Combustion synthesis of molybdenum disilicide (MoSi2) fine powders

Article abstract of DOI:10.1111/j.1551-2916.2008.02544.x

The combustion characteristics of a MoO₃+SiO₂+Mg system diluted with sodium chloride have been studied in order to synthesize molybdenum disilicide (MoSi₂) fine powders. The effect of processing variables, including the sodium chloride content and the reactants proportion on the phase composition and microstructure of the final products, was obtained. Both the combustion temperature and the wave velocity decreased with an increase in the amount of sodium chloride. With the appropriate processing parameters, the reacted product consisted of solid crystalline MoSi₂, and an MgO phase was prepared in the molten sodium halide matrix at 1100°-1350°C. The final product purification resulted in MoSi₂ powder with a particle size of 100-500 nm. The mechanism of the MoSi₂ formation in the sodium chloride matrix is discussed by analyzing the phase evaluation in the extinguished combustion wave.

Full text of DOI:10.1111/j.1551-2916.2008.02544.x

J. Am. Ceram. Soc., 91 [9] 2802–2807 (2008)
DOI: 10.1111/j.1551-2916.2008.02544.x
r 2008 The American Ceramic Society
ournal
J
Combustion Synthesis of Molybdenum Disilicide (MoSi2) Fine Powders
Hayk H. Nersisyan,w Hyung Il Won, and Chang Whan Won
RASOM, Chungnam National University, Daejeon 305-764, South Korea
The combustion characteristics of a MoO31SiO21Mg system
diluted with sodium chloride have been studied in order to syn-
thesize molybdenum disilicide (MoSi2) fine powders. The effect
of processing variables, including the sodium chloride content
and the reactants proportion on the phase composition and mi-
crostructure of the final products, was obtained. Both the com-
bustion temperature and the wave velocity decreased with an
increase in the amount of sodium chloride. With the appropriate
processing parameters, the reacted product consisted of solid
crystalline MoSi2, and an MgO phase was prepared in the mol-
ten sodium halide matrix at 11001–13501C. The final product
purification resulted in MoSi2 powder with a particle size of
100–500 nm. The mechanism of the MoSi2 formation in the so-
dium chloride matrix is discussed by analyzing the phase eval-
uation in the extinguished combustion wave.
The current work studies the effects of the sodium chloride
addition on the combustion synthesis characteristics and MoSi2
formation from the MoO31SiO21Mg system.
II. Experimental Procedure
MoO3, SiO2, Mg, and NaCl reactant powders were used, and
their specifications are shown in Table I. These powders were
first weighed and then thoroughly mixed by ball-milling for at
least 12 h. The mixed powder (150–200 g) was hand pressed into
a metallic cup 5 cm in diameter and 10 cm in height. The ex-
perimental density of the pellets was 1–1.5 g/cm3. The Ti1C
powder, about 3 g, was placed at the top of the raw material
mixture as an ignition agent.
