Materials Transactions, Vol. 51, No. 2 (2010) pp. 288 to 291
Special Issue on Development and Fabrication of Advanced Materials Assisted by Nanotechnology and Microanalysis
#2010 The Japan Institute of Metals
Preparation and Thermoelectric Properties
of Bi-Doped Mg2 Si0:8 Sn0:2 Compound
Weijun Luo, Meijun Yang, Fei Chen, Qiang Shen* , Hongyi Jiang and Lianmeng Zhang
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology,
Wuhan 430070, P. R. China
The Bi-doped Mg2 Si0:8 Sn0:2 single phase compound is prepared by a solid state reaction (SSR)-spark plasma sintering (SPS) method. The
effect of the Bi content on the thermoelectric properties of the Bi-doped Mg2 Si0:8 Sn0:2 compound is mainly investigated. The results show that
the thermoelectric properties of the obtained samples are sensitive to the Bi content. With the increase in Bi content, the electrical conductivity
() and Seebeck coefficient () of the samples are increased, while the thermal conductivity () is decreased slightly between 300 K and 850 K.
When the Bi content is greater than 3.0 at%, the sample shows a maximum figure of merit (ZT) value (1:17 0:05) at 850 K.
[doi:10.2320/matertrans.MC200908]
(Received August 31, 2009; Accepted November 19, 2009; Published January 14, 2010)
Keywords: Bi-doped Mg2 Si0:8 Sn0:2 , solid state reaction (SSR)-spark plasma sintering (SPS), thermoelectric properties
1.
Introduction
Thermoelectric (TE) materials have promising applications in cooling and power generation. Compared to the
commonly used refrigeration device, a TE device has many
advantages, such as no noise, no pollution, long life and
free from maintenance. However, it is still not widely
utilized due to its low efficiency. The efficiency of the TE
device for cooling and power generation is represented by
the figure of merit ZT, where T is the absolute temperature,
Z is given by Z ¼ 2 =, where is the Seebeck
coefficient, is the electrical conductivity and is the
thermal conductivity. Many efforts have been applied to
improve the ZT in the past decades. It was reported that
the ZT has exceeded 1 for Sb-doped Mg-Si-Sn solid
solutions.1–4) Compared to all the thermoelectric materials
with ZT > 1, Mg2 Si-based compounds possess many
interesting advantages, such as the great abundance of their
elemental content in the earth’s crust and the non-toxicity of
their processing by-products.5–7)
The conventional process for a Mg2 Si1x Snx compound
was direct melting metallurgy1,8) and BMA-HP.9) Due to the
high vapor pressure, the oxidation of Mg, as well as the
different melting points of Mg, Si, Sn, it is extremely difficult
to synthesize the pure compounds and control the structure
and properties. Thus, the solid state reaction combined with
SPS sintering is introduced to synthesize these solid solutions, involving the low temperature and large diffusion
velocity of SPS at the low temperature.
Mg2 Si1x Snx (0 x 1:0) thermoelectric materials had
been prepared by SSR-SPS techniques in the previous
work.10) It was found that when x ¼ 0:2, the highest ZT of
0.1 at about 490 K was obtained. Simultaneously, as Y.
Isoda et al. reported,4) the value of the ZT for Sb-doped
Mg2 Si0:5 Sn0:5 has exceeded 1. Considering that the VB group
atoms have more extra-nuclear electrons than other doping
atoms such as Al and Cu, and Bi belongs to the same group
*Corresponding
author, E-mail: sqqf@263.net
as Sb, Bi-doped Mg2 Si0:8 Sn0:2 might have lower thermal
conductivity because Bi has a larger radius and heavier mass.
Meanwhile, according to reports,11–13) the Bi-doped Mg2 Si
samples showed the highest thermoelectric properties among
doping atoms such as Sb, P, Al and Cu. Moreover, there is
no research concerning the thermoelectric properties of Bidoped Mg2 Si0:8 Sn0:2 . Thus, in this study, the single phase Bidoped Mg2 Si0:8 Sn0:2 was prepared and the effect of Bi doping
concentration on the thermoelectric properties of Bi-doped
Mg2 Si0:8 Sn0:2 was mainly investigated.
2.
Experimental Procedures
Bi-doped Mg2 Si0:8 Sn0:2 solid solutions were prepared by
SSR-SPS. Considering the evaporation of Mg, the nominal
excess Mg content of 4 mol% deviating from the stoichiometric composition was used. Mg, Si, Sn and Bi powders
with a purity of >99:99% and particle size of 30 mm were
mixed homogeneously in an agate mortar and dry-pressed
into cylinders, and were then placed in a furnace for solid
state reaction under argon atmosphere via long time annealing. Finally, the compound after solid state reaction was
sintered at 970 K for 10 min at 40 MPa in a graphite die
(20 mm in diameter) under argon atmosphere by the SPS
method. The density of the samples was greater than 99.9%
of the theoretical value.
