Decomposition reactions in CaCu3Ti4O12 ceramics(2)

II.ExperimentalProcedure

D.Johnson—contributingeditor

ManuscriptNo.21586.ReceivedMarch14,2006;approvedMay4,2006.WethanktheEPSRCfor nancialsupport.w

Authortowhomcorrespondenceshouldbeaddressed.e-maild.c.sinclair@Shef eld.ac.uk

High-purityCaCO3,CuO,andTiO2reagents(all99.99%pure,AldrichChemicalCo.,Milwaukee,WI)atamolarratioof1:3:4weremixedinaplanetaryballmill(FritschGMBHmodelpul-verisette6,Albishein,Germany)withacetoneusinganagatepotandballs(amixtureof5and10mmballs)at250rpmfor20min.Afterdrying,thepowder(B10g)wasreactedinairover-

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2834JournaloftheAmericanCeramicSociety—Adamsetal.Vol.89,No.9

nightat10001ConaPtfoilandthenmilledagainat250rpmfor30minbeforeasecondreactionat10001C.Thepowderwasthenplanetaryballmilledat250rpmfor60min.

XRDwasperformedonpowdersamplesusingahigh-reso-lutiondiffractometer(StoeStadiP,StoeandCieGmbH,Dar-mstadt,Germany)operatedat50kVand30mA(stepsizeofscan0.021andscanrate21/min)toassessphasepurity.XRDonpelletsurfaceswasperformedusingaSiemens(Karlsruhe,Germany)X-raydiffractometerwithCuKa1radiation.Particlesizeanalysis(ModelCoulterLS130,Beckmann,HighWyco-mbe,UK)showedabimodalparticlesizedistributioninthepowderatB0.4and6mmandad50valueofB3.4mm.

CaCu3Ti4O12powdercompactswerepressedina10mmsteeldieat0.5tonandsinteredat11151Cinairforeither3or24h,andfurnacecooledtoRT.Pelletdensitieswerecalculatedfromthemassanddimensionsofthepelletsandallwere495%ofthetheoreticaldensity.

Ceramicsforscanningelectronmicroscopy(SEM)werepre-paredbymountinginresinandpolishingsectionsperpendiculartothemajorpelletfaces.ThepolishedceramicswerecarboncoatedandanalyzedusingaJEOLJSM-6400SEM(JeolLtd.,Tokyo,Japan)equippedwithaLINKenergy-dispersiveX-ray(EDS)detectorandancillaryelectronicsoperatingat20kV.Theas-sinteredsurfaceof24hceramicswasanalyzedbyXRD,andthenpolishedwithSiCpaperandre-measured;theprocesswasrepeateduntilthesamplewasB80%oftheoriginalthickness.Theceramicmicrostructuresofpelletssinteredfor3and24hconsistofaveragegrainsizesofB5and4100mm,respectively.Detailsoftheceramicmicrostructureshavebeenreportedpreviously.4

Hydrogen-reductionthermogravimetry(TG)in5%H2/95%Ar(heatingrate101C/min,referenceAl2O3)wasperformedonpowder(B50mg)fromacrushed24hpellet.

As-sintered3and24hceramicswereplacedonaPtfoil,in-sertedintoatubefurnace,andheattreatedinoxygen-freeN2for6hat10001C(heatingrate51C/min,coolingrate2.51C/min).A3hheat-treatedceramicwasanalyzedbyXRDasafunctionof

pelletthicknessasdescribedabove.Finally,samplesof3and24hheat-treatedceramicswerepreparedforSEMasdescribedabove.

III.ResultsandDiscussion

Low-andhigh-magni cationbackscatteredelectronimages(BEI)ofas-sintered3and24hceramics,viewedasacrosssec-tionperpendiculartothemajorpelletfaces,areshowninFig.1.Atlowmagni cation,the3hsampleappearshomogeneouswithoutlarge-scale aws,suchas ssures,orporesover5mmindiameter,Fig.1(a),whereasthe24hsamplerevealsacoarserporestructurewithporesizesupto30mm,Fig.1(b).Athighermagni cationa ne-grainedtexture(o10mminsize)isevidentforthe3hsample,andtheporestructure,observedasblackvoids,isclosed.Asecondaryphasewasobservedasbrightpre-cipitatesinanumberofporesand,inparticular,atgrainbound-aryjunctions,Fig.1(c).Thegrainstructureofthe24hsamplewasmuchcoarser,althoughitwasnotpossibletoassessaccu-ratelyasthesampleswerenotetched,butpreviousresultshaveshownthatthegrainsaretypicallybetween100and300mminsize.4Asecondaryphasewasdetectedasbrightprecipitatesus-ingBEI,Fig.1(d),consistentwiththatobservedforthe3hce-ramic.Anadditionalsecondaryphaseforthe24hsamplewasobservedinBEIasdarksphericalprecipitatesofB5mmdiam-eter,Fig.1(d).

AtypicalEDSspectrumofthemainCCTOphaseinboth3and24hsamplesisshowninFig.2.Theobservedpeakscor-respondtoknownpeakpositionsforCa,Cu,andTi.AlthoughtheEDSdatadonotprovideadirectmeanstoquantifythecompositionofaparticularregion,thedataarerepresentativeofthebulkCaCu3Ti4O12phase,consistentwithanalysisperformedbyEPMAandreportedpreviously.8Figure2(b)showstypicalEDSdataforthebrightphaseobservedatthegrainboundaryjunctionsinthesamples.ThetwomajorpeakscorrespondtoknownpeakpositionsforCuandarecommensuratewith

the

Fig.1.Backscatteredelectronimagesscanningelectronmicroscopyshowingcrosssectionsof(a)3hand(b)24hCaCu3Ti4O12ceramicsatlowmagni cationandhighmagni cation(candd,respectively).

September2006DecompositionReactionsinCaCu3Ti4O12Ceramics2835

Fig.2.Typicalenergy-dispersiveX-rayspectraof(a)bulkCaCu3-Ti4O12phaseandsecondaryphasesobservedbybackscatteredelectronimagesas(b)brightprecipitatesand(c)darksphericalprecipitates.

Cu2Ophase,alsodetectedbyEPMA.8ThesecondarypeakscorrespondtoSi,Ca,andTi,whichmayarisefromtheprecip-itateitself,orfromsurroundingmaterialiftheinteractionvol-umeexceedsthevolumeoftheprecipitateduringtheEDSanalysis.TheEDSdatainFig.2(c)aretypicalofthedarkphaseobservedinthe24hsample,withmajorpeakscorrespondingtoSi,Ca,andTi,andaminorpeakcorrespondingtoCu.Themorphologyandcomposition(accordingtotheEDSdata)areconsistentwithCaTiSiO5(sphene)precipitatesobservedbyEPMA,asreportedpreviously.8TheCusignalmayarisefromincorporationofCuintothesphenelatticeorasaresultofin-teractionoftheprimarybeamwiththesurroundingarea.ThepresenceofSiarisesduetounintentionalcontaminationfromthemillingmediausedinthepowderprocessing.

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