The combustion synthesis reactions are:
I. Introduction
HE properties of molybdenum disilicide (MoSi2) have been
T
established, including a high melting point (20301C), excel-
lent high-temperature oxidation resistance, and high electrical
and thermal conductivity. Because of its superior property com-
bination, MoSi2 has received considerable research attention in
recent years as a potential candidate material for high-temper-
ature structural applications.1 A number of studies have been
attempted to prepare MoSi2 powder, using many processing
routes including solid-state reaction,2 spray forming,3 mechan-
ical alloying,4 and self-propagating high-temperature synthe-
sis.5,6 However, like other intermetallics, conventional coarse-
grained MoSi2 is brittle at normal environmental temperatures.1
Refinement of the grain sizes to nanometer dimensions is pre-
dicted7,8 to improve the ductility, the fracture toughness, and the
strength of intermetallics by inducing fundamental changes in
the strengthening and deformation mechanisms.9 Before the
present work, nanocrystalline MoSi2 was prepared using the
sonochemical synthesis method.10 In the sonochemical method
co-reduction of MoCl5 and SiCl4 with NaK alloys in hexane was
carried out using 600 W, 20 kHz irradiation. The precipitate was
annealed at 9001C, and nanocrystalline MoSi2 with a particle
size 16–31 nm was obtained. Generally, the sonochemical
method is limited in scale up, and only small amounts (a few
grams) are typically produced by this method. One of the routes
to prepare nanocrystalline MoSi2 powder is the mechanical al-
loying of silicon and molybdenum.11 In this method a-MoSi2
powder was obtained after 50 h of milling time. When the mill-
ing time was further increased to 96 h, a complete transforma-
tion of a-MoSi2 to b-MoSi2 was found. The authors concluded
that the phase-transformation phenomenon is determined by the
crystallite size of a-MoSi2. When the size of previously formed
a-MoSi2 decreases to a certain critical particle dimension (ap-
proximately 8 nm), it transforms to the b-MoSi2.
Here, a is the mole number of NaCl, and its content in all the
samples of the current study was 5–14 mol. b shows the mole
fraction of SiO2, and it was varied in the 2rbr3 interval. The
change in b affects the Mg content, because SiO2 and Mg are
balanced with a 1:2 ratio. b52 represents the stoichiometric
composition for MoSi2 synthesis. However, along with the
MoSi2 phase, a sufficient amount of Mo5Si3 (10–20 wt%) re-
sulted also by the combustion reaction at b52. By including an
additional portion of SiO212Mg mixture in the initial system,
(b 53) both the silicon concentration and the reactivity of the
process increased, which led to the elimination of Mo5Si3 from
the reaction product. Also, when b 53, a defined amount of un-
reacted silicon was detected in the final products. After the ex-
periments, the reaction products were subjected to a hydrochloric
acid treatment and were washed by distillated water to eliminate
MgO and NaCl. Free silicon containing the reaction products
were additionally treated by a 10% solution of NaOH.
The details of the experimental setup are shown in Fig. 1.
Briefly, the setup included a combustion chamber and an auto-
mated ignition and data acquisition system. The metallic cup
with a green mixture was loaded into the combustion chamber
by placing it directly on top of a nickel–chromium coil. Ignition
was achieved by using power input into the nickel–chromium
coil in a high-purity argon atmosphere of 2.0 MPa. The data
acquisition system (DASTC, Keithley) continuously recorded
the thermocouples’ time histories. Two tungsten–rhenium ther-
mocouples (W/Re-5 vs W/Re-20, 50 and 100 mm in diameter),
coated by a thin layer of Al2O3, were used for temperature
measurements. The combustion parameters examined were the
combustion temperature (Tc) and the combustion wave propa-
gation velocity (Uc). The combustion velocity was determined
from the temperature profiles and the known spacing between
the thermocouples.
B. Derby—contributing editor
The final products were characterized by using a CuKa, Sie-
mens D5000 X-ray diffractometer (Siemens, Karlsruhe, Ger-
many). Scanning electron microscopy (SEM; JSM 5410, JEOL,
Tokyo, Japan) was used to examine the morphology, shape, and
Manuscript No. 24122. Received December 19, 2007; approved May 14, 2008.
wAuthor to whom correspondence should be addressed. e-mail: haykrasom@
hotmail.com
2802

September 2008
Molybdenum Disilicide (MoSi2) Fine Powders
2803
Table I. Reactant Properties
3000
2250
1500
750
0
10
Particle size
Reactant
Manufacturer
(mm)
Purity (wt%)
8
6
4
2
0
Silicon
dioxide
Magnesium
Junsei Chemical Co. o10.0
499.0
Ltd
Daejung Chem. and
Metals, Korea
50–200
98.5
Molybdenum Yakuri Pure
oxide
o1.0
499.0
Chemical
Co. Ltd, Japan
Samchun Pure
Chemical Co. Ltd,
Korea
Sodium
chloride
o100
99.5
0
2
4
6
8
10
size of the synthesized MoSi2 powder. The specific surface area
and the oxygen concentration measurements were performed
using ‘‘Coultert SA3100 Surface Area Instrument’’ (Beckman
Coulter Inc., Fullerton, CA) and the oxygen analysis technique.
