High-temperature Electric Properties of PolycrystallineJinle Lan1, Yuanhua Lin1†, Ao Mei1, Cewen Nan1†, Yong Liu2, Boping Zhang2 and Jingfeng Li1 1 State Key Laboratory of New Ceramics a
Trang 1High-temperature Electric Properties of Polycrystalline
Jinle Lan1), Yuanhua Lin1)†, Ao Mei1), Cewen Nan1)†, Yong Liu2), Boping Zhang2)
and Jingfeng Li1)
1) State Key Laboratory of New Ceramics and Fine Processing, Department 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 June 30, 2008, in revised form January 20, 2009]
Polycrystalline La-doped CaMnO3 ceramics have been prepared by a solid-state sintering method Analysis
of microstructure and phase composition indicates that the addition of La can prohibit the further growth of grain, and no impurity phase appears The results revealed that the La doping can lead to a large change of the activation energy (from 0.22 to 0.02 eV), and thus result in a marked increase in electric conductivity of
2–4 orders of magnitude The power factor can reach about 1.5×10 −4 W·m −1 ·K −2 in a wide temperature range, which potentially make them attractive for n-type high-temperature thermoelectric materials
KEY WORDS: CaMnO3; Thermoelectric; Activation energy; Seebeck coefficient
1 Introduction
Thermoelectric material can directly convert heat
into electric energy and vice versa through the
ther-moelectric phenomena in solids, which make it a
potential way for clean energy generation by
trans-forming the heat into electricity[1] Normally, as for
the thermoelectric materials, the conversion efficiency
can be well characterized by the dimensionless figure
ZT =S2σT /κ (where T , S, σ, and κ are the absolute
temperature, thermoelectric power, electrical
conduc-tivity, and thermal conducconduc-tivity, respectively)
There-fore, high electrical conductivity σ, large Seebeck
co-efficient S, and low thermal conductivity κ are highly
expected for practical development of thermoelectric
materials and related devices[2]
As for the conventional non-oxides materials (e.g.,
Bi-Te, Bi-Se system)[3], there exist some
shortcom-ings such as surface oxidation and vaporization at
high temperature, and limit further applications of
these materials Oxides, owing to their low
ther-mal conductivity and high-resistance of oxidation at
high temperature, have recently attracted
consider-able attention Recently, oxides-based
thermoelec-tric materials have been explored (e.g., SrTiO3, NiO,
TiO2)[4–6] Some CoO2-based oxides with layered
structures (e.g., Ca3Co4O9) have been reported to
show good p-type thermoelectric properties at high
temperature in air[7,8], the much-needed
correspond-ing n-type oxides with good thermoelectric
proper-ties for power generation are still a challenge for
high thermoelectric performance Some previous
studies on the CaMnO3 perovskite system suggested
that CaMnO3 could be a potential candidate as an
n-type thermoelectric material [9,10] As we know,
CaMnO3 is an electron-doped compound, which
be-longs to the perovskite structure with a=0.5278 nm,
b=0.7448 nm, and c=0.5268 nm Some previous
stud-ies indicated that the CaMnO3 perovskite system
can exhibit colossal magnetoresistance properties at
† Corresponding author Tel.: + 86 10 62773741; E-mail
ad-dress: linyh@tsinghua.edu.cn (Y.H Lin);
cwnan@tsinghua.edu.cn (C.W Nan).
low temperature[11] Funahash et al.[12] have inves-tigated the structure and thermoelectric properties
of polycrystalline samples Ca1−xA xMnO3 (A=Yb,
Tb, Nd, and Ho), and reported that the Yb-substituted CaMnO3 showed good thermoelectric
properties (ZT ≈0.16 at 1000 K).
