Abstract:

Optical fiber communication technology is a promising multi-channel communication means owing to the advantages of large communication capacity, low transmission loss and good security performance. As one of the related technologies, silicon-based electron gas SiCl₄ is used to prepare optical fiber preform by chemical vapor deposition. But, the metal and hydrogen impurities in SiCl₄ increase the absorption loss of optical fiber transmission due to the great vibration absorption of photons. In fact, the purity of SiCl₄ for optical fiber is very high, and the content of methylchlorosilane should be reduced to 5mg/kg or less. So far, photochlorination combined with distillation is the most suitable method to prepare high purity SiCl₄ for optical fiber. In this paper, reactive molecular dynamics simulation of the photochlorination process for the removal of methylchlorosilane impurities was carried out at the molecular level. At first, the removal effects of Cl₂, Cl radical, as well as reaction temperature on methylchlorosilane were compared and analyzed. Then, the main intermediate products formed in different simulation systems and the main conversion paths were explored. This research provided the basic chemical reaction mechanism and process improvement direction for the photochlorination technology. The simulation results showed that the removal efficiency of methylchlorosilane could reach double times of that of Cl₂ under the same simulation conditions when Cl radical was introduced into the reaction system. Besides, it was not monotonous of the relationship between the reaction temperature and the removal efficiency of methylchlorosilane. As a result, there was an optimal reaction temperature (373K) to maximize the removal efficiency of methylchlorosilane. The C—H bond and C—Si bond in methylchlorosilane molecule were difficult to break because of the large bond energy. For example, the C—H bond could be observed only when the reaction temperature in the system rose to a certain value (423K), and the C—Si bond can be observed only when the active Cl radical was introduced into the system.

Key words: chemical reaction, photochlorination, methylchlorosilane, molecular simulation, micro chemical reaction mechanism

摘要：

光纤通信技术具有通信容量大、传输损耗低、保密性能好等优点，是一种极有前途的多路通信手段。相关技术中利用硅基电子气SiCl₄经化学气相沉积法制备光纤预制棒，但是SiCl₄中的含金属杂质和含氢杂质因对光子产生很大的振动吸收而增加光纤传输中光的吸收损耗。光纤用SiCl₄对纯度的要求极高，其中含氢杂质甲基氯硅烷的含量需降低到5mg/kg或者更低。通过光氯化法结合精馏工艺提纯SiCl₄是目前为止制备光纤用高纯SiCl₄最合适的方法。本文针对光氯化法去除甲基氯硅烷杂质的工艺过程进行分子层面的反应分子动力学模拟研究，重点比较分析了Cl₂与Cl自由基及反应温度对甲基氯硅烷的去除效果，并探究了不同的模拟体系中形成的主要的中间产物及其主要的转化路径，为光氯化除杂反应提供了基础的化学反应机理及工艺的改进方向。模拟结果表明，在反应体系中引入Cl自由基后对甲基氯硅烷的去除效率能够达到相同模拟条件下Cl₂的2倍；体系的反应温度与甲基氯硅烷的去除效率之间的相互关系并不是单调变化的，存在最佳反应温度（373K）使甲基氯硅烷的去除效率达到最大；甲基氯硅烷分子中的C—H键及C—Si键因具有较大的键能而难断裂，只有当体系内反应温度升高到一定值后（423K）才可观察到C—H键的断裂，只有在体系内引入活泼的Cl自由基后才可观察到C—Si键的断裂。

关键词: 化学反应, 光氯化, 甲基氯硅烷, 分子模拟, 微观化学反应机理