Silica nano-powder production by MW plasma using TE011 mode cavity
M. Kogoma1, K. Tanaka1, T. Okamoto2 and Tuyoshi Naito3
1
Department of Chemistry, Faculty of Science and Technology, Sophia University
7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan
2
IDX corporation, Tekeuchi Bld.1-4-3, Kajicho, Chiyoda-ku, Tokyo,Japan
3
Tech International, 2100-99, Kusabana, Akiruno, Tokyo, Japan
Abstract: A new type high temperature ICP system using a TE011 mode cavity
and a 2450 MHz magnetron with a frequency feedback control circuit has been
developed. By using the microwave ICP, we tried silica nano-powder production.
In the ICP, near the cavity center, the electrons rotate around the discharge zone
and do not contact the electrode or the cavity wall, because the crossed magnetic
fields create a ring-shaped high electric field. From the results, we obtained high
temperature plasma in a small zone of the discharge tube. We confirmed the
nano-size silica powder production by the decomposition and oxidation of
chlorotrimethylsilane (CTMS) through the discharge system using Ar-O2
microwave (MW)-ICP.
Keywords: microwave –ICP, TE011 mode cavity, silica nano-powder
1. Introduction
Recently, we reported the silica nano-powder
production using a high temperature RF-ICP system
in an atmospheric pressure Ar-O2 mixture [1]. But
the state of the discharge was relatively sensitive to
small fluctuations of the operation conditions. On
the other hand, the microwave atmospheric pressure
discharge system is well known as a relatively dense
and stable plasma production system in a reactor
smaller than that used with an RF discharge system.
But most MW discharge systems use a surface wave
type or a condenser coupled type discharge mode [2].
Normally, such mode discharges are not suited to
produce a high temperature plasma system in the
atmospheric pressure. For example, the electric field
shows semi-sinsoidal shape in the end of the wave
guide, so the highest field that causes the breakdown
has no sharp peak to concentrate the discharge zone.
Thus the condenser coupled type discharge is easily
dispersed in the discharge zone by a spread parasitic
glow discharge and consequently we cannot realize
the ICP in the discharge zone. On the other hand, a
well-designed atmospheric pressure MW-ICP
system that will have a sharp and strong electric
field due to the center crossing magnetic fields, has
not been yet reported. So, we have tried designs to
make a TE011 mode new type cavity which has a
cylindrical shape with an inner plunger. The plasma
will be produced in the double quartz tube inserted
along to the vertical center axis. An atmospheric
pressure Ar and oxygen mixture is used as a carrier
gas and chlorotrimethylsilane (CTMS) is used as
SiO2 precursor.
2. Experimental
2-1. Cavity and the system
Figure 1 shows the cylindrical cavity (TE011 mode)
and the discharge tube. Ar carrier gas is introduced
to the double quartz discharge tube (１０ and 18 mm
φ) surrounded by a third cooling tube (24 mmφ)
which is inserted along the vertical axis of the
cylinder center. The third tube is used to cool the
discharge tube by flowing of fluorinert oil as the
cooling medium. Figure 2 shows the magnetic field
contour lines in the half of the cavity cross section
calculated with boundary mode which included the
center quartz dielectric tube. The thick magnetic
field contour lines will be found in the center of the
cylindrical cavity, so the highest electric field should
Gas in (Ar + O2 + CTMS)
be attained near the cavity center because the electric
field is crossed to the magnetic field.
For the 2450 MHz micro wave generator, we used
two types of systems: a conventional type magnetron
system and a magnetron system tuned by a
frequency feedback control circuit, called an
injection control microwave system. Figure 3 shows
the circuit diagram of the conventional type
magnetron system and the injection control system
using a solid state signal generator. Inside the dashed
line of figure 3, the signal of the solid state MW
generator is amplified and injected to the magnetron
path through the isolator LP and HP-a; then the
frequency-controlled microwave is generated in the
magnetron. The frequency-controlled microwave
will pass through the HP-a, HP-b, power monitor,
stub-tuner and will finally be absorbed in the cavity.
The discharge power is 0.8 to 1.2 kW at 2450 MHz.
