2024年3月13日发(作者:苹果22年发布会新品)
维普资讯
第24卷第6期
无 机 化 学 学 报
2008年6月
Vo1.24 No.6
CHINESE JOURNAL OF INORGANIC CHEMISTRY
926—930
层状钴氧化物Bi2-xAgxSr2Co2Os_8的高温热电性能及XPS研究
李中华 陈 刚 裴 健 刘璞生
(哈尔滨工业大学应用化学系,哈尔滨 150001)
摘要:采用固相反应法制备了组成为BihA r2CozO。 =O.0,O.4,0.8,略写为BAC.222)的层状钴氧化物陶瓷。利用X.射线光电
子能谱考察该类化合物的电子结构,结果表明钴离子以CC+和c04+混合价态形式存在,n 的比例随着Ag掺杂的量增加而
增加。Ols光电子谱显示在所有样品中均存在点阵氧和吸附氧。热电性能测试结果显示,随着Ag掺杂量的增加,电导率显著增
加而Seebeck系数几乎保持不变,Ag的引入极大的影响了BAC.222的电子输运性质,其功率因子在1 123 K时达到了1.23 ̄
10-4 W・m ・ ,是一种具有很好应用前景的热电材料。
关键词:层状钴氧化物陶瓷;热电;XPS;BC.222
中图分类号:O612.5;0614.22;0614.23 ̄2;0614.81 ̄2 文献标识码:A 文章编号:1001-4861(2008)06.0926.05
lIigh-temperature Thermoelectric Properties and X-ray Photoemission Spectra
of Layered Co-based Oxides Bi r2Co2o
LI Zhong—Hua CHEN Gang PEI Jian LIU Pu—Sheng
(Department ofApplied Chemistry,Harbin Institute foTechnology,Harbin 150001)
Abstract:Layered Co・based ceramics with a nominal composition of Bi2_ ̄Ag,Sr2Co2O8 =0.0,0.4,0.8,denoted by
BAC一2221 were prepared using conventional solid state reaction method.X—ray photoemission spectroscopy(XPS)was
used to investigate their electronic structures.The cobalt ions are existed in mixed states of Co¨and Co4+.It is found
that the n |n ratio increases with the Ag doping content.The Ols photoemission spectra show that there are
lattice oxygen and chemical absorbed oxygen in all the samples.Meanwhile,the conductivity and Seebeck
coefifcient of Bi2喈A r2Co2o8 were studied.The substitution of Bi by Ag results in drmaatically increase of the
electrical conductivity while the Seebeck coefficient is kept almost unchanged.This abnormal phenomenon means
that Ag ions play an important role in the electrical trnasport property of BAC一222.The power factor of
Bil g r2Co20 reaches 1.23x10_4 W・m-1・KI2 at 1 123 K,which suggests that Bi2喈A r2Co2o8 are good potential
oxides for thermoelectric application.
Key words:layered Co--based ceramic;thermalelectric;XPS;BC--222
hTermoelectric generation systems can convert
utilize exhausted waste heat.Metal oxides are very
heat energy into electric energy directly without using
suitable for this purpose comp ̄ed with conventional
moving parts nad wihtout producing carbon dioxide gas,
thermoelectirc materials【 .due to their high thermal
radioactive substances,or other emissions.Studies on
and chemical stability at high temperatures in air for
this subject have been accelerated by the global need to
thermoelectric conversion.Among various metal oxides,
收稿日期:2008.O1.21。收修改稿日期:2008.03-28。
国家自然科学基金(N0.20571019),黑龙江省归国留学基金(N0.LC06C13),哈尔滨市学科带头人基金(N0.2006RFXXG001),哈尔滨工业大
学优秀青年教师培养计划(No.HffQ ̄S.2006、O28)资助项目。
通讯联系人。E—mail:gchen@hit.edu.cn
第一作者:李中华,男,32岁,博士,副教授;研究方向:材料化学和计算化学。
维普资讯
第6期 李中华等:层状钴氧化物Bi:喈A r2Co20 的高温热电性能及XPS研究
layered cobalt oxides such as NaCo2O4 fCo一1 24) ,
Ca2Co2O5 or Ca3Co409(Co一349)[4-61,and Bi2Sr2Co20,(BC一
222)[7一 have attracted a great deal of interest.The con—
version efifciency of a thermoelectric material is
expressed by the ifgure of merit,Z,defined as s |lp ,or
ZT,where S,P,K,and T are the thermoelectric power,
electrical resistivity,thermal conductivity, and
temperature,respectively.Large S coexisting with
lowpvalues is necessary to the realization of excellent
thermoelectric performance,therefore,much effort has
been made to increase the power factor l0、of
thermoelectric materials.
The single crystal of BC一222 exhibits good
thermoelectric performance(ZT>1.1 at 1 000 K1【8J.which
is competitive with that of conventional degenerate
semiconductors such as Bi21re3,PbTe,and Si1 Gex【2】.
Howerver,it is diifcult to grow single crystals large
enough for the construction of a thermoelectric device.
