Thermoelectric Properties and Transport Mechanism of
Pure and Bi-Doped SiNWs-Mg
Xiaomeng Yang, Shaoping Chen,* Hua Zhang, Feng Lv, Wenhao Fan,*
Wenxian Wang, and Zuhair A. Munir
Through the use of nano-Si wires as additive, a significant improvement in the
thermoelectric (TE) properties of Mg
Si is achieved. SiNWs-Mg
Si materials are
prepared by a wet etching method followed by field activated pressure assisted
synthesis (FAPAS). The results show that the presence of SiNWs successfully
decouples the relationship between the electrical resistivity and the Seebeck
coefficient of Mg
Si. The effect of doping with Bi is also investigated. The
results show that the addition of Bi changed the scattering mechanism, and a
zT value of 0.47 is determined at 800 K for Mg
presence of MgO as an impurity and its effect on zT are discussed.
It is estimated that about 90% of the power generated in the world
is by engines using combustion of fossil fuel with efﬁciencies of
<40%. Consequently, about 10
Watts are produced as waste heat
to the environment.
Thermoelectric (TE) functional materials
can be utilized to directly convert waste heat energy into electric
The success of the TE materials in the utilization of
waste heat is determined by the dimensionless ﬁgure of merit zT,
which is deﬁned as zT ¼ α
respectively, the Seebeck coefﬁcient, electrical conductivity, total
thermalconductivity, andabsolutetemperature. Efforts to increase
the zTvaluesfocus, obviously,onincreasing
. Investigations aimed at increasing zT for several classes of
TE materials have been made. These include silicides,
and tellurides (e.g., those of Bi,
). Of the silicides, that of magnesium, Mg
Si, and its solid
solutions have received a great deal of
attention as TE materials for energy conver-
sion in the mid-to-high temperature range.
This is, in part, due to their low cost, non-
toxicity, lightweight, and the abundance of
their constituent elements.
As indicated above, increasing the ﬁgure
of merit requires increasing both the
, and the electrical
. However, for materials with
band-like transport, there is an inverse
there is need to devise methods to decouple
these parameters. Using silver nanowires
(AgNWs) in bulk conducting polymers, Liu
reported such a decoupling, showing an increase in
without a noticeable decrease in
. This observation is consistent
with previous reports showing that the use of low-dimensional
structures, such as superlattices and nanowires, could lead to
higher zT values.
Such a beneﬁt can also be the result of a
signiﬁcant reduction in the thermal conductivity, another factor
in increasing zT, as has been reported by Hochbaum et al.
Boukai et al.
through the use of silicon nanowires (SiNWs), and
by Hasegawa et al.
for Bi and Si wires.
Recently, Yi et al.
investigated the TE properties of Mg
which silicon nano-dots were introduced and found a signiﬁcant
reduction in its thermal conductivity. It was also shown that a
zT value of 0.6 could be achieved at room temperature in rough
SiNWs of %50 nm diameter.
The achieved zT value is 60 times
that of bulk silicon. Thus, it seems reasonable to expect that the
addition of SiNWs to bulk Mg
Si, to form micro-nano composites,
would lead to enhancement of the ﬁgure of merit. In this paper we
report results of an investigation on the effect of incorporating
SiNWs on the TE properties of pure and Bi-doped Mg
2. Experimental Section
To prepare SiNWs, we started with n-type silicon wafers
with <100> crystalline orientation and a resistivity,
cm. Each wafer was etched for 2.5–3.0 h in a bath
containing a mixture of 0.035 mol L
and 20 wt.% HF.
Then the wafer was placed in a solution of 60 wt.% nitric acid
) and held there for 30 min so that the resulting silver
wires, as by-product of wet etching, could be removed from the
surface. Then the wafer was washed in an ethanol bath for 5 min
under the action of an ultrasonic agitation. During this step, the
SiNWs fell off the silicon wafer. The collected SiNWs were then
dried under vacuum.
X. Yang, Prof. S. Chen, H. Zhang, F. Lv, Prof. W. Wang
College of Material Science and Engineering
Taiyuan University of Technology
Taiyuan 030024, China
Dr. W. Fan
College of Physics and Optoelectronics
Taiyuan University of Technology
Taiyuan 030024, China firstname.lastname@example.org
Prof. Z. A. Munir
Department of Material Science and Engineering
University of California
Davis, CA 95616, USA
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/pssa.201700742.
Thermoelectric Material www.pss-a.com
Phys. Status Solidi A 2018, 215, 1700742 © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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