The solid solutions phases were examined by X-ray
diffraction (D/Max-IIIA, Japan) analysis with Cu K radiation. The microstructures were observed by scanning
electronic microscopy (S-3004N Hitachi, Japan). The samples were cut into rectangular bars with a size of 3 mm 3 mm 10 mm for the measurement of and using the
ULVAC ZTM-1 system (Japan) from room temperature to
850 K. The thermal diffusivity (D) and the specific heat
capacity (Cp) were measured by the laser flash method, using
a thermal constant analyzer (ULVAC TC-7000, Japan) in
vacuum. The was calculated according to the following
formula: ¼ DCp , where is the bulk density, which was
measured by the Archimedes method.
Preparation and Thermoelectric Properties of Bi-Doped Mg2 Si0:8 Sn0:2 Compound
289
(220)
(111)
(311)
(400) (331)
(422)
3.0 at.%
(511)
(440)
Intensity (a.u.)
2.5 at.%
2.0 at.%
1.3 at.%
0.75 at.%
0.5 at.%
no doped
20
30
40
50
60
2θ, degree / °
70
80
90
Fig. 1 The X-ray diffraction patterns of Bi-doped Mg2 Si0:8 Sn0:2 samples
with different Bi contents.
3.
Results and Discussion
Figure 1 shows the X-ray diffraction patterns of Bi-doped
Mg2 Si0:8 Sn0:2 samples with different Bi contents. The major
peaks of the patterns can be indexed to an anti-fluorite-type
structure (space group, Fm 3m), indicating that the single
phase of the Bi-doped compound has been successfully
synthesized. Simultaneously, all the peaks gradually shift
right with the increase of Bi content. Jun-ichi T.11) revealed
that Bi atoms are expected to primarily locate at the Si sites in
Mg2 Si based on first-principles calculation. According to
Bragg’s equation, because the ionic radius of Bi is larger than
that of Si and Sn, the small shifting indicates the solubility of
Bi. The microstructures of the fractured surface of the
samples are shown in Fig. 2. It is clearly seen that essentially
no pores exist after SPS processing. Also, based on the BSE
images, no impurity is detected, indicating the formation of
homogeneous solid solutions after sintering by SPS.
Figure 3(a) shows the effect of temperature on the of Bidoped Mg2 Si0:8 Sn0:2 . It is observed that is increased with
the Bi content and that the sample with 3.0 at% Bi content
shows the highest . When the temperature is below 500 K,
the Bi-doped Mg2 Si0:8 Sn0:2 samples show semiconducting
behavior, while when the temperature is over 500 K, the
samples exhibit metal behavior. According to the report of
Jun-ichi T.,11) the increase in is attributed to the substitution
of univalent Bi5 for quadrivalent X4 (X ¼ Si, Sn), which
causes the electron concentration to increase with the Bi
content. It is known that is determined by the combination
of carrier concentration (n) and carrier mobility (). With the
increase in temperature, the scattering of carriers is
enhanced; thus the mobility distance is shortened and the
carrier mobility is reduced. However, more and more
electrons transit from the impurity level to the conduction
band driven by the thermo-power, and as a result, the n
increases. Because the effect of n is more dominant than that
of , is increased with the temperature below 500 K. When
the temperature is above 500 K, the effect of is more
dominant than that of n, so the samples show metal behavior.
However, the Bi-doped Mg2 Si0:5 Sn0:5 samples show a lower
Fig. 2 (a) SEM and (b) BSE images of Mg2 Si0:8 Sn0:2 samples, respectively.
compared to Sb-doped compounds. According to the report
on Sb-doped Mg2 Si0:5 Sn0:5 ,4) the Sb atoms can be fully
ionized at low temperature with a low doping content,
suggesting that it is much more difficult for Bi to transit to the
conduction band than Sb, due to the higher barrier energy
between the impurity level and the conduction band. The
effect of temperature on the of Bi-doped Mg2 Si0:8 Sn0:2
compound is shown in Fig. 3(b). It is observed that the value is negative, Bi atoms act as donors, and is increased
with the Bi content. On the other hand, the of non-doped
Mg2 Si0:8 Sn0:2 is as large as 420 50 mVK1 at 320 K,
and when the Bi content is increased to 3.0 at%, it reaches
100 7 mVK1 at 320 K.
The effect of the Bi content on is given by:
kB 3
þ n
n ¼ ð1Þ
q 2
Ec Ef
n
ð2Þ
¼ ln
n ¼
Nc
kB T
where kB is the Boltzmann constant, Ec is the conduction
band level, Ef is the Fermi level, n is the carrier concentration, and Nc is the density of state.
