Fabrication and characterization of thermoelectric CrSi2 compound by mechanical alloying and spark plasma sintering

Article abstract of DOI:10.1166/jnn.2015.10379

A mixture of elemental Cr-Si powders has been subjected to mechanical alloying (MA) at room temperature to prepare CrSi₂ thermoelectric compound.The MA powders were sintered at 800-1000?°C Cunder 60 MPa using spark plasma sintering (SPS) technique. Due to the observed larger loss of Si relative Cr during ball milling, the starting composition was modified to Cr₃₀Si₇₀, Cr_31.5Si_68.5 and Cr₃₃Si₆₇ to get a single phase of CrSi₂ compound. The single phase CrSi₂ has been obtained by MA of Cr_31.5Si_68.5 mixture powders for 70 h and subsequently sintered at 1000?°C. X-ray diffraction data shows that the SPS compact sintered at 1000?°C consists of only nanocrystalline CrSi₂ compound with a grain size of 250 nm. The value of Seebeck coefficient of CrSi₂ compound increases with temperature and reaches maximum value of 245 ??V/K at 300?°C.

Full text of DOI:10.1166/jnn.2015.10379

Article
Journal of
Nanoscience and Nanotechnology
Vol. 15, 5070–5073, 2015
Copyright © 2015 American Scientific Publishers
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Fabrication and Characterization of Thermoelectric CrSi2
Compound by Mechanical Alloying and
Spark Plasma Sintering
Chung-Hyo Lee
Department of Advanced Materials Science and Engineering, Mokpo National University, Muan-gun, Chonnam 534-729, Korea
A mixture of elemental Cr–Si powders has been subjected to mechanical alloying (MA) at room tem-
ꢀ
perature to prepare CrSi thermoelectric compound.The MA powders were sintered at 800–1000 C
2
Cunder 60 MPa using spark plasma sintering (SPS) technique. Due to the observed larger loss of
Si relative Cr during ball milling, the starting composition was modified to Cr Si , Cr31ꢀ5Si68ꢀ5 and
3
0
70
Cr Si to get a single phase of CrSi compound. The single phase CrSi has been obtained by MA
3
3
67
2
2
ꢀ
of Cr31ꢀ5Si68ꢀ5 mixture powders for 70 h and subsequently sintered at 1000 C. X-ray diffraction data
shows that the SPS compact sintered at 1000 C consists of only nanocrystalline CrSi compound
ꢀ
2
with a grain size of 250 nm. The value of Seebeck coefficient of CrSi compound increases with
2
ꢀ
temperature and reaches maximum value of 245 ꢁV/K at 300 C.
Delivered by Publishing Technology to: Florida State University, College of Medicine
Keywords: Thermoelectric CrSi Compound, Mechanical Alloying, Spark Plasma Sintering.
2
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Copyright: American Scientific Publishers
1
. INTRODUCTION
properties. The effect of starting compositions on the for-
mation of a single CrSi compound was also investigated.
Chromium disilicide, CrSi is reported to be thermally
2
2
stable in air up 1050 K and therefore can be a can-
didate for thermoelectric power generation at elevated
temperatures. In general, the so-called figure of merit of
thermoelectric material can be improved by grain refine-
ment due to decreasing thermal conductivity.2–4
2
. EXPERIMENTAL DETAILS
1
ꢀ2
The MA was carried out at room temperature for a mixture
of Cr (99.9%, 60 ꢁm in size) and Si (99.999%, 150 ꢁm in
size) powders with the composition of Cr Si , Cr31ꢂ5Si68ꢂ5
3
0
70
Mechanical alloying (MA) based on solid state reaction
through severe plastic deformation at room temperature
has been proved to be an advantageous method for the
synthesis of nano-structured alloy, amorphous phase, qua-
and Cr Si . A planetary ball mill (Fritsch Pulverisette 5)
3
3
67
was used with its vial rotation of 200 rpm. The vial and
balls are made of the hardened steel (SKD11) and stainless
steel (SUS304), respectively. The total mass of powders
was about 15 g and the ratio of balls to powders was 7:1.
The structural changes of ball-milled powders have
been studied by ordinary X-ray diffraction of continuous
and step scanning mode with Cu-Kꢃ radiation. Consol-
idation of the MA powders was performed in a spark
plasma sintering (SPS) machine using graphite dies up
5
–8
sicystal and semiconducting thermoelectric compounds.
The method even allows the formation of those alloys
which are difficult to obtain via solidification processes.
The resulting powder material appears to be very homo-
geneous and, due to the large amount of induced grain
boundaries and accumulation of crystal defects, compacts
of MA thermoelectric materials show a comparable low
ꢀ
4
to 800–1000 C under 60 MPa with a heating rate of
100 C/min.The sintered compacts were subjected to den-
thermal conductivity. Therefore, MA process is believed
ꢀ
to be very effective to get CrSi thermoelectric material
2
with high performance.
sity measurement by Archimedes method.
In the present work, CrSi thermoelectric material has
The average grain size of CrSi
2
compound was also
2
been prepared by MA coupled with spark plasma sintering
evaluated by the so-called Hall plot method using a diffrac-
9
ꢀ12
(
SPS). In particular, the milling parameters and SPS condi-
tion line-width.
X-ray diffraction line-broadening from
tions are optimized so as to obtain the best thermoelectric
the equipment was calibrated with the standard Si
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J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 7
1533-4880/2015/15/5070/004
doi:10.1166/jnn.2015.10379

