Thermoelectric Properties of Bulk Yttrium Silicide (YSi2) Fabricated by Arc Melting and Spark Plasma Sintering

Article abstract of DOI:10.1002/pssa.201700769

Yttrium silicide (YSi₂) nanoparticles in SiGe are reported to enhance the thermoelectric figure-of-merit (ZT) to the recorded value (≈1.81). However, the thermoelectric properties of bulk YSi₂ has never been reported. In this work, the thermoelectric properties of YSi₂ is studied for the first time. The bulk YSi₂ is fabricated by arc melting highly pure Si and Y raw materials. The ingot is crushed to powder and compacted to form a highly dense pellet by spark plasma sintering. By optimizing the processing parameters, the single phase with the AlB₂-type structure (P6/mmm) is formed. The Hall measurements show that YSi₂ exhibit a metallic-like behavior with a very high electron concentration. This result in the thermoelectric properties with a very large electrical conductivity (≈18 × 10⁵ Ω^?1 m^?1) but a small Seebeck coefficient (?26 μV K^?1) at room temperature, and decrease with temperature. The thermal conductivity is around 14–19 W m^?1 K^?1 for all temperature range. The maximum ZT value is 0.026 at 423 K.

Full text of DOI:10.1002/pssa.201700769

a
ORIGINAL PAPER
Spark Plasma Sintering
www.pss-a.com
Thermoelectric Properties of Bulk Yttrium Silicide (YSi2)
Fabricated by Arc Melting and Spark Plasma Sintering
Suphagrid Wongprakarn, Supree Pinitsoontorn,* Sora Tanusilp,
and Ken Kurosaki
determined by the dimensionless figure-of-
2
Yttrium silicide (YSi ) nanoparticles in SiGe are reported to enhance the
merit, ZT ¼ (S σ/κ)T, where S, σ, κ, and T
are Seebeck coefficient, electrical conduc-
tivity, thermal conductivity, and absolute
temperature, respectively. The present
state-of-the-art TE materials around room
temperature and mid-range temperature
2
thermoelectric figure-of-merit (ZT) to the recorded value (ꢀ1.81). However,
the thermoelectric properties of bulk YSi has never been reported. In this
2
work, the thermoelectric properties of YSi is studied for the first time. The
2
bulk YSi is fabricated by arc melting highly pure Si and Y raw materials. The
2
[
1,2]
ingot is crushed to powder and compacted to form a highly dense pellet by
spark plasma sintering. By optimizing the processing parameters, the single
are Bi Te3 and PbTe.
However, these
2
materials have a certain draw back from
their thermal instability and consisting of
toxic and rare elements.
Metal silicides are a class of materials
which has been considered as candidates
for TE materials. In recent review papers,
several types of silicides for TE applications
were summarized, including Mg
MnSi , CrSi , β-FeSi .
doping, these silicides showed promising
phase with the AlB -type structure (P6/mmm) is formed. The Hall measure-
2
ments show that YSi exhibit a metallic-like behavior with a very high
2
electron concentration. This result in the thermoelectric properties with a
5
ꢂ1 ꢂ1
very large electrical conductivity (ꢀ18 ꢁ 10 Ω
m ) but a small Seebeck
ꢂ1
coefficient (ꢂ26 mV K ) at room temperature, and decrease with tempera-
ꢂ1 ꢂ1
2
Si,
ture. The thermal conductivity is around 14–19 W m
range. The maximum ZT value is 0.026 at 423 K.
K
for all temperature
[4–6]
With proper
x
2
2
TE performances with a relatively large ZT
(>0.5).
TE properties of silicide nanoparticles in
[
7]
1
. Introduction
Si or SiGe matrix was theoretically calculated by Mingo et al. In
their work, several types of silicide nanoparticles were
incorporated in the matrix and a fivefold increase in the ZT
values were observed as a consequence of a significant
suppression in thermal conductivity. This work has stimulated
many experimental works in exploiting silicide nanoparticles as
Thermoelectric (TE) materials can directly convert wastes heat
into electrical power.
[1–3]
The performance of TE materials is
S. Wongprakarn
Materials Science and Nanotechnology Program
Faculty of Science
Khon Kaen University
Khon Kaen 40002, Thailand
2
phonon scattering sources. For instance, Mg Si and FeSi2
nanoinclusions in SiGe matrix showed the reduced thermal
conductivity compared to the single-phase SiGe alloy, and hence
the maximum ZT of 1.3. Furthermore, the Si/WSi nano-
2
composites showed 40% reduction in lattice thermal conductiv-
[8]
Dr. S. Pinitsoontorn
Integrated Nanotechnology Research Center
Department of Physics
Faculty of Science
Khon Kaen University
Khon Kaen 40002, Thailand
E-mail: psupree@kku.ac.th
[9]
ity and the ZT enhancement of 30%.
