Investigations of Zr(IV) in LiF-CaF2: stability with oxide ions and electroreduction pathway on inert and reactive electrodes

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To link to this article : DOI: 10. 1016/j.electacta.2013.02.022

URL : http://dx.doi.org/10.1016/j.electacta.2013.02.022

To cite this version : Gibilaro, Mathieu and Massot, Laurent and

Chamelot, Pierre and Cassayre, Laurent and Taxil, Pierre

Investigations of Zr(IV) in LiF-CaF2: stability with oxide ions and

electroreduction pathway on inert and reactive electrodes. (2013)

Electrochimica Acta, vol. 95 . pp. 185-191. ISSN 0013-4686

Any correspondence concerning this service should be sent to the repository

Investigation

of

Zr(IV)

in

LiF–CaF

2

:

Stability

with

oxide

ions

and

electroreduction

pathway

on

inert

and

reactive

electrodes

M.

Gibilaro

_,

_L.

_Massot,

_P.

_Chamelot,

_L.

_Cassayre,

_P.

_Taxil

UniversitédeToulouse,UPS,CNRS,LaboratoiredeGénieChimique,118RoutedeNarbonne,F-31062Toulouse,France

Keywords: Electrochemistry Zirconium Moltenfluoride Zirconiumoxifluorides Zr–Cuintermetalliccompounds

a

b

s

t

r

a

c

t

Inthiswork,adetailedelectrochemicalstudyofthemoltenLiF–CaF2–ZrF4systemisprovidedinthe

810–920◦_{Ctemperaturerange,allowingthedeterminationofthereductionpotential,thediffusion}

coef-ficientandthereductionmechanismofdissolvedZr(IV)onaninertTaelectrode.AdditionofCaOinthe

moltensaltisshowntocauseZr(IV)precipitationintoanequimolarmixtureofsolidcompounds,most

likelyZrO2andZrO1.3F1.4.UnderpotentialdepositionofZronCuandNielectrodesisalsoevidencedand

GibbsenergyofformationofCu–Zrcompoundscalculatedbyopencircuitchronopotentiometry.

1. Introduction

Pyrochemicalrouteforpartitioningandtransmutationstrategy isconsideredasoneofthemostpromisingoptionfornuclearfuel cycle,andmoltensaltelectrorefiningiswelladaptedformetallic spentfuelreprocessing,suchasU–ZrandU–Pu–Zr[1–3],where theclassicalhydrometallurgicalprocesses arenomoreefficient duetothezirconiumelement.Itspresenceinthemoltenbathis commonlyconsideredasanannoyancetotheactinide-lanthanide separation,asthezirconium(IV)reductionpotentialismore pos-itivethanuraniuminchloride[4–7]andinfluoridesalts[8–10], interferingintheactinidesselectiverecovery[11].Inthespecific caseofmoltenfluoridesolvents,agoodknowledgeofbothZrF4

chemistryandelectrochemistryisthusrequired.

ZrF4 ismainlypreparedbychemicalreactionbetween

zirco-niumoxideZrO2andafluorinatingagent,suchasfluorinegas[12],

hydrogenfluoride[13],ammoniumbifluoride[14]orbromium tri-fluoride[15]. Thesesynthesesevidencetheexistenceofa great numberofzirconiumoxifluoridessolidphases.Afulldescription oftheZrO2–ZrF4phasediagramisgivenbyPerpierniketal.,who

gathereddataforthewholesystem[16].

Thezirconiumelectrochemicalbehaviour,intheformof chlo-ride, has been widely investigated and no consensus on its reductionpathwayinchloridemeltshasbeenfoundyet.Authors reportedseveraloxidationstates(0,+I,+IIand+IV)duetoacomplex chemistryinchlorideandinchloro-fluoridemelts[17–19].

∗ Correspondingauthor.Tel.:+33561557219;fax:+33561556139. E-mailaddress:[email protected](M.Gibilaro).

