Journal of Asian Ceramic Societies

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Journal of Asian Ceramic Societies

jo u r n al h om ep ag e :w w w . e l s e v i e r . c o m / l o c a t e / j a s c e r

Full Length Article

Strong and anisotropic magnetoelectricity in composites of

magnetostrictive Ni and solid-state grown lead-free piezoelectric BZT–BCT single crystals

Haribabu Palneedi

^a^,^b

, Venkateswarlu Annapureddy

^b

, Ho-Yong Lee

^c

, Jong-Jin Choi

^b

, Si-Young Choi

^b

, Sung-Yoon Chung

^a

, Suk-Joong L. Kang

^a,d

, Jungho Ryu

^b,∗

aKoreaAdvancedInstituteofScienceandTechnology(KAIST),Daejeon34141,RepublicofKorea

bKoreaInstituteofMaterialsScience(KIMS),Changwon51508,RepublicofKorea

cSunmoonUniversity,Asan31460,RepublicofKorea

dKoreaInstituteofCeramicEngineeringandTechnology(KICET),Jinju52851,RepublicofKorea

a r t i c l e i n f o

Articlehistory:

Received23July2016

Receivedinrevisedform3November2016 Accepted26December2016

Availableonline5January2017

Keywords:

Lead-free Magnetoelectric Composites Anisotropic Piezoelectric Singlecrystal

a b s t r a c t

Aimedatdevelopinglead-freemagnetoelectric(ME)compositeswithperformancesasgoodaslead (Pb)-basedones,thisstudyemployed(001)and(011)oriented82BaTiO3-10BaZrO3-8CaTiO3(BZT–BCT) piezoelectricsinglecrystals,fabricatedbythecost-effectivesolid-statesinglecrystalgrowth(SSCG) method,incombinationwithinexpensive,magnetostrictivebasemetalNickel(Ni).Theoff-resonance, directMEcouplinginthepreparedNi/BZT–BCT/Nilaminatecompositeswasfoundtobestronglydepen- dentonthecrystallographicorientationoftheBZT–BCTsinglecrystals,aswellastheappliedmagnetic fielddirection.LargerandanisotropicMEvoltagecoefficientswereobservedforthecompositemade usingthe(011)orientedBZT–BCTsinglecrystal.TheoptimizedMEcouplingof1V/cmOewasobtained fromtheNi/(011)BZT–BCTsinglecrystal/Nicomposite,inthed32modeofthesinglecrystal,whena magneticfieldwasappliedalongits[100]direction.Thisperformanceissimilartothatreportedfor theNi/Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3(PMN–PZT)singlecrystal/Ni,butlargerthanthatobtainedfrom theNi/Pb(Zr,Ti)O3 ceramic/Nicomposites.Theresultsofthisworkdemonstratethattheuseoflead- freepiezoelectricsinglecrystalswithspecialorientationspermitstheselectionofdesiredanisotropic properties,enablingtherealizationofcustomizedMEeffectsincomposites.

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

1. Introduction

Magnetoelectric(ME)materialscontinuetodrawalotofatten- tionduetotheirwidespectrumofpossibleapplications,including sensing, transduction, memory, and energy harvesting systems [1–4].Comparedtootherheterostructures,laminatestructuredME compositesconsistingofmagnetostrictiveandpiezoelectriclayers areeasiertofabricateandhavebeenfoundtoexhibitsuperiorME responses[5].Lead(Pb)-basedferroelectricssuchasPb(Zr,Ti)O₃ (PZT), Pb(Mg_1/3Nb_2/3)O3-PbTiO3 (PMN-PT), Pb(Zn_1/3Nb_2/3)O3- PbTiO₃ (PZN-PT), and Pb(Mg_1/3Nb_2/3)O₃-Pb(Zr,Ti)O₃ (PMN–PZT) areoftenemployedin MEcompositesduetotheirlargepiezo- electriccoefﬁcients(d_ij,g_ij)andelectromechanicalcouplingfactors

∗Correspondingauthor.Fax:+82552803392.

E-mailaddress:[email protected](J.Ryu).

