H2O、CO2和CO组成的冰在耐火材料表面形态的透射电镜研究
Transmission Electron Microscopy Study of the Morphology of Ices Composed of H2O, CO2, and CO on Refractory Grains
摘要：
人们一直认为，分子云和原行星盘中的颗粒上的冰是由均匀的层形成的，而不管它们的成分或结晶度如何。为了验证这一假设，我们观察到的H2O沉积到耐火基板和结晶的无定形冰（H2O，CO2和CO）使用超高真空透射电子显微镜。在H2O沉积实验中，我们发现在130 K以上的温度下形成了三维的结晶冰岛（Ic）。晶化实验表明，均匀的薄膜的非晶CO和H2O成为三维岛的多面体晶体，非晶CO2，另一方面，成为一个薄膜的纳米结晶CO2覆盖的非晶H2O。我们的观察表明，晶体形态强烈地依赖于不仅对冰的组成，但也对基板。利用有关沉积冰的结晶度和非晶冰的结晶时间尺度的实验数据，我们说明了冰的结晶度在空间上的标准，并概述了分子云中的冰粒的宏观形态如下：无定形H2O覆盖的耐火颗粒均匀，CO2纳米晶体嵌入在无定形H2O，和多面体CO晶体附着在无定形H2O。此外，在原行星盘的晶粒形态的变化。这些结果对分子的化学演化、非热脱附、冰粒的碰撞和烧结具有重要意义。
英文摘要：
It has been implicitly assumed that ices on grains in molecular clouds and proto planetary disks are formed by homogeneous layers regardless of their composition or crystallinity. To verify this assumption, we observed the H2O deposition onto refractory substrates and the crystallization of amorphous ices (H2O, CO2, and CO) using an ultra-high-vacuum transmission electron microscope. In the H2O-deposition experiments, we found that three-dimensional islands of crystalline ice (Ic) were formed at temperatures above 130 K. The crystallization experiments showed that uniform thin films of amorphous CO and H2O became three-dimensional islands of polyhedral crystals; amorphous CO2, on the other hand, became a thin film of nano crystalline CO2 covering the amorphous H2O. Our observations show that crystal morphologies strongly depend not only on the ice composition, but also on the substrate. Using experimental data concerning the crystallinity of deposited ices and the crystallization timescale of amorphous ices, we illustrated the criteria for ice crystallinity in space and outlined the macroscopic morphology of icy grains in molecular clouds as follows: amorphous H2O covered the refractory grain uniformly, CO2 nano-crystals were embedded in the amorphous H2O, and a polyhedral CO crystal was attached to the amorphous H2O. Furthermore, a change in the grain morphology in a proto-planetary disk is shown. These results have important implications for the chemical evolution of molecules, non-thermal desorption, collision of icy grains, and sintering.
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