Although electron-doped CaMnO3 as a member
of the large family of perovskite oxides has been sug-gested to be potential n-type thermoelectric materi-als, up to now, only a few systematical studies on the high-temperature thermoelectric properties have been reported In this work, we fabricated La-doped CaMnO3-based ceramics, and reported the effect of
La substitution on the phase composition, microstruc-ture and high-temperamicrostruc-ture thermoelectric properties
2 Experimental Polycrystalline ceramic samples of Ca1−xLaxMnO3
(x=0, 0.02, 0.04, 0.06, 0.08) were synthesized via a
conventional solid-state reaction Analytical purity CaCO3, MnCO3, and La2O3 were used as raw ma-terials, which were weighted in stoichiometric ratio and mixed by ball mill for 24 h, and then the mixed powders were pre-calcined at 1373 K for 6 h Fi-nally, La-doped CaMnO3 ceramics can be obtained
by sintering at 1473 K for 10 h
X-ray diffraction (XRD) with a Rigaku
D/MAX-2550V diffractometer (CuKα radiation) and scanning
electron microscopy (SEM) were employed to reveal the microstructure and phase composition of the as-sintered Ca1−xLaxMnO3 ceramic samples The sam-ples for the measurements of thermoelectric proper-ties were cut out of the sintered bodies in the form
of rectangular bars of 4 mm×4 mm×20 mm with a
diamond saw, and silver paint electrodes were formed
on both sides of the sintered ceramic discs for elec-trical measurements The temperature dependence of electric conductivity was measured in the tempera-ture range from room temperatempera-ture to 800◦C by four-probe method Thermoelectric power was obtained
from the slope of the linear relation between ∆V and
∆T , where ∆V is the thermoelectromotive force
Trang 2pro-20 30 40 50 60 70
(a) (b) (c) (d) (e)
Fig 1 XRD patterns of as-sintered samples:
(a) CaMnO3, (b) Ca0.98La0.02MnO3,
(c) Ca0.96La0.04MnO3, (d) Ca0.94La0.06MnO3,
(e) Ca0.92La0.08MnO3
duced by temperature difference ∆T
3 Results and Discussion
The XRD patterns shown in Fig 1 indicate that
all the samples are single phase CaMnO3 with an
or-thorhombic perovskite-type structure (ICSD#82211)
No impurity phase can be observed in these
ce-ramic samples, which reveals that La ions can
re-place the Ca sites The substitution of the Ca
sites with trivalent La ions will lead to the
varia-tion of electric properties, and the detailed results
will be given in the following content Figure 2
shows the lattice parameters a, b and c as a
func-tion for the changes with La doping The
parame-ters a, b and c all increase monotonously This result
5.26 5.28 5.30 5.32 7.460 7.465
La content, x / mol
a
b
c
Fig 2 Lattice parameters of the Ca1−xLaxMnO3 series
as a function of La substitution content
can be well understood based on the fact that the ionic radius of La3+ is larger than that of Ca2+ Figure 3 shows the SEM images of the surface
of the as-sintered La-doped CaMnO3 samples Ob-viously, in pure CaMnO3 ceramic sample, the grain
size is about 3–5 µm, and larger than that in the
La-doped samples, which indicates that the addition
of La can act as the grain growth inhibitor and pro-hibit the further growth of CaMnO3 grain The sim-ilar behavior has also been observed in the La-doped Na0.5Bi0.5TiO3 ceramic samples[13] It can be ob-served that the samples sintered at 1473 K have small pores, and these pores can reduce the thermal conduc-tivity, which can improve thermoelectric properties The effect of the pores will be investigated in future work
Figure 4 gives the temperature dependence of elec-trical conductivity of these La-doped CaMnO3 sam-ples in the temperature range from room
tempera-Fig 3 SEM images of the surface of as-sintered samples: (a) CaMnO3, (b) Ca0.96La0.04MnO3,
Trang 3300 400 500 600 700 800 900 1000 1100
0
20
40
60
80
100
120
140
(a)
(b)
(c)
(d)
(e)
-T / K
Fig 4 Electrical conductivity as a function of
temper-ature for: (a) CaMnO3, (b) Ca0.98La0.02MnO3,
(c) Ca0.96La0.04MnO3, (d) Ca0.94La0.06MnO3,
(e) Ca0.92La0.08MnO3
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0.00 0.05 0.10 0.15 0.20
(a)
(b)
(c)
(d)
(e)
(1000/T) / (1/K)
x / mol
Fig 5 lg(σT ) vs 1000/T plots for: (a) CaMnO3,
(b) Ca0.98La0.02MnO3, (c) Ca0.96La0.04MnO3,
(d) Ca0.94La0.06MnO3, (e) Ca0.92La0.08MnO3
ture to 1075 K The undoped CaMnO3 is a typical
n-type semiconductor, and shows a low
conductiv-ity (∼0.02 S/cm) at room temperature However,
the La doping, even as low as x=0.02, can cause a
marked increase in conductivity of 2–3 orders of
mag-nitude, especially when x=0.08 (∼100 S/cm), and the
value changes slightly with increasing the
tempera-ture This similar electrical behavior has also been
