A study of the effect of aluminum on MoSi2 formation by self-propagation high-temperature synthesis

Article abstract of DOI:10.1016/j.jallcom.2010.04.159

In this study, the self-propagation high-temperature synthesis (SHS) for the formation of MoSi₂ in the presence of aluminum was investigated. Test specimens with nominal compositions including (Mo + 2Si) with stoichiometric ratio and (Mo + 2Si) + 5 wt% Al were employed. Each of the different samples was tested for SHS to determine the thermal profile and differential thermal analysis (DTA) under air atmosphere. The registered thermal profiles resulted for the two samples showed the maximum temperature increase (about 400 °C) in the combustion front in the presence of aluminum. The DTA results also showed that the area of silicon melting point on the DTA curve was deleted in the presence of aluminum. On the other hand, the X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) studies indicated that the SHS reaction between Mo and Si was done in a more complete manner in the presence of aluminum and aluminum particles were all oxidized.

Full text of DOI:10.1016/j.jallcom.2010.04.159

Journal of Alloys and Compounds 502 (2010) 80–86
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
A study of the effect of aluminum on MoSi formation by self-propagation
2
high-temperature synthesis
∗
S. Hasani, M. Panjepour , M. Shamanian
Department of Materials Engineering, Isfahan University of Technology, 84156-83111 Isfahan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, the self-propagation high-temperature synthesis (SHS) for the formation of MoSi2 in the
presence of aluminum was investigated. Test specimens with nominal compositions including (Mo + 2Si)
with stoichiometric ratio and (Mo + 2Si) + 5 wt% Al were employed. Each of the different samples was
tested for SHS to determine the thermal profile and differential thermal analysis (DTA) under air atmo-
sphere. The registered thermal profiles resulted for the two samples showed the maximum temperature
Received 10 January 2010
Received in revised form 13 April 2010
Accepted 23 April 2010
Available online 5 May 2010
◦
increase (about 400 C) in the combustion front in the presence of aluminum. The DTA results also showed
Keywords:
Combustion synthesis
MoSi2 formation
Self-propagation high-temperature
synthesis
Aluminum
that the area of silicon melting point on the DTA curve was deleted in the presence of aluminum. On the
other hand, the X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spec-
trometer (EDS) studies indicated that the SHS reaction between Mo and Si was done in a more complete
manner in the presence of aluminum and aluminum particles were all oxidized.
© 2010 Elsevier B.V. All rights reserved.
The oxidation of aluminum
1
. Introduction
tion as an enforcing agent constituent in MoSi /Al O composite or
2
2
3
as an intermetallic factor Mo(Si1 ,Alx) . However, no attention has
−x
2
Properties of molybdenum disilicide (MoSi ) such as high heat
been paid to the role of this element during the SHS process so far.
So, it has been tried in this research to perform X-ray diffraction
(XRD), scanning electron microscope (SEM), differential thermal
analysis (DTA), and on the other hand, the thermal profile to study
the manner of aluminum affecting the process of the performance
of MoSi2 by SHS as a thermal source by employing microscopic
studies.
2
◦
resistance, high melting point (2020 C), high electrical conduc-
tivity, and excellent corrosion resistance at high temperatures all
have resulted in an ever-expanding application range of this mate-
rial in different industries [1–4]. The production of this material is
done through different techniques. Of these techniques which have
been of interest to many researchers, is the self-propagation high-
temperature synthesis (SHS). In fact, intense heat resulting from the
reaction between molybdenum and silicon provides the conditions
for the production of molybdenum disilicide by this technique [5].
Many research activities have been performed on Mo–Si system
SHS without or with the presence of a third element from different
aspects so far. For example, in 1995, Seetharama [6] studied the
2
.
Materials and experimental methods
All the specifications of raw materials such as molybdenum, silicon, and alu-
minum powders are presented in Table 1. Two sample groups were prepared to
study the effect of the presence of aluminum on the process of SHS for the forma-
tion of MoSi2. One group with stoichiometric mixture of Mo + 2Si (Mo: 33.3, Si: 66.7
in atomic percent) and another group with powder mixture of (Mo + 2Si) + 5 wt% Al
named (MSA-0) and (MSA-5), respectively. The samples were compacted within a
penetration reactions in MoSi by SHS process. Jo et al. [7] in 1996
2
studied and investigated the SHS performing mechanism in Mo–Si
system. The structure of combustion wave during the production
2
cylindrical steel frame under 180 kg/cm . After compacting the samples, the average
diameter and height of the produced discs were 12.3 mm and 20 mm, respectively
Fig. 1). Also, oxy-acetylene flame was employed to provide the ignition tempera-
of MoSi , by stimulated (mechanically) SHS process was studied by
2
(
Gras et al. [8] in 2006. Alongside with these studies, the synthesis of
ture for the initiation of the reaction. The samples were first placed on a metal tripod
and then were exposed to oxy-acetylene flame from the higher end. With the start
of the reaction in the sample, the flame was turned off and due to the propagation
of the combustion front in the sample, the reactive materials were transformed into
products. According to Fig. 1, the thermal profile of the samples was obtained by a
B-type thermocouple during the SHS. A data acquisition with a frequency of 100 kHz
was employed for the collection of the thermocouple signals and their transfer to a
computer.
Mo(Si,Al) /SiC, MoSi /Al O , MoSi /WSi and, MoSi /Mo5Si were
2
2
2
3
2
2
2
3
also studied [9–12]. But what is important is that during all the
studies performed on the formation of MoSi by SHS process in the
2
presence of aluminum, this element was only taken into considera-
In the next stage, the samples were studied for phase identification by XRD anal-
ysis (MPD-XPERT, Philips) and for microstructure by SEM (XL30 SERIES, Philips). The
SEM was equipped with an energy dispersive spectrometer (EDS). The samples also
∗ Corresponding author. Tel.: +98 3113915739; fax: +98 3113912752.
E-mail address: panjepour@cc.iut.ac.ir (M. Panjepour).
0
925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.04.159