(k_ij)[6].Althoughtheseceramicshavebeenfoundtoshowgood MEperformancewhencombinedwithmagnetostrictivemetallic alloys(Terfenol-D,Metglas),becauseofenvironmentalconcerns over theuseoftoxic and hazardousPb containingmaterials,it is now necessary to develop eco-friendly MEcomposites with lead-freepiezoelectricmaterials.Inaddition,replacingexpensive magnetic alloys with thewidely available basemetals suchas Nickel(Ni)isofinterestfortheeconomicalproductionofMEcom- posites.

Mostoftheinvestigatedlead-freeMEcompositeshavebeen basedon(K,Na)NbO₃(KNN),Na_0.5Bi_0.5TiO₃(NBT),andBaTiO₃(BT).

But,duetothelowpiezoelectricity(d₃₃<300pC/N,inmostcases) oftheselead-freeceramics,thecorrespondingMEcompositeshave beenfoundtoexhibit weaker MEresponses(onthe orderof a few hundred mV/cmOe) than those of the Pb-based ME composites [6].Recently, a newlead-freeceramic systembased on aBa(Zr0.8Ti0.2)O3-(Ba0.7Ca0.3)TiO3 solidsolutionwasreportedto

http://dx.doi.org/10.1016/j.jascer.2016.12.005

2187-0764/©2017TheCeramicSocietyofJapanandtheKoreanCeramicSociety.ProductionandhostingbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

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exhibit an elevatedpiezoelectric coefﬁcient (d₃₃ ∼620pC/N) at optimalcomposition,whichiscomparabletothatofhigh-endPZT (d₃₃∼500–600pC/NforPZT-5H)[7,8].Hence,MEcompositespre- paredusingBZT–BCTceramicsareexpectedtoexhibitgood ME performance.Nevertheless,therearehardlyanyreportsavailable ontheBZT–BCTceramicbasedMEcomposites[9].

In comparison, singlecrystal piezoelectric materialsdemon- stratebetterpiezoelectricperformancethantheirpolycrystalline counterparts due to their uniform dipole alignment [10].

Relaxor-basedferroelectricsinglecrystals(PMN-PT,PZN-PT,and PMN–PZT), when cut and poled along speciﬁc crystallographic directions, are known to offer excellent strain responses and piezoelectricproperties.Theserhombohedralstructured,domain engineered single crystals display greatly enhanced piezoelec- triccoefﬁcientsandelectromechanicalcouplingfactorsalongthe [001]and[011]non-polarorientations,ascomparedtothe[111]

spontaneouspolarizationdirection[11].Resultshaveshownthat designingMEcompositeswithspecialorientedsinglecrystalsisan efﬁcientapproachtooptimizingtheMEcoupling[12–14].Inter- estingrelationshavebeenidentiﬁedbetweenthedirectionalME couplingand the material constantsof the differently oriented piezoelectricsinglecrystals.Althoughtheprocessingofpolycrys- tallineBZT–BCTceramicshasbeenconsiderablywellreported,the synthesisof BZT–BCTsinglecrystalshasrarelybeenattempted, since it is expensive as well as challenging to grow lead-free singlecrystalsofusablesizeswithuniformcomposition[15,16].

Recently,singlecrystalsof BZT–BCTwithgoodchemical homo- geneityandbetterpiezoelectricpropertieshavebeenproducedvia thesolid-stateconversionofpolycrystalsintosinglecrystals,using aninnovativetechniquedevelopedbyCeracompCo.,Ltd.,Korea [17].

