Chemical Reaction Considered Numerical Simulation on Preparation of. AlN Nano Powder by Non-transferred Thermal Plasma. Tae-Hee Kima, Sooseok Choib, ...
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
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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).
References [1] H. Ageorges, S. Megy, K. Chang, J.M. Baronnet, J.K. Williams, C. Chapman, Plasma Chem. Plasma Proc. 13 (1993) 613. [2] S. M. On, D. W. Park, Thin Solid Films. 316 (1998) 189. [3] J. M. Park, K. S. Kim, T. H. Hwang, S. H. Hong, IEEE Tran. Plasma Sci. 32 (2004) 479. [4] S. Choi, H. S. Lee, S. W. Kim, D. W. Park, S. H. Hong, J. Korean Phys. Soc. 55 (2009) 1819.
