Electrical resistivity and Seebeck coefficient ofSr6Co5O15

JournalofAlloysandCompounds377(2004)

272–276

ElectricalresistivityandSeebeckcoef cientofSr6Co5O15

KoutaIwasakia, ,MasahikoShimadab,HisanoriYamanec,JunichiTakahashib,

ShunichiKubotab,TakanoriNagasakia,YujiAritaa,JunjiYuharaa,

YoshimasaNishia,TsuneoMatsuia

a

DepartmentofQuantumEngineering,GraduateSchoolofEngineering,NagoyaUniversity,Furo-cho,Chikusa-ku,Nagoya,Aichi464-8603,JapanbInstituteofMultidisciplinaryResearchforAdvancedMaterials,TohokuUniversity,2-1-1Katahira,Aoba-ku,Sendai,Miyagi980-8577,Japan

cCenterforInterdisciplinaryResearch,TohokuUniversity,Aramaki,Aoba-ku,Sendai,Miyagi980-8578,Japan

Received22September2003;accepted21January2004

Abstract

PolycrystallinesamplesofSr6Co5O15(m=1,n=1)andCa3Co2O6(m=1,n=0),whicharethemembersofhomologousseries(A3Co2O6)m(A 3Co3O9)n(A,A =Ca,Sr,Ba),werepreparedbysolidstatereactioninair.Thetheoreticaldensityratiosofobtainedpellets,89%forSr6Co5O15and95%forCa3Co2O6,sinteredbythesparkplasmasintering(SPS)methodincreasedcomparedtothoseofSr6Co5O15(60%)andCa3Co2O6(67%)preparedbysolidstatereaction,respectively.Electricalresistivity(ρ)ofSr6Co5O15andCa3Co2O6exhibitedsemiconductingbehaviorin300–1120K.ThetemperaturedependenceofelectricalresistivityforSr6Co5O15showedahysteresis-likebehavior.ElectricalresistivityofSr6Co5O15(6.1×10 2to4.6 cm)andCa3Co2O6(4.1×10 2to1.2×103 cm)sinteredbytheSPSmethodwerelowerthanthoseofSr6Co5O15(1.3×10 1to8.0 cm)andCa3Co2O6(1.5×10 1to4.5×103 cm)preparedbysolidstatereaction,respectively.Seebeckcoef cient(S)ofSr6Co5O15andCa3Co2O6werepositiveandmorethan100 V/Kin300–1120K,andbothcompoundsshowedthedifferenttemperaturedependences,respectively.Thepowerfactor(S2/ρ)tendedtoincreasewithincreasingtemperature;themaximumvaluesofthepowerfactorwere1.4×10 5W/(mK2)forSr6Co5O15and1.6×10 5W/(mK2)forCa3Co2O6preparedbysolidstatereaction,and3.0×10 5W/(mK2)forSr6Co5O15and5.2×10 5W/(mK2)forCa3Co2O6sinteredbytheSPSmethod.©2004ElsevierB.V.Allrightsreserved.

Keywords:X-raydiffraction;Other:thermoelectric,Sr6Co5O15,Ca3Co2O6

1.Introduction

(A3Co2O6)m(A 3Co3O9)n(A,A =Ca,Sr,Ba)arethehomologousserieswhichhavetheCa3Co2O6typestruc-ture(m=1,n=0)[1]and2H-BaCoO3(perovskite)typestructure(m=0,n=1)[2]astheendmem-bers,andseveralcompoundshavebeenreportedsofar[3–10].(A3Co2O6)m(A 3Co3O9)nhavethepseudo-one-dimensionalstructureconsistingofCo–OchainsinwhichCoO6octahedraandCoO6triangularprismsareconnectedsharingtheirfacesinthedirectionofc-axisandalkalineearthatomsisolatingtheCo–Ochains.AsshowninFig.1,CoO6octahedraandCoO6trigonalprismsarealternatelyconnectedinCa3Co2O6(m=1,n=0,spacegroupR-3c,a=9.0793(7)Åandc=10.381(1)Å,averagevalence

Correspondingauthor.Tel.:+81-52-789-4689;fax:+81-52-789-3779.

