Synthesis, characterization and thermal stability of solid solutions Zr (Y, Fe, Mo)O2

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

Synthesis, characterization and thermal stability of solid solutions Zr (Y, Fe, Mo)O ₂

Felipe Legorreta-García

^a

, Leticia Esperanza Hernández-Cruz

^a

, Maricela Villanueva-Ibá ˜nez

^b

, Marco Antonio Flores-González

^b^,∗

aÁreaAcadémicadeCienciasdelaTierrayMateriales,UniversidadAutónomadelEstadodeHidalgo,CarreteraPachuca–Tulancingo, Km4.5s/n,Col.Carboneras,MineraldelaReforma,Hgo.,México42184,Mexico

bLaboratoriodeNanotecnologíayBio-electromagnetismoAplicadoLaNBA,UniversidadPolitécnicadePachuca,CarreteraPachuca–Cd.

SahagúnKm.20,RanchoLuna,Ex-HaciendadeSta.Bárbara,ZempoalaHgo.43830,Mexico

a r t i c l e i n f o

Articlehistory:

Received9December2014 Accepted5May2015 Availableonline4June2015

Keywords:

Stabilizedzirconia Synthesis Combustion ZrO₂/Y/Fe/Mo

a bs t r a c t

ThesynthesisofFe³⁺,Mo⁴⁺andY³⁺fullystabilizedzirconiabythenitrate/ureacombus- tionrouteandthermalstabilityinairwasinvestigated.Thesolidsolutionobtainedwas characterizedbyXraydiffraction(XRD),scanningelectronmicroscopy(SEM)andusedthe BETmethodfordeterminingspeciﬁcsurface.Theceramicpowdersobtainedwerecalcined at1473Kinairatmosphereinordertodeterminetheirthermalstability.Thescanningelec- tronmicroscopy(SEM)resultsshowedahomogeneousgrainsurface,measuringseveraltens ofmicrometersacross.Thecrystallographicstudyrevealedthatbythismethoditwassuc- cessfullyachievedzirconiadopedwithFe³⁺,Mo⁴⁺andY³⁺ionsinthezirconiatetragonal monophase,evenaftercalcinations.

Síntesis,caracterizaciónyestabilidadtérmicadelasolucionessólidasZr (Y,Fe,Mo)O2

Palabrasclave:

Zirconiaestabilizada Sintesis

Combustión ZrO2/Y/Fe/Mo

r e su m e n

Seinvestigalasíntesisde zirconiatotalmenteestabilizada,incorporandolosionesFe³⁺, Mo⁴⁺,Y³⁺porlarutadecombustiónnitrato/urea,asícomosuestabilidadtérmicaenaire.

LasoluciónsólidaobtenidasecaracterizópordifracciónderayosX,(DRX),microscopía electrónicadebarrido(SEM)ysedeterminólasuperficieespecíficaporelmétodoBET.Los polvoscerámicosobtenidossecalcinarona1473Kenatmósferadeaireconelfindedeter- minarsuestabilidadtérmica.Losresultadosdemicroscopíaelectrónicadebarrido(SEM) mostrarongranosconunasuperficiehomogénea,devariasdecenasdemicrómetrosde diámetro.Elestudiocristalográficorevelóqueporestemétodoselogróconéxitoobtener

∗ Correspondingauthor.

E-mailaddress:ﬂ[email protected](M.A.Flores-González).

http://dx.doi.org/10.1016/j.bsecv.2015.05.004

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zirconia dopadacon iones Fe³⁺, Mo⁴⁺, Y³⁺ en lamonofase zirconia tetragonal, incluso despuésdelacalcinación.

EsteesunartículoOpenAccessbajolalicenciaCCBY-NC-ND (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction

Zirconia powder (ZrO₂) is a very important technological material with a wide variety of applications due to their excellent refractoryproperties, good chemicalstability and high mechanical strength. Zirconia occurs in three differ- entpolymorphicforms:monoclinicm-ZrO2(RT), tetragonal t-ZrO₂(≈1463K)andcubicphasec-ZrO2(≈2643K).Thispoly- morphismisaccompaniedwithvolumechangesthatrestrict theuseofpurezirconiapowdersforseveralapplications[1].

