Synthesis and thermal stability of nano-crystalline vanadium disilicide

Article abstract of DOI:10.1016/j.jallcom.2003.09.014

Nano-crystalline vanadium disilicide was successfully synthesized using vanadium tetrachloride and silicon as starting materials via reduction-silication route at 650°C in the molten salt solution of magnesium chloride and sodium chloride in an autoclave. X-ray powder diffraction patterns indicated that the product was hexagonal VSi₂ (a=4.572A?, c=6.372A?). Transmission electron microscopy images showed that the particle size of the product was in the range of 40-60nm in diameter. There was a strong absorption peak at 271nm in the UV-Vis absorption spectra. The oxidation of nano-crystalline VSi₂ began to proceed at the temperature of 400°C in air. But the product had high thermal oxidation stability below 1000°C. It can be used as an antioxidation coating material.

Full text of DOI:10.1016/j.jallcom.2003.09.014

Journal of Alloys and Compounds 370 (2004) 281–284
Synthesis and thermal stability of nano-crystalline vanadium disilicide
Jianhua Maa,b, Yunle Gua, Liang Shia, Luyang Chena, Zeheng Yanga, Yitai Qiana,∗
a
Structure Research Laboratory, Department of Chemistry, University of Science and
Technology of China, Hefei, Anhui 230026, PR China
Wenzhou University, Wenzhou, Zhejiang 325027, PR China
b
Received 2 July 2003; received in revised form 2 September 2003; accepted 2 September 2003
Abstract
Nano-crystalline vanadium disilicide was successfully synthesized using vanadium tetrachloride and silicon as starting materials via
reduction–silication route at 650 ◦C in the molten salt solution of magnesium chloride and sodium chloride in an autoclave. X-ray powder
diffraction patterns indicated that the product was hexagonal VSi2 (a = 4.572 Å, c = 6.372 Å). Transmission electron microscopy images
showed that the particle size of the product was in the range of 40–60 nm in diameter. There was a strong absorption peak at 271 nm in the
UV-Vis absorption spectra. The oxidation of nano-crystalline VSi2 began to proceed at the temperature of 400 ◦C in air. But the product had
high thermal oxidation stability below 1000 ◦C. It can be used as an antioxidation coating material.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Nano-structures; Vanadium disilicide; Chemical synthesis; X-ray diffraction; Thermal property
1. Introduction
as starting materials in a high purity argon atmosphere
above 1390 ◦C, crystal growth of vanadium silicide [7]
from high-temperature tin and copper solutions (at 1400 ◦C)
in an argon atmosphere using vanadium metal chips and
powder as starting materials, and solid-state synthesis [8]
via reaction of calcium and magnesium silicide with vana-
dium oxides at 1000 ◦C. But all these methods require high
temperature, and also cannot form nano-sized products.
In this paper, we have developed a molten salt route
to prepare nano-crystalline vanadium disilicide via
reduction–silication reaction of metallic sodium with vana-
dium tetrachloride and silicon at 650 ◦C. It can be described
as follows:
Due to their technologically relevant characteristics,
which are high melting point, high hardness, strength,
thermal stability, excellent wear and corrosion resistance,
the transition metal silicides have received considerable
attention in the last few decades. They, in fact, find appli-
cations in high-temperature devices [1], microelectronics
(where they are commonly used as interconnects in very
large scale integration applications) [2] and catalysis [3].
Among these silicides, vanadium disilicide is an important
material. In addition to these common properties of metal
silicides, they also have low electrical resistivity and so is
used in semiconductor devices [4]. However, there have
been few reports on the composition, structure and proper-
ties of nano-crystalline VSi2. It is meaningful to synthesize
nano-crystalline VSi2 as the nano-sized powders can reduce
the sintering temperature [5] of bulk materials.
VCl4 + 2Si + 4Na → VSi2 + 4NaCl
Reactions in molten salts provide an original method for
the preparation of solids. Molten salts can accelerate the ki-
netics of the reaction by enhancing diffusion as the diffusion
coefficients in liquid state are higher than those in solid state.
Therefore, they can serve as an inorganic solvent medium
to control the reaction speed and the particle size. Because
vanadium tetrachloride attains gaseous state at 152 ◦C [9], it
is necessary to carry out the whole reaction in an autoclave.
The product becomes free of oxide impurities by restraining
the reaction in an anaerobic environment.
Traditionally, VSi2 can be synthesized through various
methods: field activated combustion synthesis [6] of vana-
dium disilicide by using vanadium and silicon powders
∗
Corresponding author. Tel.: +86-551-3601589;
fax: +86-551-360742.
E-mail address: ytqian@ustc.edu.cn (Y. Qian).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2003.09.014