Fig. 2. Thermodynamic analysis of the MoO312SiO217Mg1aNaCl
system (P5 2.0 MPa).
by the heating, melting, and evaporation of NaCl, therefore, the
adiabatic combustion temperature drops from a. Also, Fig. 2
shows that at ao4 the combustion temperatures are not only
higher than the melting points of most raw materials, but also
higher than the melting point of MoSi2 (Tm 5 20301C). At that
multiphase reaction, products consisting of Mo5Si3, Mg2SiO4,
SiO, Mg, MgO, and NaCl phases have been predicted at ao4.
When more than 6 mol NaCl is added, the formation of MoSi2
phase with MgO and NaCl become thermodynamically favored.
A series of the combustion experiments was conducted in the
MoO312SiO217Mg1aNaCl system to reveal the effect of add-
ing NaCl on the phase purity and morphology of MoSi2. X-ray
diffraction (XRD) patterns of purified combustion products
prepared from 5, 7, and 10 mol NaCl containing green mix-
tures and the corresponding values of experimentally measured
combustion temperatures are shown in Fig. 3. All of the major
XRD peaks were found to match with standard peaks of MoSi2.
However, the XRD patterns of the final products show a visible
amount of Mo5Si3, also, regardless of the salt concentration and
the combustion temperature.
III. Results and Discussion
(1) The Combustion of the MoO312SiO217Mg1aNaCl
System
The behavior of the NaCl-assisted combustion reactions is dif-
ferent, essentially, from those of the traditional combustion pro-
cess. In our recent publications, the preparation of the
nanocrystalline powders of transition metals (Ti, W, Ta) and
appropriate carbides (TiC, WC) were reported using a sodium
halide-assisted combustion reaction.12–14 The effect of sodium
chloride on the combustion characteristics of the MoO31
2SiO217Mg1aNaCl system was investigated for the first time.
Before the experiments, the MoO312SiO217Mg1aNaCl sys-
tem was analyzed (in a preliminary fashion) by a commercially
available thermodynamic program called ‘‘THERMO’’ that is
specially designed for combustion processes.15 The program cal-
culates the equilibrium composition, the phase, and the adia-
batic combustion temperature for multi-component systems at a
set of defined temperature and/or pressure conditions. Figure 2
shows the equilibrium phases and the corresponding adiabatic
combustion temperatures (Tad). The initial temperatures of the
reagents and the reaction pressure were set to be 251C and 2.0
MPa, respectively. As shown in Fig. 2, the adiabatic combustion
temperature of the MoO312SiO217Mg ternary system (a 5 0) is
predicted to be 26171C. After adding NaCl, the heat released per
unit volume decreased. The reaction heat is partially consumed
The SEM micrographs of combustion products derived with
5, 7, and 10 mol NaCl and leached by acid are shown in Fig. 4.
As seen by comparing Figs 4(a), (b), and (c), the product mor-
phology is influenced significantly by the concentration of salt.
As a result, the MoSi2 powder obtained from 10 mol NaCl
(containing mixture) has smaller characteristic size dimensions
compared with the MoSi2 powders obtained from 5 and 7 mol
1
1
1
2
1
1
3
1
1
(c)
(b)
1
2
2
1
2
2
2
2
1
2
1
2
2
Temperature profiles
t
Thermocouple
(a)
4
5
20
30
40
50
60
70
80
2θ (°)
Fig. 3. X-ray diffraction patterns of the MoSi2 powders produced from
the MoO312SiO217Mg1aNaCl mixture: (a) a 5 5; (b) a 57; (c)
a 5 10.
