High temperature thermoelectric properties of dy doped camno3 ceramics

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J. Mater. Sci. Technol., 2014, 30(8), 821e825

High Temperature Thermoelectric Properties of Dy-doped
CaMnO3 Ceramics
Bin Zhan1), Jinle Lan1), Yaochun Liu2), Yuanhua Lin1)*, Yang Shen1), Cewen Nan1)
1) State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering,
Tsinghua University, Beijing 100084, China
2) School of Materials Science and Engineering, University of Science and Technology Beijing,
Beijing 100083, China
[Manuscript received May 22, 2013, in revised form June 26, 2013, Available online 25 January 2014]

Dy-doped CaMnO3 ceramics have been synthesized by co-precipitation method combined with the solid-state
reaction. Phase composition and microstructure analysis indicate that high density and pure CaMnO3 phase
can be achieved. The electric conductivity can be enhanced by Dy doping, and result in a slight increase of
the thermal conductivity. The highest dimensionless figure of merit ZT of 0.15 has been obtained at 973 K for
x ¼ 0.02 sample, which is about 4 times larger than that of the pure CaMnO3, which indicate that CaMnO3
can be a promising candidate for n-type thermoelectric material at high temperature.
KEY WORDS: CaMnO3; Thermoelectric properties; Thermal conductivity

1. Introduction
Environment friendly thermoelectric materials have attracted
widespread interests for potential applications in space exploration, exhaust recycling, and clean cooling[1e3]. The energy
conversion efficiency is related to the materials intrinsic properties which can be characterized by the dimensionless figure of
merit ZT ¼ S2sT/k, where T is the absolute temperature, S is the
Seebeck coefficient, s is the electrical conductivity, and k is the
thermal conductivity. The three parameters S, s, and k are
correlated to each other. Therefore, it is difficult to optimize all
parameters simultaneously. The good thermoelectric performance needs high power factor (PF, S2s), and low thermal

conductivity k. Normally, alloy semiconductors, such as Bi2Te3,
PbTe and SiGe[4e7], their ZT values can exceed 1.0, and show a
good practical prospects. For oxides-based thermoelectrics, their
high chemical and thermal stability makes them to be promising
candidates at high temperature application. Recently, various
oxides such as Ca3Co4O9[8,9], SrTiO3[10], ZnO[11] and BiCuSeO[12,13] have been investigated in detail to enhance the ZT
value.
CaMnO3 (CMO) is a typically n-type oxide thermoelectric
materials[14e18]. However, low electric conductivity leads to
Corresponding author. Prof., Ph.D.; Tel.: þ86 10 62773741; Fax: þ86
10 62771160; E-mail address: (Y. Lin).
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poor performance in CMO system. The s of pure CMO is just
w10 S/cm and ZT is less than 0.04 at 900 K. In the previous
work, the polycrystalline ceramics of La-doped CaMnO3 were
synthesized by conventional solid-state reaction (SSR)[14], and
showed a porous structure. Lan et al.[19] reported that the ZT
value can been improved to 0.24 at 973 K though reducing the
thermal conductivity in fine grain size (200e400 nm) at a low
sintering temperature. Wang et al.[20] studied the electron-doped
CaMnO3 by rare earth, which shows that the Seebeck coefficient
is determined by the carrier concentration, while the electric
conductivity and thermal conductivity can be tuned by the ion
radius and ion mass, respectively.
In this work, we attempted to control the microstructure and
optimize thermoelectric properties. The cold isostatic pressing

(CIP) was used to achieve high density sample and Dy as a
dopant to optimize the electrical properties. Our results indicate
CIP is available to enhance the density and Dy dopant is
effectively to improve the ZT value.
2. Experimental
Polycrystalline ceramic samples of Ca1ÀxDyxMnO3 (x ¼ 0,
0.02, 0.04, 0.06, 0.08, 0.10) were synthesized via a chemical coprecipitation method and solid-state reaction. CIP was used to
control the porosity. For the co-precipitation method, Dy2O3
(99.90%) was carefully dissolved in nitric acid as a kind of
starting material, and then mixed together with Ca(NO3)2$4H2O,
Mn(NO3)2 aqueous solution in deionized water to make the nitrate stock solution. NH4HCO3 and ammonia solution were used
to control the reaction pH value in the range of 7.5e9.0 to make


