J. Am. Ceram. SOC., 72 [12] 2377-80 (1989) journal Electrical Conductivity of

Cadmium Oxide-Antimony Oxide System Ceramics

Biaorong Li and Jingli Zhang Department of Solid-state Electronics, Huazhong University of Science and Technology,

Wuhan, People's Republic of China

T h e e l e c t r i c a l c o n d u c t i v i t y of Cd,Sb,O,,, and Cd,Sb,O,, semi- conductive ceramics was examined from -78" to 1000°C under oxygen, nitrogen, argon, air, methane, and CO /COz atmospheres. The existing fo rm of excess CdO in the crystal lattice structure, the relationship between oxygen vacancies and the atmosphere, the conduction mechan- ism in different temperature regions, the associated defect structure, and phase-transition properties were ana- lyzed. The activation energy and mobility of the carriers were calcu- lated. [Key words: electrical proper- ties, electrical conductivity, cadmium, antimony, defects.]

OME cadmium antimony oxide com- Spounds have been studied concerning mainly their synthesis, substitution by other elements, crystal lattice parameters, and optical properties. However, little is known about the electrical conductivity and gas-sensitive properties of CdO-Sb205 ceramics because little research has been done in these areas.

Vandenborre e t a l . I synthesized a pyrochlore-type Cd2Sb207 using 2Cd0+ Sb203 at about 600"C, and Moisan et al. obtained the same compound using CdO+ CdSb206 at 900°C. It was reported4 that CdO-Sb203-W03 semiconductive ceram- ics possess linear resistance-temperature characteristics; however, its electrical con- duction mechanism is not clear yet.

In this investigation, using CdO and Sb205 as raw materials, we obtained two semiconductive ceramics - Cd2Sb206.8 and Cd6Sb2OI0. Their defect structure, conduction mechanism, and gas-sensitive properties are discussed.

EXPERIMENTAL PROCEDURE Specimens of varying molar ratios of

CdO:Sb205 of 2 to 6 were prepared using

W. Schulze-contributing editor

Manuscript No. 198515. Received March 17,

Supported by the Chinese Science Foundation

*Model 2000 Atomscan, Jarrell-Ash, Waltham,

Model JSM-35C, JEOL, Ltd., Tokyo, Japan.

1989; approved June 27, 1989.

under Grant No. 84-035.

MA.+

the procedure explained later. The raw materials CdO (99.9% pure) and Sb205 (99.9% pure) were mixed, dried, pressed at 15 MPa, and calcined at about 800°C for 2 to 5 h. Rectangular bars (18 mm X 5 mm X 4 mm) were formed by dry press- ing and were sintering in air at about 1200°C for 2 to 5 h. To ensure electrical contact, Pt electrodes were fused to four Pt leads, which, in turn, were connected to a four-terminal dc multimeter to measure the dc resistance. The measurements were made in a controlled-atmosphere tube fur- nace using oxygen, nitrogen, argon, meth- ane, and CO/C02 gases. For the electrical conductivity measurement, some samples were heated to 850" to 900°C and then quenched i n d i f fe ren t a tmospheres (sample 21 was quenched in air at 900°C).

The resistance of the samples was measured from -78" to 1000°C using a microcomputer system. To ensure that the reaction between the sample and ambi- ent atmosphere had reached equilibrium for each datum point, a sufficient time (30 min to 1 h) was allowed for the resistance to reach a stable value. For thermoelectric power measurement, two thermocouples were attached to the two ends of the specimen and the temperature difference was controlled using the tem- perature gradient in the furnace.

X-ray diffraction (XRD) and scan- ning electron microscopy (SEM) studies were used to establish the presence and structure of the Cd2Sb2O6 and Cd6Sb2010 phases. The elements and their contents in the samples were detected using induc- tively coupled plasma emission spectos- copy,* energy-dispersive X-ray analysis,+ and quantitative chemical analysis.

RESULTS AND DISCUSSION For molar ratios of CdO : Sb205 of 2

to 6, only two compounds of Cd2Sb2068 and Cd6Sbz010 were found. When the mo- lar ratio was 2 and the samples were cal- cined at 8OO"C, a cubic pyrochlore structure of Cd2Sb2068 was obtained. If the sintering temperature was increased to about 1200"C, a tetragonal pseudo- pyrochlore structure of Cd2Sb2O6 was obtained. When the molar ratio was 6, pure Cd6Sb2010 was obtained. The com- position analysis indicated that the purity of the samples was about 99.5%.