ToobtainalesshygroscopicformandtodecreaseZr(IV) volatil-ity[20],numerousstudieshavebeenperformedonthefluoride form,ZrF4,andweredevotedtothedepositioninfluoridemedia

of coherent Zr metaland Zr alloys preparation on Ni,B and C

[8,21–25].Inthesemedia,theelectrochemicalreductionofZr(IV)is aone-stepprocessexchanging4electrons,asithasbeenprovedby manyauthors[9,10]andmorerecentlydemonstratedinLiF–NaF byGroultetal.[8].Anyway,nostudyhasbeenperformedyetin LiF–CaF2medium.

ThispaperpresentsadetailedelectrochemicalstudyofZr(IV) ionsbehaviourinLiF–CaF2inthe810–920◦Ctemperaturerange

oninertelectrode(Ta)usingseveralelectrochemicaltechniques (cyclic and square wave voltammetries, chronopotentiometry). Verysensitivetooxide,oxideions(CaO)have beenaddedinto theLiF–CaF2–ZrF4 mixturetoinvestigatethezirconium

precipi-tationbyformationofZr–O–Fsolidphases.Finally,Zr(IV)hasbeen depositedonreactiveelectrodes(Ni,Cu)and,afterdetermination ofphasescompositionbyEDS–SEM,theGibbsenergyofformation oftheseintermetalliccompoundswereestimatedbyopencircuit chronopotentiometryontheCu–Zrsystem.Theexperimentaldata werecomparedtoexistingthermochemicaldata.

2. Experimental

Thecell wasavitreouscarbon crucibleplaced ina cylindri-calvesselmadeofrefractorysteelandclosedbyastainlesssteel lidcooledbycirculatingwater.Theinsidepartofthewallswas protectedagainstfluoridevapoursbyagraphiteliner.The experi-mentswereperformedunderaninertargonatmosphere(lessthan oneppm O2), previouslydehydratedand deoxygenatedusing a

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2

j/

A

c

m

Fig.1. CyclicvoltammogramsonTaoftheLiF–CaF2systemat100mVs−1and840◦C:withoutZrF4(grey)andwithZrF4additionof0.05molkg−1(black).Inset.Variationof

thepeakcurrentdensityandthepeakpotentialversusthesquarerootofthepotentialscanrate.Workingel.:Ta(S=0.31cm2_{);auxiliaryel.:glassycarbon;comparisonel.:}

Pt.

purificationcartridge(AirLiquide).Thecellwasheatedusinga pro-grammablefurnaceandthetemperaturesweremeasuredusinga chromel–alumelthermocouple.Amoredetaileddescriptionofthe set-upcanbefoundin[26].

TheelectrolyticbathconsistedofaeutecticLiF–CaF2(Appolo

99.99%)mixture(79.5/20.5 molarratio), initiallydehydratedby heatingundervacuum(10−5bar)toitsmeltingpoint(762◦C)for 72h.Zirconiumionswereintroducedintothebathintheformof ZrF4pellets(Cerac99.99%)andoxideionsintheformofCaOpowder

(Cerac99.9%).

Tantalum,copperandnickelwires(Goodfellow99.99%,1mm diameter) wereused asworkingelectrodes. Theauxiliary elec-trodewasavitreouscarbon(V25)rod(3mmdiameter)withalarge surfacearea(2.5cm2_).

Thepotentialswerereferredtoaplatinumwire(0.5mm diame-ter)immersedinthemoltenelectrolyte,actingasaquasi-reference electrodePt/PtOx/O2−[27].

Theelectrochemicalstudyandtheelectrolyseswereperformed with an Autolab PGSTAT30 potentiostat/galvanostat controlled withtheGPES4.9software.

ThecompositionandmicrostructureofZrdepositsobtainedon reactiveelectrodeswerecharacterizedbyscanningelectron micro-scope(SEM)coupledwithanenergydispersivespectroscopy(EDS) probe.