Stabilizedzirconiahasmanymechanical,electronic,biomed- ical and chemical applications. It is well know that some cations (e.g. Ca²⁺, Mg²⁺, Y³⁺ and some rare earth cations) fromzirconiasolidsolutionsstabilizethetetragonalorcubic phase.Theinﬂuenceoftransitionmetals Co²⁺ and Fe³⁺ as stabilizers was studied early [2,3]. These works demon- stratedthatsolubility inthestabilizedsolidsolutionbegins todecreaseattemperaturesaround1148Kand898Krespec- tively, causing the progressive transformation into the monocliniczirconiaphase.Theserelatively limitedthermal stability,haveledtointroducetheadditionalincorporationof yttriumions.TheY2O3–ZrO2systemhasbeenstudiedexten- sivelybyYashimaetal.[4].Theyidentifytwoformsofthe tetragonalphasethattheycallq aformclosetothemono- clinicformandqwhichisclosetothecubicphase.Ingeneral, acontentofbetweenabout3and8mol%ofY2O3inthezirco- nia(i.e.5.8–14.87mol%YO1.5)stabilizes,moreorlessstrongly, the tetragonal phase. Stabilizationof cubic and tetragonal formsis gaining importance because ofits excellent ther- malstability,chemicalresistance,mechanicalpropertiesand oxygenconductivity.Thepreparationofhigh-quality yttria- stabilized zirconia (YSZ) is important to the fabrication of high-performancesolid-stateionicdevicessuchassolidoxide fuelcells(SOFCs)andgassensors.TheincorporationFe³⁺ions inthezirconialatticewereinvestigatedusingseveralprepara- tionmethods.Davisonetal.[5]studiedtheformationofsolid solutionsbetween␣-Fe2O3andZrO2usingthenitratedecom- positionand calcinationsroute. Aftercalcinations inairat 873K,theyfoundatetragonalsolidsolutionfor2.5and5at%

Fe³⁺.Forhigherironcontents,hematiteisalsodetected.Berry etal.[6,7]usedasimilarrouteandshowedthattheincorpo- rationofFe³⁺ionswithint-ZrO2latticeat773Kstabilizesthe cubicformandinhibitsitstransformationintothemonoclinic form.TheincorporationofFe³⁺andMo²⁺ionsinthezirconia anditspossibleroleinthestabilizationoftetragonalorcubic structureisnotmentionedintheliterature.

The interest in studying zirconia-supported molybde- numoxideismainlyduetotheirperformanceascatalysts [8–12]. Powder synthesized byMoO3/ZrO2 withamounts of Mo varying from 1 to 12wt% prepared by incipient wet- ting,subsequently driedandcalcinedinairat773Kfor6h

was reported by Bhaskar et al. [8]. Brown et al. incorpo- ratemolybdenumamountscorrespondingto2.5and10wt%

MoO3/ZrO2, preparedby coprecipitation and calcination in staticairat1073K for12h[9].Theyreportedthattheaddi- tionofhigheramountsofMoO₃increasedtheproportionof t-zirconia,butthepowderwasnotfullystabilized.Theincor- poration ofwt 11% Moby impregnation,calcined inairat 873Kfor3h,producesonlyt-ZrO2[10,11].TheMo/ZrO2and CoMo/ZrO2catalystswerepreparedbyimpregnationandcal- cinationat673^◦CbyKalu ˇzaetal.[12].Theauthorsachieve highhydrodesulphurizationwiththeCoMo/ZrO2 solidsolu- tion. Recently, Samantary et al., demonstrated that t-ZrO2