Figures 1 to 3 show the R-T curves of C d z S b 2 o 6 under different atmos-

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pheres. Three temperature regions in these curves can be identified. In the high- temperature region (T>56OoC) and the low-temperature region (T<20OoC), the logarithm of electrical conductivity (log a) is nearly a linear function of 1/T. In the middle-temperature region, d(1og u)/d( 1 /T) varies continually with the temperature. Moreover, some features of these curves in the high-temperature region are that ( 1 ) the conductivity increases when the ambient atmosphere changes from oxidiza- tion to reduction at the same temperature, (2) the slope of log u-1/T curves and the corresponding effective activation energy (Ec) decrease when oxygen concentration decreases, and (3) the samples (especially the air-quenched samples) with the higher molar ratio of Cd:Sb possess higher elec- trical conductivity.

Figure 4 shows the R-T curve of Cd6Sb2Olo ceramics. Three tempera- ture regions can also be distinguished in Fig. 4. Conduction in the high- and low- temperature regions are similar to those of Cd2Sb2O6 However, in the middle- temperature region (240" to 450°C), the conductivity shows almost no temperature dependence.

The sign of the observed Seebeck coefficient measured in air indicates that Cd2Sb206.8 and Cd6Sb2010 are n-type semiconductors.

These phenomena are mainly con- cerned with the defect structure, con- duction mechansim, and phase-transition properties of CdzSb206 and Cd6Sb2Olo. The details of these influences are discussed. Electrical Conduction at High Temperature

The formation and ionization of oxygen vacancies make Cd2Sb206.8 and Cd6Sb20 become n -type semiconduc- tors. At high temperature, the conduction is mainly related to the defect structure and carrier concentration. Influence of Ambient Atmosphere: Cd2Sb206,8 has the AzB207 pyrochlore- type structure in which six of every seven oxygen atoms consist of the octahedron net, and the one remaining oxygen atom at the center of the octahedron net can easily escape from the structure of the A2B207 pyrochlore. Therefore, oxygen vacancies may be described by

0,- v, + ; 0,

2378 Comniunications of the American Ceramic Society Vol. 72, No. 12

Applying the law of mass action, we get

P O P b': = K I LO01 (2)

where the brackets indicate the activity or concentration of the enclosed species. For simplicity, we take oxygen concentration as a constant; then, the oxygen vacancy concentration ([V,]) is

[Vo]=KPg: (3)

where K , and K are constants. Because the carrier concentration is

proportional to [ Vo], the samples possess higher electrical conductivity under a reduction atmosphere. Effective Activation Energy (Ec) and Mobili ty ( p ) : At high temperature (T>560°C), intrinsic carriers are dominant and the electrical conductivity can be described by

u=uo exp(-Ec/kT) (4)

where k is the Boltzmann constant. The values of u, Ec, and p of the

samples under various atmospheres in the high-temperature (600°C) and low- temperature (40°C) regions are calculated and listed in Table I.

According to Table I and Figs. 1 and 2, sample 20 possesses the maximum activa- tion energy (Ec=0.58 eV) because of the lowest defect concentration and maximum slope of the log u-l/Tcurve. In this situa- tion, the grain-boundary barrier is negli- gible, especially in the high-temperature

region. This is based on the following facts: (1) The electrical conductivity does not change when voltage applied to the sample alters. (2) The electrical conduc- tivity is considerably high although Ec is very low (Ec=0.03 eV in Fig. 3, curve 1). Therefore, this value of Ec (Ec=0.58 eV) is directly related to the intrinsic forbid- den band width Eg, and Eg is equal to 2Ec (1.16 eV).

Moreover, in the other curves in Figs. 1 to 3 , the value of Ec decreases from 0.58 to 0.55 eV, 0.5 to 0.45 eV, and 0.35 to 0.26 eV, respectively, when the defect concentration increases. This phe- nomenon may be explained as follows: When the defect concentration increases, the dopant energy level has already broad- ened into an energy band which may enter the conducting band and form a new de- generate conducting band. The intrinsic forbidden band width Eg as well as Ec will decrease.