Cyclic voltammetry, chronopotentiometry, square wave voltammetryandopencircuitchronopotentiometrywereusedfor theinvestigationofthezirconiumelectroreductionprocess. 3. Resultsanddiscussion

3.1. Zr(IV)reductionmechanismoninertelectrode 3.1.1. ZrF4inLiF–CaF2

3.1.1.1. Cyclic voltammetry. A series of cyclic voltammograms has been plotted on an inert tantalum electrode. A represen-tative voltammogram is presented in Fig. 1 in LiF–CaF2–ZrF4

(c0=0.05molkg−1)at840◦Cand100mVs−1,whereasinglepeak

isobservedinthecathodicrunataround_−1.15VvsPt.Thispeakis associatedwithareoxidationpeakataround_−0.9Vvs.Pt,whose shapeistypicalofthedissolutionofametaldepositedduringa cathodicrun(strippingpeak).AspresentedintheFig.1inset,the quasi-reversibilityofthesystem,characterizedbyapeakpotential independentofthescanrate,isverified.

Thecathodicpeakcurrentincreasedlinearlywiththe concen-trationofzirconium(IV)ions(Fig.2),confirmingthatthispeakcan beattributedtotheZr(IV)reductionreaction.

Then,theinfluenceofthescanrateonthepeakcurrentwas stud-ied(Fig.1inset)toverifytheBerzinsDelahayerelationship,valid forareversiblesoluble/insolublesystemandadiffusion-controlled reaction[28]: Ip=−0.61nFSc0





wherenisthenumberofexchangedelectrons,FtheFaraday con-stant(96,500C),Stheelectrodesurfaceareaincm2_{,Dthediffusion}

-0.4 -0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 jp /A cm -2 [ZrF₄]/mol kg-1

Fig.2. LinearrelationshipbetweenthecathodicpeakcurrentdensityandtheZrF4

M.Gibilaroetal./ElectrochimicaActa95 (2013) 185–191 187

Fig.3.VariationofthechronopotentiogramsoftheLiF–CaF2–ZrF4(0.08molkg−1)systemat840◦CwiththeappliedcurrentonTa.Inset.variationofI1/2vs.thecurrentat

840◦_{C.Workingel.:Ta(S=0.31cm}2_{);auxiliaryel.:glassycarbon;comparisonel.:Pt.}

coefficientin cm2_s−1_{,c0 the soluteconcentration inmolcm}−3_,

T the absolutetemperature in Kand

Thelinearrelationshipbetweenipand

insetconfirmsthepreviousassumptions:

-thereactionyieldsaninsolubleproduct,likelyzirconiummetal -theelectrodeprocessisdiffusioncontrolled.

Theslopeofthislinearequationis: ip

v

0.002 A s1/2V−1/2_cm−2 ₍₂₎

att=840◦_{Candc0=0.05molkg}−1_.

3.1.1.2. Chronopotentiometry. Chronopotentiometry was per-formed on Ta at 840◦_{C to confirm that the electrochemical}

processiscontrolledbyzirconiumionsdiffusioninthemelt.In

Fig.3,chronopotentiogramsplottedat variouscurrentdensities at0.08molkg−1 _{exhibit a singleplateauatabout−1.15V vs Pt}

correspondingtothepotentialoftheZr(IV)reductionintoZrmetal evidenced in Fig. 1. The transition time  decreased when the appliedcurrentincreased,ingoodaccordancewiththeSand’slaw

[29]validfordiffusion-controlledreactions: i1/2

c0 =0.5n

0.5_FD0.5 ₍₃₎

whereisthetransitiontimeins.

ThedataplottedintheFig.3insetarenotinfluencedbyc0,in

accordancewithEq.(3);thevalidityofthisequationwascheckedin the810–920◦_{Ctemperaturerangeandat840}◦_{Candtheconstant}

valueis: i1/2

c0 =−880.8±0.2

As1/2 (4)

The reversal chronopotentiogram presented in Fig. 4

(c0=0.08molkg−1andI=±150mA)exhibitsananodictransition

timeequaltothecathodicone(ox=red=1.6s).Thisresult,

char-acteristicofaninsolublecompound formationontheelectrode, confirmsthatthereactionyieldsZrmetalontheworkingelectrode.

