8th WorkshooThermoelectric.Cracow2004
SodiumCobalt Oxide as FunctionalThermoelectricMaterial
A. Mrotzeka,E. Miillera,
J. Plewal'2,K. Kozlowskal'3,M. Sopicka-Lizer3,
L Brunetsr,H. Altenburgr'2
IFH Muensterfuniversityof Applied Sciences,Steinfurt,Germany
'SIMa, Steinfrrter Initiative fuer Materialforschung,Steinfurt,Germany
3Silesian
Universityof Technology,Katowice,Polen
"DuetscheLuft- und RaumfahrtAnstalt, Koeln, Germany
Abstract.
Cobaltitecompoundswith a layeredstructurehavea functionalproperties;LiCoO2as
electrodeandNa"CoO2asthermoelectricmaterial.ln recentyearsthey havebeenextensive
studiedasnew electroceramic
materials[1-2].Na*CoOzfor instancewasdiscovered
to posses
a very high electricconductivityandlargeSeebeck
coefficient.
The greatability ofcobalt to adoptvariousoxidation statehasinfluencedon the physicaland
chemicalpropertiesand speciallyon the thermoelectricalproperties.In this respect
determination
of cobaltvalenceis interestfor monitoringof qualityof samples.
NaxCoO2powderhasbeenpreparedby the solid statereactionof CqOq and carbonateon
multi-step calcinationand presinteringprocess.Productswere investigatedby XRD and
TG/DTA analysis.The contentof cobalt on different oxidation statein sodiumcobaltiteswere
measuredwith t}ree chemicalmethods:iodometric,complexometricwith Cr3*andwith
EDTA. Resultsdeterminedby different redox reaction(iodometric,chromaticand H2
reduction)werein a goodaccordance.
The all samplescontentofCo3* andCoa*,somesample
(secondqualitysamples)alsoCo2*.Theratio of Co3*/Co4*
areclosebetween1,1and 1,8.
Thesevaluesarecorrelatedwith electricalproperties[3].
properties:thermopower
Na*CoOzexhibitspromisegoodthermoelectric
up to 150pV/K,
electricalconductivity 300 S/cmandthermalconductivityup to 2 Wm*K. The Pb substituted
properties[4].
samplesshowsenhancement
of thethermoelectric
Introduction
Thermoelectricpower generation,direct energyconversionfrom heatto electricalpower,has
very goodperspectivesin applications,if high+emperatureheat sourcemakesthe output
power and efficiency raisedby largetemperaturedifference.
Cracow2004
8th WorkshopThermoelectric,
Coba.ltiteswith delafossite-typestructuresshowgood chemicalstability up to high
temperaturecombinedwith a large Seebeckcoe{ficientand small resistivity as well asthermal
andCa:CozOo
are
conductivity.Fourof thesecobaltites:Na*CoOz,CuCozOs,Ca:Co+Og
known to be goodthermoelectrics,becauseof their metallic conductivitesandhigh
thermoelectricpower. Theselayeredcobaltitesshowa commonstructuralcomponent:the
CoO2planes,in which a two-dimensional-triangularlattice of Co ions is formed by a network
of edge-sharingCoO6octahedra.The athactivephysicalpropertiesof layeredcobaltatesare
strongly dependentupon overall oxygencontentaswell aslocal, or longer range,ordering.
Electronicand ionic conductivity is known to be strongly affectedby the relatedCo3*/Co4*
ratio and distribution.
Progressin the applicationof thermoelectriccobaltitesfor thermogeneratoris dueto the
improvementof good quality materials.Manufactudngof technicallyapplicable
thermoelectriccobaltitesrequireswell-definedpreparationtechlique to obtain chemical
homogen,denseceramicbulk matedals.
Preparationand characterizationof sodiumcobaltate
werepreparedby a conventional
solid-state
reaction.
Polycrystallinesamplesof NaxCoO2
Startingmaterials,Na:CO: (Merck) and Co:O+(Chempur)were mixed in a molar ratio of
Na:Co:0,6-1,0 : 1, calcinedat 750 "C for 12h, repeatedmixedandsinteredat 800oCfor 12
h. Black homogeneitypowderwas obtained.After sinteringthe samplesconsistedonly
Na*CoOzphasefor the startingx-value0,8-1,0andwith smallamountof Co:O+for the
startingx-value0,6-0,7(fig.1).
DTA,pV
130
100
40 2 Theta
800
850
900
950 1000 1050'c
Fig.1.X-ray dif&actionandDTA curvesof the samplesNa*CoOzfor startingx-valuebetween
0,6 and 1,0.Sinteringtemperature
900 oC,duration12h.
8th WorkshopThermoelectric"
Cracow2004
by thermalanalysis(fig.2).
The stabilityof Na€oOz (startingx-value0,8) was investigated
Na*CoOzwas not stableat the temperature
higherthan 1000'C and showeddecomposition
reactionwasobservedat about1000'C, the weightlossduring
andmelting.The endothermic
this step can be attributed to the oxygen. The thermographfor the pure phasedid not show
peaksfrom Co:O+(main impurities of cobaltities).TheseTG curve indicateda rapid loss of
weight at round 1000 oC (about Lfir=8,9 %), which can be attributed to the desorptionof
oxygen and a sodium evaporationfrom the Na*CoO2phaseduring the heatingprocess.These
valueof weightlossto conespondofx:0,78 (for ignoringofvalatilizationof the sodium).
