Mechanochemical reduction of MoO3 powder by silicone to synthesize nanocrystalline MoSi2

1. Mechanochemical Reduction of MoO3 Powder by Silicone to Synthesize
Nanocrystalline MoSi2
H. Ramezanalizadeha
and S. Heshmati-Maneshb
School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran
a
hralizadeh@ut.ac.ir, b
sheshmat@ut.ac.ir
Key words: Nano-crystalline, Mechanical Alloying, Mechanochemical reduction, MoSi2.
Abstract. Molybdenum disiliside is known as a ceramic material with attractive properties for high
temperature structural applications. In this study, mechanical alloying was used to produce MoSi2
powder directly from molybdenum oxide. Mixture of MoO3 and Si powders with commercial purity
were exposed to high mechanical activation in a planetary ball mill. The ball to powder mass ratio
was selected to be constant at 33:1 and the rotation speed (cup speed) was 600 rpm during the
milling operations. Crystallite sizes and structural evolutions during milling were investigated by X-
ray diffraction analysis. The morphology of the mechanically alloyed powders was evaluated with
scanning electron microscope (SEM). From XRD results, it was observed that within 6 hours of
milling MoO3 was reduced and fully converted to MoO2. After 17 hours of milling MoO2 also
began to reduce and peaks of MoSi2 (both α and β phases) and Mo were detected. Further milling
resulted in a gradual decrease in MoO2 peak intensities because of its continuous reduction. Peaks
of MoO2 were also broadened due to refinement of MoO2 crystallite sizes. Scherrer and
Williamson-Hall methods using XRD patterns were employed to calculate the mean crystallite size.
Calculations indicated that in the sample ball milled for 50 hours, MoSi2 crystallite sizes were less
than 100 nm.
Introduction
Recently, silicides of various metals have been the focus of several investigations, due to their
excellent high temperatures properties. It is because of new aerospace needs for high temperature
resistance materials with properties that cannot be met by conventional ceramics or super alloys [1,
2].
MoSi2 is an attractive material for high temperature applications. Also, it has high melting point
(2020o
C) with excellent oxidation resistance and average density (6.24 gr/cm3
). However, its
performance is limited by weak toughness at low temperatures and high creep rate at elevated
temperatures. For these reasons, numerous efforts have been focused on designing and fabricating
MoSi2- based composites for applications in oxidizing and aggressive environments [3].
MoSi2 is an intermetallic compound with 67.7 at%. Si and 33.3 at%. Mo. It has tetragonal C11b
structure (low temperature phase α) below 1900o
C and hexagonal C40 structure (high temperature
phase β) between 1900o
C and its melting point of 2020o
C. High temperature phase β melts in
congruent form at 2020o
C (Fig.1) [4]. Three Mo-Si stoichiometric are found which are Mo3Si
(cubic), Mo5Si3 (tetragonal) and MoSi2 (tetragonal).
Various methods have been introduced for synthesis of MoSi2 and its composites including arc
melting, powder metallurgy, hot pressing, combustion synthesis, mechanical alloying, chemical
vapor deposition and shock synthesis [3]. Due to high melting point of MoSi2, there will be a lot of
problems during fabrication of MoSi2 by conventional methods such as: arc melting, casting,
powder metallurgy, etc. Mechanical alloying is a high-energy ball milling which has been used to
synthesize alloys, amorphous alloys, intermetallic compounds, etc [5, 6]. Numerous investigations
have been performed on the synthesizing MoSi2 composites with different second phases such as
Ta, Nb, W, ZrO2, SiC, Si3N4, WSi2, Mo5Si3, TiB2, Al2O3, etc [7, 8]. In this investigation MoO3
powder was used as molybdenum starting material and Si was used as reducing agent to synthesize
MoSi2-SiO2 nano-composite.
Advanced Materials Research Vols 264-265 (2011) pp 1364-1369 Online: 2011-06-30
© (2011) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMR.264-265.1364
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2. Fig. 1. Equilibrium phase diagram of Mo–Si [4].
Experimental
In this investigation, mechanical alloying was performed under argon atmosphere in a high
energy ball milling at room temperature and speed of rotation (cup speed) of 600 rpm. MoO3 (99%,
100 µm) and Si (99%) powders were used as starting materials. The weight ratio of ball to powder
was selected as 33:1. The balls and vial were made of hardening steel. Samples for analysis were
removed by interrupting the ball mill at various intervals. Investigation of phases and products
evolution during ball milling has been performed by using a X-ray diffractometer (Philips Xpert
Professional) with 30 kV, 40 mA, 0.02 step size and 1 second per step with CoKα radiation (λ=1.78
Å). Also, crystallite size was evaluated using the scherrer and Williamson-Hall methods from the
line broadening of the diffraction lines, as follow [9]:
t =
. λ
θ
(Scherrer) (1)
BCosθ =
. λ
+ 2ηSinθ (Williamson-Hall) (2)
Where t is the crystallite average size (nm), λ the CoKα wave length (nm), B the diffraction peak
width at half maximum intensity (radian) and θ is the diffraction Bragg angle. The morphology of
the mechanically alloyed powders was monitored with a Scanning Electron Microscope (SEM).
Results and Discussion
Starting materials (MoO3 and Si powders) were accurately weighed according to the following
reaction and then mixed together and milled.
2MoO3 + 7Si → 2MoSi2 + 3SiO2 (3)
10 %mol. excess Si was added to compensate the probably losses during handling and milling.
Reaction (3) is in fact sum of the following partial reactions:
2MoO3 + Si → 2MoO2 + SiO2 (4)
2MoO2 + 2Si → 2Mo + 2SiO2 (5)
2Mo + 4Si → 2MoSi2 (6)
Advanced Materials Research Vols. 264-265 1365

