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