuK␣radiationusingaBrukerAXSD8Discover diffractometerequippedwithGöbel-mirrorandascintillation detector.Energydispersive X-rayspectroscopy (EDS,Rontec Si(Li)detectorwithultrathinwindow)wasusedforelemen- talidentiﬁcationofthepowdermixturesafter5hofmilling.

The morphology and the characterization of the powders werestudiedbyScanningElectronMicroscopy(SEM,LEO1540 XB)andTransmissionElectronMicroscopy(TEM,PhilipsCM- 20) and HighResolution Transmission Electron Microscopy (HRTEM, JEOL 3010) operating at 200 and 300kV, respectively.Ramanspectroscopyisawell-known,non-destructive

40 μm

2 μm 20 nm

400 nm

a

c d

b

Fig.1–Morphologyofthestartingmaterialsandpowders.(a)SEMimageofYSZasreceived(SE,MAG500×HV5kV), (b)SEMimageofMWCNTasreceived(SE,MAG10000×HV5kV),(c)TEMimageofMWCNTasreceived(at200kV) and(d)SEMimagemilledYSZafter5h(SE,MAG5000×HV5kV).

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Pleasecitethisarticleinpressas:S.Lamnini,etal.,Theroleoftheattritionmillingonthegrainsizeanddistributionofthecarbonnanotubes

20 40 60

(311)

(220)

(200)

(111)(111)(-111)

80 2θ

Intensity ua.

YSZ - 90

monoclinic zirconia (JCP2:00-037-1484) cubic zirconia (JCP2:00-049-1642) Si

CNT

YSZ - 95

YSZ - 99

YSZ - 100 (reference)

Fig.2–XRDpatternsofYSZreferenceandYSZ/MWCNTpowders.

methodtoinvestigatetheatomicbondingpropertiesandthe microstructure.Renishaw1000Bmicro-Ramanspectrometer attachedtoaLeicaDM/LMmicroscopewasusedtoexamine Ramanspectroscopyatroomtemperatureinthewavenum- berrangeof150–3500cm⁻¹with488nmlaserexcitation.The spectralresolutionofthesystemis2.5cm⁻¹andthediameter oftheexcitationspotis1␮m.

Results and discussion

TheYSZbasedpowdermixtureswithvariousMWCNTaddi- tives(1,5and10wt.%)werepreparedbyattritormilling(Fig.1 andTable1).Theresultofthephaseexaminationoftheref- erenceYSZandmilledpowdermixturesisshowninFig.2.

Themilledpowderswereconsistedmainlyofcubiczirconia (JCP2:00-049-1642) and minor monoclinic zirconia (JCP2:00- 037-1484)phases(Fig.2).Thecubiczirconiaisindicatedby the mainlines appearedat2Â of30.24^◦, 34.88^◦, 50.17^◦ and 59.69^◦.While,theminormonoclinicphasewasobservedat 2Â of28.36^◦ and 31.12^◦.As forﬁrst observation, the milling processdidnotresultedinphasetransformationonlygrain decreaseoccured.YSZreferencebasematerialexhibitedwell dispersedparticleswithsharpandirregularshape(Fig.1).The morphologicalstudyofMWCNTdemonstratedtheiragglom- eratedbundlestructure.Ontheother hand,theadditionof MWCNTstozirconiamatrixintroducedasigniﬁcantchange inthesurfacetopologyandthedistribution ofthe zirconia grains,leadingtovariousformsandshapescomparedtothe basematerial(Fig.3).

The EDS elemental analysis illustrated the features of MWCNTsdistributioninzirconiamatrix (Fig. 4).Thisstudy conﬁrmed that the MWCNTs formed agglomerations with increasingtendencywithMWCNTsaddition increase(YSZ- 99→YSZ-95→YSZ-90).TheMWCNTsagglomerations(C-red, Fig. 4) were surrounded by ultraﬁne rounded zirconium

(Zr-blue).Structuralinvestigationsofzirconiapowderbefore andaftermillingprocesswerecarriedoutbyTEM(Fig.5).The TEMinvestigationdemonstratedthedecreaseofYSZparticle sizeinaverage50␮mto400nm.

