The Seebeck coefficient was measured in the same temperature interval, and its concentration dependence was analyzed using the high-temperature HT thermopower theory proposed by Marsh–Par
Trang 1properties of Ca1−xPrxMnO3−δ (0⩽x<1)
ARTICLE in PHYSICA B CONDENSED MATTER · OCTOBER 2004
Impact Factor: 1.32 · DOI: 10.1016/j.physb.2004.06.033
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Vietnam National University, Hanoi
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Japan Advanced Institute of Science a…
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Hanoi University of Science
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Available from: Quoc Thanh Phung Retrieved on: 11 April 2016
Trang 2Physica B 352 (2004) 18–23
High-temperature thermoelectric properties of
a Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai, Hanoi 844, Viet Nam
b School of Materials Science, JAIST, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan
Received 2 June 2004; accepted 15 June 2004
Abstract
Ca1xPrxMnO3d(x=0, 0.05, 0.15, 0.1, 0.2, 0.4, 0.67; d=0.02) samples were prepared by a solid-state reaction method X-ray diffraction analysis showed that all samples prepared were of single phase with orthorhombic structure Electrical resistivity measurements from room temperature to 1300 K showed that a metallic conducting tendency dominated at high temperatures The hopping nature of the charge carriers was well interpreted in the framework of polaron theory The Seebeck coefficient was measured in the same temperature interval, and its concentration dependence was analyzed using the high-temperature (HT) thermopower theory proposed by Marsh–Parris The thermal conductivity and the figure of merit of the prepared samples were also compared with those of other similar perovskite compounds The observed figure of merit of the sample with x=0.15 was Z=1.5 104K1at T=1100 K, indicating a good potential for application as a HT thermoelectric material
r2004 Elsevier B.V All rights reserved
PACS: 72.20.Pa; 72.80.Ga
Keywords: Thermoelectric properties; Ca 1x Pr x MnO 2.98 perovskite; Electrical conductivity; Seebeck coefficient; Thermal conductivity
1 Introduction
Perovskite compounds with chemical formula,
A1xA’xBO3(where A is a rare earth metal, A’ is
an alkaline earth metal, and B is a transition metal
like Mn or Co), have attracted much attention of
researchers by their interesting physical
phenom-ena The current interest is concentrated not only
on colossal magnetoresistance (CMR) and magne-tocaloric effects (see for example Ref.[1]), but also
on thermoelectric properties Manganese perovs-kites, with various substitutions for calcium,
Ca1xAxMnO3; are considered as promising new materials for high-temperature (HT) thermoelec-tric energy conversion with a sufficiently large power factor and figure of merit over the wide temperature range 600–900C[2] Recently, some
*Corresponding author Tel.: 45582216; fax:
+84-48589496.
E-mail address: congbt@vnu.edu.vn (B.T Cong).
0921-4526/$ - see front matter r 2004 Elsevier B.V All rights reserved.
doi:10.1016/j.physb.2004.06.033
Trang 3(0pxp0:2) [3] and found the figure of merit of
Z ¼ 1:63 104K1 for x ¼ 0:2 at 1273 K The
aim of this contribution is to investigate the HT
that the ionic radii of Pr3+ (1.179 (A) and Ca2+
(1.180 (A) are almost the same[4] It is well known
that the transport properties of perovskites depend
strongly on the average size of the A cation (/
Ca1xPrxMnO3 solid solutions can exist in the
suitable for studying the role of charge carriers in
transport phenomena Most HT thermoelectric
investigations were performed in the
interesting to extend this to another, hole doping,
region According to Ref.[4], the magnetic phase
effect exists in both electron- and hole-doping
regions, around xB0.9 and xB0.3, respectively
The other rare-earth mixed-valence systems like
Sr1xPrxMnO3 and Sr1xSmxMnO3 have not this
symmetric behavior Inside and close to CMR
concentration intervals, the electrical conductivity
is sufficiently large, and then the HT power factor
of Ca1xPrxMnO3can also be expected to be large
The structure of Ca1xPrxMnO3was studied using
X-ray diffraction by Pollert et al [5] and using
neutron diffraction by Jirak et al.[6] In their work
[6], it was shown that for 0:6pxp0:7 the Seebeck
coefficient of this material changes sign from
negative to positive when the temperature becomes
low enough The HT behavior of these materials
above room temperature is the subject of the
present study
2 Experimental
Ca1xPrxMnO3d (x ¼ 0; 0.05, 0.15, 0.1, 0.2,
0.4, 0.67) perovskite samples were prepared by a
solid-state reaction method using as starting
materials powders of CaCO3, M nO2, and Pr2O3
with high purity The sample preparation method
was similar to the one described in our previous
work [3] The oxygen content in all samples was
oxidation-reduction method (the oxygen deficiency
is d ¼ 0:02) The obtained samples were identified
by X-ray diffraction using a RIGAKU
RINT-2500 V The electrical conductivity was carried out
in air in the temperature range 300–1273 K, using
a four-point probe method The Seebeck coeffi-cient was calculated from the linear gradient DV/
DT from the measured thermoelectromotive force and temperature difference, in the same tempera-ture range The thermal diffusivity of the samples was measured by a laser flash method using a ULVAC TC-7000 The thermal conductivity, l, was calculated using the method given in Ref.[3]
3 Results and discussion The quality of the prepared samples was
shows the X-ray diffraction (XRD) patterns for all samples, taken at room temperature The powder XRD patterns show a single phase of the
Pr-doping concentration The crystals have an orthorhombic structure belonging to the same
x = 0.67
x = 0.40
x = 0.20
x = 0.15
x = 0.10
x = 0.05
x = 0
2 (degree)
Fig 1 X-ray diffraction patterns for Ca 1x Pr x MnO 2:98
(x ¼ 0 0:67) samples.