E-mailaddress:k-iwasaki@nucl.nagoya-u.ac.jp(K.Iwasaki).0925-8388/$–seefrontmatter©2004ElsevierB.V.Allrightsreserved.doi:10.1016/j.jallcom.2004.01.060

stateCo3+[1]),andfourconsecutiveCoO6octahedraandoneCoO6prismareconnectedinSr6Co5O15(m=1,n=1,spacegroupR32,a=9.5035(2)Åandc=12.3966(4)Å,averagevalencestateCo3.6+[3]).Somecompoundsin(A3Co2O6)m(A 3Co3O9)nhavetheincommensurateperi-odsinthedirectionofc-axis,andallthecrystalstructuresof(A3Co2O6)m(A 3Co3O9)narenotnecessarilyanalyzed.TheCo–CodistancesinCo–Ochainarelessthan2.6Å,andtheaveragevalencestateofCoionschangedfrom+3to+4withdecreasingm/nratio.Therefore,theelectricalpropertiesarealsoofinterestinadditiontothemagneticproperties,however,theelectricalpropertieshavenotbeenstudiedexceptforCa3Co2O6[11–15]and2H-BaCoO3[16,17].

Wehavestudiedtheelectricalresistivity(ρ),Seebeckco-ef cient(S)andthepowerfactor(S2/ρ)ofpolycrystallinesamplesofCa3Co2O6(m=1,n=0,Co3+)inthetemper-aturerangeof400–1170Ktoexplorethepossibilityofitsuseasathermoelectricmaterial[15].Themaximumvalue

K.Iwasakietal./JournalofAlloysandCompounds377(2004)272–276

273

Fig.1.CrystalstructuresofCa3Co2O6andSr6Co5O15.

ofthepowerfactorwas1.6×10 5W/(mK2)at1150K,whichwasrathersmallcomparedtoastandardvalueof1×10 3W/(mK2)forpracticalmaterials.However,judgingformtheanisotropyofelectricalresistivity,itispointedoutthatthepreferredorientationofgrainsimprovesthepowerfactorofCa3Co2O6sincetheelectricalresistivityinthedi-rectionparalleltoCo–Ochains(c-axis)is1/10,000com-paredtothatinthedirectionperpendiculartoCo–Ochains[12].

Inthisstudy,weinvestigatedtheelectricalresistivity,Seebeckcoef cientandthepowerfactorofpolycrystallinesamplesofSr6Co5O15(m=1,n=1,Co3.6+)inthetemperaturerangeof300–1120Kinair.Theeffectofthesparkplasmasintering(SPS)method,whichencouragethegraingrowth,ontheelectricalpropertiesofSr6Co5O15andCa3Co2O6wasalsoinvestigated.Inthispaper,Sr6Co5O15andCa3Co2O6sinteredbytheSPSmethodareabbreviatedasSr6Co5O15-SPSandCa3Co2O6-SPS,respectively.2.Experimental

SrCO3(>99.99%,RareMetallicCo.),CaCO3(99.99%,RareMetallicCo.)andCo3O4(99.95%,KantoChemicalCo.)powderswereusedasstartingmaterials.Thesepowderswereweighedwiththeappropriateamounts(metalratioswereSr:Co=6:5forSr6Co5O15andCa:Co=3:2forCa3Co2O6).Thepowdersweremixedinanagatemortarwithethanol,andpressedintoapelletform.Thepelletswereputonaplatinumsheetandheatedat1123KforSr6Co5O15andat1243KforCa3Co2O6for70hinair.Thecalcinedsamplesweregroundintopowders,andreheatedunderthesameconditionsfor70h.Sr6Co5O15wasquenchedatroomtemperatureinairaftertheheating.Whensinteringsamples,thesparkplasmasintering(SPS)method(SumitomoCoalMiningCo.,SPS-511S)wascarriedout.Sr6Co5O15andCa3Co2O6preparedbysolidstatereactionweregroundintopowders,andtheywere lledinthegraphitecellandheated

at1123KforSr6Co5O15andat1173KforCa3Co2O6for30minunder50MPa.

