Decomposition reactions in CaCu3Ti4O12 ceramics(3)

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.

RepresentativeXRDdataforas-sinteredandpolishedsurfacesofCCTOceramicspreparedinairat11151CareshowninFig.3.Allre ectionsfromtheas-sinteredpelletsurfacewiththeexceptionofasmallpeakatB36.51correspondtothepatternreportedintheliterature10andICDDcardnumber75-2188forCaCutheas-sintered3Ti4O12.TheadditionalminorpeakobservedatB36.51inpelletsurface(seetheinsetinFig.3)wasremovedonpolishingthepelletsurfacesandmaycorre-spondtoCu2OprecipitatesatthesurfaceasthemostintensepeakintheXRDpatternforCuthe(111)re ectionat36.5212O(ICDDcardnumber77-199)is1.Unfortunately,thesecondmostintensepeak(200re ection)forCu2Ooccursat42.4231,whichisclosetothe(111)re ectionforCCTO;seetheinsetinoFig.3.AllremainingpeaksforCu2Ophase,30%,whichandgivenwaslimitedthesmallonlyvolumetotheas-sinteredfractionhaverelativeofintensitypellettheimpuritysurface,

Fig.3.TypicalX-raydiffractionpatternsobtainedfromanas-sinteredpelletsurface(bottomtrace)andthen,sequentially,fromlayer-by-layerpolishedpelletsurfaces(upper

traces).

itisnotpossibletoattributeunequivocallytheextrare ectiontoCuLow-magni cation2O.

BEIimagesof3and24hsamplesheattreatedinN2for6hat10001CareshowninFig.4.TheimagecontrastrevealstwocompositionallydistinctlayerssurroundingthebulkCaCu3Ti4O12materialinbothsamples;theselayersappeartoforma‘‘decompositionzone,’’Fig.4(a)and(b).Ahighermagni cationimagefora3hsampleshowstheinterfacebetweentheoutertwolayers,Fig.4(c)(asimilarimage,notshown,wasobtainedfora24hsample)andtheinterfacebetweentheinnerlayerandbulkmaterial,Figs.4(d)and(e).Theoutermostlayerwasahighlyporous(B50%dense),‘‘coral-like’’structureofB1mmdiameterinterconnectedstrands.Thisstructureformsthematrixofaninnerlayerinwhichtheporesare lledwithaphasethatappearsbrighterthanthematrix.Inbothsamples,thethicknessofthelayersvariesgreatlysuchthat,for3hsamples,theouterlayercanextendB100mmintothesampleor,conversely,theinnerlayercanextendfromthebulkinterface,throughtheouterlayerandtothepelletsurface.Theoverallthicknessofthedecompositionzonewasrelativelyconstantinbothsamples.Forthe3hsample,thedecompositionzonethicknesswasB150mmatthemajorpelletfaces,increasingtoB200–300mmalongthepelletcircumfer-ence.B100–150ThedecompositionTypicalEDSmm.

zonewasthinnerinthe24hsample,dataforthebrightprecipitatephaseandmatrixphaseobservedinthedecompositionzoneareshowninFigs.5(a)and(b),respectively.Thebrightphasecorrespondstoacopper-richphase,whichiscon rmedbyXRD(seelater)tobeCu2O.ThepresenceofCaandTiintheEDSdatamay,asbe-fore,arisefromtheirincorporationintotheCu2Olatticeand/orfrombeaminteractionswithsurroundingmaterial.ThedarkmatrixcontainsCa,Ti,andSi,andXRD(seelater)isusedtoshowevidenceofseveralphases,includingCaTiO3andTiO2.Forbothsamples,theinterfacebetweentheinnerlayerandthebulkmaterial,Figs.4(d)–(e),isclearlyde nedbya netex-tureofCuand,below2OstriationsoriginatingfromtheCaCutheinterface,themicrostructureiscomparable3Ti4O12grainsbothintermsofgrainsize,porosity,anddistributionofsecondaryphase(s)tothatoftheas-sinteredsamplesinFig.1.Thetextureisclearlylessstriatedinthe3hsample,suchthatthestriationsextendfor5–10mmbeforecoalescingintoamore‘‘globular’’morphology,Fig.4(d).Theinnerlayerofthe24hsample,how-ever,wasentirelystriatedbetweentheinterfacewiththeouterlayerandtheinterfacewiththebulkmaterial,Fig.4(e).

ThecrystallinephasescontainedwithinthedecompositionzoneandthebulkmaterialforsamplesheattreatedinN1CwerecharacterizedbyXRD;layer-by-layerand

typical

2at1000

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