NON-CARBON PREPARATION OF SILICON BY
MECHANICALLY ACTIVATED THERMAL SYNTHESIS
T.F. Grigorieva1, T.L. Talako2, A.I. Letsko2, V. Šepelák3, V.G. Scholz4,
M.R. Sharafutdinov1, I.A. Vorsina1, A.P. Barinova1, P.A. Vitiaz2,
N.Z. Lyakhov1
1
Institute of Solid State Chemistry and Mechanochemistry, Kutateladze
str., 18, Novosibirsk, 630128, Russia. grig@solid.nsc.ru
2
Powder Metallurgy Institute, Platonov str., 41, Minsk 220005, Belarus
3
Inst. of Nanotechnology, KIT, Eggenstein-Leopoldshafen, 76344, Germany;
4
Inst. of Chemistry, Humboldt Univ. Berlin, 12489, Germany
Introduction
In industrial processes, the production of Si is based on the
reduction of silicon dioxide by carbon at a temperature of about 1800 C.
[1]. However, the coke applied to the reduction can be hardly refined
from the most dangerous for silicon impurities like boron, phosphorus,
arsenic and antimony. That is why development of non-carbon routes for
silicon production is a topical problem of a silicon industry. Reduction
of oxides with magnesium and aluminum by the method of selfpropagating high-temperature synthesis (SHS) has been used in industry
for a long time [2]. As such reactions are highly exothermal, they can be
also organized with the use of mechanochemistry, for instance, reduction
of the copper oxide by aluminum. Mechanochemical reduction of iron
oxide by aluminum, aimed at obtaining precursors with different
compositions for intermetallide/oxide SHS composites, has been also
considered [3–6].
SiO2 + Al reaction is not high exothermic enough to organize the
SHS without preliminary heating [7]. Mansurov et al. [8] reported
creation of ceramic composites in several stages: first, the silicon oxide
was mechanochemically treated in an organic compound environment,
then the resultant material was annealed (carbonized) at ~ 850 C, and
finally the mixture of the carbonized silicon oxide with aluminum was
subjected to SHS. However, as-formed product included silicon carbide.
The objective of activities described in this paper is to study the
possibility of using mechanochemical treatment for obtaining
silicon/aluminum oxide composites by the SHS and thermal synthesis at
considerably lower temperatures with the following removal of alumina.
53
Sample preparation and examination procedures
The PA-4 aluminum powder and the silicon oxide with a particle
size of ~ 3 nm were used in our experiments.
A stoichiometric mixture of the silicon oxide with aluminum was
processed in a high energy planetary ball mill (drum volume 250 cm3,
ball diameter 5 mm, mass of the balls 200 g, mass of the sample 10 g,
and velocity of rotation of the drums around a common axis ~1000 rpm).
The IR spectra were recorded by a Specord IR 75 spectrometer;
the samples for this study were pressed with annealed potassium
bromide.
The 27Al (I = 5/2) NMR spectra were recorded on a Bruker
Advance 400 spectrometer corresponding to a 27Al resonance frequency
of 78.2 MHz. MAS experiments were realized with a high speed probe
using 2.5 mm zirconia rotor. The spinning speed was 20 KHz. The
magnetic field strength (in frequency unit) was set to 104.262 MHz. The
excitation pulse duration was chosen equal to 1 s. The recycling delay
between each acquisition was fixed to 1 s. To see weak signals in the AlO region in mechanically activated samples, we applied accumulations
numbers up to 56000 (i.e. measurement time of 15 hours).
The dynamics of the SHS process was studied with the use of
diffraction of synchrotron radiation and an OD-3 single-coordinate
detector. The samples for SHS were prepared in the form of pellets 20
mm in diameter and 1–2 mm thick by pressing at a pressure of 200 atm.
The resultant samples were placed onto a ceramic plate so that they were
in the center of the goniometer. The process was initiated by a nichrome
spiral. The OD-3 detector was triggered to operate in the “fast filming”
mode simultaneously with the beginning of pellet burning. The time of
one “frame” was 0.5 sec, and the number of “frames” was 128. The
radiation wavelength was 1.527 Å.
For investigation of mechanically activated thermal synthesis the
samples were heated up to 650 C in the reaction chamber XRK 900 in
air with a heating rate 10 /min. The OD-3 detector was also used for
studying the process dynamics, though time of one “frame” was 1 min.
