10:45 AM - 11:00 AM
[PCG19-01] Nucleation efficiency of dust approaching from microgravity experiments
★Invited Papers
Keywords:nucleation, dust, microgravity, nanoparticle
The aim of this study is understanding the formation process of dust particles via homogeneous nucleation in the gas outflow from an evolved star. We have proceeded an international project named DUST (Determining Unknown yet Significant Traits) and investigated three important properties (the surface free energy, sticking probability, and infrared spectrum) of nanoparticles as dust analogues by experimentally reproducing the nucleation processes of cosmic dust in a microgravity environment. To obtain the three properties, the generated dust analogues in a microgravity environment must be analyzed in the laboratory. At the microgravity level of an aircraft (0.01 – 0.1 G), the convection caused by the difference in density cannot be suppressed sufficiently and, therefore, the physical constants cannot be determined precisely. Because of these reasons, we use overseas rockets, which can provide a high-quality microgravity environment of ~10-4 G and can recover the produced samples.
The international project was started in 2017 with a cooperation of JAXA/ISAS. First experiment was conducted by launching MASER 14, a sounding rocket of the Swedish Space Corporation, from Esrange Space Center in June 24, 2019 with the supports by German Aerospace Center and Prof. J. Blum of Technische Universität Braunschweig. In addition, the sounding rockets Black Brant IX 343 and 365 were launched from the White Sands Missile Range in October 2019 and September 2020, respectively, and performed microgravity experiments in collaboration with Dr. J. A. Nuth III of NASA/GSFC. Including non-recovery sounding rocket experiment in Japan, we have conducted five flights and a total of 20 nucleation experiments under microgravity environment with strong supports of Prof. Y. Inatomi, JAXA/ISAS. Seven kinds of materials were used so far: iron, tungsten oxide, alumina, silica, magnesium silicate, iron silicate, and titanium carbide.
Originally designed two different kinds of experimental devices (interferometer and IR spectrometer) were mounted on the rocket. The nucleation chamber (about 500 cc) with an evaporation material was prepared in each device. After reaching microgravity, the evaporation source was heated to generate high-temperature gas. The gas cooled down and nucleation occurred homogeneously to form dust analogues. These processes were observed in-situ.
In these experiments, several results have been obtained that differ from the commonly accepted theory. For example, homogeneous nucleation from the gas phase requires much larger supersaturation than expected. One of the major hindrances for nucleation was found to be dimer formation. For example, the nucleation of iron occurred only when the size of critical nucleus was smaller than dimer. The sticking probability was very small, only about 0.002% for iron [1]. It was also found that the final stable crystal was not nucleated directly from the gas phase, but via a liquid phase. In case of large supersaturation, the nucleation temperature is lower. Then, it seems that the solid phase is directly nucleated. However, in fact, a two-step nucleation process is widely observed: droplets nucleate at first, and crystals nucleate in the supercooled droplets. This two-step nucleation easily explained by considering the depression of melting point of a nanoparticle. We reported that this two-step nucleation process is important for understanding the formation process of alumina dust [2]. In addition, the same nucleation process has been observed in various materials in ground-based experiments. Here, in addition to these results, we will present the latest findings concerning nucleation efficiency of titanium carbide, silica and silicate dust based on the nucleation experiments, which are currently being analyzed.
[1] Y. Kimura, et al., Science Advances, 3 (2017) e1601992.
[2] S. Ishizuka, Y. Kimura et al., Nature Communications, 9 (2018) 3820.
The international project was started in 2017 with a cooperation of JAXA/ISAS. First experiment was conducted by launching MASER 14, a sounding rocket of the Swedish Space Corporation, from Esrange Space Center in June 24, 2019 with the supports by German Aerospace Center and Prof. J. Blum of Technische Universität Braunschweig. In addition, the sounding rockets Black Brant IX 343 and 365 were launched from the White Sands Missile Range in October 2019 and September 2020, respectively, and performed microgravity experiments in collaboration with Dr. J. A. Nuth III of NASA/GSFC. Including non-recovery sounding rocket experiment in Japan, we have conducted five flights and a total of 20 nucleation experiments under microgravity environment with strong supports of Prof. Y. Inatomi, JAXA/ISAS. Seven kinds of materials were used so far: iron, tungsten oxide, alumina, silica, magnesium silicate, iron silicate, and titanium carbide.
Originally designed two different kinds of experimental devices (interferometer and IR spectrometer) were mounted on the rocket. The nucleation chamber (about 500 cc) with an evaporation material was prepared in each device. After reaching microgravity, the evaporation source was heated to generate high-temperature gas. The gas cooled down and nucleation occurred homogeneously to form dust analogues. These processes were observed in-situ.
In these experiments, several results have been obtained that differ from the commonly accepted theory. For example, homogeneous nucleation from the gas phase requires much larger supersaturation than expected. One of the major hindrances for nucleation was found to be dimer formation. For example, the nucleation of iron occurred only when the size of critical nucleus was smaller than dimer. The sticking probability was very small, only about 0.002% for iron [1]. It was also found that the final stable crystal was not nucleated directly from the gas phase, but via a liquid phase. In case of large supersaturation, the nucleation temperature is lower. Then, it seems that the solid phase is directly nucleated. However, in fact, a two-step nucleation process is widely observed: droplets nucleate at first, and crystals nucleate in the supercooled droplets. This two-step nucleation easily explained by considering the depression of melting point of a nanoparticle. We reported that this two-step nucleation process is important for understanding the formation process of alumina dust [2]. In addition, the same nucleation process has been observed in various materials in ground-based experiments. Here, in addition to these results, we will present the latest findings concerning nucleation efficiency of titanium carbide, silica and silicate dust based on the nucleation experiments, which are currently being analyzed.
[1] Y. Kimura, et al., Science Advances, 3 (2017) e1601992.
[2] S. Ishizuka, Y. Kimura et al., Nature Communications, 9 (2018) 3820.