1. Intermetallics 19 (2011) 1024e1031
Contents lists available at ScienceDirect
Intermetallics
journal homepage: www.elsevier.com/locate/intermet
Enhanced performances of melt spun Bi2(Te,Se)3 for n-type thermoelectric legs
Shanyu Wang, Wenjie Xie, Han Li, Xinfeng Tang*
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 December 2010
Received in revised form
22 February 2011
Accepted 5 March 2011
Available online 29 March 2011
In this article, a rapid and cost-effective melt spinning (MS) subsequently combined with a spark plasma
sintering (SPS) process was utilized to prepared n-type Bi2(Te1ÀxSex)3 (x ¼ 0.0e1.0) solid solutions from
high purity single elemental chunks. The substitution of tellurium by selenium has significant impacts on
the electrical and thermal transport properties of the Bi2(SexTe1Àx)3 compounds in a manner which can
be well understood using a valence bond rule and the corresponding change in band gap. Furthermore,
the selenium substitution effectively adjusts the carrier density allowing an optimum value of w5 Â 10
À19
cmÀ3. As a result, a maximum ZT of 1.05 at 420 K was achieved for the Bi2(Se0.2Te0.8)3 sample which
also shows an improved average ZT of w0.97 in the entire measurement temperature range. By adopting
the same p-type legs, the module fabricated by the MS-SPS Bi2(Se0.2Te0.8)3 material which acts as n-type
legs shows w10% enhancement in thermoelectric conversion efficiency compared with the module
fabricated by n-type zone melted ingots.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
A. Ternary alloy systems
C. Rapid solidification processing
B. Thermoelectric properties
G. Thermoelectric power generation
1. Introduction
With increasing concerns on environmental protection and
growing energy crisis, thermoelectricity based on solid-state
physics theory draws comprehensive attention owing to its
immense advantages, such as environmental friendliness, silent
operation, absence of moving parts and high reliability [1e3]. In the
last decade, what restrict the comprehensive applications of thermoelectricity are the low thermoelectric performances of current
applied materials. Among the current thermoelectric materials,
Bi2Te3 compound and its alloys are one of the most important and
widespread applied materials used in the vicinity of room
temperature for cooling applications [4e6]. Besides, Bi2Te3-based
alloys also can be good candidates for power generation applications at low temperature range from 300 K to 500 K [7,8]. However,
due to poor thermoelectric and mechanical performances of
commercial zone melted Bi2Te3-based materials, the corresponding
power generation modules show very low thermoelectric conversion efficiency (<5%) which restricts their further applications.
Therefore, it is important to simultaneously improve the thermoelectric and mechanical performances of Bi2Te3-based materials,
especially at elevated temperature (w500 K).
Recently, a lot of experimental results indicate that the refinements of Bi2Te3-based materials can significantly enhance the
* Corresponding author. Tel.: þ86 27 87662832; fax: þ86 27 87860863.
E-mail address: tangxf@whut.edu.cn (X. Tang).
0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.intermet.2011.03.006
thermoelectric performances [9e13]. To yield refined Bi2Te3-based
materials for large scale applications, Cao et al. [9] had previously
utilized a hydrothermal method to create nano-scale Bi2Te3/Sb2Te3
starting powders and then hot-pressed the powders into a bulk
nanocomposite which shows a maximum ZT of 1.47 at about 420 K.
Besides, Poudel et al. [10] and Ma et al. [12] reported an extensive
ball milling technique in an inert atmosphere followed by a hot
pressing process yielding a p-type Bi2Te3-based nanocrystalline
material with enhanced ZT of w1.4 at approximately 380 K.
Furthermore, Xie et al. [11,13] also successfully employed a melt
spinning combined with a subsequent spark plasma sintering
technique (MS-SPS) to fabricate nanostructured p-type (Bi,Sb)2Te3based materials. They found that the MS-SPS technique can
generate multi-scale nano-inclusions in the bulk matrix which
significantly reduce the lattice contribution to thermal conductivity, and the as-prepared materials show a highest ZT of w1.50
around 300 K. Compared with the p-type Bi2Te3-based materials
exhibiting excellent thermoelectric performances, n-type Bi2Te3based materials which are essential for module fabrication have not
shown an improvement from their ZT < 1 for several decades.
