Nanoparticle synthesis using high-power modulated induction
thermal plasmas with intermittent synchronized feeding of raw materials
Y. Tanaka1 , H. Sakai1 , T. Tsuke1 , W. Guo1 , Y. Uesugi1 , Y. Sakai2 , K. Nakamura2
1 Faculty
of Electrical Engineering & Computer Science, Kanazawa University,
Kakuma, Kanazawa 920-1192, Japan
2 Research Center for Production & Technology, Nisshin Seifun Group Inc.,
5-3-1 Tsurugaoka, Fujimino 356-8511, Japan
Abstract: The titanium dioxide (TiO2 ) nanoparticles were synthesized by direct evaporation of titanium powders using Ar-O2 pulse-modulated induction
thermal plasmas (PMITP). The PMITP has been developed to control the temperature of thermal plasma and gas flow fields in time domain. In addition
to the PMITP, a system was developed for intermittent feeding of raw materials synchronized with the modulation of the coil current. Effect of the intermittent feeding was investigated on synthesized particles with the PMITP in
terms of the mean particle diameter, the morphology, and the weight fraction
of anatase-TiO2 . The delay timing of the intermittent feeding was also studied
on the synthesized particles.
Keywords: Pulse-modulated induction thermal plasmas (PMITP), TiO2
nanoparticle synthesis, intermittent powder feeding
1. Introduction
Inductively coupled thermal plasmas (ICTPs) around
at atmospheric pressure have been widely used as effective heat and chemical species sources for various
materials processings such as syntheses of diamond
films, thermal barrier coatings, and surface modifications, etc. The advantages of ICTPs are their high
enthalpy and high radical density without any contamination. Such an ICTP is also useful to synthesize
nanoparticles of various kinds [1, 2].
On the other hand, the authors previously developed a pulse modulated induction thermal plasmas
(PMITP) system and an arbitrary-waveform-modulated
induction thermal plasma (AMITP) system [3]. The
PMITP and AMITP are established by the coil current modulated into a rectangular or other waveforms.
Such a millisecond modulation can perturb the thermal plasma markedly, and then it can change the temperature and radical densities as well as the gas flow
field in the thermal plasma in time domain [3]. Through
this modulation, the temperature and radical densities can be controlled. We found actually that the
Ar-N2 PMITP provides simultaneous control of the
nitrogen radical flow and the enthalpy flow [4].
We have been trying to adopt the Ar-O2 PMITP
to nanoparticle synthesis of titanium dioxide (TiO2 )
in anatase phase. The TiO2 nanoparticles in anatase
phase are continuously receiving attention for use as
photocatalysts [5], photonic crystals [6], photovoltaic
cells [7], gas sensors [8], and a strong deoxidation
material used for producing hydrogen gas from water
for fuel cells [9]. The TiO2 nanoparticle properties
depend on the particle size, crystal structure, phase
constituents. Thus, particle size and phase constituent
control are extremely important to obtain the specified performance of TiO2 nanoparticles. In our previous works, we found that the modulation of the coil
current can control the size of synthesized nanoparticles, and that the weight fraction of anatase-TiO2 in
synthesized nanoparticles is about 85%–90% almost
independent of the modulation condition [10]. It was
also found that the size of synthesized nanoparticles
depends on the temperature decay rate in the cooling
period in the PMITP [11].
In this paper, in addition to the PMITP, a system
was developed for intermittent feeding of raw materials synchronized with the modulation of the coil
current. In our previous work, the raw materials was
continuously fed to the plasma although the coil current was modulated into a rectangular waveform [10,
11]. Effect of the intermittent feeding was investi-
Continuous
feeding
Ti
TiO2
Solenoid Valve
Solenoid Valve
Intermittent
feeding
(Closed valve)
Intermittent
feeding
(Opened valve)
Ti
TiO2
Fig. 1. Concept of intermittent and synchronized feeding
of raw material.
gated on synthesized particles with the PMITP. The
synthesized nanoparticles were analyzed using FESEM for the morphology and the size distribution,
and using XRD for the surface compounds. The influence of the delay timing of the intermittent feeding
was also studied on the synthesized particles.
