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ZnO-based varistors prepared by spark plasma sintering

Yannick Beynet, Antoine Izoulet, Sophie Guillemet-Fritsch, Geoffroy

Chevallier, Vincent Bley, Thomas Pérel, Frédéric Malpiece, Jonathan Morel,

Claude Estournès

To cite this version:

Yannick Beynet, Antoine Izoulet, Sophie Guillemet-Fritsch, Geoffroy Chevallier, Vincent Bley, et al..

ZnO-based varistors prepared by spark plasma sintering. Journal of the European Ceramic Society,

Elsevier, 2015, vol. 35 (n° 4), pp. 1199-1208. �10.1016/j.jeurceramsoc.2014.10.007�. �hal-01168805�

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Any correspondence concerning this service should be sent to the repository administrator:

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Identification number: DOI: 10.1016/j.jeurceramsoc.2014.10.007

Official URL: http://dx.doi.org/10.1016/j.jeurceramsoc.2014.10.007

This is an author-deposited version published in:

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

Eprints ID: 13984

To cite this version:

Beynet, Yannick and Izoulet, Antoine and Guillemet-Fritsch, Sophie and

Chevallier, Geoffroy and Bley, Vincent and Pérel, Thomas and Malpiece,

Frédéric and Morel, Jonathan and Estournès, Claude

ZnO-based varistors

prepared by spark plasma sintering

. (2015) Journal of the European Ceramic

Society, vol. 35 (n° 4). pp. 1199-1208. ISSN 0955-2219

O

pen

A

rchive

T

oulouse

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rchive

O

uverte (

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)

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

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ZnO-based

varistors

prepared

by

spark

plasma

sintering

Y.

Beynet

a

,

A.

Izoulet

a,c

,

S.

Guillemet-Fritsch

a

,

G.

Chevallier

a

,

V.

Bley

b

,

T.

Pérel

b

,

F.

Malpiece

c

,

J.

Morel

c

,

C.

Estournès

a,∗

aCIRIMATUMRCNRS5085,UniversitéPaulSabatier,118routedeNarbonne,31062ToulouseCedex9,France bLAPLACE,UniversitéPaulSabatier,118routedeNarbonne,31062ToulouseCedex9,France cTRIDELTAParafoudresS.A.,Boulevarddel’Adour,65202BagnèresdeBigorreCedex,France

Abstract

Varistorceramicswerepreparedbysparkplasmasintering(SPS)usingtwodopedzincoxidebasedpowders.Optimizedsinteringcycleforthe commercialpowder(mixtureofoxides)yieldeddenseparts(>99%)containingmainlyZnOgrains(1mm)andadditionalBi2O3andZn7Sb2O12

oxides.SinteringatthelowoxygenpartialpressureinherenttoSPSleadstothereductionofBi2O3 intometallicBi.Toreducethegrainsize,

improvethedistributionofdopantsandthegrainsizedistribution,weusedco-precipitationtosynthesizeapowderwiththesameformulationasthe commercialpowder.Duetothehighreactivityofthispowder,fullydensepartswereobtainedattemperatureaslowas400◦Cunderairatmosphere

usingcarbidetools.SuchconditionsledtolimitedZnOgraingrowthandbismuthreductionduringthethermalcycle.Thefinemicrostructure obtainedledtovaristanceswithtohighelectricalproperties(thresholdfieldhigherthan2300V/mm).

Keywords:Varistor;Sparkplasmasintering(SPS);DopedZnO;Electricalproperties

1. Introduction

Inordertomakethenewgenerationofaircraftmore economi-calandthusmorecompetitiveandfortheprotectionofthe envi-ronment,manufacturers havesoughttoreduceaircraftweight byintroducingalargeproportionofcomposites.Withtheuseof thesenewtypesofmaterials,theflowofchargesduringlightning strikewillbemuchlesseffectivethanwiththeuseoftraditional conductivematerials.Itisthereforenecessarytoprovidebetter protectionfortheelectronicdevicesonboard.Asaresult,more compactandefficientvaristorsmustbedeveloped.Avaristoris anelectroniccomponentwithhighlynonlinearcurrent–voltage characteristics. In 1979, Mahan et al.1 explained this highly nonlinear behaviour by electron tunnelling triggered by hole creation inthe dopedZnO. In 1987,Dorlanneet al.brought adifferentapproachtoexplainthephenomenonofconduction bytheactionofholeswithoutconsideringatunnellingeffect.2

Correspondingauthor.Tel.:+33561556109;fax:+33561556163.