Hence, it is of great importance to optimize the thermoelectric
performances of n-type Bi2Te3-based materials.
In particular, due to the large consumptions (particularly in CdTe
photoelectric industry [14,15]) and a low reserves of tellurium
element, it is vital and necessary to explore tellurium-absence or
low tellurium-bearing thermoelectric materials to replace the
traditional Bi2Te3-based materials [16]. For n-type Bi2Te3-based
materials, owing to the same crystal structure and similar electronic

2. S. Wang et al. / Intermetallics 19 (2011) 1024e1031
structure between Bi2Te3 and Bi2Se3, selenium substitution of
tellurium in Bi2Te3 alloy seems to be an effective approach to reduce
the usage of tellurium element. Furthermore, selenium substitution
is also an operative choice to avoid intrinsic conduction and adjust
carrier concentration, along with the resultant enhancement of
thermoelectric performances [17]. It has been reported that with
increasing the selenium content in ternary Bi2(Te1ÀxSex)3
compounds, the band gap and carrier concentration of the solid
solution are both increased gradually. Simultaneously, carrier
mobility and lattice thermal conductivity can be reduced owing to
the intensified alloy scattering for electrons and phonons [17,18]. In
the last decade, though the alloying effects of selenium on the
thermoelectric properties of Bi2(SexTe1Àx)3 solid solutions had been
investigated, it can be noted that the reported results mainly focus
on the low selenium content range (x
0.2), as well as the thermoelectric properties of compounds with high selenium content
(x > 0.5) are seldom researched or reported [17e19].
In this work, we report a characterization of thermoelectric
transport properties for the ternary Bi2(SexTe1Àx)3 (x ¼ 0.0e1.0)
solid solutions prepared by a rapid and cost-effective MS-SPS
technique. We found that the selenium content (namely x value)
has significant impact on the thermoelectric transport properties in
a manner that corporately modulates the transport properties of
both electrons and phonons. As a result, the compound
Bi2(Se0.2Te0.8)3 shows the highest ZT of 1.05 at 420 K, and a relatively high average ZT of 0.97 between 300 K and 500 K which
improves by w30% compared with the traditional n-type zone
melted ingot.
2. Experiments
High purity elemental chunks of bismuth (5N), tellurium (5N)
and selenium (4N) were weighed according to stoichiometry of
Bi2(Te1ÀxSex)3.06 (x ¼ 0.0e1.0) where the additions of excess tellurium and selenium are used to compensate their volatilization, and
then sealed into glass tubes under high vacuum. The glass tubes
with elemental chunks were put into a furnace and heated to
1123 K for 60 min, then quenched to room temperature to get
starting ingots for melt spinning. The obtained ingots were put into
quartz tubes with a 0.35 mm diameter nozzle, and placed under the
protection of an argon atmosphere, the ingots were melted and
ejected under a pressure of 0.02 MPa Ar onto copper roller rotating
with linear speed of 30 m/s, respectively. The obtained ribbons
were pulverized and sintered by the SPS method at 723 K for 3 min
with a pressure of 15 MPa to get compact bulk Bi2(Te1ÀxSex)3.06
samples which are designated as MS-SPS-x for convenience.