Lee
Fabrication and Characterization of Thermoelectric CrSi Compound by MA and Spark Plasma Sintering
2
powders. More details are described in Refs. [9, 12]. Ther-
moelectric properties, such as electrical conductivity ꢀ and
Seebeck coefficient ꢁ were measured.
3
. RESULTS AND DISCUSSION
Figure 1 shows the X-ray diffraction patterns for a mixture
of Cr and Si powders with the composition of Cr Si
3
3
67
ball-milled for various time intervals. It can be seen that
the diffraction lines associated with Cr and Si remain
essentially unchanged after 20 hours of MA without any
evidence for the formation of CrSi compound. On the
2
other hand, CrSi compound begins to appear gradually
2
after 50 hours of MA. Indeed, it can be seen that the for-
mation of single phase CrSi compound essentially com-
2
pleted after 100 h of MA. No trace of an impurity phase
or secondary phase was observed. Webelieve that a sub-
stantial decrease in intensity coupled with broadening of
Figure 2. The variation of shrinkage during SPS of Cr Si MA pow-
ders heated up to 800 C, 900 C and 1000 C, respectively.
33 67
ꢀ
ꢀ
ꢀ
the diffraction lines of CrSi compound is caused by the
2
refinement in grains and also by the accumulation of
It is found that the average grain size of CrSi compound
by MA is estimated to be 50 nm by broadening of the
diffraction line-width.
2
9
defects and strains. An average grain size of CrSi may be
2
evaluated from the so-called Hall plot using the half-width
9
ꢂ12
of the diffraction lines in Figure 1.
Here the half-width
Spark plasma sintering of the Cr Si powders ball-
3
3
67
ꢃ of the diffraction line is expressed as:
milled for 70 h was carried out at various temperatures.
ꢀ
The shrinkage behaviors during heating up to 800–1000 C
ꢃcosꢄ/ꢅ = ꢆ2ꢇ sin ꢄ/ꢅꢈ+ꢆ1/ꢉꢈ
where ꢄ is the Bragg angle, ꢅ is the X-ray wavelength,
Care shown Figure 2. The shrinkage of MA samples dur-
ing SPS consolidation process increases gradually with
ꢀ
ꢇ is the inter nDa el l si vt rea ri en da bn yd Pꢉ u bi sli sa hn in agv eT r ea gc eh ng or laoi ng ys it zo e: .Florida State University, College of Medicine
increasing temperature and significant at about 550 C. It
IP: 104.221.103.237 On: Thu, 03 Dec 2015 21:22:38
ꢀ
can be seen that compact body sintered at 1000 C has
Copyright: American Scientific Publishers
a high relative density above 96% with metallic glare on
Figure 1. X-ray diffraction patterns of Cr Si MA powders as a func-
tion of total milling time.
Figure 3. X-ray diffraction patterns for SPS sintered body of Cr Si
MA powders heated up to 800 C, 900 C and 1000 C, respectively.
3
3
67
33 67
ꢀ ꢀ ꢀ
J. Nanosci. Nanotechnol. 15, 5070–5073, 2015
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Fabrication and Characterization of Thermoelectric CrSi Compound by MA and Spark Plasma Sintering
Lee
2
the surface. Optical observations of the sample sintered at
compound is 250 nm even after sintering at a temperature
of 1000 C.
Figures 5(a)–(c) show the measured electrical conduc-
tivity ꢂ, Seebeck coefficient ꢃ and power factor PF for
ꢀ
ꢀ
1
000 C also showed no voids on the polished sections.
Figure 3 shows the XRD patterns for the SPS sintered
ꢀ
ꢀ
body of Cr Si MA powders heated up to 800 C, 900 C
and 1000 C, respectively. It is seen from Figure 3 that
all diffraction lines of SPS compacts sintered at 800 C
3
ꢀ
3
67
ꢀ
ꢀ
and 900 C can be identified as CrSi compound without
2
any trace of other phase. However, it is rather surprising
that apparently the diffraction line of Cr remains even after
ꢀ
SPS processing at 1000 C. It is clear from Figure 3 that a
single CrSi phase cannot be formed from a Cr Si MA
2
33 67
powders. So we investigated the dependence of the starting
composition on the formation of a single CrSi compound.
2
Figure 4 shows X-ray diffraction patterns for SPS sin-
tered body of Cr Si , Cr31ꢀ5Si68ꢀ5 and Cr Si MA pow-
3
0
70
33 67
ꢀ
ders sintered at 1000 C. It is seen that a single CrSi
2
compound can be formed from Cr31ꢀ5Si68ꢀ5 MA powders
ball-milled for 70 h, while the diffraction lines associated
with Si and Cr still remains for Cr Si and Cr Si MA
3
0
70
33 67
powders, respectively. This suggests that Si element rela-
tive to Cr tend to loss during MA process. The diffraction
ꢀ
lines of single CrSi compound sintered at 1000 C are
2
still broad, suggesting that the grain size of the sintered
body remains very fine. The average grain size of this sam-
ple, estimated from Hall plot of line-width is found to be
50 nm.9 It is worth noting that the grain size of CrSi2
ꢁ12
2
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Figure 5. Temperature dependence of (a) Electrical conductivity,
Figure 4. X-ray diffraction patternsfor SPS sintered body of Cr Si ,
(b) seebeck coefficient and (c) power factor for SPS compacts of Cr Si ,
3
0
70
30 70
ꢀ
Cr31ꢀ5Si68ꢀ5 and Cr Si MA powders sintered at 1000 C.
33 67
Cr31ꢀ5Si68ꢀ5 and Cr Si MA powders ball-milled for 70 h.
33
67
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J. Nanosci. Nanotechnol. 15, 5070–5073, 2015