Therefore,many research group focus on the increase of
ZT values of polycrystalline bulk material for practical
applications.For polycrystalline BC一222【9J.K value
(1.6 W‘m ・K )is almost the same as single crystal
(about 2.0 W・m ・K ),S value(180 V・K )is more
than half of single crystal(about 300 txV・K ),P value
(17 mSq)is more than five times of single crystla(3 mSq)
at 1 000 K.As mentioned above,reducing p value looks
like the most effective way to improve the
thermoelectric property of polycrystalline BC一222.
Recently,it has been reported that the addition of
Ag metal to polycrystalline NaxCo2O4【ll Jand AgO to Co一
349[ can successfully reducepand consequently impr—
ove the thermoelectirc power factor(S |p or S2o-).In the
present study,we examined the effect of Ag substitution
on the thermoelectric properties of BC一222 bulk m
ateria1 s.
1 Experimental
Polycrystalline samples of Bi2 Ag r2Co2O8 were
prepared from reagent grade Bi203,SrCO3,Co3O4 and
AgNO3 powders in stoichiometric ratios.The mixtures
were pressed in the form of dish—shaped pellets and
heatedin alumina crucibleiIi air at 1 123 Kfor 12 h.
X—ray diffraction(XRD)patterns of the powders
were obtained on Rigaku D/max一2000 diffractometer
operated at 40 kV and 100 mA with Cu K radiation
(A=0.1 54 06 nm)using curved graphite monochromator
to examine the sample purities.Electrical conductivity
(or)was measured by a dc four—probe technique.Therm—
opower(o')was calculated from the thermoelectric volt—
age and the temperature difference between the two
ends of sample.XPS experiment was carried out on VG
ESCALAB MKⅡspectrometer using Al K radiation
(hv=l 486.6 eV).The binding energy scale was calibra
ted by assigning 284.6 eV to the C ls signa1.
2 Results and discussion
In the synthesis of Bi2Sr2Co2O8 ,the effects of sint—
ering temperature and time were examined.Fig.1(a)
shows XRD patterns of the Bi2Sr2Co208 ̄products synth—
esized by conventional soli
;要^】
d st
磊口。首
ate reaction method at
20/(。)
●
号
0
磊
20/(。)
Fig.1 XRD patterns of Bi2 Ag,Sr2Co2Os ̄synthesized at
different temperatures without Ag doping(a)and
Ag doped at 1 123 K,x=O,x=O.4 and x=0.8(b).
The inset shows axis enlarged figure and the
peaks shift can be seen clearly
维普资讯
无机化学学报 第24卷
different sintering temperatures for diferent periods of
time.Whenthe sample is sintered at 1 073 Kf0_r 12 h
.
we
caIl obtain Bi2Sr2Co20¨and unreacted Bi203 in the pro.
ducts.When the sintering time increases from 12 h t0
24 h,the unreacted Bi2O3 still exists.
hWereas,there is nearly no Bi2O3 impurity in the
products obtained at 1 123 K for 12 h.It means that
sintering temperature,instead of sintering time,is a
critical factor for the preparation of single phase
Bi2Sr2Co208每Therefore,Ag substituted Bi2
-
 ̄
Ag,Sr2Co2
O ̄x=0.0,0.4,0.8)were all prepared at 1 123 K.XRD
analysis eo/ ̄/'ms that the Bi2喑A&sr2C020 samples are
crystallized in a monoclinic structure as shown in Fig
.
1
.
XRD patterns of Bi2 r2C02Os-s powders are slig—
hfly shifted to higher 20 value with the increase in the
amounts of doped silver as shown in the inset of Fig.1
(b).The shitf indicates that a part of silver at least is
homogeneously doped into the Bi
一一 .>3\lu I。匿8u芬 qa ∞
2Sr2Co2O lattice.
Fig.2 shows the temperature dependence of
conductivity for Bi2 Ag r2Co208 samples.Ag substitu—
tion is effective in increasing the conductivity with all
three samples showing metallic—like behavior(i.e.,do'/
dT<0)within the measured temperature rnage.For the
x=0.4 sample the conductivity is as high as 62.1 S・cm一
at 333 K,which is about 1.47 times of that of undoped
polycrystalline smaples(42.2 S・cm ).The high conduc—
tivity may originate from the substitution of monovalent
Ag for trivalent Bi¨.It could increase hole concen.
