Enhanced thermoelectric performance of high pressure synthesized Sb-doped Mg2Si

Article abstract of DOI:10.1016/j.jallcom.2018.01.260

n-type Mg₂Si_1-xSb_x compounds were prepared within a short duration by using high pressure synthesis followed by spark plasma sintering. The structural and compositional characterizations indicated highly densified single-phase products with no obvious oxidation or volatilization of Mg. High pressure synthesized samples showed greatly enhanced thermoelectric performance compared with ambient-pressure synthesized ones. For the optimal Mg₂Si_0.985Sb_0.015, the power factor reached 3300 μWm^?1K^?2 at 773 K meanwhile the lattice thermal conductivity was suppressed to 1.0 Wm^?1K^?1 at temperatures higher than 700 K. As a result, the highest ZT of 0.94 was achieved at 873 K. High pressure synthesis thus provides us an alternative and effective pathway to synthesize magnesium silicide based thermoelectric materials.

Full text of DOI:10.1016/j.jallcom.2018.01.260

Journal of Alloys and Compounds 741 (2018) 1148e1152
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Enhanced thermoelectric performance of high pressure synthesized
Sb-doped Mg2Si
Jianghua Li, Xiaopu Li**, Bowen Cai, Chen Chen, Qian Zhang, Zhisheng Zhao, Long Zhang,
Fengrong Yu, Dongli Yu, Yongjun Tian, Bo Xu*
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, Hebei 066004, China
a r t i c l e i n f o
a b s t r a c t
Article history:
n-type Mg2Si1-xSbx compounds were prepared within a short duration by using high pressure synthesis
followed by spark plasma sintering. The structural and compositional characterizations indicated highly
densified single-phase products with no obvious oxidation or volatilization of Mg. High pressure syn-
thesized samples showed greatly enhanced thermoelectric performance compared with ambient-
Received 27 October 2017
Received in revised form
31 December 2017
Accepted 18 January 2018
pressure synthesized ones. For the optimal Mg2Si0.985Sb0.015
, the power factor reached 3300
m
Wmꢀ1Kꢀ2 at 773 K meanwhile the lattice thermal conductivity was suppressed to 1.0 Wmꢀ1Kꢀ1 at
temperatures higher than 700 K. As a result, the highest ZT of 0.94 was achieved at 873 K. High pressure
synthesis thus provides us an alternative and effective pathway to synthesize magnesium silicide based
thermoelectric materials.
Keywords:
Magnesium silicide
Thermoelectric materials
High pressure synthesis
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
materials [12,13]. Compared with rival PbTe and skutterudites,
Mg2Si made from constituent elements of high abundance and low
Thermoelectric materials, which can convert heat directly into
electricity and vice versa, have attracted great research enthusiasm
during the past several decades [1e4]. The wide applications of
thermoelectric materials for energy conversion depend on their
thermoelectric performance, which is characterized by the
cost is environment-friendly with advantages of high melting
temperature, low density, high hardness, and low coefficient of
thermal expansion [14,15]. Diverse fabrication methods have been
employed to synthesize Mg2Si-based thermoelectric materials [11],
such as solid state reaction [16e18], direct or arc melting [19e21],
mechanical alloying [22,23], self-propagating high-temperature
synthesis [24], etc. Usually, the synthesis of Mg2Si suffers from the
easy oxidization and volatilization of Mg as well as large melting
temperature difference between Mg and Si, which may bring
adverse effects on the thermoelectric properties of Mg2Si. For
example, it was reported that a small amount of impurity MgO can
reduce the thermoelectric performance of Mg2Si by 30% [25]. It is
thus necessary to explore other fabrication method that can
respond to these issues.
The effectiveness of high pressure in synthesizing thermoelec-
tric materials have been demonstrated for different materials sys-
tems [26e31]. High pressure can lower the reaction temperature
and facilitate the reaction by shifting reaction equilibrium [32]. In
addition, the high-pressure sealed reaction environmental can
avoid the unwanted loss of raw materials. Mg2Si was previously
synthesized under high pressure [33], showing absolute Seebeck
coefficient one fourth of that for ambient-pressure synthesized
sample. In this work, we prepared Mg2Si1-xSbx compounds by using
dimensionless figure of merit, ZT ¼ S2T=rk. Here S, T,
r, and k are the
Seebeck coefficient, temperature, electrical resistivity, and thermal
conductivity, respectively. Different approaches, such as electronic
band engineering [5,6], phonon-glass electron-crystal strategy [7],
and nanostructuring [8e10], etc., have been applied to thermo-
electric materials for higher ZT values. Still, other factors including
abundance, cost, toxicity and environmental impact of the
compositional materials as well as ease of fabrication should also be
considered during the selection of thermoelectric materials for
practical applications.
