While the planetary nebula phase is thought to last for around 10, 000 years, astronomers estimate the age of the outer filamentary portions of this halo to be 50, 000 to 90, 000 years.
虽然行星状星云阶段持续约1万年，天文学家估计其外晕的丝状物质年龄在5万至9万年间。
The application of phase transfer catalysis in fluorination of halo-aromatic compounds, mechanism, variety and reaction condition of fluorination are reviewed in this article.
从相转移催化剂的种类、原理、反应条件以及它在卤代芳香族化合物氟化反应中的应用等方面对相转移催化剂的作用进行概括和总结。
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