17:15 〜 19:15
[MIS13-P01] 気相から形成されるナノ粒子の均質核生成における非古典的経路の寄与
キーワード：ロケット、ダスト、結晶成長、微小重力
To understand the formation process of cosmic dust, we have been conducting experiments to synthesize dust analogues of iron, tungsten oxide, alumina, silica, silicate, titanium carbide, and carbon from the gas phase using the microgravity environment obtained by sounding rockets. In microgravity experiments, it is possible to suppress the density difference convection that occurs when the evaporation source is heated in the gas. In microgravity, it is possible to synthesize dust analogues without disturbing the nucleation environment. In such a low-fluctuation environment, it is known that nucleation occurs when the supersaturation is extremely high. This is easy to understand when you consider the phenomenon that supercooled water in a static state does not begin to freeze, but when you shake the bottle, it immediately begins to freeze. The dust analogues that are formed remain in the vacuum chamber for a long time, allowing in-situ measurement of the spectral changes.
So far, we have conducted six sounding rocket experiments, two in Japan and four overseas. The sounding rockets used in Japan sink into the Pacific Ocean immediately after the experiment, therefore neither the samples nor the experimental equipment can be recovered. As a result, it is not possible to analyze the particles produced. The analysis was carried out by applying the latest nucleation theory to the time-development data on nucleation temperature and concentration using an interferometer. In the first iron nucleation experiment conducted in Japan, it was found that homogeneous nucleation only occurs when the degree of supersaturation is so high that the size of critical nucleus becomes as small as 1 atom. This is because the formation of dimer is the greatest barrier. When the sticking probability was calculated, it was found to be an extremely small value of 0.002%. In the second domestic sounding rocket experiment, it was found from the time variation of the infrared spectrum that alumina droplets nucleate from the vapor phase, and then crystallize due to supercooling. This can be understood based on Ostwald's rule of stages. On the other hand, it is well known that the melting point of nanoparticles is lower than that of corresponding bulk, and since the size of the nuclei is very small, on the order of nanometers, it can be thought that the temperature at the time of nucleation is higher than the melting point of alumina nanoparticles, and then they become supercooled.
In the experiments on silica and titanium carbide carried out using sounding rockets overseas, the generated particles were successfully recovered and observed using a transmission electron microscope (TEM). The sticking probability and surface free energy can be calculated from the nucleation temperature, nucleation concentration and cooling time scale obtained using an interferometer and particle size measured by TEM. Here, the particle size was very large compared to possible volume, and the sticking probability was found to be over 100%. This indicates that the generated nuclei cannot simply grow by absorbing the surrounding vapor. It is well known that when nanoparticles contact each other, they fuse together to reduce surface energy. Therefore, it is thought that the sticking probability exceeding 100% is due to the fusion growth of the particles. Looking again at the TEM image, it was found that there were many small particles with diameters of a few nm. We analyzed the data assuming that these fine particles grew directly from the nucleus. As a result, we obtained reasonable sticking probability and surface free energy.
The result that fusion growth between particles is occurring suggests that non-classical nucleation must be considered in understanding dust formation. In the presentation, we will discuss the generality of the multi-step nucleation process, which is one of non-classical nucleation processes.
We thank J. A. Nuth and F. T. Ferguson of NASA and J. Blum of Technische Universität Braunschweig for their help in the microgravity experiments. Microgravity experiments were conducted with the supports of NASA Sounding Rocket Program and the Swedish Space Corporation (SSC) with financial support from the German Space Agency (DLR) with funds provided by the Federal Ministry for Economic Affairs and Climate Action (BMWK), and the Institute of Space and Astronautical Science (ISAS), the Japan Aerospace Exploration Agency (JAXA). Development of the experimental system was supported by the Technical Division of the Institute of Low Temperature Science, Hokkaido University, and the Advanced Machining Technology Group of JAXA. This work was supported by JSPS KAKENHI Grant Number 20H05657.
So far, we have conducted six sounding rocket experiments, two in Japan and four overseas. The sounding rockets used in Japan sink into the Pacific Ocean immediately after the experiment, therefore neither the samples nor the experimental equipment can be recovered. As a result, it is not possible to analyze the particles produced. The analysis was carried out by applying the latest nucleation theory to the time-development data on nucleation temperature and concentration using an interferometer. In the first iron nucleation experiment conducted in Japan, it was found that homogeneous nucleation only occurs when the degree of supersaturation is so high that the size of critical nucleus becomes as small as 1 atom. This is because the formation of dimer is the greatest barrier. When the sticking probability was calculated, it was found to be an extremely small value of 0.002%. In the second domestic sounding rocket experiment, it was found from the time variation of the infrared spectrum that alumina droplets nucleate from the vapor phase, and then crystallize due to supercooling. This can be understood based on Ostwald's rule of stages. On the other hand, it is well known that the melting point of nanoparticles is lower than that of corresponding bulk, and since the size of the nuclei is very small, on the order of nanometers, it can be thought that the temperature at the time of nucleation is higher than the melting point of alumina nanoparticles, and then they become supercooled.
In the experiments on silica and titanium carbide carried out using sounding rockets overseas, the generated particles were successfully recovered and observed using a transmission electron microscope (TEM). The sticking probability and surface free energy can be calculated from the nucleation temperature, nucleation concentration and cooling time scale obtained using an interferometer and particle size measured by TEM. Here, the particle size was very large compared to possible volume, and the sticking probability was found to be over 100%. This indicates that the generated nuclei cannot simply grow by absorbing the surrounding vapor. It is well known that when nanoparticles contact each other, they fuse together to reduce surface energy. Therefore, it is thought that the sticking probability exceeding 100% is due to the fusion growth of the particles. Looking again at the TEM image, it was found that there were many small particles with diameters of a few nm. We analyzed the data assuming that these fine particles grew directly from the nucleus. As a result, we obtained reasonable sticking probability and surface free energy.
The result that fusion growth between particles is occurring suggests that non-classical nucleation must be considered in understanding dust formation. In the presentation, we will discuss the generality of the multi-step nucleation process, which is one of non-classical nucleation processes.
We thank J. A. Nuth and F. T. Ferguson of NASA and J. Blum of Technische Universität Braunschweig for their help in the microgravity experiments. Microgravity experiments were conducted with the supports of NASA Sounding Rocket Program and the Swedish Space Corporation (SSC) with financial support from the German Space Agency (DLR) with funds provided by the Federal Ministry for Economic Affairs and Climate Action (BMWK), and the Institute of Space and Astronautical Science (ISAS), the Japan Aerospace Exploration Agency (JAXA). Development of the experimental system was supported by the Technical Division of the Institute of Low Temperature Science, Hokkaido University, and the Advanced Machining Technology Group of JAXA. This work was supported by JSPS KAKENHI Grant Number 20H05657.