E-mailaddress:estournes@chimie.ups-tlse.fr(C.Estournès).

Moreover,Emtage3hasshownthatthemeanbreakdown volt-agepergrainintheceramicislessthanthatofanisolatedgrain boundary.

Itwasthereforenecessarytoprepareceramicswithathinner microstructure thanthosealready available onthe market.4–6 Indeed, suchamicrostructureyieldsalargernumberofgrain boundariesperunitvolumeandhenceallowstheproductionof componentswithidenticalperformancestothosecurrentlyon themarket,butwithalowerweightandvolume.

Toachievethistarget,sparkplasmasintering(SPS),known toincreasethekineticsofmaterialdensification,toreducethe sinteringtemperatureandasaconsequencetocontrolthegrain growth,hasbeenconsidered.7–10Kougoetal.studiedthespark plasmasinteringofnon-dopedzincoxide.11Theyshowedthat thegrainsizeoftheceramicsobtainedincreases(1to100mm) withsinteringtemperature(450to1200◦C)andconsequently their electricalresistivitydecreases.Misawaetal.studied the influenceofcurrent flowthroughZnOsamplesduringSPS.12 Theinternaltemperatureofthesampleanditselectrical resis-tanceweremonitoredduringthesinteringcycle.Itwasshown that a temperature difference of 200K occurs between the http://dx.doi.org/10.1016/j.jeurceramsoc.2014.10.007

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specimen(Ts)andthesinteringdie(Td)duringthetimeof

max-imumtemperature(Ts>Td).Ningetal.usedanextrusiontool

withSPS toobtain ZnO films,witha c-axispreferred orien-tation, in order toimprove the component’s performances.13 Usingthistechnique,theymanagedtoreachalargenumberof grainboundariesperunitlengthsuggestingfuture implementa-tionofceramicsoflowerthicknessbutwithidenticalelectrical properties.

UsingSPS,someauthorshavealreadydensifiedpureZnO andBi2O3dopedZnOnanopowders.14TheyobservedaBi2O3 reduction phenomenawhenthe sinteringtemperaturereached 600◦C,resultingintheappearanceofBimetalmainlyatgrain boundaries.Theauthorsfoundoutthatthisreductionisperfectly reversible bypost-annealingthe ceramicsinairat650◦Cfor 10h.Thecoefficientofnonlinearityofthesesampleswaslow (between3.6and5.8)as aconsequence of thesimple binary composition.

Toinducethe“varistor”behaviourinZnO-basedceramicsa heavyelementwithalargeionicradiushastobeintroduced.15 Thisheavyelementisabletosegregateatthegrainboundary and toform an electronic conduction barrier between n-type semiconductiveZnOgrains.Bismuth,adopantpresentinmost commercialvaristors,actsatthegrainboundary,16 duetothe emergenceofaliquidphaseofBi2O3atapproximately820◦C thatpromotesgraingrowthduringsintering.Particularattention hastobepaidwithbismuthbecauseofitshighvolatility dur-ingthesinteringprocess.Antimony,meanwhile,isadopantthat inhibitsgraingrowththroughtheformationofspinelZn7Sb2O12 usually located at grain boundaries.17 Both nickel and anti-monyimprovethestabilityofthevaristor.Cobaltandmanganese oxidescauseanincreaseinthebarrierheightandtheresistivity ofthematerial.18,19

Inthepresentstudy,aZnOpowderdopedwithSb2O3,Bi2O3, Co3O4,NiOandMn2O3as inthepreparation ofcommercial varistorsbyTrideltaParafoudresCo.Ltd.wasdensifiedbySPS. Asecondpowder,ofthesamecomposition,butsynthesizedby coprecipitation,wasstudied.Theaimofthisworkwastoobtaina dopedpowderwithasmallerZnOgrainsizeanddistributionand betterdopanthomogeneity.BothpowdersweresinteredbySPS andthestructureandmicrostructureofthetwosetsofceramics wasdeterminedandcorrelatedtotheirelectricalproperties. 2. Experimentalprocedures