TheX-raypowderdiffraction(XRD)patternswererecordedbyadiffractometer(RigakuRINT2000)usingCuK radiationwithapyrolyticgraphitemonochromator.Electricalresistivitywasmeasuredbythedirectcurrentfour-probemethodinthetemperaturerangeof300–1120Kinair.Seebeckcoef cientwasmeasuredbytheleastsquaremethodoftheplotof T(temperaturedifference)– V(elec-tromotiveforce)inthetemperaturerangeof300–1120Kinair,andthecontributionofleadwires(Pt)wassubtracted.

3.Resultsanddiscussion

3.1.SynthesesofSr6Co5O15andCa3Co2O6

ThesinglephaseofpolycrystallineSr6Co5O15wasob-tainedbyquenchingthesampleatroomtemperatureinairaftertheheatingat1123K.WhenSr6Co5O15wascooledintheheatingfurnaceaftertheheating,theobtainedsampleshowedashoulderpeakat2θ=28.648 ((113)peak)asshowninFig.2.Fromourpreliminaryexperiment,itwasfoundthatSr6Co5O15isnotnecessarilystablebelow1123Kinairsinceothercompound,whichwasnotidenti ed,

was

Fig.2.XRDpatternsofSr6Co5O15:(a)quenchedatroomtemperature;(b)cooledintheheatingfurnace.

274K.Iwasakietal./JournalofAlloysandCompounds377(2004)272–276

obtainedaftertheheatingofSr6Co5O15at773Kinair.TheXRDpatternofthecompoundwassimilartothatofSr6Co5O15andshowedthepeakat2θ=28.155 .Theshoulderpeakat2θ=28.648 forSr6Co5O15(Fig.2(b))wasduetothepeakofthecompoundobtainedinthelowertemperatureregion,andthisresultalsoindicatedthattheresidualheatintheheatingfurnacecausedtheformationofthecompound.

WhenSr6Co5O15wassinteredbytheSPSmethod,theobtainedsampleincludedacompound,whichwasdifferentfromthecompoundobtainedinthelowertemperatureregionmentionedabove,inadditiontoSr6Co5O15,andSr6Co5O15wasnotthemainphase.ThecompoundshowedthemainXRDpeakat2θ=28.155 ,however,thephasewasnotidenti ed.Afterthesinteredsamplewasheatedat1123Kfor70hinair,Sr6Co5O15wasformedagain.ThesampleheatedaftertheSPSmethodisabbreviatedasSr6Co5O15-SPS.ThetheoreticaldensityratiooftheobtainedpelletforSr6Co5O15-SPS(89%)increasedcomparedtothatforSr6Co5O15(60%)preparedbysolidstatereaction.

Fromtheresults,Sr6Co5O15tendstochangetoothercompoundsdependingontheheatingconditions.Inaddition,thethermogravimetricanalysisforSr6Co5O15,carriedoutinair,showedtheweightlossof1.5%inthetemperaturerangeof673–1123K,suggestingtheformationofoxygendefects.ThelatticeparametersofSr6Co5O15quenchedatroomtemperatureinairwerea=9.4879(4)Åandc=12.3977(10)Å.Thea-axiswassmallerandthec-axiswaslargerthanthoseofSr6Co5O15(a=9.4977(5)Åandc=12.3758(15)Å)cooledintheheatingfurnace,respectively.ThechangeofthelatticeparametersofSr6Co5O15wouldbeduetotheoxygendefects.

ThesinglephaseofpolycrystallineCa3Co2O6wasobtainedbysolidstatereaction.AfterthesinteringofCa3Co2O6bytheSPSmethod,theobtainedsample,abbre-viatedasCa3Co2O6-SPS,includedatraceamountofCaO(rock-salttype)inaddition toCa3Co2O6(Fig.3(b)).TheXRDpeakat2θ=37.460wasindexedasthe(200)peakofCaO.Thesamplecontactswiththegraphitecellduringtheheating,thusthereductionatmospherewouldcausethedecompositionofCa3Co2O6.AssumingthattheexistenceofCaOwasneglected,thetheoreticaldensityratiooftheobtainedpelletforCa3Co2O6-SPSwas95%,which

was

Fig.3.XRDpatternsofCa3Co2O6:(a)heatedat1243Kinair;(b)sinteredbytheSPSmethod.

largerthanthatforCa3Co2O6(67%)preparedbysolidstatereaction.

