Download Preparation of silica-iron oxide composite particles by microwave plasma

st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Preparation of silica-iron oxide composite particles by microwave plasma
Dong-Wook Kim1, Satoshi Kodama1, Hidetoshi Sekiguchi1 and Dong-Wha Park2
Department of Chemical Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo, 152-8552,
Japan
2
Department of Chemical Engineering and Regional Innovation Center for Environmental Technology of Thermal
Plasma (RIC-ETTP), INHA University, Yonghyun-dong, Nam-gu, Incheon, 402-751, Republic of Korea
1
Abstract: The silica-iron oxide multi-composite particles were prepared by the decomposition of vapor precursors under atmospheric Ar-O2 microwave plasma. The morphologies of
the prepared particles were varied by controlling the experimental conditions. Thermodynamic approach proposed that partial pressure, Fe/Si ratio and injecting position of precursors
have a crucial influence on the morphologies of the final particles.
Keywords: Microwave plasma, Particle, Silica, Iron oxide
1. Introduction
For some decades, numerous researches reported particle preparation processes and those mechanism under
thermal plasma and microwave plasma [1, 2]. Especially
recent researches frequently deal with multi-component
particle such as doped ceramic, coated material and spinel
since their unique properties have attracted large interest
in application field [3-6]. Compared to single-component
system, multi-component system would lead to the formation of more various product by altering the experimental conditions (e.g. ratio of components, injection
method). However the particle generation phenomena
were not yet explained due to difficulties in predicting
behaviors of various chemical species in high temperature
region. To prepare the desirable multi-composite particles
from plasma processing, further understanding of the
multi-component system in the plasma is required.
In our previous research, silica coated iron oxide particles were prepared by decomposition of iron
pentacarbonyl [Fe(CO)5] and tetraethyl orthosilicate
[C8H20O4Si] under DC thermal plasma [7]. In the present
research, the same experiment was performed using the
microwave plasma and the results were compared with
that using thermal plasma. The microwave plasma has
relatively lower temperature (generally Tmax<4000K), in
which most of phase-transition temperature of iron species and silicon species is distributed, than the thermal
plasma. Therefore application of the microwave plasma
was expected to cause the different particle generation
mechanism from that in the thermal plasma. To explain
the experimental results obtained from both plasmas and
to investigate the mechanism, the thermodynamic approach was introduced in terms of nucleation temperature
based on gas-liquid equilibrium and phase diagram of
Fe-Si-O system.
2. Experiments
The silica-iron oxide composite were prepared by the
microwave Ar-O2 plasma. The microwave plasma was
generated with 2.45 GHz microwave generator and the
power was 900W at all experiments. The experimental
apparatus was described in Fig.1. The vapor precursors,
iron pentacarobonyl [Fe(CO)5] and tetraethyl orthosilicate
[C8H20O4Si] were injected into quartz tube with argon
carrier gas. The molar ratio of Fe to Si was controlled by
changing the temperature of the water bath and the flow
rate of the carrier gas. The total argon flow rate of the
swirling gas and the carrier gas was maintained as 9.3
L/min. In addition, the precursors were injected from two
injecting positions; axial injection and swirl injection. The
prepared particles were collected at the filter. The detailed
experimental conditions were summarized in Table 1.
Fig.1. Experimental apparatus
Table 1. Experimental conditions
Name
Fe/Si
ratio
Injecting
position
Ex1
3
swirl
Ex2
3
axial
Ex3
1
axial
Ex4
0.333
axial
Power
Ar flow rate
O2 flow
rate
900 W
9.3 L/min
(discharge gas
+ carrier gas)
0.7
L/min
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
3. Results and discussion
Fig.2 showed TEM images of the products obtained
from the thermal plasma (a) and from the microwave
plasma (b)-(e). A particle shown in Fig. 2(a) has external
layer and core. EDS analysis implied that the external
layer was almost composed of Si and O, meaning the
presence of the silica whereas the core part was almost
composed of iron oxide. Although the presence of few Fe
in external layer and Si in core left the possibility of the
presence of fayalite (Fe2SiO4), brownish color of the
products assured that the majority is γ–Fe2O3. Although
γ–Fe2O3 is metastable phase, some researches have reported that γ–Fe2O3 can be obtained by microwave plasma [8, 9]. The similar morphologies were observed in
Fig.2(c) although the thickness of the silica layer is thinner than that in Fig.2(a). The particles in Fig. 2(b), (d)
and (e) did not have any external layer but the mixture of
the individual particles of the silica and the iron oxide are
observed.
