Citation for published version:Zhang, Y, Xie, M, Roscow, J & Bowen, C 2019, 'Dielectric and piezoelectric properties of porous lead-free0.5Ba(Ca

0.8Zr

0.2)O

3-0.5(Ba

0.7Ca

0.3)TiO

3 ceramics', Materials Research Bulletin, vol. 112, pp. 426-431.

https://doi.org/10.1016/j.materresbull.2018.08.031

DOI:10.1016/j.materresbull.2018.08.031

Publication date:2019

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Download date: 21. Jul. 2020

Dielectricandpiezoelectricpropertiesofporouslead-free0.5Ba(Ca0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3ceramics

YanZhang,MengyingXie,JamesRoscow,ChrisBowenDepartmentofMechanicalEngineering,UniversityofBath,BA27AY,UK

Abstract:Porous barium calcium zirconate titanate 0.5Ba(Ca0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3 (BCZT) lead-freeferroelectricceramicswerefabricatedviaaburntpolymerspheres(BURPS)techniquebyintroducingcornstarchasthepore-formingagent.Theeffectofporosityonthemicrostructure,dielectric,andpiezoelectricpropertiesoftheporousmaterialswere investigated.An increase inporosityvolumefractionfrom10%to25%resulted inan increase inthehydrostaticchargecoefficient(dh),whichwas140to560%higherthanthatofthedenseBCZTmaterial.Anincreaseinporosityfractionfrom10%to25%alsoleadtoadecreaseinrelativepermittivitythatwas16.7%to60.4%lowerthanthedense material. These two changes in properties provided a significant enhancement of thehydrostatic figure of merit (dh·gh) for the porous ceramic; for example the dh·gh of the 25 vol.%porousBCZTceramicwas158timesmorethanthedenseceramicanddemonstratesthepotentialofporous lead-free ferroelectrics for piezoelectric transducer devices. Reasons for the significantenhancementinpiezoelectricperformanceoftheporousceramicsarediscussed.Highlights:

ü Porouslead-freepiezo-ceramicswerefabricatedbyintroducingpore-formingagents.ü Anincreaseinporosityincreasedhydrostaticchargecoefficientanddecreasedpermittivity.ü Hydrostaticparametersghanddh·ghoftheporousceramicwere140%to560%higherthan

thedensematerial.

IntroductionPb(Zr,Ti)O3-based (PZT) piezoelectric ceramics are commonly utilized in a variety of sensors,actuators,andultrasonictransducers.Duetolegislationthataimstorestricttheuseof leadbasedproductsandenvironmentalconcernsregardingthetoxicityoflead/leadoxideanditsrecyclinganddisposalwhenusedduring the production process, there is an increasing demand for replacing itwith lead-free alternatives. Amongst the variety of lead-free materials, barium calcium zirconatetitanatewhose chemical formula is 0.5Ba(Ca0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3 (hereafter abbreviated asBCZT) is one of the most widely studied lead-free piezoelectric ceramic [1-3]. The material is ofinterest since its piezoelectric properties are comparable, and even superior, to other lead freecompoundssuchastheconventionallyusedlead-basedpiezoelectricceramicsintermsofthe‘hard’and‘soft’PZTs,PMN-PT(leadmagnesiumniobate-leadtitanate)andPZN-PT(leadzincniobate-leadtitanate). The piezoelectric coefficient (d33) of the BZCT ceramic with the stoichiometric ratiohighlightedabovewasreportedtobeintherangeof550-640pC/N[4];whichisamongthehighestvalues to-date for lead-free materials. This promising result has attracted increasing scientificinterest for the employment of this lead-free ceramic in transducer applications and replacingcurrentlead-basedferroelectrics.To date, the majority of the literature has focused on tailoring the properties of dense BCZT bydoping[5, 6], texturing[7, 8] and optimising the processing conditions[9, 10] to increase its Curie

temperature (Tc) or further improving its piezoelectric performance in terms of piezoelectriccoefficient and permittivity. An effective way to assess the performance and properties offerroelectric materials for practical piezoelectric and pyroelectric applications is to compareappropriate figures of merit (FoM) which contain combinations of relevant physical properties.TheseFoMs havebeenutilisedwidely for assessmentofmaterials for a rangeof applications, for

example !!"!

!!!!for piezoelectric energy harvesting, !!

!!!!×(!!)!for pyroelectric energy harvesting, and

(!!!!!!!")!