B.T Cong et al / Physica B 352 (2004) 18–23 19
Trang 4space group Pnmb, and the lattice constants are
given inTable 1
Fig 2shows the temperature dependence of the
electrical resistivity, r, of the prepared samples in
the temperature range from room temperature to
1273 K Samples with x ¼ 0; 0.2, 0.4, 0.67 show a
typical semiconducting behavior in the
investi-gated temperature region Low doping with
praseodymium (x ¼ 0:05; 0.1, 0.15) causes an
essential decrease of resistivity and its value does
not change much with increasing temperature
Further doping with Pr (x > 0:15) leads to an
increase of the resistivity again There are several
concepts used for the interpretation of conducting
phenomenon in perovskites The temperature
dependence can be described using the small
this theory, the resistivity is expressed by, r
Ea kBT
; where C is given by
Ne2a2xð1 xÞnphexpð2gRÞ:
Here, e is the absolute value of the electron charge, N is the number of ion sites per unit cell volume (Mn sites), a is an average intersite distance for polaron hopping obtained from the relation a=(1/N)1/3, g is the electron wave function
fre-quency, x is the fraction of available sites occupied
concentration), and Eais an activation energy for hopping conduction
By plotting log(r/T) as a function of 1/T, one can determine the activation energy, Ea, in the temperature range from 300 to 700 K, as seen in
Fig 3a Fig 3b shows the activation energy of doped samples as a function of x in the
Ca1xPrxMnO2:98 solid solutions
A tendency of the activation energy to increase with increasing doping Pr concentration (x) is seen
inFig 3b This increase indicates that an increase
formation of polarons in this temperature interval The well-observed jump of Eaat xB0:2 indicates that the small polaron is more stabilized for xX0:2:
Fig 4shows the temperature dependence of the
(x ¼ 0 0:67), and reveals that the dominating electrical carriers at room temperature are elec-trons for all samples except for x ¼ 0:67:
At high temperatures, the conducting character
is n-type for the whole system (including both electron- and hole-doping samples) This is an interesting feature of HT behavior in comparison with the symmetric property at temperatures
conducting type prevails in the case of xo0:5 (or
x > 0:5) The dominating electron conducting character shows that the carrier mobility rather than their concentration governs HT transport
Table 1
Lattice constants of Ca1xPrxMnO2:98(x=00.67) samples
Samples a ( ( A) b ( ( A) c ( ( A) V ( ( A)3
CaMnO 3 5.273 5.267 7.451 206.922
5.279 5.264 7.448 a
Ca 0.95 Pr 0.05 MnO 3 5.280 5.278 7.462 207.947
Ca 0.9 Pr 0.1 MnO 3 5.295 5.292 7.480 209.622
Ca 0.85 Pr 0.15 MnO 3 5.306 5.305 7.495 210.965
Ca 0.8 Pr 0.2 MnO 3 5.321 5.318 7.512 212.580
Ca 0.6 Pr 0.4 MnO 3 5.381 5.377 7.568 218.947
Ca 0.33 Pr 0.67 MnO 3 5.452 5.426 7.661 228.611
a
Data taken from Ref [7]
0
2
4
6
8
-20 0 20 40 60
-2 Ω.cm)
-2 Ω.cm)
T (K)
Ca1-xPrxMnO2.98
x=0 x=0.05 x=0.10 x=0.15 x=0.20 x=0.40
x=0.67 (right axis)
Fig 2 Electrical resistivity of Ca 1x Pr x MnO 2:98 (x ¼ 0 0:67).