BulkMEcompositesareusuallydevelopedastrilayeredlam- inates, in which the piezoelectric layer is arranged between two magnetostrictive ones.They are normally operated in the transversely(in-planedirection)magnetizedandperpendicularly (thicknessdirection)polarizedmode.Thisconfigurationintensifies thestrainalongtheplanardirectionandminimizestheinfluence ofdemagnetizingfieldsfromthicknessdirection,contributingto abetterMEoutputinlowmagneticbiasranges.ForadirectME effectinthismode,becausethemagnetostrictivephasedeformsin thein-planedirection,thepiezoelectricphase,duetotheinterfacial coupling,alsohastodeformsynchronouslyalongtheplanardirec- tionandinduceanelectricvoltagealongthethicknessdirection [12–14].Thisrequiresthepiezoelectricphasetopossessalargein- planestrainresponse(i.e.,transversepiezoelectriccoefficients,d₃₁ andd₃₂)inordertogenerateamaximizedMEoutput.Toexploit thisideafurtherinthecaseoflead-freepiezoelectricsinglecrys- talsincombinationwithinexpensiveNi,(001)and(011)oriented 82BaTiO₃-10BaZrO₃-8CaTiO₃ (BZT–BCT)singlecrystalswithhigh transversepiezoelectriccoefficients(d31ord32)werechosenfor thisstudy.Herein,theoff-resonance,directMEresponsesoftri- layeredcompositesofNi/(001)BZT–BCTsinglecrystal/Ni,Ni/(011) BZT–BCTsinglecrystal/Ni,Ni/polycrystallinePZT/Niarecompared.

TheeffectsofthedifferentorientationsoftheBZT–BCTsinglecrys- talsontheMEoutputwasalsoinvestigated.Thepresentworkis anattempttodevelopeco-friendlyMEcompositeswithdesired sensitivitybyreplacingthePb-basedpiezoelectricceramicswith lead-freeones.

Fig.1. (a)Photographoftheas-receivedBZT–BCTsinglecrystals(SCs),(b)and (c)schematicdiagramsoftheNi/(001)BZT–BCT/NiandNi/(011)BZT–BCT/NiME composites,respectively.Here,thearrowsontheNiandBZT–BCTSCsindicatethe magnetizationandpolingdirections,respectively.

2. Experimentalprocedure

MEtrilayeredcompositesofNi/BZT–BCT/NiandNi/PZT/Niwere prepared using square (10mm×10mm) plates of Ni (0.25mm thick),BZT–BCT(0.5mmthick), andPZT(0.5mmthick). Apho- tographofthe(001)and(011)orientedBZT–BCTsinglecrystals andtheschematicsoftheNi/BZT–BCT/Nilaminatesarepresented inFig.1.ThepropertiesofthedifferentlyorientedBZT–BCTsingle crystalsandthePZTceramicaresummarizedinTable1.Highqual- itysinglecrystalsofBZT–BCTwhichwerepreparedbytheSSCG techniqueandcuttomaketheirplanevectorsparalleltothe[001]

and[011]directions,werecommerciallyobtainedfromCeracomp Co.,Ltd.,Korea(LTEXY1,LTEXY2).ThepolycrystallinePZTplatewas machinedandwirecutfromaPZTpellet,whichwaspreparedby pressingandsintering(1250^◦C,2h)commerciallyavailablePZT granulesintendedforuseinhighperformancepiezoelectricsen- sors and transducers. The X-raydiffraction (XRD, D/Max 2200, RigakuCorporation,Japan) patternsof the(001)and (011)oriented,rhombohedral82BaTiO3-10BaZrO3-8CaTiO3singlecrystals andthepolyscrystallinePZTusedinthisstudyareshowninFig.2.

TheBZT–BCTsinglecrystalswerethicknesspoledbyapplyingadc electricﬁeldof1.4kV/mmatroomtemperaturefor10min.ThePZT platewaspoledunder4kV/mmat120^◦Cfor20min,initsthickness direction.

Toformthetrilayerlaminates,magnetostrictiveNiplates(Alfa Aesar,99.5% metals basis)werebonded tothetop andbottom surfaces of the piezoelectric layer using epoxy adhesive (3M Scotch-Weld^TM,DP-460)andcuredat80^◦Cfor4h.FortheNiplate, thesaturated in-plane magnetostriction(11)was measuredto be−40ppm[18].Thein-planestrain-electricﬁeld (S–E)behav- ioroftheBZT–BCTandPZTsampleswasstudiedusingastrain measurementmethod,whichinvolvesamicrostraingaugewith Wheatstone bridge. A strain gauge of 5mm gauge length and 350nominalgaugeresistance(MFLA-5-350-11-1LS,TokyoSokki KenkyujoCo.,Ltd,Japan)wasmountedonthesamplesurfacewith M-bond200adhesive.Aunipolarelectricﬁeld(<3kV/mm)oftrian-

Table1

DielectricandpiezoelectricpropertiesoftheBZT–BCTsinglecrystalsandthePZTceramicusedinthisstudy.