3.1.1.3. Numberofexchangedelectrons. TheZrformationby reduc-tionofZr(IV)inonestepat_−1.1VvsPtwasfinallyevidencedby calculatingthenumberofexchangedelectronsduringthecathodic process.

Twomethodswereusedforthiscalculation:

- combination of cyclic voltammetry and chronopotentiometry measurements,

-squarewavevoltammetry.

ThefirstmethodallowstheuncertaintyontheZr(IV) concen-trationanddiffusioncoefficienttobeignoredbycoupling(1)and (3):

Ip/√

I√ =74.173

q

T (5)

Fromtheseequations,thecalculatednumberofexchanged elec-tronswasfoundtobe3.8_±0.2.

The other technique used to determine the number of exchangedelectronsisthesquarewavevoltammetry[30].Inthis

-1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 E v s Pt/V t/s

Fig.4.ReversedchronopotentiogramoftheLiF–CaF2–ZrF4(0.08molkg−1)system

at840◦_{C;appliedcurrent=±0.15A.Workingel.:Ta(S=0.31cm}2_{);auxiliaryel.:}

-0.09 -0.08 -0.07 -0.06 -0.05 -0.04 -0.03 -0.02 -0.01 0 0.01 -1.3 -1.1 -0.9 -0.7 -0.5 -0.3 -0.1 jp /A c m -2 E vs Pt/V -2 -1.8 -1.6 -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0 2 4 6 8 10 Ep vs Pt/V jp /A c m -2 f1/2_/Hz1/2

Fig.5.SquarewavevoltammogramoftheLiF–CaF2–ZrF4(0.05molkg−1)systemat9Hzat840◦C.Blackline:deconvolutedsignal.Inset.Variationofthepeakcurrentdensity

andthepeakpotentialversusthesquarerootofthefrequency.Workingel.:Ta(S=0.31cm2_{);auxiliaryel.:glassycarbon;comparisonel.:Pt.}

method,derivedfromcyclicvoltammetry,thescanningofpotential proceedsstepwisewithsuperimpositiononeachstepofthe stair-caseoftwopotentialpulses,directandreverse,withequalvalues. Plottingthedifferentialcurrentmeasuredateachstepbetweenthe successivepulsesversusthepotentialassociatedtoeach electro-chemicalreaction,aGaussianshapedpeakisobtained.Inthecase ofareversiblesystem,amathematicalanalysisofthepeakyieldsa simpleequationassociatingthehalf-widthofthepeak(W_1/2)and thenumberofexchangedelectrons:

W1/2=3.52RT_nF (6)

A typical square wave voltammogram in LiF–CaF2–ZrF4

(c0=0.05molkg−1)isshowninFig.5atT=840◦Candf=9Hz.The

curveexhibitsonepeakatabout−1.1VvsPtcorrespondingtothe Ep/2ofthecyclicvoltammogram.Beforehand,thevalidityofEq.(7)

wasverifiedinFig.5inset,asfarasalinearrelationshipisobtained betweenthepeakcurrentandthesquarerootofthefrequency[31]. Asmentionedinpreviouspapers, thedistortion ofthepeak, in comparison withtheclassicalGaussian peak valid for a sol-uble/solublesystem, is due tothecurrentlessnucleationphase duringthemetaldeposition.Concerningquasi-reversiblesystem withasymmetricpeak,thereference[32]indicateshowto deter-minethehalf-peakwidth(W1/2).FromW1/2 measurements,the

numberofexchangedelectronsisequalto4.0±0.1.

Thus, bothmethods confirmthat Zr(IV)/Zr(0) reduction pro-ceedsinasinglestep,accordingto:

Zr(IV)4e−_{=Zr (7)}

3.1.1.4. Diffusioncoefficientdetermination. UsingEq.(1)or(3)and n=4, the Zr(IV) diffusion coefficient (D) was calculated to be (6.8±0.1)×10−6_cm2_s−1 _at840◦_{C, whateverthe determination}

method.Inthe810–920◦Ctemperaturerange,thelinearvariation oflnDversustheinverseabsolutetemperature,plottedinFig.6, followsanArrhenius’lawas:

ln D=−2.22(±0.03)−10765.10(±0.03)_T (8) FromEq.(8),theactivationenergyis89.5±0.2kJmol−1_.