In many researches, the supersaturation was introduced
to explain the particle generation in the plasma [8]. In
contrast, the thermodynamic approach was introduced to
explain the difference of the particle morphologies in this
study. Here the condensation temperature of the vapor Feor Si-related species was considered as the nucleation
temperature. First, typical equilibrium diagram of
75Fe(CO)5-25C8H20O4Si-700O2-9200Ar was calculated
using the commercial program Factsage 5.3.1 and was
sketched in Fig.3. FeO(liq) is formed as a prior condensed
iron species and silica is nucleated as SiO2(liq). It is noted
that only the species related to the nucleation was
sketched in Fig.3. In this calculation, the partial pressure
is substituted for mole fraction.
The condensation temperatures were calculated to
evaluate the effect of the Fe/Si ratio and the mole fraction
of the precursors. Fig.4(a), (b) and (c) showed the condensation temperatures as a function of the mole fractions
of the precursors when Fe/Si=3, 1 and 0.333. In the experiments the injected flow rate of the precursors was
maintained as 0.012 L/min and that of the carrier gas
ranged from 0.5 to 0.8 L/min, that is the mole fraction of
the precursors in the carrier gas was 0.016~0.025.
When Fe/Si=3 (Fig.4(a)), FeO has slightly higher condensation temperature than SiO2 at low mole fraction of
the precursors. However the difference increases as the
mole fraction increases. As the rapidly quenched system
from the high temperature region was considered in the
present research, the higher nucleation temperature means
a prior nucleation. Once FeO is condensed, the partial
pressure of FeO around the condensed particle decreases.
Therefore Si-rich phase becomes easily condensed on the
surface of the as-condensed particle. As a result, the final
particle has Si-rich external layer and Fe-rich core as
shown in Fig.2(a) and (c). However Ex1(Fig.2(b)) did not
show any external layer in spite of Fe/Si=3. It’s because
precursors couldn’t pass the high temperature region of
the plasma. The tangentially injected precursors was considered to pass close to the inner surface of the quartz
tube. The gas temperature of the plasma jet was shown in
Fig.5. Since the inner radius of the quartz tube was
4.5mm, the temperature of that position might be further
lower than 2700K. If the temperature of the injecting zone
is comparable to the nucleation temperature of SiO2, SiO2
and FeO may nucleate simultaneously, forming the individual condensed particles.
Fig.2. TEM images of the silica-iron oxide composite particles obtained from (a) thermal plasma, (b)
Ex1, (c) Ex2, (d) Ex3 and (e) Ex4
Fig.3. Thermodynamic equilibrium diagram of
75Fe(CO)5-25C8H20O4Si-700O2-9200Ar system
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fig.5. Gas temperature of the microwave plasma
jet calculated from OH spectrum.
(z : axial distance from waveguide [mm])
When Fe/Si=1 (Fig.4(b)), the differences of the condensation temperature are not so large at the all of mole
fraction regions. Therefore the similar result with Ex1
was obtained by the simultaneous condensation of FeO
and SiO2.
When Fe/Si=0.333 (Fig.4(c)), SiO2 has higher nucleation temperature than FeO against all mole fractions.
Therefore the formation of the iron oxide coated silica
particles was expected by the early condensation of SiO2.
However it was difficult to confirm the iron oxide external layer by our facility problem. Thus further analysis is
required to confirm the presence of the Fe-rich external
layer.
In summary the microwave plasma in Fe-Si-O system
prepared the various silica-iron oxide composite by controlling Fe/Si ratio or the injection position of the precursors. Although the further research is required, it will be
worthwhile to predict the final product in the multi-component through the thermodynamic consideration.
Fig. 4. Condensation temperature of FeO and SiO2
with Fe/Si ratio of (a) 3, (b) 1, (c) 0.333 and difference of T(FeO) between T(SiO2): dash line : the
mole fraction of the precursors at the experimental
point
4. Conclusions
The silica-iron oxide composite particles were prepared
by the atmospheric Ar-O2 microwave plasma. The molar
ratio of Fe to Si in the precursors and the injecting position of the precursors were controlled. When only Fe/Si=3
with axial injection, the iron oxide particles having thin
silica layer were observed. The silica coating was considered as a result of prior condensation of the iron oxide
followed by condensation of silica-rich phase on the
as-condensed iron oxide core. The thermodynamic calculations indicated that the difference of the condensation
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
temperature between iron oxide and silica is obtained in
presence of high Fe/Si ratio and high mole fraction of the
precursors when Fe/Si=3. Nevertheless the silica coating
wasn't performed with the swirl injection of the precursors in spite of the proper conditions. It is considered that
the temperature of the region where the swirl-injected
precursors passed is lower than the nucleation temperature of the silica, so that the silica and the iron oxide simultaneously nucleated without the prior nucleation of the
iron oxide. When Fe/Si=1, the mixture of the individual
silica particles and iron oxide particles were prepared by
the simultaneous condensation of FeO and SiO2. When
Fe/Si=0.333, the preparation of iron oxide coated silica
particles were expected but further discussion is required
to confirm it.
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