Material d31(pC/N) d32(pC/N) S^E₁₁(pm²/N) S^E₂₂(pm²/N) k31 k32 g31(10⁻³mV/N) g32(10⁻³mV/N)

(001)BZT–BCT −403 44.1 0.41 −19

(011)BZT–BCT 242 −627 15 47.9 0.55 0.77 17.4 −45

PZT −112 15.3 0.36 −13.67

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Fig.2.XRDpatternsofthe(a)(001)orientedand(b)(011)orientedBZT–BCTsingle crystalsand(c)polycrystallinePZT.

gularwaveshapeformedbyafunctiongeneratorandamplifiedby ahighvoltagepoweramplifierwasappliedtothesampleatafre- quencyof0.1Hz.Thechangeinresistanceofthesample,underthe appliedfield,wasrecordedusingadigitaldataacquisitionsystem (USB-2404-UI,TracerDAQ,MeasurementComputingCorporation, USA)interfacedtoacomputerandthedatagatheredwasfurther convertedintoaS–Eloop.Thepolarizationvselectricfield(P–E) hysteresiswascharacterizedbyaferroelectrictestingsystem(Pre- cisionLCII,RadiantTechnologiesInc.,USA)atroomtemperature.

TheMEoutputvoltagegeneratedfromthecompositesamples,in responsetoasuperimposedacanddcmagneticfieldappliedatan off-resonancefrequencyoff=1kHz,wasmeasuredusingalock- inamplifier(SR-850,StanfordResearchSystems,USA).Thevalue oftheMEvoltagecoefficient(˛ME)wascalculatedbydividingthe

measuredoutputvoltagebythethicknessofthepiezoelectriclayer andtheappliedacmagneticﬁeld(H_ac=1Oe).

3. Resultsanddiscussion

AslistedinTable1,the(001)orientedBZT–BCTsinglecrystal possessesisotropictransversepiezoelectricproperties,whilethe (011)orientedBZT–BCTsinglecrystaldisplaysanisotropicpiezo- electricproperties.ThecrystallographicorientationsofBZT–BCT singlecrystalsandtheconﬁgurationsofthed₃₁andd₃₂modesin themaredepictedinFig.3.Herethevoltagegeneratedalongthe 3-axespertheunitforceappliedeitheralongthe1-or2-axesis thesame(d31=d32)forthe(001)orientedBZT–BCTsinglecrystal (Fig.3(a))andisdifferent(d₃₁=/ d₃₂)inthecaseofthe(011)ori- entedBZT–BCTsinglecrystal(Fig.3(b)and(c)).Suchisotropyor anisotropyinthepropertiesofthesecrystalscanbeattributedto thedifferencesinthenumberofdegeneratestatesoftheirmul- tidomainstructures,produceduponpoling[19,20].Analogousto thePb-based rhombohedralferroelectric singlecrystalssuchas PMN-PT,PZN-PT,andPMN–PZT,thespontaneouspolarizationlies alongthe[111]direction,witheightpossibledipoleorientationsfor theBZT–BCTsinglecrystalsinrhombohedralsymmetry.Anelec- tricpolingﬁeldappliedtothecrystalsalonganon-polardirection ([001]or[011])createsamacrosymmetricmultidomainstructure inthem.

TheengineereddomainconfigurationsfortheBZT–BCTsingle crystalsorientedandpoledalong [001]and[011]directionsare schematicallyshowninFig.4.Herethedashedarrowsrepresentthe domainstatesinducedbypolingandtheirswitchinguponrever- salofappliedelectricfield.The(001)orientedandpoledBZT–BCT crystalpresentsamultidomainconfiguration,witheach domain havingoneofthefourdipoleorientations([111],[⁻111],[1⁻11],and [⁻1⁻11]).Whenthepolingfieldisreversed,thepolarizationswitches tothe[⁻1⁻1⁻1],[11⁻⁻1],[⁻11⁻1],and[11⁻1]domainvariants (Fig.4(a)).