StructuralobservationsofthemixedpowdersYSZ-99,YSZ- 95, YSZ-90are shown inFig. 6. Thisresultshows that the small zirconia grains(<100nm)and particles (>200nm) are embeddedintoMWCNTsagglomerationsandnetworks.The agglomeration,thedamageorthedefectofMWCNTshasbeen reportedasabigissueresponsibleofasignificantlackinthe reinforcementofceramiccompositeswithMWCNT.Fromthe pointofMWCNTsstructurethenon-destructivemillingpro- cesswasconfirmedbyHRTEMstudy.TheMWCNTsappeared asafinefibresformingseveralagglomerationswithanapprox- imatelengthof2.5␮m.Eachfibreconsistedofanaverageinner diameter 3.8nm, anaverage outer diameter9.13nmand a numberoflayersvariatedfrom7to9(Fig.6b).Theseresultsare inaccordancewiththestudyperformedbyMelkaetal.[22].

Theyconﬁrmedthereﬁnementofthegrainsizesupto2wt.%

ofMWCNTusedforpowdermixtureprocessing[22].Indeed, whentheagglomerationofMWCNToccurs,therealamount of MWCNTslocatedin the grain boundariesisdecreasing.

Therefore, the agglomerated CNTsintroduce heterogeneity anddecreasetheefﬁciencyofthecomposite[23–25].

Scanning Raman spectroscopy measurementswere performed toconﬁrmthe presence ofMWCNTinthe powder mixtures and determine structural changes caused by the millingprocess.FourtypicalRamanspectraareshowninthe 150–3500cm⁻¹wavenumberregion,whichweremeasuredon powdermixtureswithdifferentMWCNTcontent(Fig.7).In caseofpure YSZ(reference, YSZ-100,blackline) scattering peaksat189,268,341,380,483,633cm⁻¹relatedtoyttriasta- bilizedzirconiawhichareinaccordancewithliteratureworks [26,27].CharacteristicRamanpeaksofYSZarelessintensive or completely absent in case of YSZ/MWCNTs powders (YSZ-99,YSZ-95,YSZ-90)inthe100to700cm⁻¹wavenumber

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Pleasecitethisarticleinpressas:S.Lamnini,etal.,Theroleoftheattritionmillingonthegrainsizeanddistributionofthecarbonnanotubes 400 nm

400 nm

a b

c

Fig.3–SEMimagesofmilledpowders.(a)YSZ/1wt.%MWCNT,(b)YSZ/5wt.%MWCNTandc)YSZ/10wt.%MWCNT (SE,MAG5000×HV5kV).

a

SE SE

SE

Zr Zr

Zr

C C

C

c

2 μm

b

Fig.4–EDSmappingshowingthedispersionofMWCNTsinYSZ.(a)YSZ/1wt.%MWCNT,(b)YSZ/5wt.%MWCNT and(c)YSZ/10wt.%MWCNT.Colours:C-redandZr-blue.

region.Additionalstrongpeakshavebeen observedaround 1590cm⁻¹(Gband)and1355cm⁻¹(Dband),whicharerelated totheMWCNT[28].TheGbandisrelatedtothestretching vibrationofthesp²C–Cbonds,whiletheDbandisassociated withthepresenceofstructuraldefectsintheMWCNT.Raman spectraalsoexhibitedawidebandathigherwavenumbers,

the so called 2D band, which appears at 2687, 2684 and 2690cm⁻¹for1,5,10wt.%MCWNTcontent,respectively.The 2D band feature originatesin a two-phonon,second-order Raman scattering process in carbon nanotubes [29]. The intensityratioofDandGband(ID/IG)hasbeenwidelyused to investigate the structural purity (structural defects and

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Pleasecitethisarticleinpressas:S.Lamnini,etal.,Theroleoftheattritionmillingonthegrainsizeanddistributionofthecarbonnanotubes

200

a b

220 200

220 111

111

200 nm 200 nm

Fig.5–TEMimagesofYSZwithSAEDdiffractionininsert.(a)Referencebeforemillingand(b)referenceaftermilling.The SAEDconﬁrmedthecubiczirconia(111,220,200)inbothcases.

MWCNTs

c

a b

Zirconia particles Zirconia grains

200 nm

200 nm 50 nm

5 nm

Fig.6–TEMimagesofYSZ/MWCNTs.(a)YSZ/1wt.%MWCNT,(b)YSZ/5wt.%MWCNTwithdetailofMWCNTininsertand (c)YSZ/10wt.%MWCNT.

disordering)ofMWCNTsor thepresenceofagglomerations [30,31], hence the intensityofD bandis defectdependent.