Thisis ingood agreementwiththeactivationenergy deter-minedbyGroultet al.(76.2kJmol−1)inastudydevoted tothe LiF–NaF–ZrF4system[9].

3.1.2. ZrF4inLiF–CaF2–CaO

Fig.7presentscyclicvoltammogramsplottedonaTaelectrode at 100mVs−1 _{in LiF–CaF2–ZrF4 (0.11molkg}−1_{) aftersuccessive}

CaOadditions:theZr(IV)peakcurrentdensitydecreases gradu-allywiththeoxidecontent.Afteradditionof2.4molofCaO,no moreelectrochemicalsignalrelatedtotheZrsystemisobserved onTa.

Thenormalizedquantityofzirconiumionsn(Zr)hasbeen plot-tedversusthenormalizedoxideionscontentn(oxide),calculated as: n(Zr)=nZrF4 n0 ZrF4 and n(oxide)= nO2− n0 ZrF4 where n0

ZrF4 is the initial quantity of Zr(IV) in mol, nZrF4 the

remainingquantityofZr(IV)inmoldeterminedbycyclic voltam-metry,n_O2−theaddedquantityofO2−inmol.

-12.4 -12.2 -12 -11.8 -11.6 -11.4 -11.2 -11 0.82 0.84 0.86 0.88 0.9 0.92 0.94 ln ( D /c m ² s -1) (1000/T)/K-1

Fig.6.Variationofthelogarithmofthediffusioncoefficientversustheinverseof theabsolutetemperature.

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 j/ A c m -2 E vs Pt/V (A) (B) (C) (D)

Fig.7. VariationofthecyclicvoltammogramsoftheLiF–CaF2–ZrF4(0.11molkg−1)

systemwithoxideionsadditioninthesolutionat100mVs−1_and840◦_C.(A):

beforeoxideionsaddition;(B)oxideionsaddition:0.058molkg−1_{;(C)oxideions}

addition:0.143molkg−1_{;(D)oxideionsaddition:0.178molkg}−1_{.Workingel.:Ta}

(S=0.31cm2_{);auxiliaryel.:glassycarbon;comparisonel.:Pt.}

The data, presented in Fig. 8, exhibit a linear relationship betweenthequantityofaddedoxideandthedecreaseofZr(IV) con-centration.Asreportedinpreviousworkinthecaseofrareearth

[33],thisbehaviourisanevidenceoftheformationofasolid com-pound.ThefirstassumptionwasthatinsolubleZrO2wasformed

duringoxideadditions.However,asevidencedbythetheoretical slope ofZrO2 formationplotted in Fig.8, thisassumption does

notfitwithexperimentaldata.Itisthusverylikelythat oxyflu-orideZr–O–Fcompoundswereformedduringthereaction.Atthe operatingtemperature(840◦C),accordingtotheZrF4–ZrO2phase

diagram[16],therearethreenon-stoichiometricstableoxyfluoride phases:Zr(F,O)_4−x(0.25<x<0.34),Zr(F,O)3+x(0.33<x<0.50)and

Zr(F,O)_3−x(x_∼0.30).Thecompositionofthesephasesisreported inFig.8.Fromthesedata,itisthusbelievedthatadditionsofCaO intoaZrF4-containingLiF–CaF2saltleadtotheformationofaclose

toequimolarmixtureofsolidZrO2andZrO1.3F1.4.

3.2. Zr(IV)reductionmechanismonreactiveelectrode

This section is dedicated to the study of Zr(IV) reduction on reactive cathodes in LiF–CaF2 at 840◦C. Cu and Ni

elec-trodeswereselectedsincebothCu–ZrandNi–Zrsystemspresent many intermetallic compounds at the operating temperature

[34,35].