54
Results and discussion
First, we made an attempt of direct mechanochemical reduction of
the silicon oxide by aluminum. The study of this process showed that the
chemical reaction of SiO2 reduction does not occur within 6 min of
mechanical activation. The IR spectrum of the initial mixture contains
clear absorption bands with the maximums at 1005 and 480 cm−1
(valence and deformation oscillations of the Si–O bond of the SiO4
tetrahedra of the silicon–oxygen skeleton) and two maximums in the
range of 900–670 cm−1 due to oscillations of the Si–O–Si bridges. The
phenomena observed in the course of mechanical activation were a
gradual decrease in intensity
and broadening of the characteristic bands of the Si–O bond (Fig. 1).
Fig. 1. IR spectra of the SiO2 + Al mixture
before mechanical activation (1) and after
mechanical activation during 0.5 (2), 1 (3)
and 6 (4) min
Fig. 2. Microphotograph of the
mechanocomposite after 1 min
activation in Si characteristic
radiation
An electron-microscopy study of the SiO2/Al composite obtained
after 1 min of mechanical activation in characteristic radiation revealed a
55
very small grain size and a very uniform distribution of the components
in the mechanocomposite (Fig. 2).
Based on the data of the differential thermal analysis (DTA), even
short-time activation of this mixture appreciably affects its thermal
characteristics. For the initial mixture, the real chemical interaction
occurs at a temperature T > 1000 C (Tmax = 1083.6 C) (Fig. 3 a), i.e.,
substantially higher than the melting point of aluminum, whereas the
situation is different for the mixture subjected to mechanical activation
during 20 sec. Two clearly expressed exothermal peaks appear: the first
peak at 621.7–648.6 C (Tmax = 632.7 C) and the second peak at 992.1–
1075.9 C (Tmax = 1029.2 C) (Fig. 3 b). For the mixture activated for 40
sec, the first peak is at 604.5–636.6 C (Tmax = 612 C), and the second
peak is extremely broad and smeared in the range of 816.1–1111.7 C
(Tmax = 1038.1 C).
These observations can be explained by the fact that a tight
contact is created between some part of the ultrafine non-plastic silicon
oxide and plastic aluminum already within 20 sec of mechanical
activation; the silicon oxide is “wetted” by aluminum; as a result, some
part of the silicon oxide starts to interact with aluminum at a temperature
T = 621.7C, which is lower than the melting point of the latter. As
mechanical activation is continued, aluminum becomes also dispersed to
nanoparticles, greater amounts of the components of the mixture are
involved into the contact, and the temperature of the interaction
beginning decreases: after 1 minute of activation, the interaction begins
at T = 539.9 C and ends at T = 630.3 C (Fig. 3 c).
The curve for this sample obtained by the method of differential
scanning calorimetry (DSC) has only one exothermal peak, i.e., the
entire process proceeds at a temperature lower than the melting point of
aluminum. Longer activation further decreases the temperature of
reaction beginning (Table 1), but there are no any further significant
changes in the system parameters determined by DSC.
The duration of mechanochemical treatment was limited to 6 min
for the following reasons:
- the IR spectra are so smeared already after 4 min that do not provide
any new information (see Fig. 1);
- the DTA study does not reveal any significant changes in the thermal
characteristics after 1 min of mechanical activation (see Table 1);
56
- mechanochemical actions should be always minimized to ensure the
minimum possible contamination of the products by milling.
Fig. 3. Results of differential scanning calorimetry (DSC) and thermogravimetry
(TG) studies of the SiO2 + Al mixture before (a) and after mechanical activation
during 20 (b) and 60 sec (c).
57
Table 1. Parameters of Exothermal Peaks on DTA Curves of SiO2 + Al
Samples after Mechanical Activation
Temperature, C
Duration of activation
1 min
beginning of the
reaction
593.0
2 min
587.1
624.3
4 min
586.7
629.1
6 min
587.0
625.8
27
end of the reaction
630.3
Al MAS NMR spectra of the nanostructured SiO2/Al
mechanocomposites are dominated by a broad resonance associated with
the presence of nanostructured Al matrix (Fig. 4). The interesting
observation is that additional resonance lines appear in the spectra of
mechanoactivated samples corresponding to AlO4, AlO5 and AlO6
polyhedra. Their content is slightly increasing with increasing milling
time, however, the relative intensity of AlOx polyhedra compared with
the Al matrix spectral intensity is even after the longest milling period
very low. It can be assumed that these nonequilibrium local
coordinations of aluminium atoms are located on the SiO2-Al interfaces
[9]. The intensity of the resonance lines belonging to various polyhedra
relative to the total spectral intensity allows us to calculate the volume
fraction of interface regions in the nanocomposites. Furthermore,
assuming a spherical shape of SiO2 nanoparticles, the thicknees of the
interface regions was calculated their known volume fraction.