First, ZnO powder doped with Bi, Sb, Co, Mn and Ni was prepared by co-precipitation of an oxalate precursor. A schematic representationof the synthesis isshown inFig. 1. The startingmaterials for the synthesis of doped Zn oxalate precursor were zinc nitrate (Zn (NO3)2·6H2O, 98%, Alfa Aesar),bismuth nitrate (Bi(NO3)2·5H2O,98%,Alfa Aesar), cobaltnitrateCo(NO3)2·6H2O,98%,AlfaAesar),manganese nitrate (Mn(NO3)2·6H20, 98%, Alfa Aesar), nickel nitrate (Ni(NO3)2·6H2O,98%,Fluka)andantimonytrichloride(SbCl3, 99% Merck).Solution 1 was prepared by dissolving Zn, Bi, Co, MnandNisaltsindeionized water.Bismuthnitrate was dissolved in 1M HNO3 before adding it to solution 1. The acidicpH avoidedprecipitationof basicbismuth nitrate. The

Doped ZnO powder

Nucleation( 1 minute)

Crystal growth (15 min)

Calcination (500°C/1 hour, staticair) DopedZn oxalate Drying (80°C/22 hours) Sb solution Zn, Bi, Co, Mn, Ni solution (NH4)2C2O4.H2O (pH=9.3) solution 1stprecipitate (NH4)2C2O4.H2O 2ndprecipitate

Fig.1. Schematicrepresentationofthesynthesis.

metalconcentrationwasfixedat4M.Solution2wasprepared bydissolvingantimonychloridein4Mhydrochloricacid.Two solutionsof0.2Mammoniumoxalate((NH4)C2O4·H2O)were used as precipitants. ThepH of oneof them wasadjusted to 9.3byaddingNaOH.ThisgaveapH of around7 atthe end ofthe precipitationreaction whichpromotesthe smallestand mosthomogeneousZnOgrainsize.20 Solutions 1and2were introducedsimultaneouslyintothisammoniumoxalatesolution toinitiatenucleation.Afterstirringfor1min,thesecondsolution ofammoniumoxalatewasadded.Thissecondprecipitation pro-motedcrystalgrowth.Afterstirringfor15min,theprecipitates werewashedwithdeionizedwateranddriedat80◦Cfor22h. IPCanalysesconfirmthatthepowdersynthesizedhadthesame compositionas the commercialpowder. The precursors were calcinatedinambientatmosphereat500◦Cfor1hinstaticair. Thecalcinatedpowderwasgroundandsieved.

The samples were sintered using a Dr Sinter 2080 spark plasmasintering apparatus(SumitomoCoal MiningCo.Ltd., Japan)availableatthe“PlateformeNationaleCNRSdeFrittage Flash”(PNF2-CNRS)locatedatToulouse(France).The pow-derwasintroducedinan 8or 20mm innerdiametergraphite or tungstencarbide die linedwith0.2mm thick graphitefoil (Papyex-Mersen) then pre-compacted and placed in the SPS chamber.Apulsesequenceof12:2wasappliedtoheatthe sam-ple.Thetemperaturewasmonitoredusingathermocoupleina hole1.8mmindiameter and3mmdeep locatedatthe exter-nalsurfaceof thedie. Uniaxialpressurewas appliedatroom temperatureoratthedwelltemperature.

The structure andthe nature of the phases present in the powderandtheceramicsweredeterminedbyX-raydiffraction (XRD)usingaBrukerD4EndeavorX-raydiffractometer.The powderandtheceramicswereobservedbyscanningelectron microscopy(SEM).Thegrainsize,themorphologyofthe pow-deras wellasthephasedistributionandthemicrostructureof theceramicsweredeterminedusingaJeol6510LVmicroscope.

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50 45 40 35 30 25 20 In te n si ty ( a .u .) 2θ(°)

▫ ▫

*

*

l

l

Fig.2.X-Raydiffractogramofthecommercialpowder(dZnO, Bi2O3, Sb2O3, Co3O4, NiO).(Forinterpretationofthereferencestocolourinthisfigure legend,thereaderisreferredtothewebversionofthisarticle.)

Therateofdensificationoftheceramicsaftersinteringwas evaluatedbyusing ahydrostatic balance(Kern ARJ220-4M) andbygeometricmeasurements(whenthesamplehada densi-ficationlowerthan92%).