couldincreasetheefﬁciencyofcatalysis.Theyobtainedfully t-ZrO2powdersbysolutioncombustionmethod,usingglycine as fuel,when 5or 10wt% Mowas incorporated [13–15]. In thepresentpaper,itisproposedtoinvestigatethistopicby preparingastabilizedZr0.86Y0.1Fe0.02Mo0.02O_1.94−␦ solidsolu- tionusingthenitrate/ureacombustionroute,whichuseslow costreactants,producesgram-scalequantitiesofpowdersat laboratory scale and can be readily scaled up. The aimto elaborate stable solid solutions to high temperatures is to have a rawmaterialin order tostudy the productionofa compositeZirconia-Metal-CNT.Inapreviouswork,hasbeen reportedthesynthesisofcompositepowderscontainingwell dispersedcarbonnanotubesbyselectivereductioninH2±CH4

ofoxidesolidsolutionsbetweenanonreducibleoxidesuchas Al2O3orMgAl2O4andoneormoretransitionmetaloxide(s).

Thereductionproducesverysmalltransitionmetal(Fe,Co, Niandtheiralloys)nanoparticlesatatemperatureofusually

>800^◦C.Quantitiessmallerto0.01%ofMoandFeascatalysts havebeenefﬁcienttoproduceCNTsinsitusolidsolutionsas MgO,SiO₂,Al₂O₃butneverwithZrO₂[16,17],duehispolymor- phism.Inasecondpartofthestudy,thethermalstabilityin airofat-Zr0.86Y0.1Fe0.02Mo0.02O_1.94−␦ solidsolutionhasbeen investigated.

Experimental

The combustion method was adapted from that described by Patil [18,19] and applied to obtain a lot of solid solutions [2–4,18–26]. The ceramic powders ZrO2, Zr0.96Fe0.02Mo0.02O1.94−␦andZr0.86Y0.1Fe0.02Mo0.02O1.94−␦were synthesized. However, for the sake of simplicity, the so- obtained powders will hereafter be denoted as ZrO₂, ZF2M2 and ZYF2M2 respectively. Theappropriate amounts of ZrO(NO3)2·4H2O, Y(NO3)3·6H2O, Fe(NO3)3·9H2O and (NH4)6Mo7O24·4H2O were dissolved in deionized water (20mL, 343K). The syntheses were made by combustion undertheconditionsdevelopedabove,thatistosay,witha proportionofurean=7,inordertoproduce5gofsolidsolu- tions.Allsamplesweresynthesizedinthesameconditions.

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Combustionproductswere calcined inairat873K fortwo hourstoremovetracesofcarbon.Theureaproportionswere calculated by considering the total oxidant valences (VO) and reducing valences(VR) ofthe differentspecies [18,27].

Thetotaloxidantvalencesofthe nitratesisVO=10.26.The reducing valence of urea is VR=6. The molar quantity of urea isﬁrst calculated so thatthe so-called stoichiometric ratio ϕ=mVR/VO is equal to unity, which gives m=1.71.

Was studiedearly the synthesis ofFe³⁺-stabilized zirconia bythenitrate/ureacombustionroute[3].Thesework deter- minetheappropriateamountofureaism=6–9thatallows to obtain a totally iron-stabilized nanocrystalline ZrO₂. In thiswork mwas ﬁxed=7.However,asdiscussedbyZhang andStangle[22],thismethodinvolvessomeapproximations.

Firstly,O2fromtheairatmosphereisnottakenintoaccount, whichaffectsVO.Secondly,thenitrogen-containingproducts areconsidered asbeingonlyN₂ (thus, zeroistakenasthe valence of nitrogen element) despite that nitrogen oxides and/orNH3canbeformedinusuallyundeterminedpropor- tions.ThisaffectsbothVOand VR.Thecontainerwiththe solutionwasplacedinafurnacepre-heatedat873K,keeping the door of the furnace open. After water evaporation, a combustionreactiontakesplaceaccordingtoaredoxreaction betweenthenitrates and urea,producing anoxidepowder (hereafternamedas-preparedpowder).