Trang 5behavior The Seebeck coefficients of the small
polaron conduction system at HT can be
inter-preted by Marsh and Parris’s theory[9], developed
for a strong coupling system This theory is
applied for the case that the B-site transition
electron, n, in the 3d manifold, 3pnp5: We used
the following formula for the Seebeck coefficient in
the HT limit, assuming that the energies of the Jahn–Teller (DJT) and the Coulomb interaction
a ¼ kB
e ln
3 r0 x
r0 1 þ x
:
Here, r0 is the number of egelectrons per Mn3+ site, and x is the doping concentration The comparison between theoretical and experimental values for the concentration dependence of
agreement is observed for r0¼ 1:3: The HT theory
[9]appears to describe our experiments well
Fig 6demonstrates the temperature dependence
of the thermal conductivity, l The contribution from the electronic thermal conductivity, le, is calculated by using Wiedemann–Franz’s law as
than the electronic one
Fig 7shows the temperature dependence of the power factor, sa2; calculated from the measured Seebeck coefficient and the electrical conductivity
temperature increases, and reaches the value of
quantity is sufficiently small for the hole-doping samples with x > 0:5 (for x ¼ 0:67; power factor is near zero)
0.5 1.0 1.5 2.0 2.5 3.0 3.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
10 3 /T (K -1 )
-1 Ω cm)]
Ca1-xPrxMnO2.98
x=0
x=0.05
x=0.10
x=0.15
x=0.20
x=0.40
x=0.67
0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.05
0.10
0.15
Ea
x
(a)
(b)
Fig 3 (a) log(r/T) vs T1 for Ca 1x Pr x MnO 2:98
(x ¼ 0 0:67) (b) Activation energy, E a , as a function of
doping Pr concentration for Ca1xPrxMnO2:98(x ¼ 0 0:67) in
the temperature range from 300 to 700 K.
200 400 600 800 1000 1200 1400 -250
-200 -150 -100 -50 0
-1 )
T (K)
Ca1-xPrxMnO2.98
x=0 x=0.05 x=0.10 x=0.15
x=0.20
x=0.40 x=0.67
Fig 4 Temperature dependence of the Seebeck coefficient, a, for Ca 1x Pr x MnO 2:98 (x ¼ 0 0:67) sintered bodies.
B.T Cong et al / Physica B 352 (2004) 18–23 21
Trang 6The temperature dependence of the figure of
merit, Z, in this system is plotted in Fig 8 Z
increases with increasing praseodymium fraction
from 0 to 0.15 A doping level x > 0:15 leads to a
strong reduction of Z This quantity is near zero
for x ¼ 0:6:
4 Conclusions
prepared and their thermoelectric properties were
investigated in the high-temperature region It was
shown that the observed HT transport properties
are much different from those in the region below
room temperature In view of application as a
large figure of merit of Z ¼ 1:5 104K1
for x ¼ 0:15 at T ¼ 1100 K indicates good possibilities
Acknowledgements The author (B.T Cong) thanks the JAIST-HUS collaboration program for supporting his short visit at JAIST, where a part of this work was done The help of the VNU Asia Research Center is also acknowledged
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
-400
-300
-200
-100
ρ 0 = 1.05
-1 )
x
Ca1-xPrxMnO2.98
573 K
1073 K
0=1.3 0=1.05
Fig 5 The concentration dependence of the Seebeck coefficient
for Ca 1x Pr x MnO 2:98 (x ¼ 0 0:67).
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
-1 )
T (K)
Ca1-xPrxMnO2.98
x=0 x=0.05 x=0.10 x=0.15 x=0.20 x=0.67
Fig 6 Thermal conductivity, l, of Ca 1x Pr x MnO 2:98
(x ¼ 0 0:67).
0.0 0.5 1.0 1.5 2.0 2.5
2 (x 10
-4 W
-1 K
-2 )
T (K)
Ca1-xPrxMnO2.98
x=0 x=0.05 x=0.10 x=0.15 x=0.20 x=0.40 x=0.67
Fig 7 Temperature dependence of the power factor, sa 2 , obtained from the measured Seebeck coefficient and electrical conductivity data.
0.0 0.3 0.6 0.9 1.2 1.5
-4 K
-1 )
T (K)
Fig 8 Figure of merit, Z, of Ca 1x Pr x MnO 2:98 as a function of temperature.
Trang 7[1] C.N.R Rao, B Raveau (Eds.), Colossal Magnetoresistance,
Charge Ordering and Related Properties of Manganese
Oxides, World Scientific, Singapore, 1998.
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[3] P.X Thao, T Tsuji, M Hashida, Y Yamamura, J Ceram.
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[4] C Martin, A Maignan, M Hervieu, B Raveau, Phys Rev.
B 60 (1999) 12191.
[5] E Pollert, S Krupicka, E Kuzmicova, J Phys Chem Solids 43 (1982) 1137.
[6] Z Jirak, S Krupicka, Z Simsa, M Dlouha, S Vratislav,
J Magn Magn Mater 53 (1985) 153.
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A Esteban, J Solid State Chem 156 (2001) 458.
[8] N.F Mott, E.A Davis, Electronic Processes in Non-crystalline Materials, Clarendon Press, Oxford, 1971 [9] D.B Marsh, P.E Parris, Phys Rev B 54 (1996) 16602 B.T Cong et al / Physica B 352 (2004) 18–23 23