3:45 PM - 4:00 PM
[MIS14-06] Nucleation of iron in highly supersaturated gas by molecular dynamics simulation
Keywords:nucleation, iron, molecular dynamics simulation
The process of condensation nucleation from the gas phase is an important process in various fields, but there is still no general theory that predicts the nucleation rate with high accuracy. Nucleation rates obtained from laboratory experiments have been reported to differ by an order of magnitude from those given by the classical nucleation theory.Molecular dynamics (MD) simulations are able to directly resolve details of the nucleation process and provide useful test cases for nucleation models. There were a lot of previous studies with MD simulations for the investigation of nucleation in a supersaturated vapor, but only limited type of substances have been examined.
In this study, MD simulations of the condensation nucleation process of iron from the gas phase were performed. Iron is a major material substance of the earth, and its condensation process is an important process for formation of solid matter in astronomical environments. Although there have been some studies on the MD simulation of nucleation of iron from the gas phase, the detailed comparison with the theory has not been made [1]. We performed the MD simulations of homogeneous nucleation process from iron vapor, by using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code developed at Sandia National Laboratories. We used the embedded atom method (EAM) for modeling the force field acting between the iron atoms [2]. The number of particles was increased (N=10000-100000) from the previous study (N=343-1331) [1], the ranges of temperature T and the supersaturation ratio S were from 300K to 1200K and from lnS=20 to 130.
The simulations were able to reproduce the phenomenon that proceeds with a nucleation rate several orders of magnitude lower than that in the previous study. In order to compare the results of the MD calculations of nucleation with nucleation theory, the thermodynamics quantities such as bulk surface tension are necessary. To estimate these quantities, we also performed a series of equilibrium MD simulations of liquid-vapor systems consisting of a liquid slab with vapor at fixed temperature. The critical nucleus is estimated to be unity under the conditions adopted in this study. The comparison with the theoretical values also shows that the nucleation rate are several orders of magnitude lower than the gas kinetic collision limit for nucleation rates. The reason for this difference between the theoretical and the MD simulations lie in the sticking probability of molecule during molecular growth. The sticking probability is proportional to the nucleation rate and is often simply assumed to be unity in the nucleation theory, but this is not obvious. The results of simulations under various temperature and pressure conditions show that the sticking probability depends on the temperature and the supersaturation ratio. Our results suggest that the dimer formation during the nucleation was inefficient. The difficulty of dimer formation has been pointed out for several materials [3,4] and is consistent with the results of recent microgravity experiments on the nucleation of iron [5].
Reference:
[1] N. Lummen & T. Kraska, Aerosol Science, 36 (2005) 1409
[2] M. I. Mendelev, S. Han, D. J. Srolovitz, G. J. Ackland, D. Y. Sun & M. Asta, Philosophical Magazine, 83 (2003) 3977
[3] A. E. Korencnecko, A. G. Vorontsov, B. R. Gelchinski & G. P. Sannikov, Physica A, 496 (2018) 147
[4] M. Lippe, S. Chakrabarty, J. J. Ferreiro, K. K. Tanaka, R. Signorell, J. Chem. Phys. 149 (2018), 244303
[5] Y. Kimura, K. K.Tanaka, T. Nozawa, S. Takeuchi & Y. Inatomi, Science advances, 3 (2017) e1601992
In this study, MD simulations of the condensation nucleation process of iron from the gas phase were performed. Iron is a major material substance of the earth, and its condensation process is an important process for formation of solid matter in astronomical environments. Although there have been some studies on the MD simulation of nucleation of iron from the gas phase, the detailed comparison with the theory has not been made [1]. We performed the MD simulations of homogeneous nucleation process from iron vapor, by using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code developed at Sandia National Laboratories. We used the embedded atom method (EAM) for modeling the force field acting between the iron atoms [2]. The number of particles was increased (N=10000-100000) from the previous study (N=343-1331) [1], the ranges of temperature T and the supersaturation ratio S were from 300K to 1200K and from lnS=20 to 130.
The simulations were able to reproduce the phenomenon that proceeds with a nucleation rate several orders of magnitude lower than that in the previous study. In order to compare the results of the MD calculations of nucleation with nucleation theory, the thermodynamics quantities such as bulk surface tension are necessary. To estimate these quantities, we also performed a series of equilibrium MD simulations of liquid-vapor systems consisting of a liquid slab with vapor at fixed temperature. The critical nucleus is estimated to be unity under the conditions adopted in this study. The comparison with the theoretical values also shows that the nucleation rate are several orders of magnitude lower than the gas kinetic collision limit for nucleation rates. The reason for this difference between the theoretical and the MD simulations lie in the sticking probability of molecule during molecular growth. The sticking probability is proportional to the nucleation rate and is often simply assumed to be unity in the nucleation theory, but this is not obvious. The results of simulations under various temperature and pressure conditions show that the sticking probability depends on the temperature and the supersaturation ratio. Our results suggest that the dimer formation during the nucleation was inefficient. The difficulty of dimer formation has been pointed out for several materials [3,4] and is consistent with the results of recent microgravity experiments on the nucleation of iron [5].
Reference:
[1] N. Lummen & T. Kraska, Aerosol Science, 36 (2005) 1409
[2] M. I. Mendelev, S. Han, D. J. Srolovitz, G. J. Ackland, D. Y. Sun & M. Asta, Philosophical Magazine, 83 (2003) 3977
[3] A. E. Korencnecko, A. G. Vorontsov, B. R. Gelchinski & G. P. Sannikov, Physica A, 496 (2018) 147
[4] M. Lippe, S. Chakrabarty, J. J. Ferreiro, K. K. Tanaka, R. Signorell, J. Chem. Phys. 149 (2018), 244303
[5] Y. Kimura, K. K.Tanaka, T. Nozawa, S. Takeuchi & Y. Inatomi, Science advances, 3 (2017) e1601992