Decomposition reactions in CaCu3Ti4O12 ceramics(4)

HydrogenreductionTGwasperformedonasmallamountofpowderfromacrushedpelletsinteredinairat11151Cfor24h.WeightlosscommencedatB3301Candwascomplete

by

September2006DecompositionReactionsinCaCu3Ti4O12Ceramics2837

Fig.5.Typicalenergy-dispersiveX-rayspectrafrom(a)thebrightpre-cipitatephaseintheinnerlayerofthedecompositionzoneand(b)thematrixphaseintheinnerandouter

layer.

Bpowder6801C,followingwithatotalthemassTGlossexperimentofB8%.revealedXRDofnothetraceremainingoftheoriginalCCTOphase.Instead,thesampleconsistedofCaTiO3,TiO2(Rutile),andmetallicCu,Fig.7,indicatingcompletede-compositionofCCTO.TheobservedmasslossofB8%2is1ingoodagreementwiththatexpectedforthereductionofCutoCumetal(7.8%massloss)foraninitialstartingcompositionofCaCu3Ti4O12.AlthoughitisclearthatCCTOceramicsarein-homogeneous,theoveralldecompositionprocessinreducingconditionscanbedescribedbythefollowingequation:

CaCu3Ti4O12!CaTiO3þ3TiO2þ3Cu

þ3=2O2ðgÞ

(1)

ThedecompositionprocessoccurringinCCTOceramicsat10001CinN2isincomplete,presumablybecauseofkineticef-fectsassociatedwiththelimitedheat-treatmenttimeofdenseceramicsfor6hinaninertatmosphereasopposedtoasmallamountofcrushedpowderinareducingatmosphere.Never-theless,theresultsfortheN2heat-treatedCCTOceramicscan

Fig.6.X-raydiffractiondataoftheceramicsurfacefora3hsampleafterheattreatmentat10001CinN2(bottomtrace)andthen,sequenti-ally,fromlayer-by-layerpolishedpelletsurfaces(upper

traces).

Fig.7.X-raydiffractionpatternofphaseassemblagefollowingTGAofCaCu3Ti4O12powderheatedto11001Cina5%H2

atmosphere.

Fig.8.SchematicillustrationofdecompositionreactionsoccurringinCaCu3Ti4O12ceramicsheattreatedat10001CinN2.

besummarizedschematicallyinFig.8.Attheinterfaceofthebulkphaseandthedecompositionzone,CaCuaphasemixtureofCaTiO3Tiposesinto4O12decom-discussionissimpli edtoexcludeoxygen3,Cu2O,andTiOlossfromthe2(thisbulkphaseand/orsecondaryphasesandthepresenceofSiOreactionproducts,2con-tamination).TheinnerlayercontainstheinwhichCaTiO3andTiO2coexistasamatrixstructureandCuwithinthatmatrix.Attheinterfacebetweenthe2Oasaprecipitateouterandinnerlayersofthedecomposition,volatilizationofCu2Ooccurs,leavingtheCaTiOouterlayer.Asstated3–TiO2matrixtoformapo-rous,‘‘coral-like’’previously,theelectricalpropertiesofthepolishedCCTOceramicsafterremovalofthesurfacelayersshowedthebulkresistivitytobeunalteredbytheheattreatmentinNdecreased2at10001C;however,thegrainboundaryresistivitybythreetofourordersofmagnitude.9Thisresultindicatesthatthecompositionofthegrainboundaryre-gionsinCCTOchangessignificantlyduringheattreatmentandpresumably,decompositioncommencesalongthegrainbound-ariesinCCTOceramicswheremassdiffusionofprimarilyCuandOissignificantlyhigherthanthatoccurringwithinthegrains.ThelackofvariationinbulkresistivityforsamplesheattreatedinNisunlikely2orOtobe2suggeststhattheoxygen-lossmodel(model2)theprimarysourceofsemiconductivityinCCTOceramics.

Suchlarge-scaledecompositionwasnotobservedinas-sinte-redsamples,althoughisolatedprecipitatesofCu2OandCaT-iSiO5wereobservedinsamplessinteredinairat11151Cfor24h.Inaddition,XRDofpelletsurfacesindicatethepossiblepresenceofCu‘‘Cu-rich’’grain2O,andotherworkershavereportedthepresenceofboundaries11

inCCTOceramicssinteredatelevatedtemperatures.IV.Conclusions

Theresultspresentedheresuggestthatlimitedreductionanddecompositionprocessesmayplayacentralroleinthedevel-opmentoftheelectricalmicrostructuresobservedforCCTOceramicssinteredinairat410501C.Furtherinvestigationsus-inganalyticaltransmissionelectronmicroscopyareinprogresstoprobenanoscalecompositionalvariationsinthegrain

and

2838JournaloftheAmericanCeramicSociety—Adamsetal.

5

Vol.89,No.9

grainboundaryregionsofCCTOceramics.Thisshouldprovidemoreinformationregardingthecomposition–electricalpropertyrelationshipsinCCTOceramics.

References

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S.Y.Chung,I.L.D.Kim,andS.J.L.Kang,‘‘StrongNon-LinearCurrent–VoltageBehaviourinPerovskite-DerivativeCalciumCopperTitanate,’’Nat.Ma-ter.,3[11]774–8(2004).3

D.C.Sinclair,T.B.Adams,F.D.Morrison,andA.R.West,‘‘CaCu3Ti4O12:One-StepInternalBarrierLayerCapacitor,’’Appl.Phys.Lett.,80[12]2153–5(2002).4

T.B.Adams,D.C.Sinclair,andA.R.West,‘‘GiantBarrierLayerCapaci-tanceEffectsinCaCu3Ti4O12Ceramics,’’Adv.Mater.,14[18]1321–2(2002).

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J.Li,M.A.Subramanian,H.D.Rosenfeld,C.Y.Jones,B.H.Toby,andA.W.Sleight,‘‘CluestotheGiantDielectricConstantofCaCu3Ti4O12intheDefectStructureofSrCu3Ti4O12,’’Chem.Mater.,16,5223–5(2004).8

T.B.Adams,D.C.Sinclair,andA.R.West,‘‘CharacterisationofGrainBoundaryImpedancesinFine-andCoarse-GrainedCaCu3Ti4O12Ceramics,’’Phys.Rev.B,73,0941241–9(2006).9

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