Magnesium silicide and related alloys crystallizing in the anti-
fluorite structure are promising thermoelectric materials working
in the mid-temperature region [11]. Currently, the highest ZT values
around 1.4 have been achieved in n-type Mg2Si-based bulk
* Corresponding author.
** Corresponding author.
E-mail addresses: lxpcn@ysu.edu.cn (X. Li), bxu@ysu.edu.cn (B. Xu).
https://doi.org/10.1016/j.jallcom.2018.01.260
0925-8388/© 2018 Elsevier B.V. All rights reserved.

J. Li et al. / Journal of Alloys and Compounds 741 (2018) 1148e1152
1149
high pressure synthesis (HPS) followed by spark plasma sintering
(SPS). The structural and thermoelectric properties were investi-
gated. Our results show HPS is effective to suppress the oxidation/
volatilization of Mg. Our HPS samples showed enhanced power
factor and reduced lattice thermal conductivity compared with
those ambient-pressure synthesized samples. The highest ZT of
0.94 was achieved at 873 K for the optimal Mg2Si0.985Sb0.015
.
2. Experimental procedure
Mg (99.8%), Si (99.99%), and Sb (99.999%) powders from Alfa
Aesar were mixed according to the stoichiometric composition of
Mg2Si1-xSbx (x ¼ 0; 0:005; 0:01; 0:015) with a slight overdose of
Mg (2.5 mol% more to compensate the possible loss of Mg during
the synthesis). The mixture was shaped into a cylinder with cold
press, inserted into an h-BN crucible, and loaded into a six-anvil
high pressure apparatus (CS-IV-D, working pressure: 1e6 GPa,
working temperature: 300e2300 K) for a two-step HPS experi-
ment. The first step of HPS was conducted at 973 K and 3 GPa for
0.5 h. The intermediate product was ground into powders under
argon protection and shaped into a cylinder for the second step of
HPS (773 K and 2 GPa for 1 h). The HPS product was grounded into
powders and consolidated into a dense bulk with spark plasma
sintering (SPS, 60 MPa and 1173 K for 15 min). Rectangular
blocks (2 ꢁ 2 ꢁ 8 mm3), disks (46 mm ꢁ 1:5 mm), and slices
(2:5 ꢁ 0:5 ꢁ 6 mm3) were cut from the final bulks for the electrical
transport, thermal conductivity, and Hall measurements,
respectively.
The crystal structure and morphology of the final products were
characterized with X-ray diffraction (XRD, Rigaku D/MAX/2500/PC)
and scanning electron microscopy (SEM, Hitachi S-4800 II FESEM),
respectively. The chemical compositions were determined with
electron probe microanalysis (EPMA, JEOL JXA-8230). The Seebeck
coefficient and electrical resistivity were measured with a ZEM-3
apparatus (Ulvac-Riko), and the thermal conductivity was
measured with a TC-9000H apparatus (Ulvac-Riko). The uncer-
Fig. 1. XRD patterns of Mg2Si after each step of HPS. Vertical tags mark the Bragg
reflections from Fm3m (red, JCPDS: 35-0773) and P63/mmc (black, JCPDS: 34-0673)
structured Mg2Si, respectively. (For interpretation of the references to colour in this
figure legend, the reader is referred to the Web version of this article.)
in highly purified samples with desired antifluorite phase. Due to
the fully enclosed reaction environment provided by high pressure,
the oxidation of Mg is effectively avoided. The restraint of MgO
formation in our HPS samples would be beneficial for the ther-
moelectric properties enhancement of Sb-doped Mg2Si.
The products from HPS were ground and consolidated with SPS
for a better machinability. XRD patterns of Mg2Si1-xSbx samples
tainty in S, r, and к measurements is less than 5%. The sample
density was estimated with the Archimedes method. The Hall co-
efficient measurements were performed at room temperature with
a Physical Property Measurement System (PPMS, Quantum Design).