TG, %
DTA, FV
Temperature,"C
Fig.2 Thermographof pure Na*CoO2sampleSinteringtemperature900 'C, duration 12h
It can be seenthat decomposition
and meltingprocessis completeat a temperature
closeto
the endothermicpeak at 983 oC as determinedby the DTA trace.The TG/DTA data of these
sampleshow that Na,CoOzwere single phaseand that from XRD data had the hexagonaly
structure.
Thermoelectricpropertiesof Na*CoO2
After intermediate milling and palletizing, the green body was sintered at 900 "C. The
samplescontaining various amountsofN4 becauseof Na evaporateduring firing of pellets.
The AAS analysiswas usedfor quantitativedeterminationof sodium.For example:the pellets
sinteredat 900 "C for 12 h to contain 14,4 VoNa (Naa,66CoO2),
after 24 h 11,4 % Na
(Nao,srCoOu),
after 168 h l2,l % Na (Nao,srCoO2),
and after 336 h only 10,2 % Na
(Nao,qsCoOz).
8th WorkshopThermoelectric,Cracow2004
Fig.3 Surfaceandbreachofthe Na,CoOztablet sinteredat 900 for 12 h
This sampleconsistsof plateJike Na*CoOzgrains,where someCo:O+round particleswere
trapped.The larger plateJike grains were producedas a results of significant grain growth
with duration time [5,6]. Unfortunately with duration of sintering time sampleslosses of
sodium.
The functionalpropertiesofNa*CoOz - ceramicsaredevelopmentat high temperature.
The themropower shown in fig.4 is less obviously affected by cobaltite modification and
oxygen content.cr,-,cr,'and y-phaseexhibitedthe samecharacteristictemperaturedependence
S(T) with an nearly linearity increasingS(T) of the y-phaseand broadminima in S(T) of the
o- *6 s'-phases [5]. The positive Seebeckcoefiicient indicatesholes as the main charge
carriers for each sample regardlessof composition and crystal structure. The Seebeck
coefiicient generally increasedwith the rising Na content from y- via cr'to cr-phase.This
successionis correlatedto the decreaseofthe electricalconductivity.
.*'o
116
350
3A
^ 25O
x
3"*
t
lq)
tm
-200 -t6
0
1@ 2s
3s
40
6@ 00r('s)
Fig.4. TemperaturedependentSeebeckcoefficient in pV/K with electricalconductivity in
S/cm andthermalconductivityr in Wm*K of the modificationsNa*CoOz
8th WorkshopThermoelectric,Cracow2004
ofthe
It is evidentthattheNa contentx affectedofthe electricalconductivityregardless
ratio is of 1,8in y-phaseandin cr-phase
oxidationsatesofCo in cobaltite.The Co3*/Co4*
the average
(xel) oxidationstateresultingof+3 for Co. Reducingx in Na*CoOzincreases
oxidation stateofCo andincreasethe electricalconductivity (createsdefectelectronsin the
for dopedsamples
valenceband[a]). The changeofelectricalconductivityaremeasured
(tab.1)
samples
Tab. 1. Electricalconductivityo(25'C) of Na*Co6,e5lvl6,65O2
M
Ii
o (Si'/cm)
r5
ll0
Cr
Mo
r50
220
l?0
Ic
NI
Ctt
.,\|
(h
Pb
It)
200
274
l/)
Ll5
2t0
The thermalconductivity of different phasesof Na*CoOzare similar (fig.4). After addition of
glassfrit the thermalconductivitywas significantly increased(andporosity decreased).
Na*CoOzceramicswith big grains(sirtering durationof 7 day) exhibits electrical
conductivity 300 S/cmandthermalconductivltyup to 2 WmK. The Pb substitutedsamples
properties[6].
of the thermoelectric
showsenhancement
Summarizesthe measuredvaluesof the thermopowerfor Na*CoO2samplesand comparition
with the partial substitutedone (Na*Co1-rMrO2)showsthat the partial substitutionof Co by
mostly due to on increasedconductivity and
Pb enhancedsignificantly the ZT of y-Naa.5CoO2
reducedthermalconductivity
Tab.2.SeebeckcoefficientandZT- valuesofNa*CoOzsamples
composition
S(500'C),pV/I(
t34
Nao.sCoOz(y-phase)
(cr-phase)
360
Nao.qCoOz
106
(y-phase)
Nao.sCoo.qsPbo.os02
(ct-phase)
270
Nao.qsCoo.qsPbo-osO:
zT(s00'c)
0,115
0,026
0,075
0,130(300'c)
Conclusion
Ceramicthermoelectricmaterialwith compositionof Na*CoOzshowedstructurewith platelike grains.Thesematerialsaredifferent Na content,becausesodiumevaporatedat high
temperuturesduring of formation of this compoundsand during of sinteringof ceramic.The
plateJike grainsgrew with sinteringdurationandthe electricalconductivitywas high. Similar
effect were obtainedby dopingwith Pb. Theseceramicshowsthe thermoelectricfigure of
meritzT(500"c)>0,1.
8th WorkshopThermoelectric,Cracow2004
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