Priortoelectricalmeasurements,thepelletwasslightly pol-ished,andthengoldelectrodes(30nmthick)weredepositedby sputteringoneachsideof thepellet.Theelectrical character-isticsofthevaristorswerestudiedusingasourcemeasureunit (SMU)Keithley2410or ahighvoltagegenerator FUGHCN 6.5kVforvoltageshigherthan1100VavailableattheLaplace laboratory.The thresholdvoltageVS wasmeasuredfor a

cur-rentof1mA.Thecoefficientofnonlinearityαisdefinedbythe followingequation:

α= Log (I2/I1)

Log (V2/V1) (1)

where V1 and V2 are the electric fields corresponding to

I1=1mAandI2=10mA.TheleakagecurrentIfwasmeasured

foravoltageequaltoVS/2.

3. Results

3.1. Commercialpowder

The commercial oxide powder was produced by Tridelta ParafoudresS.A.Thedopants presentintheZnOphasewere Sb2O3,Bi2O3,Co3O4,NiOandMn2O3.Forconfidential rea-sons,theactualpercentageofthedopantscannotbegiven.

TheX-raydiffractionpatternof theinitialpowder(Fig.2) showsthestrongdiffractionlinesofthemajorZnOphaseand theweaklinesofthefouradditionaloxides:Sb2O3,a-Bi2O3, NiOandCo3O4.Mn2O3waspresentatlevelsbelowthelimit ofdetectionofthedevice.

SEM-FEGobservations(Fig.3)showaheterogeneous mor-phologyandanobviouswidegrainsizedistribution,withgrain sizesrangingfrom0.3to1micron.

Inordertoobtain denseceramicsatlowtemperatures,the sintering was carried out by spark plasma sintering (SPS). Severalparametersofthesinteringprocedurewereoptimized,

i.e. the dwelltemperature (700<T<1100◦C),the dwelltime (0<t<15min), the compacting pressure (50<P<130MPa) applied atroom temperature or atthe dwell temperature, the sinteringatmosphere(vacuumorargon),theheatingrate(100 or200◦C/min).

Fig.4showshowtherelativedensityoftheceramicsvaried as a functionof the sintering temperature.An optimum rela-tivedensitybetween99and100%wasobtainedforasintering temperatureof870◦C.

Foreachtemperature,theotherparameterswerevaried,and thestructureandthemicrostructureoftheceramicschecked.The heatingrate,theappliedpressure,andthesinteringatmosphere (vacuumorargon)hadarelativelylowinfluenceonthe com-positionandthemicrostructureoftheceramics.Theceramics werecharacterizedbyX-raydiffractionanalyses.Threephases canbeindentified: ZnO,Zn7Sb2O12 andmetallicBi(Fig.5). MetallicBiresultsfromthereductionofBi2O3.Indeed,theuse of agraphitedieimplies workingatlow oxygenpartial pres-sure(undervacuumorinertgas)topreventitsdegradation.At hightemperaturethisthenleadstothereductionofpartofthe Bi2O3intometallicbismuth.14ThepresenceofmetallicBiisof

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90 91 92 93 94 95 96 97 98 99 100 1000 980 960 940 920 900 880 860 840 820 800 R e la 9 v e d e n sity (%) Temperature (°C)

Fig.4.Variationoftherelativedensityoftheceramicsobtainedversusthesinteringtemperature,forvariousconditions( :100MPaundervacuum5min100◦C/min Loadappliedatroomtemperature, :100MPaundervacuum5min100◦C/minLoadappliedatdwelltemperature, :100MPaundervacuum0min100C/min Loadappliedatroomtemperature,⋆:100MPaundervacuum1min100◦C/minLoadappliedatroomtemperature,+100MPaundervacuum15min100C/min Loadappliedatroomtemperature, :50MPaundervacuum5min100◦C/minLoadappliedatroomtemperature, :100MPaundervacuum0min200C/min Loadappliedatroomtemperature, :100MPaAr0min100◦C/minLoadappliedatroomtemperature, :100MPaAr5min100C/minLoadappliedatroom temperature).(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

50 45 40 35 30 25 20 In te n si ty ( a .u .) 2θ(°)

*

*

▫ ▫

Fig.5.XRDdiffractogramofceramicssinteredSPSat870◦C(•ZnO, Bi, Zn7Sb2O12).(Forinterpretationofthereferencestocolourinthisfigurelegend,the readerisreferredtothewebversionofthisarticle.)