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the metal ions precipitate completely. The resultant suspension
was aged to remove supernatant fluid and then was subjected to
suction filtration. The precursor powder was calcined at 1073 K
for 4 h in air, and then pressed to pellets. The bulks sealed in
glove and compacted by CIP at 200 MPa in oil to enhance the
density (LDJ-100/320-300 Sichuan Airlines Industry Chuanxi
Machinery Factory, Yaan, China). The samples were sintered at
1473 K for 12 h.
X-ray diffraction (XRD) with a Rigaku D/MAX-2550V
diffractometer (Rigaku, Tokyo, Japan; CuKa radiation) and
scanning electron microscopy (SEM, JSM-6460LV, JEOL,
Tokyo, Japan) were used to investigate the phase composition

and microstructure of CMO bulks, respectively. The temperature
dependence of electric conductivity was measured from room
temperature to 973 K by a four-probe method. Seebeck coefficient was obtained from the slope of the linear relation between
DV and DT, where DV is the thermoelectromotive force produced
by the temperature gradient DT. The thermal conductivity k was
determined by the following parameters: the thermal diffusivity
(a), the heat capacity (Cp), and the density (r), using the relationship k ¼ aCpr. The relative bulk density was measured by
the Archimedes method, and a Netzsch LFA 457 (Selb, Germany) laser flash apparatus was used to measure the thermal
diffusivity and the specific heat.
3. Results and Discussion
3.1. Morphology and structure
Fig. 1(a) shows the XRD patterns for Dy-doped CMO samples. All the samples are corresponded to CaMnO3 phase with an
orthorhombic perovskite-type structure in Pnma space group.
The lattice parameters have been calculated by the XRD data, as
shown in Fig. 1(b). The lattice constant a and cell volume are
increased as dopant concentration increases. This behavior is
independent of the ionic radius of Dy[21,22], in spite of the radius
of Dy3þ ions (1.08 nm, C.N. IX) is slightly smaller than that of

Ca2þ ions (1.18 nm, C.N. IX). The electron doping will induce
the presence of Mn3þ within the Mn4þ matrix for charge balance. The ionic radius of Mn3þ is larger than that of Mn4þ (r
[Mn3þ] ¼ 0.64 nm, C.N. VI and r[Mn4þ] ¼ 0.53 nm, C.N. VI),
which leads to the MnO6 octahedra to be distorted in CMO
structure.
Fig. 2 shows the SEM images of microstructure of CMO
samples. The precursor powder is typical spherical and diameter
changes from 1 to 10 mm as seen in Fig. 2(a), which are
composed of many small grains of w100 nm in size as shown in
the insert of Fig. 2(a). Fig. 2(b) shows the surface of sample
without CIP, and some pores with diameter of 1e2 mm appeared.

As shown in Fig. 2(c), the macroporous pores disappeared by
CIP technology and the density of samples can increase to 95%
after CIP as compared with the density 80% of samples without
CIP. It can be observed that the high density will deteriorate the
thermal conductivity, which can reduce thermoelectric properties. Compared Fig. 2(c) and (d), pores will further decrease with
more dopant. And the binding between grains are more closely,
which indicates that the addition of Dy can act as the sintering
aids and contribute to the formation of grains, which is helpful to
make CMO ceramic be densified.
3.2. Electric properties
Fig. 3(a) shows the temperature dependence of electric conductivity s of samples from 300 to 973 K. With increasing doping
concentration, s gradually increases and reaches the largest value
(184 S/cm) at x ¼ 0.10 at 973 K. As compared with undoped CMO
(sw10 S/cm), a significant increase of electric conductivity can be
observed by Dy doping, which is mainly derived from the variation of carrier concentration. The substitution of Dy3þ for Ca2þ
will import a large number of electron carriers and induce Mn3þ
appeared in Mn4þ matrix. The increased carrier concentration is
directly affect the s, and the presence of Mn3þ is beneficial to
electron hopping in perovskite manganites. Therefore, Dy doping
can facilitate the transport of carriers by hopping mechanism and
then enhance the electric conductivity.
The slope of seT curve is positive relationship (ds/dT > 0) at
low temperature, which indicates semiconducting behavior. And
then it shows a metallic behavior (ds/dT < 0) as the temperature
further increases. The insulator-metal (IM) transition temperature
TIM increases from 400 to 550 K with the Dy content increasing.
For semiconductor part, the temperature dependence of the
conductivity is generally described using the small polaron
model given by Mott as the following equation[23]

s¼

Fig. 1 XRD patterns (a) and the lattice constant a and cell volume (b) of
Ca1ÀxDyxMnO3 samples.



C
ÀEa
exp
;
kB T
T

where C, kB, and Ea are the pre-exponential terms, Boltzmann
constant, and activation energy, respectively. Fig. 3(b) shows
the activation energy increases with increasing content of Dy
at low temperature interval, which raising from 0.048 to
0.070 eV. With more dopant, the densifying of ceramic may be
beneficial to electronic transport. The carrier concentration
increases and more Mn3þ ions can be formed, and then a
higher s and TIM can be obtained. This indicates that the
increase in Mn3þ concentration is favorable for the formation
of polaron in this temperature range[19].
Fig. 3(c) displays the temperature dependence of Seebeck
coefficient. All samples exhibit negative Seebeck coefficient,


B. Zhan et al.: J. Mater. Sci. Technol., 2014, 30(8), 821e825

823

Fig. 2 Typical SEM images of CaMnO3 samples: (a) x ¼ 0.06 powder by co-precipitation route; (b) CMO ceramic without CIP; (c) x ¼ 0.02 ceramic by
CIP; (d) x ¼ 0.06 ceramic by CIP.