At high temperature, the intrinsic car- riers are dominant and the conduction may be described by

I I I I I 10 20 30 40 50

1/rx104 ( ~ - 1 )

Fig. 1. Conductivity-temperature properties of Cd,Sb206.8 in air: (1) sample 21 ( C d : S b = l . O l : l , quenched at 900°C); (2) sample 21 (cooled in furnace) ; and ( 3 ) sample 2 0 (Cd:Sb=0.99:1, cooled in furnace). Arrows show the process of temperature, up or down.

1 00

lo-'

10-3

10-4

P =; (@"+ pp)

If no=1019/cm3 is used for an approxi- mate calculation, the defect carriers are neglected, and the experimental data in Figs. 1 to 3 are substituted into Eqs. (4) to ( 7 ) , then the activation energy (Ec), mobility (p), and carrier concentration (n) can be calculated. The results are listed in Table I. Influence of Cadmium Contents: Compar- ing Figs. 2 and 3 and curves 2 and 3 in Fig. 1, it can be concluded that the electri- cal conductivity increases with the content of CdO. XRD data show that samples 20 and 21 belong to the pyrochlore-type s t ructure and the crystal parameter decreases from 1.023 to 1.020 nm (10.23 to 10.20 A) when the ratio of Cd:Sb changes from 1 : 1 to 1.01 : I . Therefore, Cd2+ is in the crystal lattice, but not in the place of the interstitial atom. For every Cd atom added into the crystal lattice, one Sb vacancy and 2.5 oxygen vacancies must be formed to maintain electrical neu- trality. But this type of defect, resulting from excess Cd, does not influence the con- ductivity too much because of the electro- neutrality condition and combination of opposite ion vacancies, especially in the high-temperature region. Actually, the conductivity of samples only increases to some extent, because the increase of the defect concentration may result in the increase of energy state density and the decrease of Eg, or result in the increase of the mobility of the defect conduction.

-

I I I I I I I

8 12 16 20 24 28 32 36 llJxlO4 (K-1)

Conductivity-temperature properties of CdzSb,06 Fig. 2. (sample 20) in: (1) CO/CO,=IO%; (2) NZ; and (3) 0,. Arrows show the process of temperature, up or down.

December 1989 Communications of the American Ceramic Society 2379

10’.

10.’ -

.... 10-2.

P ,525 D

10-3

i 0-4

8 12 16 20 24 28 32 36 I/TX 104 ( ~ - 1 )

Fig. 4. Conductivity-temperature properties of Cd6Sb,0,, in air.

8 12 16 20 24 28 32 36 i/rx104 ( ~ - 1 )

Fig. 3. (sample 21) in: (1) CO/COz=lO%; (2) Nz; and (3) 02. Arrows show the process of temperature, up or down.

Conductivity-temperature properties of Cd2SbzO6

Electrical Conductivity at Low Temperature

In the low-temperature region, the carrier concentration, electrical conduc- tivity, and change of the slope of the log u-l/T curve are more sensitive to the defect concentration compared with that in the high-temperature region. For ex- ample, if the conduction properties of sample 2l(quenched) are considered, we should get IT= lO-’/(n. cm) in the low- temperature region if we predicted the conduction properties based on the intrin- sic properties (Eg= 1.16 eV) at high temperature. In fact , because of the increase of the oxygen vacancies, u and Ec are 7 X 102/(f l . cm) and 0.024 eV, respectively.

In the nonstoichiometric oxide semi- conductive ceramics, metal-ion and oxy-

gen vacancies may result in p-type and n-type electrical conduction, re~pectively.~ In Cd2Sb20b.g and Cd6Sb2Oio ceramics, Sb is a multivalent metal because of the 5p3 and 5s’ bands of the Sb ions. To meet the electroneutrality condition, an Sb5+ must be altered to an Sb3+ accompanying an oxygen vacancy.

Vo+ SbS ‘-+Vo’ ’ + Sbs’”+Vo’ ‘ + Sb3+ (8)

The electron that results from the process in Eq. (8) is easily excited, and it will result in an n- type electrical conductivity.