1: SHS Reactor;
2: Data logger;
Fig. 1. Schematic representation of the experimental apparatus.

2804
Journal of the American Ceramic Society—Nersisyan et al.
Vol. 91, No. 9
Fig. 4. Scanning electron microscopy images of the MoSi2 powders produced from the MoO312SiO217Mg1aNaCl mixture: (a) a 5 5; (b) a 5 7; (c)
a 5 10.
NaCl (containing mixtures). The fine structure of MoSi2 shown
in Fig. 4(c) might be attributed more to the combustion tem-
perature, which is relatively low (13001C), than to the melting
point of silicon (14101C).
particles of molybdenum silicides and MgO (b 5 3) is clearly
seen in the micrograph as shown in Fig. 6(a). Figure 6(b) shows
the morphology of the final product particles after purification.
It is noted that large size and corrugated form agglomerates had
apparently formed due to chemical conversions, which occurred
on the surface of the melted Mg particles. These agglomerates
show porosity morphology and consist of smaller size particles
(Fig. 6(b), upper right side). These agglomerates have low me-
chanical strength, and after the short time ball milling, trans-
form to fine powder (Fig. 6(c)).
Figure 7 shows the XRD patterns of purified samples syn-
thesized at different b levels. The major diffraction peaks are
attributed to the MoSi2 phase. Some minor peaks, detected at
b 5 2 and b 5 2.5 (Figs 7(a) and (b)), were identified as the
Mo5Si3. However, when b increased, the intensity of the Mo5Si3
peaks decreased and became invisible at b 5 3 (Fig. 7(c)). Si-
multaneously, new minor peaks were found at b 5 3 and iden-
tified as the Si. The formation of silicon is attributed to an excess
of SiO2, which was used in the combustion experiment. Free Si
was easily dissolved into a 10% solution of NaOH, and phase
pure MoSi2 was obtained (Fig. 7(d)).
The effect of the NaCl concentration (a) on the combustion
characteristics of the MoO313SiO219Mg1aNaCl system and
the morphology of MoSi2 powder were also studied (Fig. 8). As
can be seen, the combustion temperature drops from 15501 to
11001C when a changes from 8 to 13.5. The diminishing char-
acter of the combustion process observed at a 5 14 had resulted
in a partially combusted sample. Above this point an attenua-
tion of the combustion occurred. Conditionally, the investigated
interval of can be divided into two zones: (1) a high-temperature
zone, in which the combustion temperature is higher than the
silicon melting point (14101C), and (2) a low temperature zone,
in which the combustion temperature is lower than the silicon
melting point. A passing from one temperature zone to the other
took place at a 5 9.5, and the combustion wave velocity showed
an abrupt increase during this transition (Fig. 8).
Mo5Si3, as a by product phase, has been reported to be pres-
ent, in a small amount, in powder processed MoSi2.16–18 In our
system, a minor content of Mo5Si3 phase in the final products
can be attributed to the initial system heterogeneity. Conse-
quently, two approaches to eliminate the residual Mo5Si3 from
the final product were tested. First, we used small particle size
SiO2 and Mg powders in the combustion experiments and, sec-
ond, we increased the content of SiO2 and Mg in the initial
mixture. Generally, no positive change in the Mo5Si3 content
was found when small size SiO2 (o1 mm) and Mg (B50 mm)
powders were used in the combustion experiments. The second
approach, which was tested on the MoO31(312b)Mg1
bSiO2110NaCl initial system, was found to be effective to re-
duce the Mo5Si3 content in the final product. First, we examined
the effect of b on the combustion parameters (Tc and Uc). The
heat released per volume increased from b, therefore, both the
combustion temperature and the wave velocity increased. These
features are shown in Fig. 5. Also, Fig. 5 shows that the com-
bustion temperatures are lower than the melting points of Mo,
Si, Mo5Si3, and MoSi2, but higher than the melting point of
NaCl. The formation of a molten matrix of NaCl with solid
1400
1350
1300
1250
1200
0.1
0.08
0.06
0.04
0.02
The average size and the specific surface area of MoSi2 par-
ticles prepared by the NaCl-assisted combustion reaction (b 5 3,
a 5 10–13) were in the 100–500 nm and 25–30 m2/g ranges, re-
spectively. The average concentration of oxygen in the MoSi2
powder was about 1.0–1.5 wt%.