The most promising experiment was demonstrated by
[
10]
Ahmad et al.
nanoparticles in SiGe matrix and obtained the recorded ZT
ꢀ1.81) for the p-type SiGe alloy. The enormous improve-
ment in TE performance was attributed to the coherent
interfaces between YSi nanoparticles and SiGe grains which
2
who incorporated yttrium silicide (YSi )
(
S. Tanusilp, Dr. K. Kurosaki
Graduate School of Engineering
Osaka University
2
strongly scattered phonon but have a minimum effect on the
power factor. This work has not only emphasized the
2
-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Dr. K. Kurosaki
Research Institute of Nuclear Engineering
University of Fukui
nanoparticle-in-alloy concept but also shed the light on YSi
as a promising TE material. Nevertheless, YSi was not
or in the calculation
2
2
1
-2-4 Kanawa-cho, Tsuruga, Fukui 914-0055, Japan
[4,5]
included in the reviews on silicides,
[
7]
Dr. K. Kurosaki
JST, PRESTO
by Mingo et al. Moreover, research on bulk YSi
reported.
2
were rarely
4
-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Consequently, in this work, the bulk YSi
2
was fabricated and
DOI: 10.1002/pssa.201700769
the TE properties were investigated for the first time.
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(Ulvac-Riko) from room temperature to 773 K. The mobility
and carrier concentration at room temperature were measured
by Hall measurement system (Toyo ResiTest8300). Thermal
conductivity (κ) was calculated from κ¼ DC
p p
d, where D, C , and
d are thermal diffusivity, heat capacity, and density, respectively.
D was measured by laser flash technique (NETZSCH, LFA457).
C
p
¼ 3nR, where n and R are the number of atoms per formula
unit and gas constant. D was calculated from the measured
weight and dimensions of the samples.
3. Results and Discussion
Figure 1 shows the powder XRD patterns of the YSi ingot and
2
the bulk samples after SPS. The main peaks of all samples agree
well with the YSi1.67 standard pattern (JCPDS 01-071-3919),
representing the single phase of the AlB -type structure (P6/
2
mmm), without any secondary phases observed. Samples 1–5
show the identical peaks indicating the reproducibility of the
fabrication technique. The lattice parameters calculated from the
XRD patterns are shown in Table 1, which are almost same with
[11]
the literature values.
The densities of the bulk samples are
also presented in Table 1. All samples possess very high density
between 93.3 and 94.5% of theoretical values which are the
fundamental requirements for good TE materials.
The SEM micrographs alongside with the EDS mappings of
the pressed surface of the bulk YSi2 sample are shown in
Figure 2. It is obviously seen the fully dense sample with
homogenous distribution of the elements. Quantitative analysis
showed that the chemical compositions of Y and Si were 37.14
and 62.86 at.%, respectively, which is equivalent to Y:Si ¼ 1:1.69.
2
Figure 1. Powder XRD patterns of the YSi samples after arc melting and
after SPS.
2
. Experimental Section
YSi
2
ingots were prepared by arc melting (AM) under Ar
2
This result showed that the nominal composition YSi was in
atmosphere, by melting highly pure Si (11N), and Y chunks
99.9%). The ingot was crushed to powder and then loaded into a
graphite die for fabricating the bulk samples by using spark
plasma sintering (SPS). The SPS was carried out at 1273 K for
fact YSi1.69, which is closely agreed with the literature value of
[11,12]
(
YSi1.67
.
The elemental analysis of all five samples show
nearly identical compositions as summarized in Table 1.
The TE properties of the bulk YSi samples are shown in
2
5
min in Ar atmosphere under a pressure of 100 MPa. To test the
Figure 3. All five samples show almost the same TE character-
istics. The Seebeck coefficients were negative throughout the
temperature range, indicating the n-type charge carrier, which
was confirmed by the Hall measurement (Table 1). The absolute
reproducibility, five samples under the same synthesis con-
ditions were prepared.
Crystallinity and morphology of the samples were observed
using the X-ray diffraction analysis (XRD; Ultima IV, Rigaku)
and scanning electron microscopy (SEM, JSM-6500F, JEOL).
The chemical composition was confirmed by energy dispersive
X-ray spectrometry (EDS) equipped in the SEM. Seebeck
coefficient and electrical conductivity were measured by ZEM-
ꢂ1
S values decreased with temperature from 26 mV K at room
ꢂ1
temperature to 14 mV K at 773 K. These values are very low
ꢂ1 [4,5]
compared to other silicide materials (100–300 mV K ).
The
reason for the low S values is attributed to the very large carrier
22
ꢂ3
concentration (n
H
ꢀ6–7 ꢁ 10 cm ), Table 1, much higher than
H H
Table 1. Density (d), relative density (%T.D.), lattice parameter (a and c), Hall carrier concentration (n ), and Hall mobility (m ) at room
temperature.