In principle, the larger the number of defects, the higher theDbandintensity.Peakpositionsandintensityofdistinct scattering bands were determined by Lorentzian ﬁtting of RamanspectrashowninFig.7.Althoughsuchprocedurehas someuncertaintybecauseofthenumberoffreeparameters,

itcanprovideadditionalinformationabouttheYSZ/MWCNTs powders.WefoundthattheID/IG ratiois0.6forYSZ-99,0.8 forYSZ-95and0.7forYSZ-90,whichvaluesareingoodaccor- dance with previous morphological observations showing the presence of structural defects and agglomerations in the YSZ/MWCNTspowders asaresultofintensivemilling.

Amongthepowdermixtures,YSZ-95containsmorestructural

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Pleasecitethisarticleinpressas:S.Lamnini,etal.,Theroleoftheattritionmillingonthegrainsizeanddistributionofthecarbonnanotubes 0,0

0,5 1,0 1,5 2,0 2,5 3,0

ID/IG=0.7

ID/IG=0.8 2D

YSZ-100 YSZ-99 YSZ-95 YSZ-90

Raman intensity (arb.units)

Raman shift (1/cm) 189

268 483

633 1463

1621 2154

D G

ID/IG=0.6

500 1000 1500 2000 2500 3000 3500

Fig.7–Ramanspectrausingalaserexcitationwavelengthat488nmforreferenceandYSZ/MWCNTs.

defectscomparedtotheothersaccordingtoitshighestID/IG

ratio(0.8).

Conclusion

TheaimofourstudywasthedevelopmentofYSZ/MWCNTsas nickelfreeSOFCmaterial.Thisworkshowedthemainresults ofﬁrst preparation step; millingand homogenization. The inﬂuenceofMWCNTadditionandmillingprocessonstruc- tureanddesignofcompositeswerestudied.ZrO2–8mol.%

Y2O3(YSZ)powdersweremixedwith1,5and10wt.%MWC- NTsandmilledbyhighattritionmillingat4000rpminethanol for5h.Thestructuralandmorphologicalinvestigationsofas- receivedzirconiapowderconsistedoftheparticleswithsharp andirregular shape.In fact,the effectofmillingonthe as receivedzirconiaresultedinaconsiderabledecreaseforpar- ticlesizewithoutphasetransformationofYSZparticles.The phaseanalysisprovedtheexistenceoftwophases;cubiczirco- niaasamainphaseandasmallfractionofmonocliniczirconia asaminorphase.Theparticlesizeshowedarefinementfrom anaverageof50␮mto400nm.However,nosignificantdam- ageorstructuralchangesaffectedMWCNTsaftermillingwere observed.TheMWCNTswereobservedasafinefibresforming several agglomerations, networkswith approximate length 2.5␮m,eachfibreconsistedofanaveragediameter3.8nm,an averageouterdiameter9.13nm.Thenumberofcarbonlayers variedfrom7to9.Ramanspectroscopy resultsshowedthe presenceoftheGandDbandsforallpowdermixturesatpeak positionsof∼1590and∼1355cm⁻¹respectively.Thesebands confirmedthestructuralintegrityofMWCNTafterthemilling process.

Thenovelresultsarefollows:

• millingprocessdidnotresultinphasetransformationof YSZonlygraindecrease;

• theadditionofMWCNTstozirconiamatrixintroducedasig- niﬁcantchangeinthesurfacetopologyandthedistribution ofthezirconiagrains;

• from the point of MWCNTs structure, non-destructive millingprocesswasconﬁrmedbyHRTEMstudy;

• thestructuralintegrityofMWCNTsafterthemillingprocess wasconﬁrmedbyRamanmeasurementstoo.

Acknowledgement

Ms. Soukaina Lamnini thanksfor Stipendium Hungaricum scholarship.TheauthorsacknowledgethesupportfromCER- ANEA(FLAG-ERA)project.Furthermore,theauthorswouldlike tothankMr.LeventeIllésforSEMandEDSmeasurements.The authorsarealsogratefultoMr.ViktorVargaforhishelpwith samplepreparation.

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