1.5
2
2.5
3
3.5
β
, mol
Fig. 5. The combustion parameters of the MoO31(312b)Mg1
bSiO2110NaCl system versus b (PAr 5 2.0 MPa).

September 2008
Molybdenum Disilicide (MoSi2) Fine Powders
2805
Fig. 6. The morphology of the final products (b 5 3): (a) after the combustion; (b) after the purification; (c) ball-milled powder.
(2) Temperature Distributions and the Formation Mechanism
of MoSi2
crease in the temperature in the reaction zone is observed. At a
higher concentration of NaCl (10–14 mol) temperature growth
is retarded, starting with 7001C. This feature of the temperature
is attributed to the consumption of reaction heat for the melting
of Mg (6501C), MoO3 (7951C), and NaCl (8101C). Note that the
propagation of the combustion wave at a 5 14 was unstable
(Fig. 9, curve 4) and, after passing about 4–5 cm distance,
stopped at the middle part of the sample. Several zones were
visually observed on the partially combusted sample. The aver-
age size of the individual zones was 2–4 mm, and we could sep-
arate them easily from each other and investigate the phase
structures by X-ray analysis. A schematic of a longitudinal sec-
tion of the partially combusted sample, with X-ray analysis
data, is shown in Fig. 10. Using these data, the sequence of
chemical reactions leading to the MoSi2 formation can be pre-
sented as follows:
The average temperature distributions over the combustion
zones of the MoO313SiO219Mg1aNaCl system are shown in
Fig. 9. These distributions are started by a thermal layer (left
side of ordinate) with an exponentially growing temperature.
Ã
The initial reaction temperature (T ) corresponds to the melting
Ã
point of magnesium (T 5 B6501C). The extent l of the thermal
layer is determined by the distance in which the temperature
Ã
Ã
(starting with T ) drops e times (l 5 T /e), and the extent L of
the reaction zone is the distance between T and Tc. Generally,
Ã
the reaction zones (L) are very wide in comparison with the
heating zones (l) (D 5 L/l44 1), which means the domains gov-
Ã
19
erning the propagation velocity are near T
and have a size
on the order of l (1.0–3.0 mm). The remaining part of the re-
action zone (about 10–15 mm) is called the afterburning zone,
where the postcombustion processes occur.
The character of temperature growth in the reaction zone
(right side of ordinate) is attributed to the NaCl content. When 8
mol of NaCl is added (Fig. 9, curve 1), a smooth and sharp in-
Zone I (B7001C): reduction of MoO3 to MoO2:
MoO3 þ Mg ¼ MgO þ MoO2
(2)
Zone II (B7001–8501C): reduction of MoO2 to metallic Mo:
1
MoO2 þ 2Mg ¼ 2MgO þ Mo
(3)
1
1
1
1
1
1
1
1
0.15
0.12
0.09
0.06
0.03
0
(d)
1
1600
1200
800
400
0
(c)
(b)
(a)
3
2
3
3
2
2
2
2
2
2
2
2
2
20
30
40
50
2θ (°)
60
70
80
7
9
11
13
15
Fig. 7. X-ray diffraction patterns of the MoSi2 powder versus b: (a) 2.0;
(b) 2.5; (c, d) 3.0.
Fig. 8. The combustion parameters of the MoO313Mg19SiO21
aNaCl system versus a (PAr 5 2.0 MPa).