3. Results and discussion
A two-step HPS procedure was used to synthesize magnesium
silicide. In Step one, higher pressure and temperature (3 GPa and
973 K) were imposed to ensure the preliminary reaction within a
short duration. The product after Step one was ground into pow-
ders under argon protection to improve the composition homo-
geneity, then subject to Step two of HPS (2 GPa and 773 K) to fulfill
the reaction. Fig. 1 shows the XRD patterns of undoped Mg2Si after
each step of HPS. Obviously, there are substantial secondary phases
coexisting with the desired antifluorite Mg2Si (Fm3m symmetry,
JCPDS #35-0773) after Step one of HPS, most of which can be
attributed to the hexagonal Ni2In-type Mg2Si (P63/mmc symmetry,
JCPDS #34-0673). The observation is consistent with the phase
transition of Mg2Si under high pressure reported previously
[34,35]. The antifluorite Mg2Si was reported to undergo a phase
transition to the hexagonal lattice at pressures of 2.5e5.5 GPa and
temperatures of 600e1200 ꢂC [34], which cover the pressure and
temperature condition employed in Step one and account for the
formation of hexagonal Mg2Si. This phase transition is reversible
[35], and the hexagonal Mg2Si formed during Step one of HPS can
transform into the antifluorite Mg2Si during Step two with lower
pressure and temperature. Similar phase evolution processes dur-
ing HPS were also observed for Sb-doped Mg2Si samples, resulting
Fig. 2. (a) XRD patterns of Mg2Si1-xSbx (x ¼ 0, 0.005, 0.01, 0.015) samples consolidated
with SPS. (b) A typical SEM image of the fractured surface of Mg2Si0.985Sb0.015 sample.

1150
J. Li et al. / Journal of Alloys and Compounds 741 (2018) 1148e1152
Table 1
electron concentration as well as decreasing electrical resistivity
and Seebeck coefficient (absolute value). The carrier mobility de-
creases after the initial Sb doping but keeps an almost constant
value of ca. 100 cm2Vꢀ1sꢀ1 for all three Sb-doped samples, similar
to those samples from self-propagating high-temperature synthe-
sis [24]. Note the carrier concentration reaches 2.35 ꢁ 1020 cmꢀ3 for
Mg2Si0.985Sb0.015, which is substantially higher than those achieved
in ambient-pressure synthesized samples with similar or even
higher Sb doping level [20,24,36]. The much higher carrier con-
centration achieved in our samples may relate to the slightly
overdosed magnesium in the raw materials and the reaction under
high pressure. Usually, excessive Mg is taken to compensate the loss
of Mg during the synthesis of Mg2Si materials under ambient
condition. In our case, the reaction is enclosed inside of the high
pressure cell and the volatilizing loss of Mg is basically eliminated,
as indicated by the EPMA determined chemical compositions in
Table 1. For the Mg-rich raw material under high pressure, excess
Mg atoms would occupy the interstitial sites of Mg2Si and donate
electrons to the system [37], contributing to a higher carrier
concentration.
Actual composition, lattice constant a (nm), density d (gcmꢀ3), electrical resistivity
r
(
mUm), Seebeck coefficient S (
m
VKꢀ1), thermal conductivity
k
(Wmꢀ1Kꢀ1), carrier
concentration n ( ꢁ 1019 cmꢀ3), and carrier mobility
m
(cm2Vꢀ1sꢀ1) for Mg2Si1-xSbx
samples at room temperature.
x
EPMA
a
d
r
S
k
n
m
0
Mg2.04Si
0.6355 1.99 72.2 ꢀ276 9.03 0.38 220
105
0.005 Mg2.04Si0.996Sb0.005 0.6356 1.98 19.3 ꢀ128 7.13 3.1
0.010 Mg2.05Si0.989Sb0.009 0.6362 2.00 3.2
0.015 Mg2.05Si0.986Sb0.017 0.6368 2.04 2.5
ꢀ78
6.34 19.4 101
6.24 23.5 106
ꢀ64
after SPS consolidation are presented in Fig. 2a. All the final samples
are dominated by the antifluorite Mg2Si, and no obvious secondary
phase, such as MgO or hexagonal Mg2Si, was detected. Due to larger
ionic radius of Sb than that of Si, the diffraction peaks systemati-
cally shift to smaller angles with increasing Sb doping level, indi-
cating the expansion of the lattice (see Table 1 for the lattice
constant). Fig. 2b shows a typical SEM micrograph of the fractured
surface of Mg2Si0.985Sb0.015. Large grains with size up to tens of
micron are well sintered without obvious micro-cracks or pores,
which is consistent with the high density of our samples as listed in
Table 1. The structure characterization indicates that highly densi-
fied single-phase Mg2Si samples can be prepared with HPS fol-
lowed by SPS.