observed in the low Pr-doped CaMnO3 ceramics[14],
which may be ascribed to the variation of the strength
of the bending of the Mn–O–Mn bond and narrowing
the conduction bandwidth
For the interpretation of conducting phenomenon
in Mn-based perovskite system, the temperature
de-pendence of the conductivity is generally described
using the small polaron model given by Mott and
Davis[15]as follows,
σ = (C/T )exp(−Ea/KBT ) (1)
where C, Ea, KB, and T are the pre-exponential terms
related to the scattering mechanism, the activation
energy, Boltzmann constant, and absolute
tempera-ture, respectively
Figure 5 illustrates the plots of lg(σT ) vs 1/T
for various La-doped samples Obviously, when the
temperature is below 600 K, all samples show a good
linearity between lg(σT ) and 1/T Additionally, for
La-doped CaMnO3 sample, this linear behavior can
-600 -500 -400 -300 -200 -100 0
(a)
(b)
(c)
(d)
(e)
T / K
Fig 6 Temperature dependence of the Seebeck coeffi-cient for: (a) CaMnO3, (b) Ca0.98La0.02MnO3, (c) Ca0.96La0.04MnO3, (d) Ca0.94La0.06MnO3, (e) Ca0.92La0.08MnO3
0.0 0.5 1.0 1.5 2.0 2.5 3.0
T / K
(a)
(b)
(c)
(d)
(e)
Fig 7 Temperature dependence of the power fac-tor for: (a) CaMnO3, (b) Ca0.98La0.02MnO3, (c) Ca0.96La0.04MnO3, (d) Ca0.94La0.06MnO3, (e) Ca0.92La0.08MnO3
be retained at higher temperature as the La-doping
concentration increases The activation energy Ea ob-tained from Eq (1) (insert in Fig 5) for pure CaMnO3
is about 0.22 eV, which is in agreement with that re-ported in previous work (0.16 eV)[16] Moreover, for La-doped CaMnO3 samples, they yield smaller
ac-tivation energies of ∼0.02–0.03 eV, which was also
observed in Ce3+-, Y3+-, or Sm3+-doped CaMnO3
samples (∼0.02–0.04 eV)[16] The variation of activa-tion energy should be attributed to the doping effect
of trivalent La ions on the Ca sites and the related hopping conduction mechanism in the CaMnO3-based perovskite oxides To understand the nature of the conduction behavior in this system, further investiga-tion is necessary and desirable
Figure 6 shows the temperature dependence of S
for the La doped CaMnO3 samples The S values are all negative, indicating n-type conduction The pure
CaMnO3 sample shows a large absolute value of S (∼550 µV/K at near 500 K), and decreases with
in-creasing the temperature, which should be related to the carrier concentration and semiconductor behavior For these La-doped CaMnO3 samples, with the tem-perature increasing (from RT to 450 K), the absolute
S values also increase It may be ascribed to the
vari-ation of Mn2+/Mn3+ These various valence states
of Mn ions will give contribution on the Seebeck co-efficients like Co2+/Co3+ in the CoO2-based layered oxides as previously reported[7,8] As the
Trang 4tempera-ture further increases, the absolute S values intend to
decrease, which may be mainly related to the
varia-tion of the corresponding electrical conductivity, and
need further work to understand these behaviors It
should be pointed out that though the S decrease, a
maximum power factor S2σ=1.5×10 −4 W·m −1 ·K −2
appears for the La-doped sample with x=0.04 in a
wide temperature range (see Fig 7), being ascribed
to the improvement in the electric conductivity
As shown in Fig 7, the power factors Sσ2
calcu-lated from the electric conductivity and Seebeck
co-efficient indicate that the La substitution has a great
influence on the power factor, which should be caused
by the significant variation of the electric
conductiv-ity and Seebeck coefficient, which can be comparable
to the other n-type La, Y, or Dy doped SrTiO3 and
Al and Ti co-doped ZnO ceramic systems[17,18] The
good thermoelectric performance and high
tempera-ture durability in air suggest that these oxides can be
potential high temperature thermoelectric materials
4 Conclusion
We fabricated polycrystalline La-doped CaMnO3
ceramics by the conventional solid state reaction, and
investigated the effect of La substitution on the
high-temperature thermoelectric properties Our results
indicate that all La-doped CaMnO3 samples show
negative Seebeck coefficients, indicating n-type
con-duction The La doping has a remarkable effect on
the electric transport properties of these
CaMnO3-based ceramics, which may be related to the carrier
concentration, defect structure, the spin-state of the
electrons caused by the La3+ ions on the Ca2+ sites
Acknowledgements
This work was financially supported by the National
Program on Key Basic Research Project (“973 Program”)
under grant No 2007CB607505, and the National High
Technology Research and Development Program of China under grant No 2009AAO3Z216
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