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J. Ma et al. / Journal of Alloys and Compounds 370 (2004) 281–284
20
30
40
50
60
70
80
2 theta / degree
Fig. 1. XRD patterns of vanadium disilicide prepared via reduction–silication route.
2. Experimental
All the reagents used were analytically pure. All manip-
ulations were carried out in a dry glove box filled with N2.
First, 10 g of MgCl2 and 10 g of NaCl were added to an au-
toclave. Then 0.02 mol of poly-crystalline Si powder (200
meshes) was put into the autoclave followed by thorough
mixing of the powder. Finally 0.01 mol of VCl4 and 0.04 mol
of Na were put into the autoclave. The autoclave was sealed
and maintained at 650 ◦C for 10 h and then cooled to room
temperature in the furnace. After that, the products were
washed with 0.1 mol/l nitric acid, diluted alkali solution, dis-
tilled water and alcohol in a sequence. The final product was
vacuum-dried at 60 ◦C for 12 h. The product obtained was a
black-colored powder.
The powder-products were analyzed by powder X-ray
diffraction (XRD) on an X-ray differactiometer (Rigaku
rA) using Cu K␣ radiation (wavelength λ = 1.54178 Å),
and transmission electron microscope (TEM) with a Hitachi
800 TEM. The UV-Vis absorption spectrum was recorded
on a Shimadzu UV-2401PC UV-Vis spectrophotometer.
The thermogravimetric analysis was performed on a ther-
mal analyzer (Model: TA-50) below 1000 ◦C in air at a
rate of 10 ◦C min−1 to study the oxidation behavior of the
nano-crystalline vanadium disilicide. The density of the
product was found out by using a pycnometer method with
toluene at 25 ◦C.
Fig. 2. TEM images of the vanadium disilicide prepared via reduction–
silication route.
6.372 Å, which are in good agreement with the values (a =
4.572 Å, c = 6.373 Å) found in the Powder Diffraction File.1
No evidence of impurities such as V, Si, V2O3, V2O5 and
SiO2, can be found in the XRD patterns. The crystallite size
is estimated to be about 50 nm in diameter, according to
the reflections by the Debye–Scherrer equation with a shape
factor of 0.9. The density using a pycnometer method with
toluene at 25 ◦C is 4.550 g cm−3, which is close to the re-
ported result (Dx: 4.624 g cm−3).
3. Results and discussion
A TEM image is shown in Fig. 2. The material exhibits
slightly agglomerated particle morphology. The primary par-
Fig. 1 shows the XRD patterns of the as-prepared vana-
dium disilicide sample. All the diffraction peaks can be in-
dexed as the hexagonal phase of vanadium disilicide. The
lattice parameters were calculated to be a = 4.572 Å, c =
1
See Powder Diffraction File No. 38-1419, International Center for
Diffraction Data, New town Square, PA, USA.

J. Ma et al. / Journal of Alloys and Compounds 370 (2004) 281–284
283
271nm
200
300
400
500
600
700
Wavelength / nm
Fig. 3. Room temperature UV-Vis absorption spectra of the vanadium disilicide prepared via reduction–silication route.
ticle size is estimated to be in the range 40–60 nm in diam-
eter, which is in agreement with the value calculated from
the Debye–Scherrer equation.
The UV-Vis absorption spectra of the VSi2 nano-crystal-
lites, which were ultrasonically dispersed in absolute
ethanol, are shown in Fig. 3. It can be seen that there is
only a strong absorption peak at 271 nm. The onset of the
absorption appears at about 350 nm. We have not found any
information about the UV-Vis absorption records of VSi2
in literatures. The width of the absorption band [10] is as-
sociated with the dispersion of the particle size. So we can
see that the quantum dimension effect of nano-crystalline
vanadium silicide results in the broadening of the band.
The oxidation process of nano-crystalline VSi2 was stud-
ied at temperatures below 1000 ◦C by TGA, as shown in
Fig. 4. The weight of the product does not increase signif-
icantly below 400 ◦C but increase gradually by about 28%
from 400 to 1000 ◦C. But when the temperature is above
600 ◦C, the increase in weight becomes even bigger. That is
to say the oxidation begins at a relatively low temperature
30
25
20
15
10
5
0
0
200
400
600
800
1000
Temperature / degree
Fig. 4. TGA curves heated in flowing air for vanadium disilicide prepared via reduction–silication route.

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J. Ma et al. / Journal of Alloys and Compounds 370 (2004) 281–284
a
a : VSi2
b : V2O5
a
a
a
a
b
b
b
b
b
b
a
a
a
a
a
b
b
a
b
20
30
40
50
60
70
80
2 Theta / degree
Fig. 5. The XRD patterns of the oxidation product of TGA testing.
of 400 ◦C and takes place slowly only on the surface of
the particles. Theoretically, the increment in weight is about
106.7% according to the calculation when VSi2 was oxi-
dized thoroughly. This result is not in agreement with the
result obtained in [7], who reported that the oxidation of
VSi2 crystals began to proceed at a measurable rate in the
temperature range of about 515 ◦C and yielded the final oxi-
dation product V2O5. This is because the VSi2 prepared us-
ing the method shown in this paper is nano-crystalline and
has a high surface energy. So the oxidation occurs at a lower
temperature.
via a reduction–silication route by the reaction of metallic
sodium with vanadium tetrachloride and silicon at 650 ◦C
in a molten salt solution using magnesium chloride and
sodium chloride in an autoclave. The molten salts serve
as reaction-medium to control the reaction rate and parti-
cle size. The particle size is estimated to be in the range
of 40–60 nm in diameter. The oxidation in air begins to
proceed at the temperature of 400 ◦C. The formed ox-
ides on the surfaces can diminish further oxidation below
1000 ◦C.
The XRD patterns of the oxidation product obtained af-
ter TGA testing are shown in Fig. 5. All the peaks in the
XRD patterns are indexed as the hexagonal VSi2 is the ma-
jority phase, while the orthorhombic V2O5, is the minority
phase. There are no SiO2 peaks in the patterns, due to the
fact that SiO2 is amorphous. As the temperature rises, oxi-
dation begins at the surfaces of the particles, gradually form-
ing vanadium oxide and silicon dioxide. The newly-formed
vanadium oxide and silicon dioxide, becomes a protective
layer, and cannot evaporate from the surfaces of the particles
and they have the power to diminish a further oxidation pro-
cess. The final product is mainly vanadium silicide. Because
the oxides (V2O5 and SiO2) were formed on the surfaces
of the particles, the as-prepared nano-crystalline vanadium
disilicide exhibits high thermal stability.
Acknowledgements
Financial support from the Chinese National Foundation
of Natural Science Research is gratefully acknowledged.
References
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4. Conclusion
As a summary we conclude that nano-crystalline hexag-
onal vanadium disilicide has been successfully prepared
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