20 25 30 35 40 45 50 In te n si ty ( a .u .) 2θ(°)

*

*

*

Fig.6.XRDdiffractogramofceramicssinteredbySPSat870◦Candannealedat750C(•ZnO, Bi2O3, Zn7Sb2O12).(Forinterpretationofthereferencesto colourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

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coursenotdesiredintheformulationbecauseitisaconductor.In ordertoreoxidizemetallicBiintoBi2O3,theceramicsarethen annealedinair.PreviousworkshowedthatmetallicBipresentin dopedZnOsinteredbySPS14canbereoxidizedintoBi2O3after annealinginairat750◦Cfor10hataheatingrateof1C/min. Infact,threephasesareobserved:ZnO,Bi2O3andZn7Sb2O12 after annealing the samples under the same conditions (Fig.6).

SEM observations of the pellets sintered at 870◦C under vacuum,with dwell times of 0, 1 and 15min, aheating rate of 100◦C/min and cold compaction at 100MPa are shown onFig.7.Thezinc oxidephase exhibitsgrainsizes ofabout 1mm,4mmand8mm,respectively,fordwelltimesof0,1and 15min. Moreover,small grains, aboutafewtens of nanome-tres, were observed and indentified by EDS analyses as the Zn7Sb2O12phase.BS-SEMobservationonpolishedsample sin-teredat 870◦C undervacuum,without dwelltime, a heating rate of 100◦C/min andcoldcompaction at100MPa (Fig.8) shows the threephases characteristics of a varistor: the ZnO grainappear indark gray, spinelphase inlight grayandthe bright elements are Bi2O3. Unfortunately the microstructure (grainsize and phasedistribution) isless visiblethan onthe fracturedsamplespresented onFig.7 ofthe article.Wehave attempted several conditions for both chemical and thermal etching on the dense samples to reveal the grain boundaries without any success due to the high volatility of the Bi2O3 species.

3.2. Co-precipitatedpowder

In ordertoreduce the grainsize andthe grainsize distri-bution and to improve the distribution of dopants, an oxide powder of the same composition as the commercial powder, wasobtainedbyco-precipitationfollowedbycalcination.Fig.9

showsthemicrostructureofthecalcinatedpowder.Thegrains havearegular,moreorlesscubicshape.Theaveragegrainsize isapproximately700nm.Athighmagnification,crystallitesof about20nmarevisibleinthegrains.

X-Ray diffraction (XRD) of the co-precipitated powder (Fig.10)showsthezincoxidephaseandtwoantimonyoxide phases(Sb2O3andSb2O4).Thetwoantimonyoxideformswere explainedbyPeiteadoetal.,astheyhaveshownthatat500◦C Sb2O3isoxidizedintoSb2O4(Sb2O3+1/2O2→Sb2O4).21In ourcase,Sb2O3wasnotcompletelyoxidized.Theotherdopants i.e.Bi2O3,Co3O4,NiOandMn2O3werenotdetectedbyX-Ray diffractionanalyses.Thelargerwidthofthepeaksofthe synthe-sizedpowder,comparedwiththecommercialpowder,indicates smallercrystallitesize.

Atfirst,thepowderobtainedbythechemicalroutewas sin-tered bySPS using the sameconditions as thoseestablished forcommercialpowder(870◦C,nodwell,appliedpressureof 100MPa,vacuum).Duringthesintering,thermalshrinkagewas monitoredbyadilatometer.Fig.11showsthesinteringprofiles ofbothpowders.Thedensificationof thecommercialpowder startsat700◦C,whiletheshrinkageofthesynthesizedpowder startsattemperaturesaslowas200◦C.Thelowertemperatureof

Fig.7.SEMmicrographsfractureofpelletssinteredat870◦Cwithadwelltime of(a)0min(b)1min(c)15min.

theonsetofdensificationstartwasexplainedbythesmallergrain sizeofthesynthesizedpowder,involvinghigherdensification.