S. Hasani et al. / Journal of Alloys and Compounds 502 (2010) 80–86
81
Fig. 1. Schematic representation of thermal profile for the samples obtained.
Table 1
sample. It is also noticed that the rate of the sample cooling after
the passage of the combustion front is less in MSA-5 relative to
that of MSA-0. The smooth thermal gradient of the temperature
profile diagram for MSA-5 sample relative to MSA-0 supports it
pretty well. With regard to these facts, it can be inferred that the
presence of aluminum effectively results in the increase of the
The specifications of raw materials.
Powder material
Particles size (␮m)
Purity (%)
Company
Si
Mo
Al
1–10
1–40
100–200
99
99.9
99
Merck
Merck
Aldrich
◦
maximum temperature at the combustion front (about 400 C), the
intense increase of the propagation rate of combustion front, and
the decrease of cooling rate.
underwent differential thermal analysis (DTA) by the STA device (L70/2171, Linseis)
to complete the results and to examine the effect of the presence of aluminum on
the above SHS more precisely. The important parameters in this experiment such as
heating rate, maximum heating temperature, and the weight of the samples were
◦ ◦
0 C/min, 1200 C, and 36 mg, respectively.
2
3
. Results and discussion
3.1. Thermal profiles
Fig. 2 depicts the recorded thermal profile during SHS reaction
for the two samples of MSA-0 and MSA-5.
As it is observed in the figure, the maximum temperatures
recorded by thermocouple for samples MSA-0 and MSA-5 are
◦ ◦
1
020 C and 1400 C, respectively. On the other hand, as it is
observed, the maximum recorded temperature for the thermal pro-
file of MSA-5 sample has been done in less time relative to MSA-0
sample. So, it can be inferred that the rate of the propagation of
combustion front in MSA-5 sample is more than that in MSA-0
Fig. 3. XRD patterns after SHS synthesis for (a) MSA-0 and (b) MSA-5.
Fig. 2. The results of thermal profile for MSA-0 and MSA-5 during SHS reaction.

