ZnO-based varistors prepared by spark plasma sintering

(1)

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

[email protected]

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

Open Archive Toulouse Archive Ouverte (OATAO)

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

makes it freely available over the web where possible.

(2)

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,∗

a_CIRIMAT_UMR_CNRS_5085,_Université_Paul_Sabatier,₁₁₈_route_de_Narbonne,₃₁₀₆₂_Toulouse_Cedex_9,_France b_LAPLACE,_Université_Paul_Sabatier,₁₁₈_route_de_Narbonne,₃₁₀₆₂_Toulouse_Cedex_9,_France c_TRIDELTA_Parafoudres_S.A.,_Boulevard_de_l’Adour,₆₅₂₀₂_Bagnères_de_Bigorre_Cedex,_France

Abstract

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

oxides.SinteringatthelowoxygenpartialpressureinherenttoSPSleadstothereductionofBi2O3 intometallicBi.Toreducethegrainsize,

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

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

∗_{Corresponding}_author._Tel.:₊₃₃_561556109;_fax:₊₃₃_561556163.

E-mailaddress:[email protected](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)_and_consequently

their electricalresistivitydecreases.Misawaetal.studied the influenceofcurrent flowthroughZnOsamplesduringSPS.12 Theinternaltemperatureofthesampleanditselectrical resis-tanceweremonitoredduringthesinteringcycle.Itwasshown

that a temperature difference of 200K occurs between the

(3)

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,_resulting_in_the_appearance_of_Bi_metal_mainly_at_grain

boundaries.Theauthorsfoundoutthatthisreductionisperfectly reversible bypost-annealingthe ceramicsinairat650◦_C_for

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 1st_precipitate (NH₄)₂C₂O₄.H₂O 2nd_precipitat_e

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◦_C_for₂₂_h.

IPCanalysesconfirmthatthepowdersynthesizedhadthesame

compositionas the commercialpowder. The precursors were

calcinatedinambientatmosphereat500◦_C_for₁_h_in_static_air.

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.

(4)

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

•

• ▫ ▫

▫

*

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 _dwell_time

(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

(5)

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/min_Load_applied_at_dwell_temperature, _:₁₀₀_MPa_under_vacuum₀_min₁₀₀◦_C/min

Loadappliedatroomtemperature,⋆:100MPaundervacuum1min100◦_C/min_Load_applied_at_room_temperature,₊₁₀₀_MPa_under_vacuum₁₅_min₁₀₀◦_C/min

Loadappliedatroomtemperature, :50MPaundervacuum5min100◦_C/min_Load_applied_at_room_temperature, _:₁₀₀_MPa_under_vacuum₀_min₂₀₀◦_C/min

Loadappliedatroomtemperature, :100MPaAr0min100◦_C/min_Load_applied_at_room_temperature, _:₁₀₀_MPa_Ar₅_min₁₀₀◦_C/min_Load_applied_at_room

temperature).(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

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

▫

*

• ▫

*

▫ ▫

▫

•

• ▫

▫

Fig.5.XRDdiffractogramofceramicssinteredSPSat870◦_C_(•ZnO, _Bi, _Zn

7Sb2O12).(Forinterpretationofthereferencestocolourinthisfigurelegend,the

readerisreferredtothewebversionofthisarticle.)

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

▫

*

•

Fig.6.XRDdiffractogramofceramicssinteredbySPSat870◦_C_and_annealed_at₇₅₀◦_C_(•ZnO, _Bi

2O3, Zn7Sb2O12).(Forinterpretationofthereferencesto

(6)

coursenotdesiredintheformulationbecauseitisaconductor.In ordertoreoxidizemetallicBiintoBi2O3,theceramicsarethen

annealedinair.PreviousworkshowedthatmetallicBipresentin dopedZnOsinteredbySPS14canbereoxidizedintoBi2O3after

annealinginairat750◦_C_for₁₀_h_at_a_heating_rate_of₁◦_C/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 ₁₀₀_MPa _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 _under_vacuum,_without _dwell_time, _a _heating

rate of 100◦_C/min _and_cold_compaction _at₁₀₀_MPa ₍_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,_no_dwell,_applied_pressure_of

100MPa,vacuum).Duringthesintering,thermalshrinkagewas monitoredbyadilatometer.Fig.11showsthesinteringprofiles

ofbothpowders.Thedensificationof thecommercialpowder

startsat700◦_C,_while_the_shrinkage_of_the_synthesized_powder

startsattemperaturesaslowas200◦_C._The_lower_temperature_of

Fig.7.SEMmicrographsfractureofpelletssinteredat870◦_C_with_a_dwell_time

of(a)0min(b)1min(c)15min.

theonsetofdensificationstartwasexplainedbythesmallergrain sizeofthesynthesizedpowder,involvinghigherdensification.

The thermalshrinkage curvesshowed that it was possible

(7)

Fig.8.BSSEMpolishedmicrographofpelletmadefromcommercialpowder sinteredat870◦_C,₁₀₀_MPa_and₀_min_under_vacuum_atmosphere.

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 ₆₀₀◦_C, _with _a _dwell _time _of ₃_min. _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◦_C_is_not_enough _to_allow

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._This_image_shows_the_ZnO_grains_in

grayandtheBi-richphasesinwhite.

4. Electricalcharacterization

4.1. Influenceofthedwelltimeduringsinteringofthe

commercialpowder

TheelectricalcharacteristiccurvelogJ(currentdensity)vs logE(electricalfield)obtainedatroomtemperatureforthepellet sinteredbySPSat870◦_C,_with_a_dwell_time_ranging_from₀_to

15min,withapressureof100MPaandsubsequentlyannealed inairat750◦_C_for₁₀_h_is_reported_in_Fig.₁₅_._A_varistor_effect

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.