Batchesof80mgofthesolidsolutionspowderwerecal- cinedbythermogravimetric(DTA,Netzsch404S)equipment in flowing air (4.8Lh⁻¹) at 1473K. A heating and cooling rateof 1473Kh⁻¹ was applied(no dwelltime atthe target temperature).X-raydiffraction(XRD)patternsofall materi- alswererecordedusingCu-K␣radiation(l=0.15418nm).The carboncontentforthepowderswasdeterminedbyflashcom- bustionwithanaccuracyof±2%.TheBETspecificsurfacearea ofthematerialswasmeasuredusingN2adsorptionatliquid N2temperatureinaMicromeriticsFlowSorbII2300apparatus (thereproducibilityoftheresultsis±3%).Thecompositepow- derswerestudiedusingfield-emission-gunscanningelectron microscopy(FEG-SEM,HitachiS4500,operatedat5kVor8kV).

Results and discussion

TheX-raydiffractionpatternsofthesolidsolutionsareshown inFig.1. Thepurezirconiaclearlyreveals signalsoft-ZrO2

andm-ZrO2.Theotherssolidsolutionsare completelysta- bilized, only peaks of the tetragonal structure are shown.

Theseobservationsindicateforstabilizethet-ZrO₂isneces- sarythepresenceofY³⁺ orFe³⁺/Mo⁴⁺ions.Wehaveshown inearlyworks,thesynthesisofFe-ZrO2powdersbycombus- tionmethodobtainingt-ZrO2phase;thiswasdeterminedby severalcharacterizationtechniques,notablyMössbauerspec- troscopy [3,22]. Nevertheless, ispossible todistinguish the (101)and(110) peaksofZF2M2powderswhichappearsto beslightly shiftedtolow2 anglescomparedwithZYF2M2 solid solution, this probably reveal that the corresponding inter-reticulardistanceislightlyhigher.Additionally,forthe stabilizedsolidsolutions,isnotdetectedanyphaseofmolyb- denumorironoxide,sothatcouldbeconsideredthatY³⁺,Fe³⁺

andMo⁴⁺ionsareinthezirconialatticefullystabilized.

(101)t (111)m

(002)m (002)m

(110)t

ZYF2M2

ZF2M2

ZrO₂ (011)m

20 25 30 35

2θ

40 45

Fig.1–XRDpatternsofthesolidsolutionsoftheZrO₂, ZF2M2,andZYF2M2.

TheDTA curve (not shown)revealed weakendothermic peaks at about 923, 1173 and 1423K (the latter extremely weak)duringheatingandasharpexothermicpeakatabout 973K.Kiminami[23]reportedweakandbroadpeaksduring heating,indicatingalow transformationrate,forapowder with1mol%␣-Fe2O3 and noobservablepeaks for10mol%.

Itwasverifiedthatthepreparedpowdercontainonlytrace amountsofcarbon.TheBETspecificsurfaceareaSw,andpro- portions oft-ZrO₂ are shown inTable1.Theproportionof thetetragonalphasewithrespecttomonoclinicphasewas calculatedbytheGravie-Nicholsonmethod[28].TheBETspe- cificareaofthepowderscontainingelementyttrium,ironand molybdenumisthehighest.HigherSwcouldreflectasmaller grainsizeduetobothstrongernatureofthecombustionpro- cessexperimentedforthesolidsolutionscontainingyttrium.

Furthermore,whenalowvalencydopantcation,suchasY³⁺, Fe³⁺, isintroduced into the ZrO2 lattice,oxygen vacancies arecreatedforthechargebalanceandgenerationofpositive holeswithlatticedefects.Theselatticedefectscanbecausea smallcrystallitesize.ThesedatawerecomparedtotheXRD averagecrystallitesizecalculatedbyapplyingtheScherrerfor- mulaonthetetragonal(101)Braggpeak.Onecannotethatthe sizeevolutionisalsoingoodagreementwiththeSwresults.

Table1–Surfacespeciﬁcresultsforthesolidsolutions synthesized.Tetragonalproportion(t(%))isthe proportionoftetragonalphaseevaluatedfromtheXRD patterns(balanceismonocliniczirconia[29]).Average crystallitesizedeterminedfromtheXRDpatternusing theFWHMofthetetragonal(101)Braggpeak.