8
2
S. Hasani et al. / Journal of Alloys and Compounds 502 (2010) 80–86
Fig. 4. (a) A SEM micrograph of MSA-0 after the SHS synthesis, (b) EDS pattern of the specified point.
3.2. Phase analysis
can be inferred that the obtained product through this SHS in real
conditions is only the MoSi phase. As it is observed, it is reasonably
2
The products of the synthesized samples were analyzed through
confirmed by XRD results (Fig. 3a and b).
XRD to study the effect of the presence of aluminum on SHS process
of MoSi2 formation. The XRD patterns for the obtained products
resulting from the synthesis of the two samples of MSA-0 and MSA-
3.3. Microstructural observations
5
are depicted in Fig. 3. In Fig. 3a concerning the results obtained
Figs. 4a and 5a present the SEM micrographs of MSA-0 and MSA-
, respectively. While, the EDS analysis for the points specified on
the SEM images are shown in Figs. 4b and 5b–d.
According to Fig. 4a and b, the equiaxed MoSi2 grains can be
deduced in the products of MSA-0 sample.
from the XRD pattern of the products related to MSA-0 sample, the
MoSi2 phase along with an amount of the remaining raw materi-
als such as molybdenum and silicon are observed. While in Fig. 3b
related to the result obtained from the XRD pattern of the products
resulting from the MSA-5 sample, the MoSi phase along with Al O
is observed and no trace of the unreacted raw materials is seen. This
can be an indication of fact that in the presence of aluminum, the
reaction takes place more completely and on the other hand, the
aluminum present in the raw materials is entirely under the oxi-
5
2
2
3
Fig. 5b–d is related to the EDS analysis for points 1, 2, and 3,
respectively. The equiaxed MoSi grains (Fig. 5b) along with Al O
2
2
3
particles (Fig. 5c) formed in the products of MSA-5 sample are fairly
observed due to these results. Furthermore, some of the MoSi par-
2
ticles imprisoned within the Al O particles are fairly observed in
dation process and is oxidized into Al O3 during the synthesizing
2
3
2
process between the particles of molybdenum and silicon.
The important thing here is the lack of intermetallic phase of
Mo5Si3 among the products. The cause of this phenomenon has
been investigated by Seetharama [6]. He showed that at heating
rates over 100 C/min, only MoSi phase is formed and it is only at
heating rates lower than 100 C/min that Mo5Si phase is formed.
But as it is obvious from the recorded thermal profiles in Fig. 2, the
heating rate is very higher than this value (100 C/min), therefore it
2
Fig. 5d. The presence of MoSi2 particles imprisoned within Al O3
in a way proves the melting condition of Al O particles when the
2
3
combustion front is passing.
Therefore, according to these figures, it can be inferred that in
the presence of aluminum, the temperature of the combustion front
◦
2
◦
at least exceeds 2054 C (melting point of Al O ) in practical con-
2 3
◦
3
ditions. In fact, this temperature increase improves the conditions
◦
for the formation of MoSi during the SHS.
2

S. Hasani et al. / Journal of Alloys and Compounds 502 (2010) 80–86
83
Fig. 5. (a) A SEM micrograph of MSA-5 after the SHS synthesis and EDS patterns of the specified points (b) 1, (c) 2 and (d) 3.