Fig.8.VariationofthenormalizedZr(IV)amountinthemeltversusthenormalized oxideionscontentaddedinthemelt.ComparisontoexistingphasesintheZr–O–F systemat840◦_C(1)Zr(F,O) 4−x(0.25<x<0.34)phase;(2)Zr(F,O)3+x(0.33<x<0.50) phase(3)Zr(F,O)3−x(x∼0.30)phase. -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 -1.5 -1 -0.5 0 0.5 1 1.5 j/ A c m -2 E vs Pt/V Ta Cu Ni

Fig. 9.Superposition of the cyclic voltammograms of the LiF–CaF2–ZrF4

(0.11molkg−1_)at100mVs−1 _and840◦_{C.Workingel.:Ta,Cu,Ni(S=0.31cm}2_);

auxiliaryel.:glassycarbon;comparisonel.:Pt. 3.2.1. Cyclicvoltammetry

ThecyclicvoltammogramspresentedinFig.9represent zirco-niumionsreductiononinert(Ta)andonreactiveelectrodes(Cu, Ni)at100mVs−1_{.Onbothreactiveelectrodes,additional}

reduc-tionwavesaredetectedataround-0.85VvsPtonCuand−0.75V vsPtonNipriortoZrmetaldepositiononTa,andareattributed toalloysformation.Theseshiftsinpotentialaredue toa depo-larisationeffectalsocalledunderpotentialdeposition:theZralloy formationoccursatmorepositivepotentialthanpureZr[36].

3.2.2. Electrolysisruns

Galvanostatic electrolyses in LiF–CaF2–ZrF4 (0.11molkg−1)

wereperformedduring30minat_−0.1Acm−2onNiandCuplates. Thecross-sectionofthesampleswasanalyzedbySEM–EDXand micrographsarepresentedinFig.10a(Cu)andb(Ni):severallayers ofintermetalliccompoundsareformedatthesurfaceofthe elec-trodes.Theircompositions,reportedfromtheinsidetotheoutside ofthestartingmaterial,are:

-Oncopperelectrode:Cu/Cu5Zr/Cu51Zr14/CuZr2

- Onnickelelectrode:Ni/Ni5Zr/Ni21Zr8/NiZr

Ni–ZrandCu–Zralloyshavethusbeenobtained,confirmingthe underpotentialdepositionofZr(IV)ionsonreactivecathodes.As previouslydemonstratedonothersystems,itwasnoticedonboth electrodesthatthecompoundwiththehighestNiorCucontentis obtainedattheboundaryoftheinitialNiorCumetalsubstrates

[37].

3.2.3. Gibbsenergycalculationsusingopencircuit chronopotentiometrydata

Zr-based compounds have been formed and identified after electrolysisrunsandopencircuitchronopotentiometrywasusedto determinetheGibbsenergyofformationofthesecompounds.This electrochemicalmethodishoweverrestrictedtobinarysystems containingintermetalliccompoundsratherthansolidsolutions.As theZr–Niphasediagramexhibitsbothsolidsolutionsand inter-metalliccompounds,thedeterminationofGibbsenergyvalueshas beenapplied totheCu–Zr systemand its6 intermetallic com-pounds:CuZr2,CuZr,Cu10Zr7,Cu8Zr3,Cu51Zr14andCu5Zr[34].

Themethodconsistsinfirstelectrodepositingasmallquantity ofZronthecathodebyashortcathodicrunandthenmeasuringthe opencircuitpotentialofthecathodeversustime.Theintermetallic diffusionofZrandCuleadstothesuccessiveformationofZr–Cu compounds,withadecreasingZrcontentfromthesurfacetothe bulkoftheCumaterial.Theopencircuitchronopotentiogramthen

Fig.10. SEMmicrographsofacrosssectionofCu(a)andNi(b)platesafterreduction ofZrF4at840◦CinLiF–CaF2–ZrF4(0.11molkg−1).Appliedcurrent=−0.1Acm−2,

time=30min.

exhibitsplateausofmetallicinterdiffusioncorrespondingto suc-cessiveintermetalliccompoundsformationatthesurfaceofthe electrode,as describedin [38,39].Eachpotentialplateauofthe chronopotentiogramcorrespondstoanequilibriumbetweentwo intermetalliccompoundsinthesolidstateanddiffusionofZrwithin thesubstrateexplainsthecathodepotentialincrease.

