Nucleation is a fundamental event to determine character, such as size, number density and morphology, of produced crystals [1].  Therefore, understanding and control of nucleation process are crucial in various fields.  Nucleation theories have been used to understand the nucleation temperature, number density and size of produced particles.  However, it has been well known that nucleation rates obtained by classical nucleation theory (CNT) and by experiments have a large difference.  The semi-phenomenological (SP) nucleation theory proposed by Dillmann and Meier (1991) [2] explains some results of laboratory experiments and agree well with results from numerical approaches based on molecular dynamics (MD) simulations of homogeneous nucleation over a wide range of temperatures for liquids and solids [3,4].  However, SP model also has a limitation; nucleation rates in experiments of relatively complex molecules [5,6] and in a system of larger size of critical nuclei [7] cannot be explained.

    Recently, we started a new project to determine the physical parameters of nanometer sized particles and evaluate nucleation theories by homogeneous nucleation experiments in vapor phase.  Nanoparticles were formed from a highly supersaturated vapor, supersaturation ratio was as high as ~5×10⁴, after evaporation by electrical heating in a gas atmosphere.  The temperature and concentration at the nucleation sites were obtained by an in-situ observation system using interferometry.  We succeeded to determine surface free energy and sticking probability of manganese nanoparticle from condensation temperature and size of produced particles, which was determined by transmission electron microscopy, based on nucleation theories [8].  The surface free energy and sticking coefficient of Mn at 1106 ± 50 K were 1.55 ± 0.10 J/m² and 0.39 (+0.39, -0.2), respectively, by CNT and 1.57 ± 0.35 J/m² and 0.42 (+0.42, -0.21), respectively, by a SP model.

In this laboratory experiment, hot evaporation source generates heterogeneity of nucleation temperature and concentration caused by strong convection of gas atmosphere.  In addition, there is a possibility of that produced nuclei collides and then fused together to be a larger particle and decreasing number density after a stage of nucleation, which called fusion growth in nanoparticles [9].  This phenomenon might be a reason of large differences of nucleation rates between experiments and theories and of very small sticking coefficient of zinc, ~10^-5, obtained by microgravity experiment [10].  If there is no convection, evaporated vapor diffuses uniformly and the temperature profile becomes concentric around the evaporation source. As the result, nucleation will occur at the same condition. Then, we can obtain physical properties with relatively smaller error bars and then we may be able to evaluate nucleation theories more precisely. Therefore, we performed a microgravity experiment using the sounding rocket S-520-28 launched on December 17^th, 2012.

    We prepared specially designed double wavelength Mach–Zehnder-type interferometers with an evaporation chamber and camera recording systems to fit the space and weight limitations of the rocket. Three systems, named DUST 1 to 3, with same configuration except evaporation source and gas pressure in the chamber were installed into the nosecone of the rocket. The evaporation source and gas atmosphere were tungsten and gas mixture of oxygen (4.0×10³ Pa) and argon (3.6×10⁴ Pa) for DUST 1, iron and argon (2.0×10⁴ Pa) for DUST 2, and iron and argon (4.0×10⁴ Pa) for DUST 3.  The experiments were run sequentially and automatically started from 100 s after launch of the rocket. The evaporation source was electrically heated under microgravity. Evaporated vapor was concentrically diffused uniformly, cooled and condensed in the gas atmosphere. The temperature and concentration at the nucleation site were determined from the movements of the fringes in the interferogram.  The nucleation occurred very far from the thermal equilibrium and the supersaturation ratio was extremely high, more than 10¹⁰.  Here, we will show the first results of the homogeneous nucleation in vapor phase under microgravity. 

[1] e.g., Vekilov, P. G. Cryst. Growth Des. 2010, 10, 5007-5019.
[2] Dillmann, A.; Meier, G. E. A. J. Chem. Phys. 1991, 94, 3872-3884.
[3] Tanaka, K. K.; Tanaka, H.; Kawamura, K.; Nakazawa, K. J. Chem. Phys. 2005, 122, 184514.
[4] Tanaka, K. K.; Tanaka, H.; Yamamoto, T.; Kawamura, K. J. Chem. Phys. 2011, 134, 204313.
[5] Hämeri, K., Kulmala, M. J. Chem. Phys. 1996, 105, 7696.
[6] Anisimov, M. P. et al. J. Chem. Phys. 2001, 115, 810.
[7] Tanaka, K. K et al. To be submitted.
[8] Kimura, Y.; Tanaka, K. K.; Miura, H.; Tsukamoto, K. Cryst. Growth Des. 2012, 12, 3278-3284.
[9] Kimura, Y.; Miura, H.; Tsukamoto, K.; Li, C.; Maki, T. J. Cryst. Growth 2011, 316, 196−200.
[10] Michael, B. P., Nuth III, J. A., Lilleleht, L. U. Astrophys. J. 2003, 590, 579-585.

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