tration of the system,which can be further approved by
XPSanalysis
Fig.2 Electrical conductivity or for Bi2. ̄Ag,Sr2Co20w
as afunction oftemperature
As shown in Fig.3,all samples show positive
values of the thermoelectirc power,suggesting p-type
Fig.3 Seebeck coefifcient(S)for Bi g o2o¨as
a function of temperature
conductors.This temperature dependence is similar to
single—crystla BC一222.Contrary to hte Ag concentration
dependence of conductivity,the addition of Ag has little
effect on S.For the three samples the values of
thermoelectric power all increase with increasing
temperature in the whole temperature rnage measured,
hTe above and S data indicate htat Ag substitution is
fairly effective for improving the BC-222 systems
thermoelectric properties. ‘
hTe thermoelectirc power factor(PF=S2・cr)was
calculated from measured and S values(Fig.4).The
PF of each sample increases with increasing
temperature.Because the addition of Ag to BC一222
increases ,and S almost keeps unchanged,the PF of
Ag doped samples are all increased.The values of
calculated power factor at 1 123 K are about 1.06,1.23,
nad 1.22xlO_4 W・m-l・K_2 for the =0,0.4,and 0.8
samples,respectively.The decrease in the conductivity
value suggests an increase in the carrier concentration,
which is in agreement with the substitution of Ag for
Bi .However,the change in thermoelectirc power value
Fig.4 Temperature dependence of PF for
Bi2喈A o20¨
维普资讯
第6期 李中华等:层状钴氧化物Bi  ̄Ag,Sr2Co O¨的高温热电性能及XPS研究 929
indicates the opposite.It should be noted that the
increase of conductivity and unchanged Seebeck
coeffic.1ent with Ag substitution are dififcult to
understand by conventional theory.
The valence states of cobalt ions in Bi2 Ag ̄Sr2Co2
O8书were examined with XPS.Fig.5 shows the spectra
for the Co2p level of the samples at x=0.2.0.4。0.5.The
three spectra are very similar and exhibit the 2p3/2 and
2p 1,2 doublets due to the spin—orbit coupling.The peaks
positions of the Co2p level of the Bi2 Ag ̄Sr2Co2O8书at
=
0.0.0.4 and 0.8 are observed as 781.4,781.2 and
78 1.1 eV,respectively.From these results,it seems to
be difficult to discern the valence states of cobalt ions
in Bi2 AgxSr2Co208书.The spectral features of these
Co2p peaks in Fig.5 are broad and the asymmetric
bands show the structure information about cobalt ions.
If the Co coexists with the Co“。it should lead to
asymmetry of Co2p peak feature[131.Here we use an
asymmetric index 8,where 8 is defined as the ratio of
half-width at the half-maximum on a high--binding--
energy side,W}b to half-width at the half-maximum on a
low—binding—energy side,Wl1.The value of increases as
1.O(x=O.0),1.1 =0.4)and 1.3(x=O.8)with in Bi2 A&
SrzCo2O8毋This indicates that Co coexists with the Co“.
From these spectra of Co2p level,we conclude that the
cobalt ions have a mixed state of Co and Co“.The
fra tinn 0f Co4 is increased with th increase 0f .
Binding energy/eV
Fig.5 Co2p photoelectron spectra of Bi2 ̄Ag r2C02O8
M。re。Ver, he。xYgen states。f
Bi2一Ag Sr2c。208书
were studied by XPS.Fig.6 shows the spectra f0r the
Ols level of Bi2 AgxSrzCo208书 =0.0,0.4 and 0.8).
Every O ls spectrum of Bi2 AgxSrzC0208书shows two
peaks.The peak at low—binding—energy side is assigned
●
8
苦
Binding energy/eV
Fig.6 O ls photoelectron spectra of Bi2 g r2C02Osz
as lattice oxygen O ,and the other peak at high—
binding—energy side is assigned as adsorbed oxygen
species .The binding eneryg of lattice(OL),adsorbed
oxygen(O and their relative intensity are shown in
Table 1.The binding energy of lattice oxygen(OL)
decreases with increasing .which indicates that O—Co
bond is strengthened by doping of Ag and the increase
of Co“.It is well known that CoO2 layers are responsible
ofr the electric conduction of layered Co—based oxides,
therefore,the strengthening of O—Co bond suggests an
increase of electric conductivity of Ag substitution Bi2
Ag ̄Sr2Co208书.On the other hand,the increase of relat—
ive intensity ratio of odo1 is a natural result of Ag
substitution for Bi¨,which means an increase of hole
concentration in the system and the increase of cond—
uctivity.
Table 1 Positions of lattice(OI and adsorbed
oxygen(O ̄for the Ols XPS spectra of
Bi Ag ̄r2Co20w =0.2,0.4,0.5),as well
as their relative intensity ratios(R)
3 C
ondusions
Bi2 Ag ̄Sr2Co208书 =0.25,0.3,0.4)ceramics were
prepared by solid—state reaction.The crystalline phases,
electronic structure and high temperature thermoelectr—
维普资讯
无机化学学报 第24卷
ic property of Bi2_ ̄Ag,Sr2Co208 have been investigated
.
XPS results show that cobalt is in mixed states of C03+
and Co4 and the n
c.
]n ratio increases with the Ag
一
o —
doping content increasing.The O-Co bond is strength.
ened by doping of Ag and the increase of Co4 .,I1le
increase of relative intensity ratio of OJOL results in
hole concentration increase in the system.which iS
responsible for the positive effective in increasing
conductivity of Ag substitution Bi2- ̄Ag,Sr2Co2O8 .Bee-
ause it provides a signiifcant increase of cond-uctivity
without changing S value,Ag substitution has been
proved to be an effective way to obtain high thermo-
electirc performance materilas.
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