1025
The phase compositions of bulk samples were determined by
powder X-ray diffraction (XRD) (PANalytical X’Pert Pro X-ray
diffraction) using Cu Ka radiation (l ¼ 1.5406 Å). The morphologies
and compositions of bulk materials were analyzed by field emission
scanning electron microscopy (FESEM) and energy-dispersive X-ray
spectroscopy (EDS) (Sirion 200 FESEM). Electrical conductivity (s)
and Seebeck coefficient (a) were simultaneously measured by
commercial equipment (ZEM-1, Ulvac Riko, Inc.) under a low
pressure inert gas (He) atmosphere from 300 K to 500 K. The
thermal conductivity (k) was calculated from the measured thermal
diffusivity (D), specific heat (cp), and density (d) using the relationship k ¼ Dcpd. The thermal diffusivity was tested by the laser
flash diffusivity method using a Netzsch LFA 457 system, and the
specific heat (cp) was measured by TA instrument: DSC Q20. All the
measurements were performed in the temperature range from
300 K to 500 K. The densities of the bulk samples were measured by
the Archimedes method and decrease from 7.82 to 7.1 g/cm3 with
increasing the selenium content, which indicates that the relative
densities of obtained bulk samples are higher than 99.0%. The Hall
coefficient (RH) and electrical conductivity (sH) at room temperature were measured by Accent HL5500PC Hall system using the van
der Pauw method with the magnetic field strength of 0.513 T, and
the corresponding carrier concentration (n) and carrier mobility
(mH) were calculated by the followed equations: n ¼ 1=ejRH j and
mH ¼ s=ne. The compressive strength of the MS-SPS samples and
ZM ingots were measured by DEBEN Microtest (MT 10121) system,
and the size of measured samples is 6 Â 3 Â 3 mm3. The thermoelectric conversion efficiency and maximum output power of
a thermoelectric module were simultaneously measured by
commercial power conversion efficiency measuring apparatus
(Model PEM-1S Ulvac Riko, Inc.). The size of the measured module
is 30 Â 30 Â 4 mm3 and each module includes 71 p-n legs.
3. Results and discussion
3.1. Phase compositions and microstructure characterizations for
bulk samples
The phase compositions of bulk samples were characterized by
powder XRD shown in Fig. 1. All the peaks visible in the XRD
patterns can be indexed to the rhombohedral lattice (space group
R3m), which indicates that all the as-prepared specimens are single
phase as well as possess the same crystal structure as the binary
Bi2Te3 compound [20]. The enlarged (205) peaks of the XRD
patterns shown in Fig. 1(b) display an apparent shift to high angle
with increasing the selenium content, which is mainly due to the
Fig. 1. (a) The XRD patterns and (b) the enlarged XRD patterns of (205) peak for the MS ribbons with different selenium content.

3. 1026
S. Wang et al. / Intermetallics 19 (2011) 1024e1031
smaller atomic radius of selenium (1.15 Å) compared with that of
tellurium (1.4 Å). This also presumably verifies that the selenium
atoms successfully enter into the crystal lattices of Bi2Te3 to form
ternary solid solutions.
Fig. 2 presents the FESEM photos of the free crack surface for the
bulk samples with selenium content x equaling to 0.0, 0.2 and 0.5. It
can be noted that the as-prepared compounds show similar
morphologies and well preserve the laminated structure of Bi2Te3based materials due to the existence of van der Waal gaps [21]. The
grains of all the samples are placed tightly and show no preferred
orientations, which are in accordance with the high relative
densities and the isotropic transport properties for the MS-SPS
samples of our previous studies [13,22]. In addition, the average
size of grains apparently decreases via the selenium substitution,
which indicates that selenium alloying is an effective approach for
crystalline refinement. Compared with the traditional zone melted
ingot or other melting ingots, the MS-SPS samples display much
more refined crystals and possess numerous fine layered structures. These structure refinements are probably originated from the
coexistences of a large amount of amorphous components and high
density strain fields in the ribbons associated with the ultrahigh
cooling rate of the non-equilibrium MS process [11,13].
Table 1
The nominal and actual composition results measured by EDS for Bi2(SexTe1Àx)3
(x ¼ 0.0, 0.2, 0.5, 1.0) samples.
Sample
x
x
x
x
¼
¼
¼
¼
0.0
0.2
0.5
1.0
Nominal composition
Actual composition (EDS)
Bi2Te3.06
Bi2Te2.448Se0.612
Bi2Te1.53Se1.53
Bi2Se3.06
Bi2Te3.05
Bi2Te2.44Se0.56
Bi2Te1.52Se1.41
Bi2Se2.93
In practice, bismuth, tellurium and selenium are easy to
vaporize during the fabrication processes, whereas the thermoelectric properties of Bi2(Te,Se)3-based materials are very sensitive
to their chemical composition, especially the ratio of anionic and
cationic components [4]. In order to well understand the variations
of transport properties for ternary Bi2(SexTe1Àx)3 with varying
selenium content, it is necessary to make certain what is the actual
composition of as-prepared bulk samples. Table 1 shows the
nominal compositions and actual compositions measured by EDS
for bulk Bi2(SexTe1Àx)3 (x ¼ 0.0, 0.2, 0.5 and 1.0) samples. It is clear
that when the value of x is small, the corresponding samples own
excess cationic elements and thus the ratios of anionic and cationic
Fig. 2. The FESEM figures and the corresponding high resolution figures of the MS-SPS bulk Bi2(SexTe1Àx)3 (x ¼ 0.0, 0.2 and 0.5) compounds.