(8)

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

▫ ▫

▫

*

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◦_C₅₀₀_MPa,_(b)₅₀₀◦_C₂₅₀_MPa_(c)₆₀₀◦_C₂₅₀_MPa_(•ZnO, _Bi,

(9)

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_:_breakdown_field. 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_:_breakdown_field.

Fig.13.SEMfracturemicrographsofpelletsmadefromsynthesizedpowder sinteredat600◦_C_and₂₅₀_MPa.

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)_under_air_atmosphere._Varistors_with

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 _the_case_of_varistors_sintered_at

870◦_C,_those_obtained_from_the_synthesized_powder_showed_a

Fig.14.BSSEMpolishedmicrographofpelletmadefromsynthesizedpowder sinteredat600◦_C,₂₅₀_MPa_and₃_min_under_air_atmosphere.

(10)

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◦_C_with_a_dwell_time_of:_(a)₁₅_min,_(b)₁_min_and_(c)₀_min.

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,₁₀₀_MPa,₁_min)_under_vacuum,_(e)_synthesized_powder_SPS

sintered(870◦_C,₁₀₀_MPa,₁_min)_under_vacuum,_and_(f)_synthesized_powder_SPS_sintered₍₆₀₀◦_C,₂₅₀_MPa,₃_min)_under_air_atmosphere.

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.

References

1.MahanGD,LevinsonLM,PhilippHR.TheoryofconductioninZnO varis-tors.JApplPhys1979;50:2799–812.

2.DorlanneO,TaoT.Thegrainjunctioninzincoxidevaristors.HighTechnol Ceramics1987;38:1809–17.

3.EmtagePR.Statisticsandgrainsizeinzincoxidevaristors.JApplPhys 1979;50:6833–7.

4.Gupta TK. Application of zinc oxide varistors. J Am Ceram Soc 1990;73:1817–40.

5.BartkowiakM,Mahan GD,ModineFA, AlimMA,LaufR,McMillan A.VoronoinetworkmodelofZnOvaristorswithdifferenttypesofgrain boundaries.JApplPhys1996;80:6516–22.

6.ÖzgürÜ,Alivov YI,LiuC,TekeA,ReshchikovMA,Do˘ganS,etal. A comprehensive review of ZnO materials and devices.J Appl Phys 2005;98:41301.

7.ChaimR,LevinM,ShlayerA,EstournèsC.Sinteringanddensification ofnanocrustalline ceramicoxide powders.A review.AdvAppl Ceram 2008;27:159–69.

8.EstournèsC,OquabD,SelezneffS,BoidotM,MonceauD,GrossinD, etal.Shapingofnanostructuredmaterialsorcoatingsthroughsparkplasma sintering.MaterSciForum2012;70:6–709(24-30).

9.ChaimR,MarderR,EstournèsC,ShenZ.Densificationandpreservationof theceramicnanocrystallinecharacterbysparkplasmasintering.AdvAppl Ceram2012;111(5&6):280–5.

10.OrruR,Licheria R,Mario Locci AM,CincottiA, CaoG. Consolida-tion/synthesisofmaterialsbyelectriccurrentactivated/assistedsintering. MaterSciEngRes2009;63:127–287.

11.KougoT,YoshikawaA.Fabricationandcharacterizationofpolycrystalline bulkZnOwithlargegrainsizeofsimilarto100mumbythesparkplasma sintering.PhysStatusSolidiC2006;3:785–8.

(11)

12.MisawaT,ShikataniN,KawakamiY,EnjojiT,OhtsuY,FujitaH. Observa-tionofinternalpulsedcurrentflowthroughtheZnOspecimeninthespark plasmasinteringmethod.JMaterSci2009;44:1641–51.

13.NingJL,JiangDM,KimKH,ShimKB.Influenceoftextureon electri-calpropertiesofZnOceramicspreparedbyextrusionandsparkplasma sintering.CeramInt2007;33:107–14.

14.SaintMacaryL,KahnML,EstournèsC,FauP,TrémouillesD,BafleurM, etal.Sizeeffectonpropertiesofvaristorsmadefromzincoxide nanopar-ticlesthroughlowtemperaturesparkplasmasintering.AdvFunctMater 2009;19:1775–83.

15.TsaiJK,WuTB.NonohmiccharacteristicsofZnO–V2O5ceramics.JAppl

Phys1994;76:4817–22.

16.GreuterF,BlatterG.Electricalpropertiesofgrainboundariesin polycrys-tallinecompoundsemiconductors.SemicondSciTechnol1990;5:111–37.

17.PeiteadoM,FernandezJF, CaballeroAC.Processingstrategies to con-trol graingrowth in ZnObased varistors.J EurCeram Soc 2005;25: 2999–3003.

18.OnreabroyW,SirikulratN.Effectsofcobaltdopingonnonlinearityofzinc oxide.MaterSciEngB—Adv2006;130:108–30.

19.HongYW,KimJH.TheelectricalpropertiesofMn2O3dopedZnO.Ceram

Int2004;30:1301–6.

20.MoryJE,GuyI,SchneiderD,RoussetA,LegrosR,PeigneyA.Dopedzinc oxidepowder,preparationthereoandceramicproducedtherefrom.Patent WO1994022765A1.1994.

21.PeiteadoM,DelaRubiaMA,FernandezJF,CaballeroAC.Thermal evo-lutionofZnO–Bi2O3–Sb2O3systemintheregionofinterestvaristors.J