!!!!forhydrostaticandSONARapplications,wheredij isthepiezoelectricchargecoefficient,

εr istherelativepermittivityatconstantstress(ordielectricconstant),ε0 isthepermittivityoffreespace,p is thepyroelectric coefficient,d33andd31are the longitudinaland transversepiezoelectricchargecoefficientsrespectively.WecanseeintheaboveFoMsthatthepermittivityofthematerialis included in all of the equations and is inversely proportional to eachFoM; this indicates that alowerpermittivity is beneficial to achieve ahighperformance if thepiezoelectric andpyroelectriccoefficient remainsunchanged.However, the relativepermittivityof ferroelectrics is typicallyveryhigh;forexamplefordenseBCZT,εr˃2100atroomtemperature.OneapproachtoachieveahighFoM is to introduce low permittivity pores into the densematerial [11, 12]. Therefore a detailedunderstanding of effects of porosity on ferroelectric and piezoelectric properties in suchpiezoelectriclead-freematerialsystemsisbeneficialinunderstandinghowtoenhanceandtailorthedesiredpropertiesofthesenewmaterials.Thispaperprovides the firstexperimentalmeasurementsonporousBCZTceramics tounderstandtheeffectofporosityonferroelectricandpiezoelectricpropertiesofthese lead-freeferroelectrics.Theobjectiveofthisworkistoanalysethemicrostructure,dielectric,andpiezoelectricpropertiesofporousBCZTferroelectricceramicsthathavebeenmanufacturedusingvolatileadditionsthatburnoutduringthehightemperaturesinteringprocess,knownasthe‘BURPStechnique’.Thepropertieswillbecorrelatedwithporosity,andtheunderlyingphysicalmechanismsfortheobservedchangesinpropertieswillalsobeexplained.

Experimental

BCZTpowdersandporousBCZTceramicswerepreparedbyasolid-statereactionapproachandtheBURPStechnique (Burnt Polymer Spheres). Analytical grade (Sigma Aldrich) BaCO3 (99%), CaCO3 (99%), TiO2(99.9%),andZrO2(99%)wereselectedasstartingmaterialsandweighedaccordingtothestoichiometricratio.After4hofball-milling,themixtureswerecalcinedat1200°Cfor3handfollowedbyadditionalball-millingfor24h.Themilledpowdersweremixedfor12hwithethanolanddifferentadditionsoftheporeformingagent,cornstarchpowder(0,5,16,25,34,48wt.%basedonthecalcinedpowders),followedbymixingwith1wt.%PVAbinderforbothdenseandporoussamples.Afterdrying,thepowderswereuni-axially cold-compacted to form pellets of 13 mm in diameter and 1.5mmin in thickness. The cold-compactedpelletswerefirstheatedto600°Cfor3htoremovetheorganicadditivesandthensinteredat1400°Cfor4htoachievethefinalsamples.ThephasestructureoftheceramicswasexaminedbyaX-raydiffractometer(BRUKERD8-Advance,USA)withCuradiationwith2θrangingfrom20°-70°.Thesinteredsamplesweresectionedbya low-speedprecisiondiamondsaw(Buehler,IsoMetLS)toachievearepresentativecross-sectiontoinvestigatethemicrostructure by scanning electron microscopy (SEM, JSM6480LV, Tokyo, Japan). The apparentporosity,bulkdensityandapparentsoliddensityofthesinteredspecimenswasmeasuredusingtheArchimedes’ principle based on British Standard (Advanced technical ceramics-Monolithic ceramics-Generalandtexturalproperties-Part2:Determinationofdensityandporosity,BSEN623-2:1993).Forevaluationoftheelectricalproperties,silverpaintwascoatedonbothfacesofthesinteredsamplestoform electrodes. For piezoelectric measurements, the ceramics were electrically poled using thecoronapolingbyapplyingaDCvoltageof14kVappliedfroma35mmpointsourcefor30minat

60 °C. Maximum polarization, remnant polarization and coercive field, were measured using aRadiantRT66B-HViFerroelectricTestsystemoninitiallyunpoledmaterialswithafrequencyof5Hzandadouble-bipolarinputastheelectricalsignal.Thetemperatureandfrequencydependenceofthedielectricconstantwasstudiedusinganimpedanceanalyzer(Solartron1260,Hampshire,UK)inatemperatureandfrequencyrangeof30-150°Cand1-105Hz.Thelongitudinalpiezoelectricstraincoefficient (d33) and the transverse piezoelectric strain coefficient (d31) were measured using aBerlincourtPiezometer(PM25,TakeControl,UK)atatestfrequencyof97Hz.Resultsanddiscussions

