层状钴氧化物Bi2-xAgxSr2Co2O8-δ的高温热电性能及XPS研究

层状钴氧化物Bi2-xAgxSr2Co2O8-δ的高温热电性能及XPS研究


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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