Journal
J.Am.Ceram.Soc.,89[9]2833–2838(2006)
DOI:10.1111/j.1551-2916.2006.01174.xr2006TheAmericanCeramicSociety
DecompositionReactionsinCaCu3Ti4O12Ceramics
TimothyB.Adams,DerekC.Sinclair,wandAnthonyR.West
DepartmentofEngineeringMaterials,TheUniversityofShef eld,MappinStreetS13JD,U.K.
CaCu3Ti4O12(CCTO)ceramicssinteredinairat11151Cfor3and24hhavebeenheattreatedinN2at10001C.Surfacelayersdevelopontheouterregionsoftheceramics,andacombinationofX-raydiffractionandanalyticalelectronmicroscopyhasbeenusedtoestablishthephasecontentofthelayers.AmodeltoexplaintheformationofthesurfacelayersisproposedbasedondecompositionofCCTOintoamixtureofCaTiO3,TiO2,andCu2O.TheroleoflimiteddecompositioninthedevelopmentofelectricallyinhomogeneousCCTOceramicspreparedatelevat-edtemperaturesinairisdiscussed.
I.Introduction
iscurrentlyconsiderableinterestinanunusualper-ovskite-relatedoxideCaCu3Ti4O12(CCTO)1–3asapoten-tialmaterialforinternalbarrierlayercapacitor(IBLC)applications.ItisnowgenerallyacceptedthatCCTOceramicspreparedinairat410001Careelectricallyheterogeneous,con-sistingofn-typesemiconductinggrainsandinsulatinggrainboundaries,3–5therebypossessingtherequiredelectricalmicro-structureofanIBLC.TheadvantageofCCTOcomparedwithothertitanateperovskitessuchas(Ba,Sr)TiO3-basedceramics(BST)forIBLCapplicationsisthattheelectricalmicrostructurecanbedevelopedinasingleprocessingstepinairandthereforeavoidthemulti-stepprocessingroutesrequiredforBST.Thedefectchemistryandmicrostructuraldevelopmentassociatedwiththemulti-stepprocessingrouteofBSTceramicsisnowreasonablywellunderstoodandhasbeendevelopedovermanyyearstooptimizetheIBLCcharacteristicsofthesematerials.Incontrast,thedefectchemistryandevolutionofceramicmicro-structureofCCTOremainpoorlyunderstood.Establishingtheoriginofthen-typesemiconductivityinCCTOandthecompo-sitionofthegrainboundaryregionsinCCTOceramicsremainchallengingandimportantproblems,especiallyifthesematerialsaretobeoptimizedforIBLCorrelatedapplicationsinthenearfuture.
Twomodelshavebeenproposedforthen-typesemicon-ductivityinCCTO.The rstinvolvesoxygenloss,viz.,CaCu3Ti4O12Àd,asiscommonlyobservedformanytitanate-basedperovskitessuchasBST6whenheatedathightempera-turesand/orunderreducingatmospheres.Althoughthelevelofoxygenlossisoftensmall,d(0.01,itissuf cienttochangeelectricallyinsulatingTi-basedperovskitessuchasBSTfromanoff-white/creamcolorwithabulkresistivityinexcessof1010OÁcmatroomtemperature(RT),intoadark-bluecolorwithanRTbulkresistivityB0.1–10OÁcm.Thebluecolorandn-typesemiconductivityareduetopartialreductionofTi41(do)toTi31(d1)associatedwiththeoxygenlossfromthelattice.
TO
HERE
T
ceramicsarecommonlysinteredintherangeB10001–11001C,andtherefore,inthismodel,reductionofCu21toCu1occursatthesehightemperatureswithchargecompensationbypartialoccupationoftheCusitebyTi41accordingtotheformula
1141
Ca(Cu21À3xCu2xTix)3Ti4O12.OncoolingtoRT,themonovalentCu1ionsarere-oxidizedtoCu21ionsandaninternalredoxprocessoccurs,causingpartialreductionofTi41toTi31ionsontheB-sitesublatticeleadingtotheformula
1414131
Ca(Cu21ÀxTix)3Ti4À6xTi6xO12,andn-typesemiconductivity.AshasbeenpointedoutbyLietal.,7onlysmalllevelsofei-theroxygennon-stoichiometry(model1)orcationnon-stoic-hiometry(model2),xo0.001,arerequiredtoinducethelevelofRTbulksemiconductivity(B10–100OÁcm)commonlyob-servedforCCTOceramics.Itisthereforelikelytobeverydif- culttodistinguishbetweenthesetwomodelsbydirectchemicalanalysis;however,someformofCu-richsecondaryphase(s)shouldbedetectedformodel2insamplespreparedfromastartingcompositionofstoichiometricCaCu3Ti4O12,assumingnegligiblevolatilityofCuathightemperatures.
Recently,wehavereportedelectronprobemicroanaly-sis(EPMA)onCCTOceramicssinteredfor24hinairat11151C.8TheresultsrevealedthepresenceofCu2Owithintheceramicsandmoresignificantly,theCCTOgrainstobeCude- cientwithanaveragecompositionCa0.98(2)Cu2.92(2)Ti4.04(2)O12(onlycationcontentsweredetermined).Inaddition,dramaticchangesinphaseassemblagewereobservedforCCTOceramicsheattreatedinN2at48001Cwiththedevelopmentofsurfacelayers.ImpedancespectroscopywasusedtoshowedthattheRTbulkresistivityofB100OÁcmobservedinCCTOceramicssin-teredinairwasinsensitivetoheattreatmentinN2orO2at8001–10001C.9Incontrast,theRTgrainboundaryresistanceshowedsubstantialvariationwithpost-heattreatment,decreasingbythreetofourordersofmagnitudeafterheattreatmentinN2.9Theresistanceofthegrainboundariescouldberecoveredtovaluesclosetotheair-sinteredvalueswhenheattreatedinO2at10001C.TheseresultshaveledustosuggestthatoriginofthesemiconductivityinCCTOismoreprobablyrelatedtocationnon-stoichiometry(model2)thantooxygenloss(model1).Tothebestofourknowledge,thethermalstabilityofCCTOceramicsundervariousheat-treatmentconditions(temperatureandoxygenpartialpressure)hasnotbeenestablished.Here,wereportphaseanalysisbyX-raydiffraction(XRD),thermalanal-ysis,andanalyticalelectronmicroscopyresultsonCCTOce-ramicsthathavebeensinteredinairat11151Candthenheattreatedinaninert( owingN2)orreducing( owing5%H2/95%Ar)atmosphereat10001C.CharacterizationofthesurfacelayersthatdeveloponCCTOceramicsundertheinertcondi-tionsprovidesfurtherinsightsintotheimportantroleofthein-stabilityofCu21inCCTOceramicssinteredinairathightemperatures.