4. S. Wang et al. / Intermetallics 19 (2011) 1024e1031
components (which can be defined as g for convenience) are larger
than the stoichiometric formula 3:2. However, with further
increasing the selenium content, g decreases gradually and finally
the samples with higher x value would show excess cationic
component (namely g < 3/2). These different composition variations are mainly due to the different vaporization capabilities of
selenium and tellurium elements (for example, at 640 K, the
saturated vapor pressures of bismuth, tellurium and selenium
elements are w10À5, 1 and 100 Pa, respectively [23]). Furthermore,
these fluctuations of chemical compositions would presumably
result in the changes in carrier concentration as well as the thermoelectric properties of bulk materials.
3.2. Electrical transport properties
Fig. 3 shows the temperature dependences of electrical transport properties for the prepared bulk materials. The electrical
conductivities decrease monotonically with increasing the
temperature which indicates a degenerate semiconductor characteristic. Moreover, with increasing the selenium content, the electrical conductivity shows a nonmonotonic variation which first
decreases and then increases. Conversely, the Seebeck coefficient
exhibits an opposite variation tendency and the MS-SPS-0.2 sample
possesses the highest value in the entire temperature range.
Nevertheless, it is worthwhile to note that the samples with high
selenium content exhibit relatively lower Seebeck coefficient. In
order to further investigate the detailed transport properties of the
prepared bulk samples, the room temperature Hall coefficients
were measured, as well as the corresponding carrier concentrations
and mobilities were calculated.
As plotted in Fig. 4(a), the carrier concentration first decreases
and then increases with increasing the selenium content. This
variation can be accounted for the conduct mechanisms of Bi2Te3based materials along with the inconsistent changes of actual
chemical composition of the prepared ternary solid solutions. In
Bi2Te3-based materials, anion vacancies and anti-site defects are
two dominating factors for carrier concentration and also its
transport properties, even for the determination of the type of
majority carriers [4,19]. With regards to anti-site defects, take
binary Bi2Te3 compound for example, excess bismuth atoms (when
g is lower than 3/2) would occupy the sites of tellurium to generate
BiTe anti-site defects acting as electron acceptors. Whereas, when
g is larger than 3/2, excess tellurium atoms could enter the sites of
1027
bismuth to generate TeBi anti-site defects and each of TeBi would
donate one electron into the material [24]. Furthermore, in the
courses of material preparation-especially in the MS process,
selenium or tellurium element would considerably vaporize and
leave a large number of lattice vacancies, namely VSe or VTe, and
each of the lattice vacancy produces two electrons. At the same
time, bismuth cations which own similar ionic radius and physical
properties with tellurium or selenium anion would occupy the
anionic lattice vacancies to generate anti-site defects BiTe or BiSe
providing holes into the material. As mentioned in our previous
works [22], VSe and BiTe are more easily formed than VTe and BiSe in
Bi2(SexTe1Àx)3 compounds, respectively. Therefore, due to the
excess additions of tellurium and selenium, the samples with small
x value possess excess anion components. These excess tellurium or
selenium would occupy the sites of bismuth to generate TeBi or SeBi,
and the corresponding samples show high n value. When x 0.4
and with increasing x value, n decreases gradually due to the
reduction of the number of TeBi which is the dominant point defect
in the lower selenium compounds. However, with further
increasing the selenium content, the number of TeBi would reduce
sharply, whereas the increase of the number of VSe along with the
simultaneous reduction of the content of BiTe predominately
contributes to the increase of carrier concentration.