Thus,thecomponentsofallfourpolarizationvectorslyingalong thebodydiagonalsofthepseudocubicunitcellofthe(001)oriented BZT–BCTareequal,sothatthedomainwallshavenoincentiveto moveunderanexternalelectricﬁeldalong[001],givingrisetoan isotropicpiezoelectricresponseintheplane(i.e.,d₃₁=d₃₂)[21,22].

OncetheisotropicBZT–BCTsinglecrystal iscombinedwiththe magnetostrictivephasetoformalaminatecomposite,theresul- tanttransverseMEresponseissupposedtobeisotropic.Whenthe BZT–BCTsinglecrystalisorientedandpoledalongthe[011]direction,allthepolarizationsarealignedtothe[111]and[⁻111]dipole orientations,whichswitchtothe[⁻1⁻1⁻1]and[1⁻1⁻1]domainvariants uponreversalofthepolingﬁeld(Fig.4(b)).Thesetwodipoleorien- tationslyingalongthediagonalsofthe(011)and(0⁻11)planestend torotatetowardthe[011]directionupontheapplicationofaposi- tiveelectricﬁeld.Suchamovesimultaneouslyinducescompressive stressalongthe[100]directionandtensilestressalongthe[0⁻11]

direction,resultingindifferentsignsandmagnitudesoftheplanar

Fig.3.Schematicrepresentationofthed31andd32modesin(001)and(011)orientedBZT–BCTsinglecrystals.

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Fig.4.Schematicillustrationofthedomainconﬁgurationsforthe(a)(001)and(b)(011)orientedandpoledBZT–BCTsinglecrystals.

Fig.5. Electricalpolarization hysteresis behaviorof (001)and(011)oriented BZT–BCTsinglecrystals.

piezoelectriccoefﬁcients(i.e.,d₃₁ =/ d₃₂)[23,24].Theseanisotropic transversepiezoelectricpropertiesofthe(011)orientedBZT–BCT singlecrystalwouldenabletherealizationofanisotropicMEprop- ertiesincombinationwithNi.

Fig.5showstheP–Ehysteresisloopsof the(001)and(011) orientedBZT–BCTsinglecrystals.Althoughthecoercivefieldsof boththesinglecrystalsarefoundtobesimilar,thehigherrema- nentpolarizationdisplayedbythe(011)orientedBZT–BCTreflects thebetterferroelectricnatureofit.Thisleadstoenhancedelec- tromechanicalresponsefromthe(011)orientedBZT–BCTaswellas efficientconversionofmagneticfieldinducedstrainintoelectrical outputfromthecorrespondingMEcomposite.Forfurtherunder- standing,theS–EbehavioroftheBZT–BCTsinglecrystalsandthe PZTceramicwasinvestigated.Theschematicofthestrainmea- surementset-upandtheunipolarstrainresponsesoftheBZT–BCT andthePZTsamplesareshowninFig.6.Thein-planestrainswere measuredalongthe[100]and[0⁻11]directionsforthe(011)ori- entedBZT–BCTandalongthe[010]and[100]directionsforthe (001)orientedBZT–BCT.ItisclearfromtheS–Ecurvesofthesam- plesthatthe(011)orientedBZT–BCT,along the[100]direction,

exhibitslargestelectricﬁeldinducedstrain(0.2%)whilethePZT showsleaststrainresponse(0.076%).The(001)orientedBZT–BCT displayedthesamemagnitudeofstrain(0.12%)alongboththe[010]

and[100]directions(theS–Ecurveisshownforthe[010]direction only).Thisindicatesthatthe(011)orientedBZT–BCTsinglecrys- tal,ind32mode,wouldgenerateahigherstraininducedelectric voltageandcontributetobettertransverseMEoutputthanthose ofthe(001)orientedBZT–BCTsinglecrystalandthePZT,fromthe respectiveMEcomposites.Theanisotropicstrainbehaviorofthe (011)orientedBZT–BCTsinglecrystalcanbeunderstoodtobethe resultofitsanisotropicpiezoelectricity.Thisfurthersuggeststhe possibilityofisotropicandanisotropicMEcouplingintheNi/(001) BZT–BCT/NiandNi/(011)BZT–BCT/Nicomposites,respectively.