8
4
S. Hasani et al. / Journal of Alloys and Compounds 502 (2010) 80–86
Table 2
Enthalpies of reactions for the formation of MoSi2 in the presence of aluminum (air atmosphere).
Reaction
ꢀHi
ꢀHi (kJ mol−1)
Ref.
[
[
[
[
[
Mo(s) + 2Si(s) + 0.28Al(s) + 0.21O2(g)]298 → [MoSi2(s) + 0.14Al2O3(s)]298
MoSi2(s) + 0.14Al2O3(s)]298 → [MoSi2(s) + 0.14Al2O3(s)]2293
MoSi2(s) + 0.14Al2O3(s)]2293 → [MoSi2(l) + 0.14Al2O3(s)]2293
MoSi2(l) + 0.14Al2O3(s)]2293 → [MoSi2(l) + 0.14Al2O3(s)]2327
MoSi2(l) + 0.14Al2O3(s)]2327 → [MoSi2(l) + 0.14Al2O3(l)]2327
ꢀH1
ꢀH2
ꢀH3
ꢀH4
ꢀH5
−371.529
202.928
85.001
1.730
[13,14]
[13]
[15]
[13]
[13]
15.551
Tad
ꢀ
l
p
l
p
[MoSi2(l) + 0.14Al2O3(l)]2327 → [MOSi2(l) + 0.14Al2O3(s)]Tad
ꢀH6
[C (MoSi2) + 0.14 ∗ C (Al2O3)]dt
[13,16]
2
327
3.4. Determination of adiabatic temperature
required thermodynamic data for enthalpy of each stage are pre-
sented in Table 3.
Theoretical computations can be used for the determination of
Mo(s)+2Si(s) → MoSi2(s)
(4)
adiabatic temperature in the presence of aluminum for an initial
prediction and then with the help of thermal analysis studies, the
manner of the effect of the presence of aluminum is also studied.
Therefore, computation of the adiabatic temperature of the reaction
in different conditions from the view of the presence of aluminum
is studied in continuation.
First, it is important to note that aluminum can play two differ-
ent roles depending on the type of the prevailing atmosphere as
follows:
With the help of thermodynamic data presented in Table 3 as
well as with the help of Eq. (5), the existing adiabatic temperature
in combustion front was equal to 2000.8 K in this state as well.
ꢀ
T
ad
s
p
ꢀ
H = ꢀH + ꢀH2 = −136.9316 +
[C (MoSi )]dt = 0
(5)
1
2
2
98
The obtained adiabatic temperature in this research is compared
with the values of other references in Table 4. As it is observed, there
is a good conformity between the results obtained from different
references and this study.
Theoretical computations of adiabatic temperature show that
if aluminum is present in the raw material mixture and oxygen is
present in the prevailing atmosphere, this parameter is intensely
increased in such a way that the combustion front temperature
3
.4.1. Air atmosphere
If the reaction takes place in air atmosphere, the reaction in com-
bustion front will take place in the manner of reaction (1). ꢀH = 0
is taken into consideration to obtain the adiabatic temperature in
the reaction (Eq. (2)). It is required to mention that the amount of
the existing aluminum by its weight percent was obtained to be
equal to 0.28 mole for every mole of molybdenum. On the other
hand, as it was observed in reaction (1), the amount of oxygen is
only considered on the basis of aluminum oxidation reaction
(
(
Tad = 3462 K) is more than the melting temperature of Al O3
2
2352 K). A microscopic study performed by SEM (as Figs. 4 and 5)
properly supports this theory.
Mo(s) + 2Si(s) + 0.28Al(s) + 0.21O2(g) → MoSi2(l) + 0.14Al O
(1)
(2)
3.4.2. Oxygen-free atmosphere
2
3(s)
If the reaction takes place in an oxygen-free atmosphere, alu-
minum particles do not enter the reaction and the reaction of SHS
takes place as per reaction (6)
ꢀH = 0 ⇒ Hreactants|298 = Hproduct|Tad
Different stages of the reaction during these conditions and pro-
cesses of the conversion of raw materials to products along with
the required data for the enthalpy at each stage are presented in
Table 2.
Under these conditions and with regard to the data presented
in Table 2, the adiabatic temperature of the system was computed
by Eqs. (2) and (3) to be 3462 K.
Mo(s) + 2Si(s) + 0.28Al(s) → MoSi2(l) + 0.28Al(l)
(6)
Different stages of the reaction and conversion of the raw mate-
rials to products along with data regarding the enthalpy at each
stage are observed in Table 5.
With regard to the enthalpy values presented in Table 5 and
with the help of Eq. (7), the temperature of adiabatic in this state is
calculated at 1736 K.
ꢀ
H = ꢀH + ꢀH + ꢀH + ꢀH + ꢀH5 + ꢀH = 0
(3)
1
2
3
4
6
But, in the absence of aluminum, the SHS reaction takes place
in accordance with reaction (4). Its different stages along with the
ꢀ
H = ꢀH + ꢀH + ꢀH + ꢀH = 0
(7)
1
2
3
4
Table 3
Enthalpies of reactions for the formation of MoSi2.
Reaction
ꢀHi
ꢀHi (kJ mol−1)
−136.932
Ref.
[14]
[Mo(s) + 2Si(s)]298 → [MoSi2(s)]298
ꢀH1
Tad
ꢀ
s
[MoSi2(s)]298 → [MoSi2(s)]Tad
ꢀH2
[C (MoSi2)]dt
[13]
p
2
98
Table 4
Compare the computed adiabatic temperature of the formation MoSi2 by SHS in different references.
Ref.
[8]
[17] and [18]
[19]
[20]
∼1943
This work
2000.8
Tad
∼1910
∼2000
∼1900