2806
Journal of the American Ceramic Society—Nersisyan et al.
Vol. 91, No. 9
is formed first, which is then enriched with liquid silicon by the
diffusional reaction of liquid silicon through the product layer of
Mo5Si3, thus forming MoSi2. Gao et al.,21 using a combustion
front quenching technique, reported that the formation mecha-
nism of MoSi2 during the combustion reaction of the Mo12Si
mixture was neither the reactive diffusion mechanism exclusively
nor the dissolution–precipitation mechanism exclusively. Cab-
ouro et al.,22 using time-resolved XRD coupled to IR thermo-
graphy, have observed a secondary formation of Mo5Si3 at the
surface of the molybdenum, due to an exothermic diffusional
reaction between solid silicon and solid molybdenum after
5001C. Then Mo5Si3, intermediate phase, and Mo were dis-
solved into Si melt, forming MoSi2 particles.
The distinguishing feature of the system investigated here is
the formation of MoSi2 below the Si melting point. As reported
in the literature, the reactive diffusion and dissolution–precipi-
tation mechanisms of the MoSi2 formation are based on liquid
Si. Consequently, a reasonable assumption that could explain
the MoSi2 formation below the Si melting point might be at-
tributed to molten NaCl. The NaCl melt makes it easier for Mo
and Si particles to slide and rotate, provides chances of particle–
particle contacts, and accelerates Si(solid)–Mo(solid) and
Si(solid)–Mo5Si3(solid) diffusion reactions to form MoSi2.
Therefore, a detailed mechanism for the salt-activated com-
bustion synthesis of MoSi2, under the Si melting point, is the
material for future studies.
1600
1200
800
400
0
1
3
2
4
T*
L
L
l
T
°
–9
–6
–3
0
3
6
9
12
15
18
X, mm
Fig. 9. The average temperature distributions over the combustion
zones of the MoO313SiO219Mg1aNaCl system: (1) a 58; (2)
a 5 10; (3) a 5 12; (4) a 5 14.
Zone III (B9001–9501C): reduction of SiO2 to Si and molybde-
num silicides formation:
SiO2 þ 2Mg ¼ Si þ 2MgO
Mo þ 2Si ¼ MoSi2
(4)
(5)
(6)
IV. Conclusion
The effects of adding NaCl to the combustion characteristics
and the MoSi2 formation of the MoO31SiO21Mg system were
studied using the combustion synthesis technique. The SiO2 acts
thermodynamically as a diluent to the combustion synthesis re-
action, thereby reducing both the combustion temperature and
the wave velocity. With appropriate processing parameters, the
reacted product, consisting of solid crystalline MoSi2, MgO
phases and the molten sodium halide matrix, was prepared at
10001–13501C. The average size and the specific surface area of
MoSi2 particles prepared by the NaCl-assisted combustion re-
action (b5 3, a5 10–13) were in the 100–500 nm and 25–30 m2/g
ranges, respectively. The average concentration of oxygen in the
MoSi2 powder was about 1.0–1.5 wt%.
5Mo þ 3Si ¼ Mo5Si3
According to XRD analysis, the volume fraction of MoSi2 in
zone III is predominant, and then Mo, Si, and Mo5Si3 follow.
This probably indicates that the main part of MoSi2 forms by
reaction (5).
Zone IV (B10001C): reaction (5) had been continued by con-
verting the remaining part of Mo and Si to MoSi2. The Mo5Si3
phase was also converted to MoSi2 by reacting with Si:
Mo5 þ 7Si ¼ 5MoSi2
(7)
The mechanism of MoSi2 formation from Mo and Si by
combustion reaction has been investigated by many research-
ers.20–22 Deevi20 reported that in the combustion process Mo5Si3
Acknowledgement
The authors acknowledge Prof. Joe K. Ballenger from Stephen F. Austin State
University, Nacogdoches, TX, USA for his valuable advice and assistance with
English grammar.
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