ThechronopotentiogramoftheCu–Zrsystemispresentedin

Fig.11where7plateaushavebeenobservedontopofpureZr.By

-1.6 -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0 100 200 300 400 500 600 700 800 900 1000 E v s P t/ V t/s

Fig.11.Open-circuitchronopotentiogramofglassyLiF–CaF2–ZrF4(0.11molkg−1)

onCuelectrode(S=0.31cm2_)at840◦_{CafterapolarizationatI=−0.2A.Auxiliary}

el.:glassycarbon;comparisonel.:Pt.

Table1

ExperimentalvaluesofGibbsenergyofformationcomparedtocalculatedonefrom SGTEdatabaseintheCu–Zrsystemat840◦_C.

Compound 1Evs.Zr/V 1rG(Jmol−1) 1fGexp(kJmol−1)

Thiswork SGTEdatabase Zr CuZr2 0.063 −24,318 −24±1 −38 CuZr 0.083 −32,038 −28±1 −28 Cu10Zr7 0.158 −60,988 −258±2 −242 Cu8Zr3 0.347 −133,942 −168±2 −148 Cu51Zr14 0.520 −200,720 −985±2 −844 Cu5Zr 0.724 −279,464 −90±2 −62

linearcombination(seereference[39]forcalculationdetails),the Gibbsenergyofformationofeachintermetalliccompoundhasbeen calculated.ThedataarecompiledinTable1.Formostcompounds,a rathergoodagreementisfoundbetweenthepresentexperimental valuesandtheonetabulatedintheSGTEdatabase[40].

4. Conclusions

TheZrF4 electrochemicalbehaviourhasbeeninvestigated in

LiF–CaF2at840◦Coninertelectrode(Ta)anditsreductiontakes

placeataround1Vfromthesolventreduction(Li+_{).Usingdifferent}

electrochemicaltechniques,itwasdemonstratedthatthe reduc-tionmechanismisaonestepprocessexchanging4electronsand controlledbytheZrionsdiffusioninthemoltensalt,inaccordance withpreviousstudiesinmoltenfluoridesolvents:Zr(IV)+4e−_=Zr.

Diffusioncoefficientshavealsobeendeterminedonawide tem-peraturerange(810–920◦_{C)anddatashowadependenceoflnD}

withtheinverseofthetemperature,asaclassicalArrhenius-type law:

lnD=−2.22(±0.03)−10765.10(±0.03)_T

Then,theeffectofoxideionsbyCaOadditioninaLiF–CaF2–ZrF4

systemhasbeenstudied.At850◦C, thezirconiumprecipitation isevidenced andtheconcentrationdecreaseofZr(IV)withCaO additionsindicatesthattheprecipitateismostprobablycomposed ofzirconiumoxideZrO2andzirconiumoxifluorideZrO1.3F1.4.On

nickel and copper electrodes, Zr(IV) reduction wasobserved at amorepositivepotentialthanpureZrdepositionandzirconium alloysformation(Ni–ZrandCu–Zr)wereevidencedbySEM–EDS. ThankstotheopencircuitchronopotentiometrydataontheZr–Cu system,theGibbsenergyofformationofthecorresponding inter-metalliccompounds(CuZr2,CuZr,Cu10Zr7,Cu8Zr3,Cu51Zr14,Cu5Zr)

wereestimatedandingoodagreementwiththeSGTEdatabase. Acknowledgement

ThisstudywaspartlyfinanciallysupportedbyCNRSwithinthe frameoftheFrenchPACENprogramme.

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