2. Concept of intermittent feeding of raw materials synchronized with the PMITP
Fig. 1 illustrates the concept of the intermittent and
synchronized feeding of raw material to the PMITP.
The PMITP can repetitively produce a high-temperature
field during the ‘on-time’ and low-temperature field
during the ‘off-time’ in thermal plasmas. Here, the
on-time means the time period with the higher current level, while the off-time is the time period with
the lower current level. If the raw materials are continuously fed to the PMITP as a conventional way,
they can be supplied to the thermal plasma both during the on-time and off-time. In this case, the supplied particles experience different temperature histories in the thermal plasma. On the other hand, if we
use the intermittent and synchronized feeding of raw
materials to the PMITP, the raw material particles
can be supplied to the thermal plasma only when the
high-temperature plasma is established in the torch
during the on-time. This may promote complete and
efficient evaporation of the supplied raw material. Furthermore, evaporated materials in the high-temperature
thermal plasma is rapidly cooled during the successive off-time in the reaction chamber. This may provide the efficient nucleation for nanoparticle, and then
prevent synthesized particles from growing up.
3. Experimental setup
3.1. The developed system for intermittent feeding of raw materials
Fig. 2 shows the whole nanoparticle synthesis system
using the PMITP used in the present work. The upper
side in Fig. 2 portrays the newly developed system
for intermittent feeding of raw material to the PMITP.
The system contains the trigger circuit, the delay circuit and a high speed solenoid valve. The pulse generator provides the modulation control signal with 0–
10 V to the gate signal circuit for the metal oxide
semiconductor field effect transistor (MOSFET) and
also the trigger circuit. The FET gate signal circuit
controls the fire angle of the MOSFET elements for
rf power supply with the pulse modulation of the coil
current. On the other hand, the trigger circuit and
then the delay circuit control opening and closing a
high-speed solenoid valve. Use of this system enables us to intermittently feed the raw material powder to the thermal plasma with controlling feed timing. The solenoid valve has a time constant of less
than 2 ms.
3.2. Plasma torch and rf power source
The plasma torch in Fig. 2 is configured identically
to that used in our previous work; its details are described in an earlier paper [10, 11]. The plasma torch
has two coaxial quartz tubes. The inner diameter of
the interior quartz tube is 70 mm; its length is 370
mm. An argon-oxygen gas mixture was supplied as a
sheath gas along the inside wall of the interior quartz
tube from the top of the plasma torch.
Titanium raw material was fed using a powder feeder
with Ar carrier gas through the solenoid valve and
a water-cooled probe inserted from the top of the
plasma torch head, as depicted in Fig. 2. Downstream of the plasma torch, the water-cooled chambers are installed in vertical and then in horizontal,
as depicted in Fig. 2. Further downstream, a powdercollecting filter is set up and then a vacuum pump.
Synthesized particles were collected in the filter.
3.3. Experimental conditions
The total sheath gas flow rate was fixed at 100.0 standard liters per minute (slpm) (=1.33 × 10−3 m3 s−1 ).
The oxygen gas admixture ratio to Ar was fixed at
10% in the gas flow rate. The Ar carrier gas flow rate
was 4 slpm. Titanium powder with a diameter less
Intermittent feeding system
Trigger
Trigger
circuit
circuit
Pulse
Pulse
Generator
Generator
Ar+Ti
powder
Solenoid
valve
Delay
Delay
circuit
circuit
FET
FET gate
gate
signal
signal
circuit
circuit
On-time Off-time
t
Watercooled tube
Ar+O2
Torch
(a) NM / IMF
(b) PM / CWF
(c) PM / IMF & td = 0 ms
(d) PM / IMF & td = 8 ms
3-phase
power
source
Rectifier
RF source circuit
Filter
MOSFET
inverter
Impedance
matching
circuit
Vacuum
pump
Fig. 2. Nanoparticle synthesis system using pulse modulated induction thermal plasma with intermittent feeding
of raw material.
than 45 µm was used as raw material. The powder
feed rate g of the Ti raw material was set from 3.3
to 4.0 g/min for any feeding condition. The pressure
inside the chamber was controlled, actually fixed at
300 Torr (=40 kPa). The time-averaged input power
to the rf power supply was fixed at 20 kW. The SCL,
which is the ratio of the lower current level to the
higher current level, was set to 80%. The on-time
and the off-time were set to 12 ms and 3 ms, respectively. In this case, the duty factor DF is 80%.