3. Gibbs free energy changes for the reaction (3) were calculated using Eq. 7 [10] and found to be
-1496.2 kJ.mol-1
at room temperature which shows the reaction (3) is thermodynamically feasible in
the milling conditions.
∆G = -1505.9 + 32.5 T (kJ.mol-1
) (7)
Fig. 2 shows XRD patterns of the un-milled and milled powders mixture after different milling
times. As can be seen, by increasing the milling time, peaks of starting materials are broadened and
their intensities are decreased. This is obviously due to gradual decrease of crystallite sizes in
powder particles and transformation of crystalline structures to amorphous state. After 6 hours of
milling, almost all of the MoO3 is consumed and a mixture of Si, SiO2 and MoO2 forms. By further
milling, the intensity of Si peaks is decreased and for those of MoO2 is increased. Concurrently,
SiO2 is formed and its peaks are overlapped with those of MoO2. After 17 hours of milling, MoO2
also began to reduce and peaks of MoSi2 (both α and β phases) and Mo were detected. Mo peaks
cannot be observed clearly because of high mass absorption coefficient of synthesized phases.
Fig. 2. XRD patterns of the un-milled powder together with samples milled for different milling times.
Variations of crystallite sizes for Si, MoO2, MoSi2 (both α and β phases) has been shown in
Table 1. For α-MoSi2 and Si, the Williamson-Hall method was used, while for β-MoSi2 and MoO2,
the scherrer method was used due to the overlapping of peaks.
As can be seen, by increasing the milling time, the mean crystallite sizes of all powder
components decrease. Decrease in the crystallite sizes of powder components is due to the line
broadening of the diffraction lines, increase in lattice micro-strains and transformation of crystalline
structure to amorphous state.
An interesting phenomenon occurred during the milling operation was the conversion of β-
MoSi2 to α-MoSi2 after 50 hours of milling. Because of its meta-stable nature, β-MoSi2 phase
transforms to α-MoSi2 phase when its mean crystallite size reaches to a certain amount (almost 11
nm) [4]. After 50 hours of milling, the mean crystallite sizes were calculated to be 11 nm and 9 nm
for β-MoSi2 and α-MoSi2, respectively.
1366 Advances in Materials and Processing Technologies II