Figure1(A)-(C)showtheX-raydiffractionpatternandscanningelectronmicroscopy(SEM)imageof the as-synthesizedBCZTpowders and the corn starchpowders.Asseen inFigure1(A), strongXRDpeakswithasmall fullwidthathalfmaximum(FWHM)valueswereobservedwhich indicatesthatthesynthesizedpowderswerewellcrystallized.Notraceofthepresenceofsecondaryphaseswas found in the diffractogram, neither reflection splitting nor super-lattice reflections wereobserved. The XRD patterns of BaTiO3-based BCZT phase showed good agreement with theconventionaltetragonalBaTiO3structurewiththeP4mmspacegroupfromtheJCPDScardnumber05-0626.Intensediffractedpeaksobservedat2θ=22.27°,31.63°,39°,44.91°,45.36°,51.02°,56.38°that correspond to the (hkl) Miller index of (100), (110), (111), (002), (200), (210), (211) planesrespectively.The electrostatic Coulomb repulsions between 3d electrons of Ti4+/Zr4+ions and 2pelectronsofO2−ionsledtothedistortedcrystalstructureintheBaTiO3-basedceramic,resultinginastabletetragonalstructure.Thesplittingofthe(200)peakofthepureBCZTinto(002)and(200)peaksinthe2θrangeof44°-46° at room temperature corresponding toCa2+ (A-site) andZr4+ (B-site) ionsdiffusingintoBa2+(A-site)andTi4+(B-site),respectivelyconfirmedthetetragonalortetragonal-dominantphaseformation[13].TheBCZTpowdersafterballmillingexhibitedan irregularshapewithanaveragesizeofapproximately1µm,ascanbeseeninFigure1(B).Figure(C)presentsthespherical-likemorphologyofthecornstarchpowderswithaparticlesizeof~10µm.

Figure1(A)XRDspectrumofsynthesisedpowders,SEMmicrographsof(B)BCZTpowdersafterballmillingand(C)cornstarchpowders.

Figure2(A)and(B-F)showSEMimagesofthecross-sectionsofthedenseandporousBCZTwithaporosityrangeof10%to25%,whichcorrespondtocornstarchadditionsfrom5wt.%to48wt.%,and the porous characteristics for the porous ceramics obtained from different corn starchadditionsareshown inTable1.AscanbeseeninFigure2,amicrostructurewithafullydensifiedpolycrystallinematrix and struts can be clearly seen for all thematerials which were sintered at1400°C; this indicates that theBCZTpowderswerewell sinteredunder thisprocessing condition.From Figure 2(A), the dense BCZT ceramic, which had no pore forming agent added, was highlydense with a measured apparent porosity of approximately 4 %. As the amount of pore-formeragent (corn starch) was increased from 5 wt.% to 48 wt.%., the number of the pores in themicrostructureincreased,whichcanbeseeninFigures2(B-F).Largeporesarelikelytobegeneratedbythecoalescenceofporesandporeformeragentagglomeration.Inaddition,thereislikelytobean increase inthe interconnectionofporesastheamountofporeformer increased,e.g. theporesizeincreasedtoapproximately20µmwhentheporosityvolumefractionwas25%,comparedwiththeporesizeof5µmforthematerialwithonly10%porosity.Themicrostructuresforall levelsofporosityexhibitedhomogeneouslydistributedandmostlyopenporesofspherical-likemorphologyin the ceramicmatrix. Since the pores are formed as a result of decomposition of the organicphase,theporesareareplicaofthecornstarchwhosemorphologywasshowninFig.1(C).Itwasalso seen that a continuous porous network was formed in the porous BCZT ceramics and thehomogeneous pore distribution led to the formation of a strong pore wall network. The smallchange in apparent solid density, and large change in apparent porosity, with an increasedadditionofthecornstarch inTable1 indicatesthattheporosity inpredominantelyopen,ratherthan closed. The increase in apparent densitywith increasing corn starch content indicates theamountofclosedporositydecreasesasthemicrostructurebecomesmoreporous.

Figure2SEMmicrographsof(A)denseandporousBCZTceramicswiththeporosityfractionsof(B)10%,(C)15%,(D)18%,(E)21%,(F)25%.

Table1PorouscharacteristicsfortheporousBCZTobtainedfromdifferentadditionsofcorn

starch.