Lattice parameter (nm)
EDS (at.%)
ꢂ3
20
ꢂ3
2
ꢂ1 ꢂ1
Sample
d (g cm
)
%T.D.
a
c
Y
Si
Carrier type
n
H
(ꢁ10 cm
)
μ
H
(cm V s )
No. 1
No. 2
No. 3
No. 4
No. 5
4.24
4.29
4.28
4.25
4.28
93.3
94.5
94.3
93.6
94.3
–
0.3843(4)
0.3844(8)
0.3845(4)
0.3846(3)
0.3844(9)
0.3843
0.4143(2)
0.4142(1)
0.4147(5)
0.4147(4)
0.4143(2)
0.4143
37.14
37.11
37.34
37.26
37.19
–
62.86
62.89
62.66
62.74
62.81
–
n
n
n
n
n
–
7.43 ꢃ 0.7
6.11 ꢃ 0.6
6.95 ꢃ 0.7
6.84 ꢃ 0.7
6.63 ꢃ 0.7
–
17.9 ꢃ 0.2
21.7 ꢃ 0.2
19.1 ꢃ 0.2
19.6 ꢃ 0.2
18.7 ꢃ 0.2
–
[
11]
4.54
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Figure 3. Temperature dependences of (a) S, (b) σ, (c) S σ, (d) κ, (e) κlat
and (f) ZT of YSi
,
2
.
the TE and Hall measurements in the present work are in
consistence with the previous electronic structure calculations of
which the Fermi energy was right in the middle of the
[14]
conduction band, representing the metallic characteristic of
YSi2.
Figure 3d shows the thermal conductivity of the bulk YSi2
samples. The κ values slightly decreased with temperature and
Figure 2. a) SEM images of the pressed surface of YiS
elemental mapping (b) Y, (c) Si.
2
along with the
ꢂ1 ꢂ1
were approximately about 15 Wm [
K
above 500 K, several
4,5]
folds higher than other silicides.
Generally, κ ¼ κlat þ κ
is the
electronic thermal conductivity which can be estimated from
Wiedemann–Franz’s law, κ
¼ LσT, where L ¼ Lorenze number
The reason for the high thermal
conductivity is due to the very large κ (dotted line). After
subtracting κ , the κ is plotted as shown in Figure 3e. This
e
,
2
0
21
ꢂ3 [4,5]
other silicide materials (ꢀ10 –10 cm ).
Normally, for
where κlat is the lattice thermal conductivity and κ
e
metals or degenerate semiconductors, the Seebeck coefficient is
[1,13]
inversely proportional to the carrier concentration.
fore, the very large carrier concentration resulted in the reduced
S values.
Figure 3b showed the decreasing electrical conductivity with
increasing temperature, illustrating a metallic-like behavior. The
observed σ values were very high throughout the measurement
There-
e
ꢂ8
2
ꢂ2 [1]
¼ 2.45 ꢁ 10 V K
.
e
e
l
implies that the thermal conductivity is mostly govern by the
electronic part which could be justified from the very high carrier
concentration. Figure 3f shows the temperature dependence of
ZT which has the maximum of 0.026 at 423. This value is
relatively low in comparison to other good TE silicides, i.e.,
5
ꢂ1 ꢂ1
temperature (18 ꢁ 10 Ω
m
at RT), about an order of
4
ꢂ5 ꢂ1 ꢂ1 [4,5]
magnitude larger than other silicides (ꢀ10 –10
Ω
m ).
The metallic-like behavior and large σ are consistent with the
high n . The mobility of YSi in the present study was nearly
Mg
YSi
silicides, for example, URu
2
Si, MnSi
in our case is comparable (or even better) to some inferior
2
Si , CeSi , SrSi ,
x 2 2
, CrSi , FeSi , with ZT > 0.5. However, the ZT of
H
2
2
ꢂ1 ꢂ1 [4,5]
double of other silicides (ꢀ10 cm V
s
).
Though with very
2
high σ, the small S led to relatively low power factor (1.28 mW
2
3
, Fe0.95Cr0.05Si
2
2
ꢂ1 ꢂ2
[5]
m
K
at RT), as shown in Figure 3c. It should be noted that
2 2 2
SrAl Si , BaSi , CaSi.
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. Conclusion
Keywords
In this work, the single phase of fully dense bulk YSi
2
was
silicides, spark plasma sintering, thermoelectric, YSi
2
successfully fabricated using a combination of arc melting and
SPS. The TEandHallmeasurementsofthe bulk YSi sample were
2
Received: October 10, 2017
Published online:
carriedout forthefirsttime. Fromtheresultsit was concluded that
the bulk YSi2 possessed the metallic characteristics. A large
concentration of electrons above the Fermi level acts as the charge
carrier. As a result, the very high electrical conductivity was found
with the relatively small Seebeck coefficient. The thermal
conductivity, dominated by the electronic contribution, was also
large. Consequently, the ZT of this material is relatively low in
comparison to other good TE silicides. Since the carrier
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Acknowledgments
This work is supported by Thailand Research Fund (TRF) in cooperation
with Khon Kaen University (RSA5980014), Royal Golden Jubilee Ph.D.
Program (PHD/0192/2556), and National Research Council of Thailand
through Khon Kaen University (6100067).
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Conflict of Interest
The authors declare no conflict of interest.
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