The composition dependence of carrier mobility is plotted in
Fig. 4(b) and shows a nonmonotonic variation. Besides the carrier
scattering mechanisms, the properties of chemical bonds should
significantly influence the value of carrier mobility. Generally,
covalent bond is more favorable for the transport of carriers
compared with ionic component [25]. Based on the traditional
Pauling empirical formula and the electronegativities of both
bonding atoms, the proportion of ionic component of the AeB bond
can be estimated by the following equation: 1 À exp[À(cA À cB)2/4],
where cA and cB are electronegativities of A and B atoms, respectively [26]. According to the electronegativities of three atoms
bismuth (2.02), tellurium (2.10) and selenium (2.55), it can be
calculated and noticed that BieSe bond (6.78%) displays much more
ionic component than BieTe bond (0.16%), therefore binary Bi2Te3
possessing more covalent component shows much higher carrier
mobility than binary Bi2Se3. Moreover, with the incorporation of
selenium into the binary Bi2Te3 compound, carrier mobility first
decreases due to intensifying alloying scattering and gradual
transformation of bond property. However, carrier mobility then
slightly increases because of subdued alloying scattering with
Fig. 3. Temperature dependences of (a) the Seebeck coefficient a and (b) electrical conductivity s for bulk Bi2(SexTe1Àx)3 (x ¼ 0.0e1.0) samples.

5. 1028
S. Wang et al. / Intermetallics 19 (2011) 1024e1031
Fig. 4. Room temperature (a) carrier concentrations n and (b) carrier motilities mH for the bulk Bi2(SexTe1Àx)3 (x ¼ 0.0e1.0) samples which were measured by the room temperature
Hall effect.
further increasing the selenium content (x > 0.5). Theoretically, the
samples with intermediate selenium content show lowest m value
which is in accordance with previous researches [4,18]. Based on
equation s ¼ nme, it is not difficult to understand that the variations
of n and m co-affect the variation of electrical conductivity.
To further understand the mechanisms of the variation of Seebeck coefficient, by assuming a charge carrier scattering distance
independent of energy and degenerate approximation, the Seebeck
coefficient can be written as Eq. (1) [27,28]:
a ¼

6. 8p2 k2 * p 2=3
B

7. E¼E ¼ 3eh2 m T 3n
F
p k2 T dlnsðEÞ

8. B

9. 3 e
dE
(1)
where kB, EF, h, m*, and T are the Boltzmann constant, Fermi energy,
Planck constant, effect electron mass and absolute temperature,
respectively. The relationship of room temperature a-n is presented
in Fig. 5(a) and the solid line demonstrates an nÀ2/3 dependence
which indicates the dominant scattering mechanism in MS-SPS
Bi2(SexTe1Àx)3 bulk samples is acoustic phonon scattering [27].
Hence, the change of the Seebeck coefficient is predominated by
the variation of carrier concentration. Therefore, though alloying
scattering has significant impact on the transport properties of
electrons and thus decreases the electron mobility, it can be
inferred that the bond property and phononeelectron interactions
exclusively dominate the transport of electrons.
The power factor (a2s) of Bi2(SexTe1Àx)3 compounds are presented in Fig. 5(b). It can be noted that the compounds with higher
selenium content (x 0.5) show very low power factors. On the
contrary, due to the high proportion of covalent bond component
and thus high carrier mobility, the compounds with lower selenium
exhibits relatively better electrical properties and may be promising candidates for thermoelectric applications. As a result, the
MS-SPS-0.2 sample possesses highest power factor in the entire
measured temperature range.
3.3. Thermal transport properties
The temperature dependences of thermal transport properties
for Bi2(SexTe1Àx)3 compounds are presented in Fig. 6. As shown in
Fig. 6(a), with increasing temperature, the thermal conductivities of
the compounds with lower selenium (x 0.5) first decrease due to
the intensified phononephonon coupling, and then increase owing
to the bipolar diffusion carriers (namely electron-hole pairs) conducting much heat. Interestingly, the thermal conductivities of the
compounds with high selenium (x ! 0.5) decrease monotonically
Fig. 5. (a) The room temperature Seebeck coefficients as a function of carrier concentration and the solid line demonstrates such an nÀ2/3 dependence, (b) the temperature
dependences of power factors (a2s) for bulk Bi2(SexTe1Àx)3 (x ¼ 0.0e1.0) samples.

