Chemical Reaction Considered Numerical Simulation on Preparation of
AlN Nano Powder by Non-transferred Thermal Plasma
Tae-Hee Kima, Sooseok Choib, Dong-Wha Parka*
a
Department of Chemical Engineering and Regional Innovation Center for Environmental Technology of
Thermal Plasma (RIC-ETTP), INHA University, 253 Yonghyun-dong, Nam-gu,Incheon 402-751,
Republic of Korea
b
Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology,4259 Nagatsuta-cho,
Midori-ku, Yokohama 226-8503, Japan
Abstract: Numerical simulation on the thermal plasma characteristics was carried
out inside of the DC non-transferred thermal plasma system to preparation of
AlN nano powder. Especially, chemical reactions were considered between
thermal plasma, Al metal bulk, and NH3 gas inside of the reactor by FLUENT
computational fluid dynamics code. In our simulations, 2-D axis-symmetric
domain was assumed, and the DPM (discrete phase model) model was used for
investigating the chemical reaction of AlN which is synthesized from evaporated
Al metal and NH3 gas.
Keywords: aluminium nitride (AlN), ammonia (NH3), thermal plasma, numerical
simulation.
1. Introduction
Metal nitride has many applications in the related
industries. Among the metal nitrides, aluminium
nitride (AlN) has been widely used for the plate and
the film materials in the semiconductor. Because, it
has high thermal conductivity of typically 200
W/m∙K, high electrical resistivity, low dielectric
constant, low thermal expansion coefficient, electric
insulation property, and the chemical stability [1,2].
The high-purity nano-sized aluminium nitride
powder offers low temperature sintering and high
thermal conductivity in the manufacture of AlN
plate for semiconductor. In order to obtain the
sufficiently dense powder, high temperature of
approximately 1900 °C is essentially required, and
careful control of impurity contents those effect on
the thermal conductivity of the powder is also
required [2].
It is well that thermal plasma offers a high
temperature environment, high enthalpy, steep
temperature gradient which enables rapid quenching
and a clean reaction atmosphere controlling
byproduct generation. The high temperature and the
high chemical reactivity of the plasma state are
utilized to provide a powerful medium to promote
high heat transfer rates and chemical reactions for
the nano particle synthesis process.
In order to understand thermal plasma synthesis of
AlN nano powder, the thermal plasma reactor was
numerically simulated with a consideration of
chemical reactions inside the reactor.
2. Simulation model
Figure 1 shows the schematic diagram of
simulated DC non-transferred thermal plasma
system for preparation of AlN particles. Plasma
torch, reactor chamber wall, and Al holder were
cooled by cooling water. Argon-nitrogen mixing
plasma jet composed of 15 L/min Ar and 3 L/min N2
was generated by the DC non-transferred arc plasma
torch. Al bulk, raw material, was fixed on the
tungsten crucible. As a reactive gas, NH3 with N2
carrier gas was introduced into the reactor through a
low temperature area of top side of the reaction
chamber to prevent its hasty decomposition into
nitrogen and hydrogen. Al bulk was evaporated just
after thermal plasma generation due a high enthalpy
environment. Vaporized Al reacted with NH 3 gas
and AlN particles were synthesized by a quenching
effect cause by the steep temperature gradient of the
14000
600
Temperature [K]
Axial velocity [m/s]
12000
500
11000
400
10000
9000
300
8000
7000
Axial velocity [m/s]
Temperature [K]
13000
200
0
1
2
Radial distance [mm]
Figure 1. Schematic diagram of modeled experimental apparatus
for AlN preparation.
Figure 2. Temperature and velocity profiles of thermal plasma
jet in the torch nozzle.
the simulation of the reactor region.
thermal plasma flame. The prepared AlN fine
powder was usually collected on the inside wall of
the reactor.
3. Simulation condition
This numerical simulation including complex
flow field and heat transfer in the reactor was
performed by using the FLUENT computational
fluid dynamics (CFD) code. Several assumptions
such as steady-state flow, two dimensional axisymmetric conditions were applied in this numerical
work. The complex high temperature thermal flow
inside the reactor was calculated according to mass
continuity, energy and momentum conservation as
governing equations. The thermal plasma flow
inside the reactor was regarded as turbulent flow due
to its high velocity and a sharp gradient of
temperature. Hence, the standard k-ε turbulence
model was employed in this simulation. Chemical
reaction for preparation of AlN was simulated by
discrete phase model (DPM) in order to take into
account different phase of solid state of Al and fluid
state of plasma and reactive gas.
Thermal plasma characteristics inside the torch
region were analyzed by using DCPTUN code, a
magnetohydrodynamic (MHD) computational code
for thermal plasma [3, 4]. From the results on the
torch region, temperature and velocity profiles at the
torch exit were predicted as shown in Fig. 2, and
they were used as the inlet boundary condition in
4. Results and discussion
The numerical simulation on complex fluid field
inside the reactor was carried out using FLUENT
code to understand flow characteristics in the AlN
powder synthesis process based on thermal plasma.