Lee
Fabrication and Characterization of Thermoelectric CrSi Compound by MA and Spark Plasma Sintering
2
−
4
2
ꢀ
SPS compacts of Cr Si , Cr31ꢀ5Si68ꢀ5 and Cr Si MA
power factor of 9ꢀ3 × 10 W/mK at 300 C. Therefore
3
0
70
ꢀ
33 67
powders sintered at 1000 C as a function of temperature.
CrSi thermoelectric compound is a good candidate for
2
2
The PF (ꢁ ꢂ) was calculated from measured ꢁ and ꢂ
intermediate and high temperature applications. Further
study is underway to examine in more detail the thermal
conductivity related to the present nano-structured CrSi2
compound.
values. The magnitude of ꢂ for SPS compacts of Cr Si
3
0
70
and Cr31ꢀ5Si68ꢀ5 increases with increasing temperature, indi-
cating the semiconducting behaviors. In addition, Seebeck
coefficient ꢁ for SPS compacts of Cr31ꢀ5Si68ꢀ5 increases
with increasing temperature and reaches maximum value
References and Notes
. M. Mikami and K. Kobayashi, J. Alloy and Compounds 466, 530
ꢀ
of 245 ꢃN/K at 300 C. The thermoelectric power factor
1
for SPS compacts of Cr31ꢀ5Si
shows similar behavior,
68ꢀ5
(2008).
−
4
2
ꢀ
reaching peak value of 9ꢀ3×10 W/mK at 300 C, which
2. Y. Isoda, Y. Imai, and Y. Shinohara, J. Jpn. Inst. Metals 67, 410
(2003).
. L. F. Mattheiss, Phys. Rev. B: Condens. Matter 45, 3252 (1992).
. M. Umemoto, Z. G. Liu, R. Omatsuzawa, and K. Tsuchiya,
J. Metastable and Nanocrystalline Materials 8, 918 (2000).
2
is comparable to conventional thermoelectric materials.
3
4
Therefore, thermoelectric CrSi compound by MA can be
2
a promising candidate for thermoelectric power generation
at elevated temperatures.
5. C. Suryanarayana and F. H Froes, Nanostructured Mater. 3, 147
1993).
(
6
7
8
. J. Eckert and L. Schultz, J. Less-Common Metals 166, 293
(1990).
. C. H. Lee, M. Mori, and U. Mizutani, J. Non-Cryst. Solids 117, 733
4
. CONCLUSION
The effect of mechanical alloying on the formation of
CrSi thermoelectric compound was investigated. The sin-
gle phase CrSi can be obtained by MA and subsequently
(1990).
2
. C. H. Lee, M. Mori, T. Fukunaga, and U. Mizutani, Mat. Sci. Forum
88, 399 (1992).
2
SPS sintering for a Si-rich composition of Cr31ꢀ5Si68ꢀ5 mix-
ture powders, suggesting the constituent Si loss during MA
9. W. H. Hall, J. Inst. Met. 75, 1127 (1948).
10. T. B. Massalski, Binary Alloy Phase Diagrams, 2nd edn., ASM
(1990).
process. It is found that the grain size of CrSi SPS com-
2
11. F. R. de Boer, R. Boom, W. C. M. Matten, A. R. Miedema, and A. K.
pact analyzed by Hall plot method is reduced to 250 nm.
Niessen, Cohesion in Metals, North-Holland, Amsterdam (1988).
12. C. H. Lee, T. Fukunaga, Y. Yamada, Okamoto, and U. Mizutani,
J. PhaseEquilibria. 14, 167 (1993).
The present CrSi sample possesses good thermoelectric
2
properties, with a Seebeck coefficient of 245 ꢃN/K and
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IP: 104.221.103.237 On: Thu, 03 Dec 2015 21:22:38
Copyright: American Scie Rn te i cf i ec i vP e ud b: l8i s hN eo rvs ember 2013. Accepted: 29 May 2014.
J. Nanosci. Nanotechnol. 15, 5070–5073, 2015
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