The thermalshrinkage curvesshowed that it was possible tosinter thesynthesizedpowder atverylowtemperatures. In

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Fig.8.BSSEMpolishedmicrographofpelletmadefromcommercialpowder sinteredat870◦C,100MPaand0minundervacuumatmosphere.

Fig. 9.SEM (a) and FEG-SEM (b) micrographs of the doped ZnO co-precipitatedpowder.

Table1

Densificationofthesynthesizedpowdersinteredunderairatmosphere. Sintering temperature (◦C) Dwelltime (min) Pressure (MPa) Density (g/cm3) Densification (%) 400 3 500 5.45 96.5 500 3 250 5.50 97.3 600 3 250 5.56 98.4

this case, three sintering temperatures were chosen: 400◦C, 500◦C and 600C, with a dwell time of 3min. Decreasing the sintering temperature involvedincreasing the compacting pressure(250MPaand500MPa)toobtaindensesamples.High compactingpressureimpliedtheuseofatungstencarbidedie, knownto bepressureresistant.Then, sintering could be per-formedunderairatmosphere,andthereductionofbismuthoxide intometallicbismuth shouldbe avoided.HowevertheX-Ray diffractionanalysescarriedoutonthepelletsstill showedthe peakofmetallicBi(Fig.12),butsmaller.Itwasthereforestill necessaryto annealthe pellet after sintering to reoxidizethe metallicBi.Moreover,theX-Raydiffractionanalysesindicated theZnOphaseandthespinelphaseZn7Sb2O12(Fig.12).Itcan benotedthat atemperature of400◦Cisnotenough toallow completereactionbetweenSb2O4andZnOtoformthespinel phase.

Thedensifications of thepellets arehigh, between96 and 98%,whateverthesinteringconditions(Table1).SEM obser-vations(Fig.13)wereperformedonthepelletsinteredat600◦C. Theceramicgrainsizerangedfrom0.3to0.6mm,whichwas lowerthanthatofpelletspreparedfromthecommercialpowder (1mm).Thisceramicwithsuchsmallgrainsizeisobtaineddue tothe moderategrowthof thecrystallitesoccurringatsolow sinteringtemperature.Fig.14BS-SEMobservationonpolished pelletsinteredat600◦C.ThisimageshowstheZnOgrainsin grayandtheBi-richphasesinwhite.

4. Electricalcharacterization

4.1. Influenceofthedwelltimeduringsinteringofthe

commercialpowder

TheelectricalcharacteristiccurvelogJ(currentdensity)vs logE(electricalfield)obtainedatroomtemperatureforthepellet sinteredbySPSat870◦C,withadwelltimerangingfrom0to 15min,withapressureof100MPaandsubsequentlyannealed inairat750◦Cfor10hisreportedinFig.15.Avaristoreffect isclear.Indeed,astrongnonlinearbehaviourisseeninawide rangeofvoltageswithasignificantchangeinslope.Themain electricalcharacteristicsarereportedinTable2.

The electrical field decreases as the grain size increases. Indeed,ifgrainsaresmaller,therearemoreZnOgrainsperunit volumeandso, morepotentialbarriersper unitvolume. This characteristicis particularlyinteresting because it shows that thethreshold fieldcanbeoptimized bycontrollingthe dwell time.

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50 45 40 35 30 25 20 In te n si ty (a .u .) 2θ( °)

▫ ▫

*

*

l

l

*

+

+

a

b

Fig.10.X-Raydiffractogramsof(a)commercialpowder(•ZnO, Bi2O3, Sb2O3, Co3O4, NiO)and(b)synthesizedpowder(•ZnO, Sb2O3,+Sb2O4).(For interpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

0 1 2 3 4 5 1000 900 800 700 600 500 400 300 200 100 0 Displacement (mm) Temperature (°C)

a

b

Fig.11.Sinteringprofilesof(a)commercialpowder(b)synthesizedpowder.