Additionofcornstarch(wt.%)

Apparentporosity(vol.%)

Bulkdensity(g/cm3)

Apparentsoliddensity(g/cm3)

Averageporesize(µm)

5 10 4.92 5.47 516 15 4.68 5.52 725 18 4.56 5.56 1134 21 4.45 5.63 1648 25 4.26 5.68 20

Figure3showsthepolarisation-field(P-E)loopofthedenseandporousBCZTceramicswithdifferentporosity levels at ambient temperature. The porous materials exhibited both a lower remnantpolarisation (Pr) and coercive electric field (Ec) compared to the dense BCZT ceramic;whichwere7.69µC/cm2and2.62kV/cm,respectivelyforthedensematerial.Thehysteresisloopsoftheporousmaterials exhibited a symmetrical shape, indicating a good ferroelectric response for all the BCZTmaterials. Both thePr andEc canbe influencedby theporosity, as thepresenceof thepores canchange the domainmovement under electric field. Figure 3 shows that as the porosity increasedfrom 10 % to 25 % the Pr decreased from 5.86 µC/cm2 to 3.84 µC/cm2, while the Ec graduallydecreased from 1.77 kV/cm to 1.51 kV/cm. The presence of the passive pore phase (air) in theporousBCZTresultedinareducedfractionoftheactiveBCZTferroelectricceramicsperunitvolumewhichledtothedecreasedremnantpolarisation.Thedecreaseofthecoercivefieldastheporosityincreasedfrom10to25%islikelyduetothereducedinternalstressinthevicinityoftheporesandhighercomplianceoftheporousmatrixwhichfacilitatestheeaseofdomainswitching.

Figure3FerroelectrichysteresisloopsofdenseandporousBCZTwithdifferentporosities.

Figure4showstherelativepermittivityanddielectriclossofthedenseandporousBCZTceramicsatarangeoffrequenciesandtemperatures.ThefrequencydependenceoftherelativepermittivityanddielectriclossofBCZTweremeasuredatroomtemperatureforthedifferentporositylevels,andarepresentedinFigure4(A)and(B).AlltheporousceramicsexhibitedalowerrelativepermittivityandhigherdielectriclossthanthatofthedenseBCZT.Forinstance,therelativepermittivityat1kHzoftheporousBCZTreducedfrom2158to1026whentheporosityfractionincreasedfrom10%to25%,whichwas16.7%to60.4%lowerthanthatofthedenseBCZTatroomtemperature(εr=2590).Itcanbe seen that in the low frequency range (≤ 500Hz) thedielectric loss of both thedense andporous BCZTs increases with decreasing frequency and the behaviour of thematerial in this lowfrequencyrangeisassociatedwithasmalllevelofdcconductivityinthematerial;i.e.aconductionloss[14].Afteraminimumlossat~1kHzthelossthenbeginstoincreasewithincreasingfrequencyabove~2kHz.Thishigh frequency loss is likely tobeassociatedwith theonsetof ionic relaxationlosses.Figure4(C)and(D)showthetemperaturedependenceoftherelativepermittivityanddielectricloss.Thepeakofpermittivity curve, or the turningpointof the tangent loss curves, correspond to theCurie temperature (TC) of the ferroelectric ceramic, where dense BCZT exhibited a TC ofapproximately106 °C.When the temperaturewas lower thanTC, the increase inpermittivitywithtemperature can be accredited to an increase in the conductivity of both the dense and porousBCZTs, and therefore an increased permittivity and dielectric loss was achieved. When thetemperature(T)washigherthanTC,therelativepermittivityofallBCZTceramicsreducedaccordingtotheCurie-Weisslaw(ԑr=C/(T-TC),andCistheCurie-Weissconstant)andiswidelyusedtodescribethe changeof thepermittivity. Thedielectric lossof theporousBCZTmaterials followeda similartrendwithequallylowvaluesasthedenseBCZTatdifferentfrequenciesandtemperaturesandthiswas attractive in terms of reducing the energy loss from the ferroelectric composite (BCZT/air).Moreover, one can see that the TC of the porous BCZT slightly increased from 107-111 °C as theporosity level increased, thiswasattributed to the stress relaxationnear theporearea, similar tothat of our porous PZTmaterialswith the porosity from 25 to 45%[12]. This suggested that theporosityrangingfrom10to25% imposedsmalleffectsonthephasetransitionandchangeofthecrystallinestructureoftheporousBCZTceramicswhenemployingcornstarchastheporeformingagent.

Figure4RelativepermittivityanddielectriclossofporousBCZTwithdifferent(A)(B)frequenciesatroomtemperatureand(C)(D)temperaturesat1KHzfrequency.