Thus, the study of mechanically activated SiO2+Al mixtures
shows that silicon reduction does not occur during mechanical activation
step except formation of some AlOx species at the interfaces, but an
exothermal reaction in activated mixtures can proceed at substantially
lower temperatures.
In the subsequent step, the nanostructured SiO2/Al
mechanocomposites were used as precursors for the preparation of
Si/Al2O3 composites via self-propagating high-temperature synthesis.
Our experience shows that combustion initiation requires sample
58
preheating approximately to 200 C (as compared with 650-860 С
reported in [7]).
Fig. 4. 27 Al MAS NMR spectra of non-activated sample (a), the sample
mechanoactivated for 1 (b) and 6 (c) minutes.
59
The overall pattern of phase transformations is illustrated in Fig. 5
a. To analyze them, however, it is more convenient to use the projection
onto the diffraction angle (β)–time plane (Fig. 5 b). As the silicon oxide
used in these experiments is amorphous to x-ray radiation, only
aluminum peaks are observed.
Fig. 5. Dynamics of phase transformations in the Al + SiO2 mechanocomposite
60 image; (b) projection onto the
in the SHS mode: (a) three-dimensional
diffraction angle–time plane.
It is clearly seen that
aluminum becomes heated
as the combustion wave
approaches: the peaks are
shifted toward smaller
angles,
i.e.,
greater
distances between the
planes. After that, the
intensity of these peaks
drastically
decreases,
which is apparently due to
melting. No crystalline
phases are observed in the
two frames (~ 1 sec). In
our opinion, corundum
(Al2O3)
peaks
appear
slightly earlier than silicon Fig. 6. Microphotograph of the SHS product
in Si characteristic radiation.
peaks. A possible reason is
the lower melting point of
silicon (1410 C), as compared with corundum (2050 C). An electronmicroscopic study of the SHS product of the SiO2 + Al system subjected
to mechanical activation during 1 min in characteristic radiation (Fig. 6)
shows a fairly uniform distribution and small size of all elements in the
system, including silicon being formed.
Previously, it was shown that chemical interaction between SiO2
and Al in the mechanocomposites formed during the mechanical
activation starts at essentially (~ 500 C) lower temperatures as compared
with the non-activated mixtures.
In the final step, we used as-formed mechanocomposites as
precursors for the preparation of Si/Al2O3 composites via thermal
synthesis. The samples after mechanical activation for 6 min were
placed into cuvette and gently prepressed to get the plane surface. Then
the cuvette with the sample was sited in the furnace. The thermocouple
was directly close to the registration area. Recording of diffractograms
was started at temperature 230 С. Dynamics of phase transformation in
Al / SiO2 composites during heating from 590 up to 660 C is presented
in Fig.7.
61
Fig. 7. Dynamics of phase transformation in Al / SiO2 composites during
heating from 590 up to 660 C.
Fig. 8 XRD-pattern of the thermal synthesis product from the mechanocomposites
activated for 6 min and heated up to 660 C.
As can be seen from the Fig. 7, the reaction products (silicon and
alumina) start to form at about 590 С. It is interesting, that corundum is
formed during the SHS and thermal synthesis after low activation time,
62
while -Al2O3 is identified in the product of thermal synthesis after
longer MA durations (Fig. 8).
Conclusions
Thus, though the silicon oxide is not reduced by aluminum
directly by mechanical activation, the use of the mechanocomposite as a
precursor for both SHS and thermal synthesis allows a fine-grain
silicon/aluminum oxide composite to be obtained. In both cases
chemical interaction starts at essentially lower temperatures as compared
with the non-activated mixtures.
Acknowledgements
This work was supported by the joint project No. 5 “Non-carbon
preparation of Si by mechanically activated thermal synthesis” of NASB
and SB RAS.
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