Electrical Conductivity in the Middle-Temperature Region

In the middle-temperature region, the conductivity of CdtiSb2010 is nearly in- dependent of the temperature. This pheno-

menon concerns the phase transition at 240°C and the scattering mechanism in this region. (The phase transition has been confirmed by high-temperature XRD and thermal analysis.) After the phase transi- tion, the crystal lattice expands and the conductivity decreases; therefore, the R-T curve become flatter. The tempera- ture dependence is determined by both the concentration and mobility terms. In this region, the scattering by the crystal lattice is dominant and the mobility decreases with the temperature. The contribution of carrier concentration and mobility make the conductivity independent of the tem- perature, which is favorable to the con- struction of gas sensors.

For the moment, whether or not the excition and mobility of carriers are related to the linear negative-temperature- coefficient (NTC) properties of CdO- Sb203-W03 ceramics still needs further investigation.

CONCLUSIONS Cd2Sb206.8 and Cd6Sb2010 ceramics

are n -type semiconductors. Their conduc- tivities are controlled by oxygen activity, temperature, and composition. In the high-temperature region, the conductivity is controlled by the temperature, and it increases with CdO content. In the low- temperature region, the conductivity remains relatively unchanged with little

Table I. Values of Ec, n, IT, and p for Cd2Sb20b,8 under Various Atmospheres at Various Temperatures T= 600°C T- 40°C

Ec 0 I*- EC U Sample Atmosphere (ev) n (/cm’) ((flxm-’) (crn2/(V.s)) (eV) ( ( f l a n - ’ ) Figure Curve

20 0.58 4.6x10’* 0.032 22 0.47 3.4X10-4 2 3

21 20

0 2 0 2

(230°C) 0.50 1.3X1016 0.15 36 0.18 1.8X10-4 3 3 0.55 6.8X10’5 0.14 64 0.23 2 . 1 ~ 1 0 - ~ I 3

Air ( 150°C) 21 Air 0.50 1.2x1016 0.22 55 0.088 4.2X10-’ 1 2 20 0.47 2.1X1016 0.068 10 0.30 7 X 2 2

21 co/co2= 10% 0.28 2.6X10’6 0.56 6.7 0.03 5 x lo-* 3 1

N2 ( 150°C) 21 Nz 0.45 2.6X10‘6 0.25 30 0.08 4.2X10-’ 3 2

5 . 6 ~ 1 0 - ~ 2 1 20 co/co2= 10% 0.35 9.7X1016 0.12 3.9 0.19

21 (quenched) Air 0.26 3.4X1Oi7 0.68 6.3 0.024 7.2X10-’ 1 1

2380 Communications of the American Ceramic Society Vol. 12, No. 12

temperature dependence as a result of phase transition at 2400c for Cd6Sb2010

ceramics, which is favorable to the con- struction of gas sensors.

sor, University of Michigan, Ann Arbor, MI, for his encouragement and help to finish this work. Part of the work reported in this paper was accomplished at the University of Michigan.

Pyrochlore Structure,” Spectrochim. Acra, Part A , 3&4, 997-1003 (1982).

*J. Y. Moisan, J . Pannetier, and 1. Lucas, “Cad- mium and Lead Antimonates: Some Substitutions,” C. R. Arad. Sci., Ser. C , 271, 403-405 (1970).

’W. L. Wanmaker, A. H. Hoekstra, and J . G . Verriet, “The Preparation of Calcium, Strontium, Cad- mium, and Manganese Antimonites,” R e d . Trav. Chim. Pays-Bas, 86, 537-44 (1967).

‘F. Mazuoka, “Thermistor Materials,” Jpn. Pat. No. 518477, 1976.

5Z . M . Jarzebski, Oxide Semiconductors; pp. 87-93. Pergamon Press, Oxford, U.K., 1973. 0

REFERENCES ‘M. T. Vandenborre, E. Husson, and 1. L. Forquet,

“Vibrational Spectra and Force Fields of Several Com- pounds with the Formulas A,B,O, and A,B,O, and

ACKNOWLEDGMENT The authors wish to thank Dr. T. y. Tien, Profes-