The temperature dependent thermoelectric properties of
Mg2Si1-xSbx samples are shown in Fig. 3. Compared with the
undoped Mg2Si, the electrical resistivities of Sb-doped samples
decrease significantly due to increasing electron concentration af-
ter Sb doping (Fig. 3a). Samples with higher Sb content (x ¼ 0.01,
The room temperature carrier concentration and mobility are
summarized in Table 1 along with other properties of Mg2Si1-xSbx
samples. All the samples show an n-type conduction behavior with
negative Seebeck coefficient. With elevating Sb doping level, more
electrons can be introduced into the system, leading to increasing
0.015) show a heavily doped semiconductor behavior and
r in-
creases monotonically with elevating temperature. The lowest
electrical resistivity achieved in Mg2Si0.985Sb0.015 sample (2.5 mU
at room temperature and 8.2 mUm at 873 K for instance) is
m
Fig. 3. Thermoelectric properties for Mg2Si1-xSbx (x ¼ 0, 0.005, 0.01, 0.015) samples as a function of temperature. (a) Electrical resistivity. (b) The Seebeck coefficient. (c) Power
factor. (d) Thermal conductivity. The inset to panel d shows the lattice thermal conductivity of Mg2Si1-xSbx samples.

J. Li et al. / Journal of Alloys and Compounds 741 (2018) 1148e1152
1151
clear that kl decreases significantly with increasing Sb content. The
lattice thermal conductivity for Mg2Si0.985Sb0.015 drops to values
around 1.0 Wmꢀ1Kꢀ1 at temperatures higher than 700 K, which is
one of the lowest values for Mg2Si systems. Substituting Sb atoms on
Si sites in Mg2Si induces point defects that can scatter phonons
effectively due to the large mass/size difference between Sb and Si
atoms. Higher Sb content means stronger phonon scattering thus
lower lattice thermal conductivity. Moreover, the high pressure
synthesis method employed in current work promotes the forma-
tion of interstitial Mg atoms [37], which can also scatter phonons
and further reduce the lattice thermal conductivity.
Fig. 4 demonstrates the temperature dependent ZT of HPS Sb-
doped Mg2Si samples based on the thermoelectric properties
measurements. Representative results of ambient-pressure syn-
thesized Sb-doped Mg2Si from literature are also presented for
comparison reason [20,24,36]. For the optimal Mg2Si0.985Sb0.015
sample, ZT increases with elevating temperature and reaches
0.94 at 873 K, with an average ZT as high as 0.49 for the whole
temperature range. As a comparison, the highest ZT values are less
than 0.7 for those ambient-pressure synthesized samples, with a
much lower average ZT values as well [20,24,36]. The better ther-
moelectric performance of our HPS samples originates from the
enhanced power factor and suppressed (lattice) thermal conduc-
tivity benefitted from high pressure synthesis. During the synthe-
sis, the sealed reaction environment ensured by high pressure
prohibits the volatilization or oxidation of Mg, which are usually
suffered by ambient-pressure synthesis methods. Moreover, high
pressure effectively promotes the formation of interstitial Mg. Both
substitutional Sb and interstitial Mg can contribute electrons to
increase the carrier concentration and scatter phonons to decrease
the lattice thermal conductivity, leading to the superior thermo-
electric performance. High pressure synthesis thus offers an alter-
native approach to synthesizing Mg2Si-based thermoelectric
materials. Note the temperature and pressure conditions for syn-
thesizing Mg2Si can readily be realized in industrial production
[41], which is advantageous for commercial applications of ther-
moelectric materials.