Sample Sw(m⁻²g) t(%) Crystallite

size(nm)

ZrO2 22.0 55 25.2

ZF2M2 32.2 100 15.8

ZYF2M2 54.3 100 14.4

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Fig.2–FEG–SEMImagesofthesolidsolutions(a)ZF2M2and(b)ZYF2M2.

Noattemptwas madetocalculateasizebased onthe Sw resultsbecausethehypothesisofnon-agglomeratedparticles isnotveriﬁedinthepresentpowders.

Typicallow-magnificationSEMimagesoftheas-prepared powdersareshowninFig.2.Thepowdersconsistingoflarge particleswithahomogeneoussurface,measuringseveraltens ofmicrometersacross, aswell asmicro-orsubmicrometric particles:furthermore,thezirconiaparticlesarequiteporous, reflectingtheescapingofgasesduringthecombustion.Note thatthismorphologyisevidentforZYF2M2powders,Fig.2(b), accordingwithsurfacespecificresults.

TheXRDpatternsofZF2M2and ZYF2M2solidsolutions aftercalcinationsinairat1473KarepresentedinFig.3.For thepowderZYF2M2,onlythepeakoffullytetragonalphaseis

(111)c

(111)m

(110)m

(112)m (120)m (111)m

(211)m (211)m (020)m

(022)m

(002)m

(013)m (002)m

(200)c

(220)c

(202)m (311)c

25 30 35

2θ

40 45 50 55 60

Fig.3–XRDpatternsoftheZF2M2andZYF2M2solid solutionsaftercalcinationsinairat1473K.

detected.Alsothe(101)veryﬁneandintensepeakaccount- ingforthetetragonalphaseismorenarrow,indicatingsome crystallitegrowth.InthecaseforZF2M2powdersareclearly the transformations in t-ZrO₂ and m-ZrO₂ phases because thesolubilitydecreases.Theseresultsareagreedwithearly studies.TheZr0.9Fe0.1O1.95 andZr0.9Co0.1O1.9 werestabilized in the tetragonal phase by the combustion method using ureatoo.Thestudyoftheirthermalstability,demonstrated to solubility in the stabilized solid solutions, show that it startstodecreaseattemperaturesaboutto1148Kand898K respectively, provoking the progressivetransformation into themonocliniczirconiaphaseforbothsolidsolutions.Free Mo3OorFe2O3couldremainundetectedontheXRDpatterns becauseofaverysmallsize.

Conclusion

ThesynthesisofthesolidsolutionsZr_0.96Fe_0.02Mo_0.02O_1.94−␦, and Zr0.86Y0.1Fe0.02Mo0.02O_1.94−␦, fully stabilized in tetrago- nalformarereportedforfirsttime.Thepowdersconsisting of large grains with a homogeneous surface, measuring several tens of micrometers across. The BET on specific area of the powders contain element yttrium, iron and molybdenum ishigherthatforpureZrO2.HigherSw could reflect a smaller grain size due to both stronger natures of the combustions process experimented for the solid solutions containing yttrium, in agreement with the XRD results. Thestudyofthe thermalstabilityinairforZF2M2 revealed that this solid solutionis unstable at 1473K, this suggest thatMo⁴⁺ and Fe³⁺ ionsleaving the solid solution provokingthemixtureoft-ZrO2andm-ZrO2phases.Theintro- duction ofthe element yttrium (ZYF2M2) provides a solid solutionofzirconiastabilizedintetragonalform,evenafter calcinationsat1473K.Materialsobtainedcouldcontributeto significantlyimprovetheionicconductivityofthesolidstate conductor.Inaddition,theseresultsofferpossibilitythatthis solidsolutioncanbeexposedtoCVDprocesstosynthesize CNT-Metal ZrO2 composites, without phasechanges ofthe zirconia.

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Synthesis, characterization and thermal stability of solid solutions Zr (Y, Fe, Mo)O2