Thevariationsinthetransverse˛MEwithanappliedbiasﬁeld (H_dc),forthetrilayercompositesmadeusingtheBZT–BCTsingle crystalsandthePZTplateareshowninFig.7(a)and(b),respectively.Itcanbeseenthat,amongallthecompositesamples,˛ME

exhibitedatypicalH_dcdependence,showingasignchangewith respecttothe reversalofthe H_dc direction.Moreover,thebias fieldpositionofthemaximum˛ME,whichis directlyrelated to thepiezomagneticcoefficient(q_ij)ofthemagnetostrictivephase (Ni, inthisstudy), remainsthesamefor allthesamples.In the caseoftheNi/BZT–BCT/Nilaminates,˛MEwasmeasuredbyapply- ingamagneticfieldalongdifferentplanardirectionsi.e.,H//[010]

and H//[100]for (001) BZT–BCT, and H//[100]and H//[0⁻11]for (011)BZT–BCT.TheNi/(001)BZT–BCT/Nicompositeshowedalmost identical˛MEcurveswithapproximatelythesamemagnitudeof maximum ˛ME (0.3V/cmOe) irrespectiveof the magnetic ﬁeld direction(H//[100]andH//[010]).Thissimilarityinmagnetoele- cricitycanbeattributedtotheisotropictransversepiezoelectricity (d₃₁=d₃₂)ofthe(001)orientedBZT–BCTsinglecrystal(Table1).

Incontrast,theNi/(011)BZT–BCT/Nilaminatedisplayedstrongly anisotropic ME coupling characteristics, withopposite signs of the˛MEcurves,anddifferentmaximum˛MEvaluesof1V/cmOe and0.6V/cmOe,obtainedfortheH//[100]andH//[011]⁻ conﬁgura- tions,respectively.SuchdirectiondependentMEresponsescanbea consequenceoftheanisotropyinthetransversepiezoelectricprop- ertiesofthe(011)orientedBZT–BCTsinglecrystal,i.e.,d32≈−3d31

(Table1).IthasbeendemonstratedinearlierstudiesbyPatiletal.

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Fig.6. (a)Schematicofthein-planestrainmeasurementsetupand(b)StrainvsE-ﬁeld(unipolar)behaviorfortheAu-electroded,differentlyorientedBZT–BCTsinglecrystals andPZTceramicsamples.

Fig.7.Magnetoelectricresponsesofthe(a)Ni/BZT–BCT/NilaminateswithdifferentBZT–BCTsinglecrystalorientationsand(b)Ni/PZT/Nilaminate.

[25]thatdifferenttheoreticalexpressionsareapplicableforeach ofthe transverseMEcoefﬁcients (˛E31=ıE₃/ıH₁ (H//[0⁻11]) and

˛E32=ıE3/ıH2(H//[100]))oftheMElaminatewithananisotropic piezoelectriccrystal,givingrisetoadifferentsignandmagnitude ofthemagnetoelectricity,consistentwiththeexperimentaldata.

FortheNi/PZT/Nicomposite,a lowervalueof␣ME,0.2V/cmOe, wasobtained,whichismuchlowerthanthosemeasuredforthe Ni/BZT–BCT/Nicomposites.TheMEresponseoftheNi/PZT/Nicom- positepreparedinthisstudywassimilartotherangeofreported MEoutputs(0.2–0.4V/cmOe)forthesametrilayercompositesfab- ricatedbydifferentmethods[26,27].

Itisnoteworthyherethatthevalueof˛ME obtainedforthe Ni/(011)BZT–BCT/Ni(whenH//[100])was5timeslargerthanthat oftheNi/PZT/Niandmorethan3timesgreaterthanthatofthe Ni/(001)BZT–BCT/Ni. These resultsare justified by thesmaller transversepiezoelectric coefficient of PZT (d31 or d32) and the largertransversepiezoelectriccoefficient(d₃₂)of(011)BZT–BCT (Table1).Thedifferencesintheelasticcompliances(sÊ₁₁ orsÊ₂₂) andelectromechanicalcouplingfactorsofthePZT(k31=k32=0.36), (001) BZT–BCT (k₃₁=k₃₂=0.41) and (011) BZT–BCT (k₃₁=0.55, k₃₂=0.77)crystalsarealsolikelytocontributetothevariedME performancesoftherespectivecomposites.Thestrain-mediated MEcouplingfollows therelation,˛ME∝d_ij. q_ij where d_ij is the piezoelectricconstant,q_ij(=dij/dH)isthepiezomagneticcoeffi- cient,andijisthemagnetostriction.Duetothenegativein-plane magnetostrictionofNi,itcontractsintheappliedmagneticfield directionandexpandsinitsthicknessdirection.SincetheNiplate issquare-shapedanditstransversedimensionsarelargerthanits thickness,itsplanarmagnetostrictionwillbeisotropicbutmore