Fig. 4. ZT of Mg2Si1-xSbx (x ¼ 0, 0.005, 0.01, 0.015) samples as
a function of
temperature.
substantially lower than those attained in ambient-pressure syn-
thesized samples [20,24,36], which can be attributed to the higher
carrier concentration in the HPS sample. As shown in Fig. 3b, all the
samples show negative Seebeck coefficients indicating n-type
conduction. The undoped Mg2Si sample shows an early onset of
intrinsic conduction occurring around 650 K due to its low carrier
concentration. The Seebeck coefficient of Sb-doped samples shows
a roughly linear dependence on temperature, a typical behavior of
heavily doped semiconductors, and the absolute value of S de-
creases with increasing Sb content. The trend of the Seebeck co-
efficient with respect to temperature and Sb content is consistent
with that of the electrical resistivity. Compared with the samples
synthesized under ambient-pressure with similar composition
[20,24,36,38], the HPS Mg2Si0.985Sb0.015 sample shows a slightly
lower Seebeck coefficient (64
its higher carrier concentration.
m
VKꢀ1) at room temperature due to
Fig. 3c shows the temperature dependent power factor
(PF ¼ S2=
r
) of Mg2Si1-xSbx samples. PF for undoped Mg2Si is rela-
tively small with a peak value of 1400
m
Wmꢀ1Kꢀ2 reached around
4. Conclusions
400 K. After Sb doping, the power factor is greatly enhanced, and
the maximal PF of 3300
m
Wmꢀ1Kꢀ2 is achieved for the optimal
Highly densified single-phase Mg2Si1-xSbx samples were suc-
cessfully synthesized with high pressure synthesis followed by
spark plasma sintering. HPS can effectively eliminate the oxidation
or volatilization of Mg, and promote the formation of interstitial
Mg. Consequently, the thermoelectric performance of our HPS
samples is significantly improved compared with those ambient-
pressure synthesized samples. The optimal Mg2Si0.985Sb0.015 sam-
Mg2Si0.985Sb0.015 at 773 K. This value is higher than those achieved
in ambient-pressure synthesized Sb-doped Mg2Si samples since
the decrease of electrical resistivity is much more significant than
the degradation of Seebeck coefficient in our HPS sample pos-
sessing higher electron concentration. Moreover, the power factor
of Mg2Si0.985Sb0.015 maintains a relatively high value (>3000
m
Wmꢀ1Kꢀ2) when temperature is higher than 650 K, which is
ple shows substantially enhanced power factor (3300 m
Wmꢀ1K2 at
beneficial for the overall thermoelectric performance.
773 K) and greatly suppressed lattice thermal conductivity (ca. 1.0
Wmꢀ1Kꢀ1 in the high temperature region), leading to a ZT value as
high as 0.94 achieved at 873 K. Through implementing the strate-
gies of nanostructuring as well as alloying Si with Ge and Sn, further
enhancement of thermoelectric performance of Mg2Si-based
thermoelectric materials is anticipated with HPS method.
The temperature dependent thermal conductivity of Sb-doped
Mg2Si is shown in Fig. 3d. The thermal conductivity for undoped
Mg2Si is qualitatively consistent with the results reported previ-
ously [36,39]. After Sb doping, the thermal conductivity decreases
mildly but does not show a markedly dependence on Sb content,
especially at high temperature. The total thermal conductivity k is a
sum of the lattice thermal conductivity kl and the electronic
contribution ke. For our HPS samples with increasing Sb content, the
rapid increase in ke offsets the decrease in kl, resulting in the thermal
conductivity almost the same across different samples. ke is gener-
Acknowledgements
This work was supported by the Natural Science Foundation of
China (51525205, 51421091, 51402254, 51722209, and 51332005),
and the Key Basic Research Project of Hebei (14961013D).
allyestimated with the Wiedemann-Franz law (ke ¼ LT=
the Lorenz number) from the measured electrical resistivity. L (in the
unit of 10ꢀ8 W ꢀ2) can be calculated from the experimentally
determined Seebeck coefficient (in the unit of
VKꢀ1) through
L ¼ 1:5 þ expðꢀjSj=116Þ [40]. kl can then be assessed by subtracting
ke from . The inset to Fig. 3d presents kl of Mg2Si1-xSbx samples. It is
r, where L is
U
K
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