4. Effect of intermittent feeding with synchronized to pulse modulation
4.1. SEM images and particle diameter distribution
Fig. 3 presents examples of FE-SEM micrographs of
particles for four conditions: (a) non-modulation and
intermittent feeding (NM/IMF), (b) pulse modulation
and continuous feeding (PM/CWF), (c) pulse modulation and intermittent feeding with delay time td of 0
ms (PM/IMF & td =0 ms), and (d) pulse modulation
and intermittent feeding with delay time td of 8 ms
(PM/IMF & td =8 ms). From these micrographs, the
particle size distribution was subsequently obtained
from observation of 200 randomly sampled particles
in Fig.4. This figure also includes the mean diameter
d̄, the median diameter d50 and the standard deviation
σ. In addition to this, we have had results for nonmodulation / continuous feeding (NM/CWF) which
Fig. 3. SEM images of synthesized TiO2 nanoparticles. PM means the pulse modulation of the coil current,
whereas NM corresponds to non-modulation. IMF means
the intermittent feeding of raw materials, and CWF indicates the continuous feeding. On-time/off-time=12 ms/3
ms.
has d̄=63.2 nm, d50 =54.5 nm, and σ=30.6 nm. First,
the comparison between NM/CWF and PM/CWF results indicates that the coil current modulation provides smaller nanoparticles. This was already obtained in our previous results [10]. NM/CWF and
NM/IMF results were compared to imply that the
intermittent feeding of raw materials also produce
smaller nanoparticles. Moreover, PM/IMF conditions,
in particular PM/IMF & td = 8 ms condition, provide
smaller nanoparticles than conditions NM/CWF, NM/IMF
and PM/CWF. Fig. 5 shows the dependence of d̄ on
the delay time of intermittent feeding of raw materials. As seen, the conditions of PM/IMF & td =6–
8 ms are ones that provide the smaller nanoparticle
synthesized in the present work. This feeding timing
of td =8 ms was found to be synchronized between
the feeding of raw material and the high temperature
timing in the PMITP.
4.2.
Weight fraction
of anatase-TiO2 analyzed us1
ing XRD
The phase constitutes of synthesized nanoparticles
were analyzed using X-ray diffraction technique (XRD).
Almost identical XRD spectra were found for particles synthesized for any operation conditions. These
XRD spectra were all related to TiO2 . Thus, this re-
(a) NM / IMF
(b) PM / CWF
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(d) PM / IMF & td = 8 ms
Mean particle diameter [nm]
Fig. 4. Particle size distribution of synthesized nanoparticles. On-time/off-time=12 ms/3 ms.
65
Non-modulation
Intermittent feeding
80%DF-80%SCL
Continuous feeding
60
80%DF-80%SCL
Intermittent feeding
Weight fraction of anatase TiO2
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Non-modulation
Intermitent feeding
1.0
80%DF-80%SCL
Continuous feeding
0.9
0.8
80%DF-80%SCL
Intermittent feeding
0.7
0.6
0
2 4 6 8 10 12
Delay time [ms]
Fig. 6. Weight fraction of anatase-TiO2 estimated from
XRD analysis.
5. Conclusions
The TiO2 nanoparticles were synthesized using the
PMITP with intermittent feeding system of raw materials. It was found that combination of the PMITP
and the intermittent feeding of raw materials provides smaller nanoparticles, keeping high weight fraction around 0.85 for anatase-TiO2 .
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Fig. 5. Dependence of mean diameter of synthesized
nanoparticles on the delay time of the intermittent feeding.
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