S. Hasani et al. / Journal of Alloys and Compounds 502 (2010) 80–86
85
Table 5
Enthalpies of reactions for the formation of MoSi2 in the presence of aluminum (oxygen-free atmosphere).
Reaction
ꢀHi
ꢀHi (kJ mol−1)
Ref.
[
[
[
Mo(s) + 2Si(s) + 0.28Al(s)]298 → [MoSi2(s) + 0.28Al(s)]298
MoSi2(s) + 0.28Al(s)]298 → [MoSi2(s) + 0.28Al(l)]933
MoSi2(s)+0.28Al(l)]933 → [MoSi2(s) + 0.28Al(l)]933
ꢀH1
ꢀH2
ꢀH3
−136.932
50.979
10.711
Tad
[14]
[13]
[13]
ꢀ
s
p
l
p
[MoSi2(s) + 0.28Al(l)]933 → [MoSi2 + 0.28Al(l)]Tad
ꢀH4
[C (MoSi2) + 0.28 ∗ C (Al)dt]
[13]
9
33
In this state, aluminum plays the role of a diluting agent,
reactions in these regions:
so that its presence only results in an adiabatic temperature
drop. Therefore, according to Merzhanov standard, the above
process cannot self-sufficiently progress under these conditions
Region I: In this region an exothermic peak related to the
solid–solid reaction is observed between molybdenum
and silicon particles which are in contact with each
other.
[
21].
Region II: In this region an endothermic peak related to the melt-
ing of a part of silicon particles is observed.
3.5. Differential thermal analysis (DTA)
Region III: An exothermic peak related to the dissolution of molyb-
Ultimately, to confirm the results obtained from the effect of
denum particles in silicon melt and formation of MoSi2
composition is also observed in this region.
aluminum on the trend of MoSi2 formation during SHS, a powder
mixture of the two samples of MSA-0 and MSA-5 went under DTA
with the results depicted in Fig. 6.
As it is observed in Fig. 6a (DTA curve represents the MSA-0 sam-
ple), the curve is divided into three basic regions with the following
Moreover, in Fig. 6b (DTA curve depicts the MSA-5 sample), the
curve is divided into two basic regions:
Region I: In this region an exothermic peak related to the
solid–solid reaction is observed between molybdenum
and silicon particles which are in contact with each other
and the oxidation of aluminum particles.
Region II: An exothermic peak related to the dissolution of molyb-
denum particles in silicon melt and formation of MoSi2
composition is observed in this region (according to
Seetharama studies [6]).
Thus, according to the curves in Fig. 6a and b, the diagrams
obtained from the two samples have some differences which are
pointed to as follows:
I. The DTA curve related to the MSA-0 sample includes three
regions, while the DTA curve related to the MSA-5 consists of
two regions. This is because of the deletion of the peak related
to silicon melt in the presence of aluminum in Fig. 6b. In fact,
it is the overlapping of the melt endothermic peak with the
peak related to the oxidation of aluminum particles (exothermic
reaction).
II. The region I in DTA curve for the MSA-5 (Fig. 6b) sample starts at
◦
lower temperatures (about 50 C) relative to the MSA-0 sample
(Fig. 6a). In fact, the cause of this phenomenon is the start of
the oxidation of aluminum particles in this region followed by
the sample temperature increase resulting in the improvement
of the conditions for the start of penetrative reaction between
molybdenum and silicon particles in solid state.
III. The peak related to the formation of MoSi for the MSA-5 sam-
2
ple (the II region in Fig. 6b) has started at lower temperatures
◦
(
about 80 C) than the MSA-0 sample (the III region in Fig. 6a).
This phenomenon is also due to aluminum oxidation reaction
and the heat liberated from this reaction in the MSA-5 dur-
ing SHS process. In other words, the liberated heat from this
reaction creates conditions for the formation of MoSi at lower
2
temperature.
Therefore, as it was observed in these results, when the condi-
tions for the oxidation reaction of aluminum particles in the MSA-5
sample are prepared, the heat resulting from this reaction can have
a role as a thermal source in the combustion front. This results
Fig. 6. Variations of DTA curve for (a) MSA-0 and (b) MSA-5.

8
6
S. Hasani et al. / Journal of Alloys and Compounds 502 (2010) 80–86
in a temperature increase in the combustion front so that it can
bring about appropriate conditions for better completion of the SHS
reaction (according to Fig. 3). In other words, aluminum has varied
• A part of the liberated heat resulted from aluminum oxidation
will provide the heat required for melting of a part of silicon parti-
cles. Another part of this liberated heat increases the temperature
of the sample resulting in the acceleration of the formation of
effects on the formation trend of MoSi during SHS process due to
2
its oxidation reaction. For example, it can be referred to adiabatic
temperature and movement rate in combustion front, reduction of
the amount of the unreacted raw materials, and the starting time
of the reactions which have been discussed in this study.
MoSi composition and this composition will form with shorter
durations.
2
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[
[
[
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. Conclusion
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[
[
[
[
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2
3
2
353–2360.
•
Intense changes will be observed in the thermal profile of the
samples in the presence of aluminum. These changes are in a
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rate of the movement of combustion front and reduction rate
of cooling of the samples after combustion are observed by an
intense increase of the maximum temperature.
[
[
12] G.-J. Zhang, X.-M. Yue, T. Watanabe, J. Mater. Sci. 35 (2000) 4729–4733.
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[
[
[
•
We will face a significant reduction in the raw materials in the
products due to the liberation of the heat resulting from alu-
[
[
minum oxidation reaction during the formation of MoSi by SHS
2
[
[
process. In other words, the presence of aluminum in the mix-
ture of raw materials and its oxidation results in an increase in
the purity of the products.