4. Table 1. Calculated mean crystallite sizes in the milled powder particles.
Table 2. shows the thermodynamic data for the starting materials and the reactions occur during
the mechanical alloying [10]. Calculation of the Gibbs free energy changes for the reactions shows
that they are all thermodynamically feasible at room temperature. Reaction (3) is extremely
exothermic and according to the ∆H298/Cp parameter, the mechanism of this reaction can be of
combustion type. It is noticeable that the exothermicity of a reaction is often characterized by the
ratio of the heat of formation to heat capacity at room temperature (∆H298/Cp). Typically,
∆H/Cp>2000 K is required for the propagation of a self-sustaining reaction [5]. This value for
reaction (3) is 5629.32 K [10]. The apparent activation energy for starting this reaction is not too
much, such that just after the beginning of the milling operation, reduction of MoO3 starts in
suitable locations of powders due to mechanical forces. Right after, due to release of the reactions
heat, temperature rises up to which the reaction between molybdenum and silicone starts concurrent
with MoO2 reduction reaction. The reaction between Mo and Si is also an exothermic one. Thus,
these factors provide the desirable thermodynamic conditions to form the β-MoSi2, because this
phase is stable at higher temperatures. However, this phase is meta stable at room temperature and
by further milling, it is expectable to transform to a stable state. This prediction is completely
consonant with XRD analysis. As can be seen from the XRD patterns (Fig. 2.), this phase (β-MoSi2)
is present until 50 hours of milling and it is expected to disappear with prolonged milling and
transform to α-MoSi2.
Table 2. Thermodynamic data for starting materials and reactions occur during the mechanical alloying at
room temperature [10].
SiO2 Si MoO3 Mo Reaction 4 Reaction 6 Reaction 3
∆S
[J/deg. mol]
41.5 18.8 77.8 28.6 -54.9 65.1 -32.5
∆H
[kJ/mol]
-910.9 ------ -745.2 ------ -930.5 -131.8 -1505.9
Cp
[J/deg. mol]
43.93 23.93 75.19 28.52 8.06 67.86 267.52
∆G
[kJ/mol]
----- ----- ----- ----- -314.14 -151.2 -1496.2
∆H298
/Cp
[K]
----- ----- ----- ----- ----- ----- 5629.32
Fig. 3 shows the SEM micrographs of the un-milled and milled powders after different milling
times. Fig 3(a) shows the morphology of un-milled sample. As seen in this image, MoO3 particles
have a blade-shaped morphology. Fig. 3(b) shows the powders morphology after 17 hours of
milling. Particles coalescence and formation of large agglomerates are obvious in this image.
MoO3/Si system is a brittle-brittle system and according to [5], the cold welding phenomenon does
not occur in these systems. Thus, the agglomeration might probably be due to Mo presence after 17
hours of milling in this system. So, the molybdenum soft particles act as welding agent and result in
Milling time
[h]
d(MoO2)
[nm]
d(Si)
[nm]
d(βMoSi2)
[nm]
d(αMoSi2)
[nm]
Strain η
[%]
6 16.6 49.2 --- ---- 0.275
12 --- 204 --- ---- 0.687
17 --- 15.2 32 48.2 ------
22 --- --- 17.6 26 0.36
28 --- --- 17.6 25.7 0.52
33 --- --- 12.5 19.2 0.64
50 --- --- 11 9 0.93
Advanced Materials Research Vols. 264-265 1367

5. the formation of agglomerates. Also, in brittle-brittle systems, when there is not any ductile
component like Mo, large agglomerates can be formed. This occurs during milling since the harder
(more brittle) component gets fragmented and embedded in the less brittle (softer) component (Fig.
3(c)). However, with prolonged milling, the agglomerates break down again into small particles
(Fig. 3(d)) because the final product (MoSi2) is formed and there is not any ductile Mo available to
result in welding of particles.
Fig. 3. SEM Micrographs of (a) un-milled sample and those milled for (b) 17, (c) 33 and (d) 50 hours.
Conclusion
The feasibility of MoSi2 synthesis by ball milling of mixtures MoO3 and Si powders at ambient
temperature was investigated. A MoSi2-SiO2 nanocomposite was successfully synthesized. From
the XRD studies on milled samples, it was shown that after 6 hours of milling, silicon can reduce
MoO3 to MoO2. Increased milling time resulted in peaks broadening and decreasing in peaks
intensity due to particles refining and increased lattice micro-strains. After 17 hours of milling,
peaks of MoSi2 (both α and β phases) and Mo were detected. Simultaneous formation of these two
phases is probably due to high intensity of milling operation and in such a condition, either α and/or
α + β molybdenum disilicide may form. An interesting phenomenon occurred during the milling
which was gradual conversion of β-MoSi2 to α-MoSi2 after 50 h of milling.
1368 Advances in Materials and Processing Technologies II

6. Acknowledgements
The authors would like to acknowledge the Iranian Nanotechnology Initiative for financial support
of this work.
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Advanced Materials Research Vols. 264-265 1369

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Mechanochemical Reduction of MoO3 Powder by Silicone to Synthesize Nanocrystalline MoSi2
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