Thermal plasma jet was injected with 7,000 ~
1,3000 K in temperature and 200 ~ 600 m/s in
velocity range as shown in Fig. 2. Thermal plasma
jet with such high velocity and temperature was
entered into the reactor where reactive NH3 gas with
N2 carrier gas are introduced. Temperature
distribution inside of reactor is indicated in Fig. 3.
Although the numerical simulation was conducted
for th entire reactor region, Fig. 3 is focused on the
plasma torch and raw material of Al on the crucible.
Figure 3. Temperature distribution inside the reactor.
High termperature flame of the plasma jet contacts
with surface of Al raw material, and it flows to the
radial direction. Surface of raw material is heated up
over 3,000 K in wide area. Since the vaporization
temperature of Al is 2,800 K, it can be estimated that
AlN synthesis process from Al raw material is fairly
occured in the present thermal plasma system. Also
in actual experiment, the center part of raw material
was mainly vaporized that contact with thermal
plasma jet.
Before the synthesis of AlN, Ar-N2 mixing plasma
gas, NH3 reactive gas, and N2 carrier gas exist in the
reactor. Ar and N2 gas were used as plasma forming
gas with their flow rates of 12 L/min and 3 L/min,
respectively. NH3 reactive gas of 30 L/min was
injected from the top side of the reactor with N2
carrier gas of 15 L/min. Figure 4. shows the mole
fraction of N2 and NH3 gas those are presented in the
left side and the right, respectively. NH3 and N2 gas
can be utilized as reactant N in the AlN preparation
process. However, it can be estimated that NH3 is
more effective reactant source than N2, because N2
have strong bonding as diatomic molecule. Although
N2 was decomposed into N radial by plasma, almost
of them rapidly return to N2 molecule. In Fig. 4,
amount of NH3 is much larger than that of N2.
Chemical reaction of Al metal with N2 and NH3
gas simulated using the Arrhenius equation. It is
defined as follow equation.
k  Ae (  Ea / RT )
where, A is pre-exponential factor, Ea is activation
energy, R is gas constant, and T is absolute
temperature, respectively. In this work, following
two chemically reactions were mainly investigated.
Al(s) + N2(g) → AlN(s) + N(g)
Al(s) + NH3(g) → AlN(s) + 3H(g)
(2)
(3)
In these reactions, each reaction coefficients were
estimated from chemical equilibrium calculations. In
the all of temperature range, chemical reaction with
NH3 gas was more dominant than that of N2 gas.
Therefore, it is revealed that using NH3 gas as
reactive gas is effective for AlN synthesis process.
Figure 5. shows mole fraction distribution of
synthesized AlN in the inside of reactor. In fig. 4.,
each mole-fraction for distribution of NH3 and N2
was similar in all of range except for plasma jet.
Temperature of every wall were fixed 300K in AlN
synthesis simulation. It can be estimated that
vaporized Al was reacted with N reactant source and
synthesized AlN particle stream run to outflow
direction. In actual experiment, synthesized and
quenched AlN was collected in the every wall.
Herewith, it was estimated that quenching effect for
synthesized AlN was disregarded in this simulation
work. Therefore, synthesized AlN flow to the
outflow with different fluid in the inside of reactor.
(1)
Figure 5. Mole fraction distribution of synthesized AlN particle
in the inside of reactor.
Figure 4. Mole fraction comparison of N2 in left and NH3 in
right inside the reactor.
5. Conclusion
It was simulated that AlN synthesis process by
DC non-transferred thermal plasma. Among other
thing, work focus on the chemical reaction carried
out in this simulation.
In the first place, vaporization possibility of Al
metal was looked by temperature distribution in the
inside of the reactor. As the results, temperature
range of the contact surface between Al and plasma
jet was over 3000K. in other words, vaporization of
Al metal is possible because vaporization
temperature of Al is 2800K.
Second, it was made a comparison with mole
fraction of N2 and NH3 in the inside of the reactor.
Among the injected gases into the reactor, amount of
NH3 was much more than N2 gas. It was indicated
that NH3 can be more effective as reactant N source
than N2.
Finally, mole fraction of synthesized AlN was
calculated through reaction rate from Arrhenius
equation. As the results, more AlN particles existed
in the area closed outflow.
The numerical simulation results were compared
with the experimental results. From this numerical
simulation work, it was provided that a better
understanding of environmental synthesis process
inside the reactor.
Acknowledgment
This work was supported by the Regional
Innovation Center for Environmental Technology of
Thermal Plasma (ETTP) at Inha University
designated by MKE (2011).
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