which indicates that electrons are the predominant charge carriers (n-type conduction). The x ¼ 0.02 sample has a very large S
value, being about À370 mV KÀ1 at 300 K. The absolute value
of S decreased obviously as Dy content increasing, which arises
from the increase of carrier concentration. For x ¼ 0.02, absolute
value of Seebeck coefficient decreases with increasing

temperature, which shows a typical characteristic of nonmetallike temperature dependence. With more Dy doping, the absolute value of S decreases with increasing temperature and exhibit
metallic behavior. This difference should be attributed to the
contribution of the oxygen deficiency[18,24]. The turning point
came in the x ¼ 0.04 sample, which did not appear in s.

Fig. 3 Temperature dependence of electric conductivity (a), Seebeck coefficient (c), and power factor (d); and (b) activation energy plotted vs Dy
content.


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B. Zhan et al.: J. Mater. Sci. Technol., 2014, 30(8), 821e825

sum of lattice thermal conductivity (kl) and electronic thermal
conductivity (ke) as k ¼ kl þ ke. The ke can be calculated by the
WiedemanneFranz law ke ¼ LTs, where L is the Lorentz constant. In order to simplify the calculation, the Lorentz constant Lo
(Lo ¼ p2kB2/(3e2) ¼ 2.44 Â 10À8 W U KÀ2) was used. The
calculated ke in the entire temperature range is quite small as
compared to kl (less than 15%), which ranges from 0.168 to

0.436 W mÀ1 KÀ1 at 973 K kl is the predominant component in
thermal conductivity, and the variation of k is mainly caused by
the change of kl as shown in the insert of Fig. 4(a). The lowest
value of k is 2.48 W mÀ1 KÀ1 at 973 K for x ¼ 0.02. As we
mentioned before, CIP process can increase the density, which is
an important reason for high k. The variation of thermal diffusion
coefficient is the dominant factor for this result, which indicates
that CIP not only makes the ceramic be densified, but also
changes some intrinsic properties of materials.
As shown in Fig. 4(b), the dimensionless ZT of
Ca1ÀxDyxMnO3 was calculated. The optimal ZT is 0.15 at 973 K
when x ¼ 0.02, which is about 4 times larger than that of the
pure CaMnO3 (w0.038). More detailed further work is desirable
to further enhance the thermoelectric performance of CaMnO3.
4. Conclusion

Fig. 4 Temperature dependence of k (a) and ZT (b) value for Dy-doped
CMO.

For materials with more than one type of charge carrier, the
diffusion Seebeck coefficient can be expressed as
S ¼

Xsi 
i

s

Si

where si and Si are the partial electrical conductivity and the
partial Seebeck coefficient associated with the ith group of
carriers, respectively. We can rewrite S of CMO as
S ¼

sin
sex;defact
S þ
S
sin þ sex;defact in sin þ sex;defact ex;defact

where sin and Sin are the contribution from intrinsic carriers;
sex,defect and Sex,defect are the contribution from extrinsic
carriers due to the oxygen defects. Since the increase of
electrical conductivity ðweÀEa =ðKB T Þ Þ is faster than the decrease
of S (wÀEa/(KBT )) for semiconductors, one could expect the
second term in above equation would increase and therefore
the absolute value of Seebeck coefficient for CMO would
increase, which should be responsible for the simultaneous
increase of the electrical conductivity and absolute value of S
with increasing temperature at low temperature interval. It
indicates that the existence of oxygen deficiency is important
for Seebeck coefficient.
3.3. Thermoelectric properties
The temperature dependence of the power factor (PF) is
shown in Fig. 3(d). At 973 K, the maximum of PF can reach
3.82 mW cmÀ1 KÀ2 when x ¼ 0.02, which is a high value for a
kind of n-type oxide thermoelectric material. Fig. 4(a) shows the
temperature dependence of thermal conductivity of CMO samples. Thermal conductivity k can be expressed generally by the

x
0.10)
In summary, high density Ca1ÀxDyxMnO3 (0
ceramic has been prepared and the microstructure and thermoelectric properties have been investigated. Nanostructured precursor powders were obtained by co-precipitation method and
the bulks can reach a high density with CIP. The electrical
conductivity can be obviously improved by Dy doping. The
maximum power factor can reach 3.82 mW cmÀ1 KÀ2 at 973 K
in sample x ¼ 0.02. The highest dimensionless figure of merit ZT
of 0.15 has been obtained at 973 K in the air for
Ca0.98Dy0.02MnO3.
Acknowledgments
This work was financially supported by the Ministry of Sci
& Tech of China through a 973 Project, under grant No.
2013CB632506, the National Natural Science Foundation of
China under Grant Nos. 51025205 and 11234012, and the
Specialized Research Fund for the Doctoral Program of Higher
Education, under grant No. 20120002110006.
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