Whirl flow gas (Ar, O2)
Coolant out
Plunger
Cooling
shroud
Cylindrical Cavity
First tube
Second tube
Wave guide
After glow
plasma
Coolant in
Powder trap
2-2. Powder formation
Chlorotrimethylsilane vapor is mixed with the
carrier gas (Ar + O2) using a bubbling system and
introduced to the discharge tube. Between the first
tube and second tube, oxygen whirl flow is
introduced to avoid the direct plasma contact with
the quartz discharge reactor. Chlorotrimethylsilane is
dissociated and oxidized immediately in the high
temperature ICP zone; then silica nano-powder will
be produced on the cooled wall of the after plasma
zone.
Figure 1. The cylindrical cavity (TE011 mode) and the
discharge tube.
Figure 2. Magnetic field contour lines in the half cross section
of TE011 mode cavity. Vertical axis: position from the center
(0mm) to outer face (80mm) of the cylinder. Horizontal axis:
height of the cylinder.
Magnetron
Isolator
HP-a
Isolator
HP-b
Solid State
Amp
Isolator
LP
Wave
Absorber
Solid State
Generator
Wave
Absorber
Power Monitor
Stub Tuner
Cavity
Figure 3. Circuit diagram of 2450 MHz microwave generation and the translation systems for the cavity. The frequency control systems
are shown inside the dotted line.
Frequency / MHz
Power / W
Figure 4. Magnetron frequency of uncontrolled (◆, ■) and controlled (▲, ×, ✳, ●) systems as a function of output power.
a)
b)
Figure 5. Wave heights as a function of the frequency of a) uncontrolled circuit and b) control circuit.
3. Results and Discussion
Figure 4 show the output magnetron characteristics
of the conventional system and the frequency
controlled system. The frequency is easily
influenced by the output power in the conventional
system; on the contrary, the controlled system is not
influenced even for a different frequency that is
controlled by the solid state MW generator.
In fact, we could not obtain any discharge when
using the conventional system but we succeeded by
using the control system. This was because in the
TE011 cavity, the Q factor is higher than 1000, the
conventional MW generator system could not
generate the accurate frequency which is needed to
absorb in the high Q cavity.
Figures 5a and 5b show the output wave height as a
function of MW frequency of the conventional and
the controlled system. In figure 5a, the difference
between the lowest to the highest frequency is about
7MHz; on the other hand, in figure 5b, only one
peak appears, which is controlled by the signal
generator. In the cavity, a ring-shaped strong electric
field will be created in the second tube. The power
monitor showed that almost all of the generated MW
power was absorbed in the cavity and was used to
create the Ar ICP plasma. For starting the discharge,
a tungsten ring wire electrode is inserted once in the
discharge zone to make a first ignition and then
eliminated after the discharge starts. Powder
formation was done in the discharge of the
frequency fixed at 2453 MHz.
Figure 6 shows the light emission photograph of the
plasma zone. In the end of first tube and in the after
plasma zone in the second tube, a strong light
emission appears. The same time, white powder
production is found on the inner tube wall of the
downstream position of the reactor tube. The
produced powder was found by XPS elemental
analysis to be almost pure SiO2.
Figure 7 shows SEM observations of the SiO2
powder. The powder particles have a round shape;
the diameter is 50 - 200 nm.
ICP discharge
Edge of first
discharge tube
4. Conclusion
We could confirm high temperature ICP plasma in
the discharge tube using TE011 mode cavity. By
using the cavity, we have succeeded in the
decomposition and oxidation of CTMS to produce
high purity SiO2 nano-powder in the discharge of
Ar-O2 mixture.
After glow plasma
Figure 6. Photo of the plasma zone at the end of the first
discharge tube.
References
[1] M. Kogoma, A. Takeda, M. Mio, H. Fukui, K.
Maeyama and K. Tanaka, “ICP Plasma Formation of
Silica Nano Powder to be used as a Biomedical
Absorber”, Proceedings of ISPC 18,700 (2007)
Kyoto.
[2] Masuhiro Kogoma and Kunihito Tanaka, “Solid
Surface Treatment Using Low Temperature
Microwave Remote Plasma at Atmospheric
Pressure”, Proceedings of HAKONE 9 (2004).
Figure 7. SEM observation photo of nano-sized silica powder
produced by the MW- ICP.