TherelativeintensitiesofXRDpeaksforSr6Co5O15-SPSandCa3Co2O6-SPSwerealmostthesameasthoseforSr6Co5O15andCa3Co2O6preparedbysolidstatereaction(Fig.3),respectively,indicatingthatthepreferredorientationofgrainsdidnotproceedbytheSPSmethod.3.2.Electricalproperties

Fig.4showstheelectricalresistivity(ρ)ofSr6Co5O15measuredinthetemperaturerangeof300–1120Kinair.TheelectricalresistivityofSr6Co5O15showedsemiconductingbehavior(1.3×10 1to8.0 cm),andthehysteresis-likebehaviorwasobservedbelow1000K.Thehysteresis-likebehaviorwasconsideredtoresultfromtheformationofthecompoundobtainedinthelowertemperatureregionandtheoxygendefectsmentionedearlier,however,thedetailwasnotclari ed.Ontheotherhand,suchahysteresis-likebehaviorwasnotobservedforCa3Co2O6.TheweightlossofCa3Co2O6wasnotalsoshownfromthethermogravimetricanalysisbelow1243Kinair[15].

Fig.5showsSeebeckcoef cient(S)ofSr6Co5O15measuredinthetemperaturerangeof300–1120Kinair.Seebeckcoef cientofSr6Co5O15wasposi-tive,anddecreasedat300–750K(130–110 V/K),in-creasedat750–1000K(110–138 V/K),anddecreasedat1000–1120K(138–129 V/K)withincreasingtemperature.ThetemperaturedependenceofSeebeckcoef cientwithincreaseintemperaturewereslightlydifferentfromthatwithdecreaseintemperaturebelow1000K.

TheelectricalresistivityofSr6Co5O15-SPSandCa3Co2O6-SPSareshowninFig.6withthoseofSr6Co5O15andCa3Co2O6preparedbysolidstatereaction.TheelectricalresistivitywithdecreaseintemperaturewasadoptedforSr6Co5O15andSr6Co5O15-SPSinFig.6.ThetemperaturedependencesoftheelectricalresistivityforSr6Co5O15-SPSandCa3Co2O6-SPSweresimilartothoseforSr6Co5O

15

Fig.4.TemperaturedependencesofelectricalresistivityforSr6Co5O15(opencircleshowsthevaluewithincreaseintemperature,and lledcircleshowsthevaluewithdecreaseintemperature).

K.Iwasakietal./JournalofAlloysandCompounds377(2004)272–276

275

Fig.5.TemperaturedependencesofSeebeckcoef cientforSr6Co5O15(opencircleshowsthevaluewithincreaseintemperature,and lledcircleshowsthevaluewithdeceaseintemperature).

andCa3Co2O6,respectively.Thesesimilartemperaturede-pendencescorrespondedtotheresultthatthepreferredori-entationsofgrainsdidnotproceedforSr6Co5O15-SPSandCa3Co2O6-SPS.TheelectricalresistivityofSr6Co5O15-SPS(6.1×10 2to4.6 cm)andCa3Co2O6-SPS(4.1×10 2to1.2×103 cm)weresmallerthanthoseofSr6Co5O15(1.3×10 1to8.0 cm)andCa3Co2O6(1.5×10 1to4.5×103 cm),whichmainlyresultedfromtheincreasesofthebulkdensity.TheelectricalresistivityofSr6Co5O15(8.0 cm)at300KwasaboutthreeordersofmagnitudesmallerthanthatofCa3Co2O6(4.5×103 cm),however,thedifferencebecamesmallwithincreasingtemperature,andtheresistivityofbothsampleswerealmostthesameabove1000K.