dominant than its out-of-plane magnetostriction. The isotropic in-plane magnetostriction behavior of Ni resultsin a constant transversepiezomagneticcoefficient.So,thequantityofmagne- tostrictivestraintransferredfromNitotheBZT–BCTwillbethe sameinalldirections,butitseffectontheresultantelectricvoltage andtheMEbehaviorofNi/BZT–BCT/Nishallbemainlyinfluenced bythesignsandmagnitudesofthetransversepiezoelectriccoef- ficients(d31ord32),theelasticcompliances(sÊ₁₁orsÊ₂₂),andthe electromechanicalcouplingfactor(k₃₁ork₃₂).

TheMEresponsesofNi/BZT–BCT/Nishowsimilartrendstothose inotherstudies[3,22,23,28].Thegreatervalueof˛ME(1V/cmOe) displayedbythelead-freeNi/(011)BZT–BCT/Nicompositeiscom- parable to those obtained for the Pb-based Ni/(011) PMN–PZT singlecrystal/Ni(1V/cmOe)andNi/(011)PMN-PTsinglecrystal/Ni (2.5V/cmOe)compositeswithsimilargeometryanddimensions [22,23].The˛ME valueofNi/(011)BZT–BCT/Niisalsosimilarto thebestreportedone(1.32V/cmOe)forthePb-freeMEcompos- iteof Terfenol-D/(001)NBT-BT singlecrystal/Terfenol-D [28]. It shouldbereiteratedherethatNiismuchcheaperthanthefre- quentlyemployedmagnetostrictivealloysTerfenol-DandMetglas andthus,itismoreeconomicaltodevelopNi-basedMEcompos- ites.Basedontheaboveresults,itcanbeemphasizedthatoptimal cutBZT–BCTsinglecrystalsarepotentialcandidatestoreplacePb- basedceramicsaspiezoelectricconstituentsinMEcomposites.

4. Conclusions

In summary, lead-free magnetoelectriclaminate composites werepreparedusingmagnetostrictiveNiand(001)and(011)ori-

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entedBZT–BCTpiezoelectricsinglecrystals.Thedifferencesinthe transversepiezoelectricpropertiesofthesesinglecrystals,dueto thedifferencesinnumberofalloweddomainstatesordipoleorien- tationsunderpolarization,significantlyinfluencedtheMEoutput of thetrilayer composites. TheNi/BZT–BCT/Ni composite made usingthe(011)orientedBZT–BCTsinglecrystalwithanisotropic transversepiezoelectricpropertiesshowedalargeMEcoefficient of1V/cmOe,outperformingtheMEresponseofcompositesformed using the (001) orientedBZT–BCT single crystal with isotropic transverseproperties,andpolycrystallinePZT.Itcanbesuggested that usinghighperformance, anisotropiclead-freepiezoelectric singlecrystalsalongwithmagnetostrictivebasemetalssuchasNi, willallowthedevelopmentofeco-friendlyandcost-effectiveME compositeswithgoodoutputasalternativestoPb-basedones.

Acknowledgments

ThisresearchworkwassupportedbytheGlobalFrontierR&D Program(Grant No. 2016M3A6B1925390)on Centerfor Hybrid InterfaceMaterials(HIM)fundedbytheMinistryofScience,ICT&

FuturePlanning,Korea;KoreaInstituteofMaterialsScience(KIMS) internalR&Dprogram(GrantNo.PNK4991);andtheU.S.Ofﬁceof NavalResearchGlobal(GrantNo.N62909-16-1-2135).

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