Figure5(A)and(B)showthevariationofthepiezoelectriccoefficientsandhydrostaticparametersoftheBCZTceramicswithporositylevel.ItcanbeseenfromtheFigure5(A)thatthelongitudinal(d33)andtransverse(-d31)piezoelectriccoefficientsdecreasedastheporosityfractionincreased.However,thehydrostaticchargecoefficient(dh)increasedastheporositylevelintheBCZTmaterialsincreased.Specifically,thed33and-d31valuesoftheporousBCZTdecreasedfrom424to285pC/Nand195to96pC/Nwhilethedhincreasedfrom34to93pC/Nwhentheporosityincreasedfrom10to25%;thiscorresponded to a 19-46% and 24-62% decrease and 140-560% increase compared the denseceramicwhichexhibitedad33,-d31anddhof524,255and14pC/Nrespectively.Thedecreaseinthepiezoelectric coefficients (d33, -d31) with increasing porosity is related to the reduced fraction ofpiezoelectricphaseperunit volumewhich leads toa reduced remnantpolarisation (Figure3) andpiezoelectric response. Secondly, it is know that during the poling process the electric fieldconcentratesinsidethelowpermittivityporeregion,alsoleadingtoreducedpolarisation,accordingtoourpreviousmodellingresults[11].Howeverthelargerdecreasein-d31,comparedtod33,leadstotheporousmaterialexhibitingahigherdh,seeFigure5(A).ItisevidentinFigure5(B)thattheporousBCZTshowsapronouncedincreaseofhydrostaticvoltagecoefficient (gh=dh/ԑ0ԑr) and hydrostatic figure of merit (dhgh), where dh is the hydrostatic chargecoefficient (dh=d33+2d31). For the dense BCZT ceramic in this work, d33 is almost equivalent -2d31which ledtoavery lowdhof14pC/N,andwhencombinedwith itshighrelativepermittivity (εr=2590) this resulted in the dense ceramic exhibiting a low dh, gh, and dh·gh, thereby limiting theirperformance as a transducermaterial under hydrostatic conditions. However the introduction ofporosityandreductionof-d31andpermittivityleadtoagreatlyincreaseddh(Figure5(A)),gh(Figure5(B))anddh·gh(Figure5(B)).Itcanbeseenthatghanddh·ghincreasedwithincreasingporositywiththehighestvalueofgh~10.2×1012Vm/Nanddh·gh~0.95×1012m2/N,this isapproximately17and158 times higher than that of denseBCZTmaterials (gh ~ 0.61×1012 Vm/N anddh·gh ~ 0.008×1012m2/N).

Figure5(A)Piezoelectriccoefficientsand(B)hydrostaticparametersofporousBCZTwiththeporosityrangingfrom10to25%.

Conclusions

Porous 0.5Ba(Ca0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3 (BCZT) lead-free ceramics were produced by usingvolatileadditivesatdifferentadditionsoftheporeformingagent.Themicrostructure,piezoelectricanddielectricpropertiesofporousPZT–PCNceramicswereinvestigatedindetailasafunctionoftheporosity. The relative permittivity of the porous BCZT decreased from 2158 to 1026,while denseceramicexhibitedarelativepermittivityof2590.Therewasalsoalargedecreaseinthelongitudinalpiezoelectric coefficient as the porosity content increased. These factors were beneficial to theperformance figures of merit for futurematerial election and design for transducer applications.Moreover, the loss tangents of the composites remained relatively low, which was preferred intermsof reducing theenergy loss fromadielectricmaterial.Enhancedpiezoelectricproperties forthe lead-free porous 0.5Ba(Ca0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3 ceramics were demonstrated with adh∼93pC/N,gh∼10.237×1012Vm/Nanddh·gh∼0.952×1012m2/Naredemonstratedatthemaximumlevelof25%porosity.Thus, theseporousBCZTwith lowdielectric lossandhighperformancearesuitable for many practical transducer applications such as piezoelectric sensors and energyharvestersandexhibitcompetitivesensitivitiescomparedtothelead-basedequivalentmaterials.Acknowledgement

Dr.Y.ZhangwouldliketoacknowledgetheEuropeanCommission'sMarieSkłodowska-CurieActions(MSCA), through theMarie Skłodowska-Curie Individual Fellowships (IF-EF) (H2020-MSCA-IF-2015-EF-703950-HEAPPs)underHorizon2020.Prof.C.R.Bowen,Dr.M.XieandMrJ.Roscowwouldliketo acknowledge the funding from the European Research Council under the European Union’sSeventhFrameworkProgramme(FP/2007–2013)/ERCGrantAgreementno.320963onNovelEnergyMaterials,EngineeringScienceandIntegratedSystems(NEMESIS).

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