10. S. Wang et al. / Intermetallics 19 (2011) 1024e1031
1029
Fig. 6. (a) The temperature dependences of the thermal conductivities k and (b) lattice thermal conductivities (kL) for bulk Bi2(SexTe1Àx)3 (x ¼ 0.0e1.0) samples. The inset in (a)
shows the composition dependence of room temperature thermal conductivity.
with increasing temperature. These apparent distinctions may be
due to the differences in energy gaps (Eg) and carrier concentrations
which decide the excitation temperature of intrinsic conduction
[29]. With increasing the selenium content (x value), the room
temperature thermal conductivity first decreases and then
increases, as shown in the inset map of Fig. 6(a). The sample whose
ratio of selenium and tellurium equals to 1:1 (MS-SPS-0.5 sample)
shows the lowest k value which is in conformity with previous
studies [4,19]. These tendencies are partially attributed to the
similar variations of electrical contributions to thermal conductivity, whereas the most important factor is the different phonon
scattering effects of selenium alloying.
The electronic contribution to thermal conductivity was calculated according to the WiedemanneFranz equation: ke ¼ LsT, where
L is Lorenz constant and can be taken to be 1.5 Â 10À8 V2 KÀ2 for
near-degenerate or degenerate semiconductor [30,31]. The lattice
contributions were then calculated by subtraction and are shown in
Fig. 6(b). With increasing temperature, the lattice thermal
conductivities show the same variation trends as those of thermal
conductivities, where the samples with high selenium content
decrease monotonically and the low selenium-bearing compounds
decrease firstly and then increase. These changes are consistent
with the variations of Eg and n with the incorporation of selenium.
Furthermore, the alloying of selenium largely reduces the lattice
thermal conductivity which is mainly attributed to the strong
phonons scattering by atom mass fluctuations. In particular, it can
be noticed that the samples, especially the ternary solid solutions
prepared by the MS-SPS technique, exhibit much lower lattice
thermal conductivities compared with the materials fabricated by
other techniques [17,19,32]. The ultra-low lattice thermal conductivities should be partially attributed to the compositional
homogenization in atomic size associated with the rapid frozen
process, which largely intensifies the alloy scattering of shot-wave
phonons. Moreover, the grain refinements, a large number of
nanostructures, point defects and strain fields generated by the
non-equilibrium MS technique also contribute much to the significant reduction of lattice thermal conductivity [11,13].
3.4. The dimensionless figure of merit ZT and the thermoelectric
conversion efficiencies of modules
The dimensionless figure of merit ZT was calculated by the
measured values of a, s and k and is shown in Fig. 7(a). Due to the
detrimental bipolar effects, the ZTs of the samples with lower
Fig. 7. (a) The temperature dependences of ZT and (b) the composition dependence of room temperature ZT for the bulk Bi2(SexTe1Àx)3 (x ¼ 0.0e1.0) samples.

11. 1030
S. Wang et al. / Intermetallics 19 (2011) 1024e1031
Fig. 8. (a) The temperature dependence of capability factors (s) for ZM ingot and the MS-SPS-0.2 samples, (b) the schematic photo of the fabricated thermoelectric module for
thermoelectric conversion efficiency measurements.
selenium content first increase and then decrease with increasing
temperature, reaching peaks around 450 K. The MS-SPS-0.2 sample
shows a highest ZT of 1.05 at 420 K. More importantly, compared
with the ZM ingot provided by Guangdong Fuxin Electronic Technology Co., Ltd whose ZT drops rapidly with temperature down to
about 0.5 at 500 K, the ZT value of the MS-SPS-0.2 sample shows
much lower temperature dependence which exhibits a relatively
high average ZT of 0.97 between 300 and 500 K. This average ZT
value is about 30% improvement compared with the ZM ingot, and
it can be inferred that the MS-SPS-0.2 material should be more
beneficial for low temperature power generation and also cooling.