Seebeckcoef cientofSr6Co5O15-SPSandCa3Co2O6-SPSareshowninFig.7withthoseofSr6Co5O15andCa3Co2O6preparedbysolidstatereaction.Seebeckcoef- cientofsampleswerepositiveandmorethan100 V/Kat300–1120K.Seebeckcoef cientofSr6Co5O15-SPSwassimilartothatofSr6Co5O15,however,Seebeck

coef cient

Fig.6.TemperaturedependencesofelectricalresistivityforSr6Co5O15,Ca3Co2O6,Sr6Co5O15-SPSandCa3Co2O6

-SPS.Fig.7.TemperaturedependencesofSeebeckcoef cientforSr6Co5O15,Ca3Co2O6,Sr6Co5O15-SPSandCa3Co2O6-SPS.

ofCa3Co2O6-SPSwassmallerthanthatofCa3Co2O6inthelowertemperatureregionandthedifferencewasabout300 V/Kat300K.Inthepreviousstudy,itisreportedthatSeebeckcoef cientofpolycrystallineCa3Co2O6ataroundroomtemperaturewassigni cantlyaffectedbythesubtledifferencesinthesynthesisconditionsofCa3Co2O6andthevaluedidnotdependonthesynthesiscondi-tionsabove1000K[15].ThetemperaturedependenceofSeebeckcoef cientforSr6Co5O15differedfromthatforCa3Co2O6althoughbothcompoundshavethesimi-larcrystalstructures.Furtherinvestigationsarenecessaryfortheunderstandingoftherelationshipbetweentheva-lencestateofCo ions(m/nratio)andthepropertiesfor(A3Co2O6)m(A3Co3O9)Thepowerfactor(Sn2.

/ρ)ofSr6Co5O15,Ca3Co2O6,Sr6Co5O15-SPSandCa3Co2O6-SPSareshowninFig.8.Thepowerfactorofsamplestendedtoincreasewithin-creasingtemperature.Themaximumvaluesofsamplespre-paredbysolidstatereactionwere1.4×10 5W/(mK2)forSr6Co5O15and1.6×10 5W/(mK2)forCa3Co2O6.ThemaximumvaluesofSr6Co5O15-SPS(3.0×10 5W/(mK2))andCa3Co2O6-SPS(5.2×10 5W/(mK2))were

about

Fig.8.TemperaturedependencesofpowerfactorforSr6Co5O15,Ca3Co2O6,Sr6Co5O15-SPSandCa3Co2O6-SPS.

276K.Iwasakietal./JournalofAlloysandCompounds377(2004)272–276

2.1and3.3timesaslargeasthoseofSr6Co5O15andCa3Co2O6,respectively.TheincreasesofthepowerfactorarisefromthedecreaseoftheelectricalresistivitybytheSPSmethod.

Insummary,theelectricalresistivity,Seebeckcoef cientandthepowerfactorofSr6Co5O15,Ca3Co2O6,Sr6Co5O15-SPSandCa3Co2O6-SPSwereinvestigatedat300–1123K.ThepowerfactorofSr6Co5O15-SPSandCa3Co2O6-SPSwerelargerthanthoseofSr6Co5O15andCa3Co2O6pre-paredbysolidstatereaction,respectively.However,themaximumvalueswereoftheorderof10 5W/(mK2),whichare3rathersmallcomparedtoastandardvalueof1×10 W/(mK2)forpracticalmaterials.Thepreferredorientationofgrainsinthec-axisdirectionisnecessarilyforthefurtherimprovementofthepowerfactor.

Sr6Co5O15changedtoothercompound,whichshowedsimilarXRDpatterntoSr6Co5O15,at773Kinair.InthecaseofBa8Co7O21(m=3,n=5),thephasetransi-tionfromorthorhombictotrigonalphasewasreportedbe-tween1148and1193K[7].Theseresultssuggestthatsuchchangesofcompoundswouldbefoundinotherphasesof(A3Co2O6)m(A 3Co3O9)oxygenn.

JudgingfromthedefectofSr6Co5O15above673Kandthesynthesisof2H-BaCoO3underthehighoxygenpressure[17],therelationbetweenthestabilityof(A3Co2O6)m(A 3Co3O9)nandoxygenpressureisalsooneofthesubjectsofinvestigationsforthesynthesisof(A3Co2O6)m(A 3Co3O9)n.Acknowledgements

K.IwasakithanksDr.Yamaguchi(GraduateSchoolofEngineering,NagoyaUniversity)forhiscooperationinther-

mogravimetricanalysis.ThisworkwaspartlysupportedbyMETI,Japan,undertheSynergyCeramicsProject.

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