Photonic crystals in biology

Poster Session, Tuesday, June 15
Theme A1 - B702
Preparation of MoSi 2 -MoC Nano-composite Powder by Mechanical Alloying
H. Ramezanalizadeh
1 and Saeed Heshmati-Manesh 1 *
1 School of Metallurgy and Materials Engineering, University of Tehran, Tehran, PO Box 14395-553, Iran
Abstract- In this study, MoSi 2 -MoC nano-composite powder was prepared by mechanochemical synthesis. MoO 3 powder and various amounts
of Si and C were mixed together and subjected to severe mechanical activation using a high energy planetary ball mill. The milling operation
resulted in carbosilicothermic reduction of MoO 3 and simultaneous formation of an ultra fine MoSi 2 -MoC powder. The synthesized p roduct
was characterized by SEM and X-ray diffraction. XRD analysis indicated that in a typical mixture of reactants, MoSi 2 is obtained after 25 h
milling. The final products after 60 h milling were -MoSi 2 -MoSi 2 and MoC and their mean crystallite sizes were calculated to be 22.5, 22.1
and 9 nm, respectively.
R
Molybdenum disiliside is an attractive material for high
MoO3+Si 2 +SiO 2 298 = -293 kJ/mol (4)
temperature applications. It is an intermetallic compound and
has tetragonal C11b
value in which case, it was expected to have SiO2 in the XRD
pattern. But it could not be detected because of occurrence of
1900 o C and hexagonal C40 structure (high temperature phase reaction (5) in the milling condition, the product of which is in
o C and its melting point. MoSi 2 has excellent gaseous state:
oxidation resistance and average density (6.24 gr/cm 3 ).
However, its performance is limited by weak toughness at low SiO 2 +Si (5)
temperatures and high creep rate at elevated temperatures. For
these reasons, numerous efforts have been focused on During the next reduction step, MoO 2 can be reduced by Si
designing and fabricating of MoSi 2 based composites for and C. Comparing the XRD patterns of the samples in similar
applications in oxidizing and aggressive environments [1]. milling times, it was found that the rate of decrease in
Various methods have been introduced for synthesis of intensity of MoO 2 peaks was faster in the sample with higher
MoSi2 and its composites including arc melting, powder C content, which means that C is the effective reducing agent
metallurgy, hot pressing, combustion synthesis, mechanical at this step. Also, the overall reduction rate was observed to
alloying, chemical vapor deposition and shock synthesis [1]. increase in samples with higher C/Si mo le ratio in the starting
Mechanical alloying (MA) has also been experienced on material. However, C acts as a lubricant which retard the
various reactants to synthesize MoSi 2 and its composites with formation of MoSi 2 as more MoSi 2 was obtained when more
different second phases [2,3]. MA has been extensively Si was added to the powder mixture.
exploited as a powerful technique to produce a variety of Typical XRD results obtained from a sample containing
stable and/or metastable crystallines, nanocrystallines and 10 mol% of excess C and 30 mol% of excess Si, indicated that
amorphous structured materials including alloys, intermetallic -MoSi2 -MoSi 2 phases appeared after 25
compounds, ceramics, etc [4].
hours of milling. When the sample was milled for 42 h, more
In this investigation, MoO3 powder was used as Mo starting intense peaks of - -MoSi 2 were appeared. At the same
material together with Si and C as reducing agents to prepare time, peak intensities of MoO 2 decreases and a peak related to
MoSi 2 -MoC nano-composite by MA which combines both MoC was detected. Increasing milling time to 60 hours
reduction and alloying into one single milling step.
resulted in complete reduction of MoO 2 . The final products of
Powders of MoO3, C and Si with commercially purity were -MoSi 2 -MoSi 2 and MoC and their
precisely weighed according to reaction 1 and milled by a mean crystallite sizes were calculated to be 22.5, 22.1 and 9
planetary ball under pure argon atmosphere.
nm, respectively.
In summary, MoSi2 was synthesized successfully by MA.
MoO 3 +2Si+1.5C 2 +1.5CO 2 (1) MoSi 2 -MoC nano-composite powder was formed by
mechanical activation of MoO 3 , Si and C powder mixtures.
Other samples containing excess C and Si powders over The rate of MoO 3 reduction reaction and formation of MoSi 2
stoichiometric ratios were also prepared to evaluate their was shown to be influenced by the Si and C contents in the
effect on the process.
initial powder mixture.
Phase composition and morphology of the samples were The financial support of this work by the Iranian
evaluated by XRD and SEM, respectively. The mean Nanotechnology Initiative is gratefully acknowledged.
crystallite size of the milled powders was calculated by
Scherrer method from the line broadening of the diffraction *Corresponding author: 1Tsheshmat@ut.ac.ir
lines [5].
XRD results showed peak broadening in patterns of the [1] Yao, J. Stiglich, and T.S. Sudarshan, ASM International,
milled samp les which was indicative of the crystallite size JMEPEG, 8, 291 (1999).
[2] S.C. Deevi and S. Deevi, Scr. Metall. Mater. 33, 415 (1995).
reduction and accumulation of micro strains in the crystal
[3] M. Sannia, R. Orru, J.E. Garay, G. Cao and Z.A. Munir, Mater.
lattices.
Sci. Eng. A, 345, 270 (2003).
During the milling operation on all samples, MoO 3 was first [4] C. Suryanarayana, Prog. Mater. Sci. 46, 1 (2001).
reduced to MoO 2 . This step is believed to occur through either [5] B.D. Cullity, Elements of X-Ray Diffraction, 2nd edn. Addisonesley
reaction 3 or reaction 4.
Publishing, (1977).
MoO 3 +C 2 +CO 2 298 = -122.3 kJ/mol (3)
6th Nanoscience and Nanotechnology Conference, zmir, 2010 346