Furthermore, the samples with intermediate selenium
(x ¼ 0.3e0.5) also show comparable average ZT with the ZM ingot
between 300 K and 500 K, and their thermoelectric properties can
be further optimized by proper carrier concentration and chemical
composition modifications. With increasing the x value, the room
temperature ZT presented in Fig. 7(b) enhances previously and then
decreases, which is consistent with the hot-press results and other
studies [19]. In practice, excellent mechanical properties are
necessary for material processing and module fabrications. The
compressive strength measurements reveal that the MS-SPS-0.2
sample (w220 MPa) shows more than 300% improvement in
compressive strength in comparison with that of ZM ingots
(w50 MPa).
The compatibility factor s is also important for thermoelectric
materials in practice and defined as [33,34]:
s ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1 þ ZT À 1
aT
(2)
Fig. 8(a) shows the temperature dependences of s for the MSSPS-0.2 sample and ZM ingot. It is obvious that the MS-SPS-0.2
sample shows lower temperature dependence, particularly at
elevated temperature, than that of ZM ingot, which indicates that
Table 2
The output powers and conversion efficiencies of modules fabricated by different
n-type materials.
Modules
Th (K)
Tc (K)
U (V)
I (A)
Pmax (W)
h (%)
ZM
MS-SPS-0.2
476
474
321
321
2.112
2.915
0.973
0.776
2.053
2.26
4.1
4.5
the MS-SPS-0.2 sample may be more favorable for n-type thermoelectric legs. In order to ascertain and further identify the
thermoelectric performances of these two kinds of n-type materials, with the same zone melted p-type legs and fabricating techniques, two modules were fabricated by these two n-type
materials, respectively. Fig. 8(b) presents the schematic photo of
a fabricated module which includes 71 p-n pairs, two Al2O3 ceramic
plates and two leg wires. By fixing the Th (the temperature of hot
side) and Tc (the temperature of cold side) which determine the
input power, the thermoelectric conversion efficiencies (h) and
maximum output powers (Pmax) of the two modules were
measured and are presented in Table 2. It can be noted the MS-SPS0.2 module shows w10% enhancement in conversion efficiency
equaling to 4.5% in comparison with that of the ZM module (4.1%).
This result also confirms that the n-type MS-SPS-0.2 sample
possesses better thermoelectric performances as well as promising
application potentials in comparison with the n-type ZM ingot.
4. Conclusions
In this work, a melt spinning combined with a subsequent spark
plasma sintering technique (MS-SPS) was successfully utilized to
synthesize n-type selenium substituted ternary Bi2(Te1ÀxSex)3
(x ¼ 0.0e1.0) solid solutions. By significantly influencing the
chemical bond component and the transport properties of electrons and phonons, selenium substitution and its content apparently impact the microstructures and thermoelectric properties of
n-type Bi2Te3-based compounds. As a result of better electrical
properties and lower lattice thermal conductivity, the samples with
intermediate selenium show improved thermoelectric performances. The maximum ZT reaches 1.05 at 420 K for the MS-SPS-0.2
sample, and the sample also shows a high average ZT of 0.97
between 300 and 500 K which is about 30% enhancement
compared with the n-type ZM ingots. Furthermore, the MS-SPS-0.2
module shows w10% improvement in thermoelectric conversion
efficiency compared with that of ZM module. Therefore, it is
worthwhile to note that the substitution of tellurium by selenium
as well as the utilization of time-saving and cost-efficient MS-SPS
technique, which results in bulk n-type Bi2(Te1ÀxSex)3 based
materials with high thermoelectric and mechanical performances,
are both of great significances for the commercial applications of
Bi2Te3-based materials.

12. S. Wang et al. / Intermetallics 19 (2011) 1024e1031
Acknowledgments
We thank the supports of the National Basic Research Program
of China (Grant No. 2007CB607501), the Natural Science Foundation of China (Grant Nos. 50672118 and 50731006), and 111 Project
(Grant No. B07040) along with the Fundamental Research Funds for
the Central Universities (Grant Nos. 2010-IV-046 and 2010-ZY-CL040). We also acknowledge Jianzhong Zhang, Guoliang Yu and
Weiqiang Cao of Guangdong Fuxin Electronic Technology Co. Ltd
for the supports in module fabrications and material supplies.
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