It is seen that with the increase in Bi content, n is increased,
causing n to be reduced, which resulted in the increase
in n .
The total of is commonly given by:
¼ ph þ el
ð3Þ
290
W. Luo et al.
(a) 100
(a)
80
60
-1
-1
Lattice Thermal Conductivity, κph / wm K
3
Electrical Conductivity, σ / 10 Sm
-1
Non-doped
0.5 at.%
0.75 at.%
1.3 at.%
2.0 at.%
2.5 at.%
3.0 at.%
40
20
0
300
400
500
600
700
800
900
Temperature, T / K
2.0
1.5
1.0
0.5
Non-doped
0.5 at.%
0.75 at.%
1.3 at.%
2.0 at.%
3.0 at.%
300
400
500
600
700
800
Temperature, T / K
(b)
-1
(b)
-1
-1
Carrier Thermal Conductivity, Κel / wm K
-100
Seebeck Coefficient, α / µVK
2.5
-150
-200
-250
-300
Non-doped
0.5 at.%
0.75 at.%
1.3 at.%
2.0 at.%
2.5 at.%
3.0 at.%
-350
-400
-450
300
400
500
600
700
800
900
Temperature, T / K
1.2
0.8
Non-doped
0.5 at.%
0.75 at.%
1.3 at.%
2.0 at.%
2.5 at.%
3.0 at.%
0.4
0.0
300
400
500
600
700
800
900
Temperature, T / K
Fig. 3 (a) The effect of temperature on the of Bi-doped Mg2 Si0:8 Sn0:2
compound; (b) The effect of temperature on the of Bi-doped
Mg2 Si0:8 Sn0:2 compound.
(c) 3.00
-1
-1
Thermal Conductivity, κ / wm K
where ph and el are phonon and carrier components of respectively. el can be calculated based on the WiedemanFranz law using the Lorentz number L and as follows:
el ¼ LT
ð4Þ
In this study, el was calculated using L ¼ 2:45 108 V2 K2 in eq. (4). Because can be tested as described
in the Experimental Procedures, ph can be calculated
according to eq. (3).
Figure 4 shows the effect of temperature on the el , ph
and of Bi-doped Mg2 Si0:8 Sn0:2 compounds. It is observed
that the el is increased while and ph are decreased with
the Bi content and temperature. The reduction in ph is
derived from the crystal distortion by doping Bi atoms
enhanced by the scattering of phonons. Compared with Sbdoped Mg2 Si0:5 Sn0:5 samples, Bi-doped Mg2 Si0:8 Sn0:2 samples show a lower , due to the larger radius and heavier
mass of Bi, which is enhanced by the scattering of phonons.
The effect of temperature on the thermoelectric figure of
merit (ZT) is shown in Fig. 5. It is observed that the value of
ZT is increased with temperature, which appears the same as
that for Bi-doped Mg2 Si.11) When the Bi content is greater
than 0.75 at% and the value of ZT > 1, the sample with
3.0 at% Bi content shows the highest ZT of 1:17 0:05 at
about 850 K, which is greater than the value of Al-doped
Mg2 Si0:9 Sn0:1 (ZT ¼ 0:68 at 864 K)14) and the reports on P,
Sb, Bi, Al, Cu-doped Mg2 Si.12,13) According to the report of
Non-doped
0.5 at.%
0.75 at.%
1.3 at.%
2.0 at.%
3.0 at.%
2.75
2.50
2.25
2.00
1.75
1.50
300
400
500
600
700
800
Temperature, T / K
Fig. 4 Carrier contribution (el ), lattice contribution (ph ) and of Bidoped Mg2 Si0:8 Sn0:2 compound.
Y. Isoda et al., the value of ZT for Sb-doped Mg2 Si0:5 Sn0:5
could reach 1.2 at 620 K; our result shows the same value as
theirs though by different preparation method.4)
4.
Conclusions
The single phase of a Bi-doped Mg2 Si0:8 Sn0:2 solid
solution has been successfully obtained by the SSR-SPS
method. The electrical conductivity, Seebeck coefficient and
thermal conductivity of the samples are strongly affected by
the Bi content. The addition of Bi significantly decreases
thermal conductivity and increases the electrical conductivity
and the Seebeck coefficient. When the doping concentration
Preparation and Thermoelectric Properties of Bi-Doped Mg2 Si0:8 Sn0:2 Compound
REFERENCES
1.25
Non-doped
0.5 at.%
0.75 at.%
1.3 at.%
2.0 at.%
3.0 at.%
Figure of Merit, ZT
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0.75
0.50
0.25
0.00
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300
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900
Temperature, T / K
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of the sample shows the highest value, 1:17 0:05 at 850 K.
Acknowledgement
This work is financially supported by the Major State Basic
Research Development Program of China (973 Program,
Grant No. 2007CB607501).
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