20 25 30 35 40 45 50 In te n si ty ( a .u .) 2θ(°)

*

*

*

*

*

*

+

a

b

c

Fig.12.XRDdiffractogramsofpelletsmadefromsynthesispowdersinteredbySPSat(a)400◦C500MPa,(b)500C250MPa(c)600C250MPa(•ZnO, Bi, Zn7Sb2O12,+Sb2O4).(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

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Table2

Electricalcharacteristicsofthecommercialpowdersinteredwithdifferentdwelltimes. Sintering

temperature(◦C)

Dwelltime(min) Pressure(MPa) Grainsize(mm) E*(V/cm) α I

f(nA) 870 0 100 1 880 27 1900 870 1 100 4 500 50 1430 870 15 100 8 260 37 1070 * E:breakdownfield. Table3

Electricalcharacteristicsofthecommercialandthesynthesizedpowdersinteredindifferentconditions. Powder SinteringT(◦C)

Dwelltime(min) Pressure(MPa)

Annealed

T(◦C)/t(h)

Grainsize(mm) E*(V/cm) α If(nA)

Commercial 870/1/100 750/10 1–2 880 27 1900

Synthesized 870/1/100 750/10 2–3 1182 28 2440

Synthesized 600/3/250 580/10 0.3–0.6 >2300

* E:breakdownfield.

Fig.13.SEMfracturemicrographsofpelletsmadefromsynthesizedpowder sinteredat600◦Cand250MPa.

4.2. InfluenceofthepowderandtheSPSparameters

In order to improve the characteristics of the varistor, a ZnO based powder was synthesized and sintered by SPS at lowtemperature(600◦C)underairatmosphere.Varistorswith verysmallgrainswereprepared.Theelectricalcharacteristics curve of these varistors is reported in Table 3 and Fig. 16. Thisfigurealso reports the characteristics of the commercial and the synthesized powder sintered at higher temperatures (870◦C).

The varistors prepared from the synthesized powders and sintered at low temperature presented an electrical field that was significantlyhigher than that of ceramics prepared from commercialpowders. A value of morethan 2300V/cm (this value was limited by the equipment used) was noted in the varistorsinteredat600◦C.In thecaseofvaristorssinteredat 870◦C,thoseobtainedfromthesynthesizedpowdershoweda

Fig.14.BSSEMpolishedmicrographofpelletmadefromsynthesizedpowder sinteredat600◦C,250MPaand3minunderairatmosphere.

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1E-09 1E-08 1E-07 1E-06 1E-05 1E-04 1E-03 1E-02 1E-01 1E+00 1000 100 10 Lo g J (A /c m ²) Log E (V/mm)

a

b

c

Fig.15.E–Jcharacteristicsofpelletssinteredat870◦Cwithadwelltimeof:(a)15min,(b)1minand(c)0min.

1E-11 1E-10 1E-09 1E-08 1E-07 1E-06 1E-05 1E-04 1E-03 1E-02 1E-01 1E+00 10000 1000 100 10 Lo g J (A /c m ²) Log E(V/mm)

d

e

f

Fig.16.E–JcharacteristicsofpelletssinteredbySPS:(d)commercialpowderSPSsintered(870◦C,100MPa,1min)undervacuum,(e)synthesizedpowderSPS sintered(870◦C,100MPa,1min)undervacuum,and(f)synthesizedpowderSPSsintered(600C,250MPa,3min)underairatmosphere.

higherbreakdownvoltagethanthosepreparedfromthe com-mercialpowder,although itsgrainsizewashigher.The more homogeneous dopant distribution in the ceramic probably explainsthisresult.

5. Conclusion

Nano-sizedZnO-dopedpowderwassynthesizedby coprecip-itation.ItwassinteredbySPSatlowtemperature,i.e.600◦C, withhighcompacturepressure(250MPa)andunderanair atmo-sphere. Sintering of small grains at low temperature yielded varistors withvery low grainsize: 300–600nm. In thiscase, thethresholdfieldwasincreasedfrom880V/mmtomorethan 2300V/mm. Controlling the grainsize allowed the threshold fieldtobeoptimized.

Acknowledgements

Theseresearchworksweremadewithintheframeworkofthe projectPREFACE(ProjectofStudyLightningonmoreElectric CompositePlane).Theauthorswouldliketothanktheprojects leaders ASTECH:Hispano-Suiza,AESE:SafranEngineering Service,PEGASE:Eurocopter,andtheDGEandRegion Midi-PyérénéesfortheirfinancialsupportunderGrantsNo. 08-2-93-0744.ThesupportofthePlateformenationaleCNRSdefrittage flashwasgratefullyappreciated.

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