[M-IS10] 結晶成長、溶解における界面・ナノ現象

[MIS10-09] Homogeneous Nucleation from Vapor in Al-O Binary System

*石塚 紳之介¹、川野 潤²、山﨑 智也³、羽馬 哲也³、木村 勇気³ (1.国立環境研究所、2.北海道大学大学院理学研究科、3.北海道大学低温科学研究所)

キーワード：均質核生成、宇宙ダスト

A gaseous molecule can have a chemical formula that is inconsistent with its ability to act as a building block for crystals; for example, hot AlO and AlO₂ vapors cannot solidify directly to crystals. The contributions of such gases to homogeneous nucleation processes are unknown. Using in situ IR spectroscopy on nucleating nanoparticles, we show that a liquid-like behavior of nucleating nanoparticles in the Al–O system induces immiscible phase segregation. In an oxygen-deficient atmosphere, aluminum oxide nanoparticles formed with a unique shape composed of an Al metal head and an anisotropic Al₂O₃ crystalline tail. The anisotropic nanoparticles are larger than Al₂O₃ nanoparticles formed in a sufficient oxygen condition. Fewer nuclei may be available in the supersaturated gas, indicating that homogeneous nucleation is initiated by oxygen-bearing species. The nuclei grow by incorporation of Al-bearing species, changing the composition of the particle as a whole toward an Al-rich material. In situ IR measurements revealed that the degree of anisotropy continues to increase, suggesting that oxygen-bearing species also formed by oxidation on the surface of the molten Al-rich head. The scenario is supported by quantum-chemical calculations indicating that oxygen-deficient (AlO)_n clusters as few as 16 molecules induce aggregation of Al atoms. Because Al metal has a significantly lower melting temperature than Al₂O₃, the immiscibility of the nucleating nanoparticles leads to vapor–liquid–solid growth. Astronomical implication will be given in the presentation.