Siloxane working fluid has good environmental performance and thermodynamic performance, widely used in high temperature heat source ORC system. During the filling process of working fluid and operation in ORC system operation, air may leak into the system. The reaction of siloxane working fluid with oxygen in high temperature environment will produce abrasive silica, which will cause serious damage to the system. The oxidative decomposition mechanism of hexamethyldisiloxane (MM) was analyzed by density functional theory (DFT) and ReaxFF molecular dynamic simulation. It is found that the initial reactions of MM oxidation decomposition can be divided into two categories. The first type is the thermal decomposition of MM itself. The second one is the collision reaction between MM molecule and O2 molecule. Comparing the two kinds of chemical reaction barriers, MM is more likely to react with O2. Pathways 2–3 involving the collision between O2 molecule and Si atom are the optimal pathway of initial reaction kinetics, and its reaction energy barrier is 204.1 kJ/mol. It was observed from ReaxFF MD simulations that the main products of MM oxidative decomposition were CH4, CH2O, CO, CO2, SiO and amorphous silica. In the process of oxidative decomposition, small free radicals such as CH3, O, H, OH played a very important role in the oxidative decomposition of working fluids, which greatly promoted the decomposition of working fluids and the formation of main products.

中文翻译：

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六甲基二硅氧烷的氧化分解机理


硅氧烷工作流体具有良好的环境性能和热力学性能，广泛应用于高温热源ORC系统。在工作流体的填充过程和 ORC 系统运行中的操作过程中，空气可能会泄漏到系统中。硅氧烷工作流体在高温环境下与氧气反应会产生磨蚀性二氧化硅，对系统造成严重损坏。通过密度泛函理论 （DFT） 和 ReaxFF 分子动力学模拟分析了六甲基二硅氧烷 （MM） 的氧化分解机制。研究发现，MM氧化分解的初始反应可分为两大类。第一种是 MM 本身的热分解。第二个是 MM 分子和 O2 分子之间的碰撞反应。比较两种化学反应势垒，MM 更有可能与 O2 反应。涉及 O2 分子和 Si 原子碰撞的途径 2-3 是初始反应动力学的最佳途径，其反应能垒为 204.1 kJ/mol。从 ReaxFF MD 模拟中观察到，MM 氧化分解的主要产物是 CH4、CH2O、CO、CO2、SiO 和非结晶二氧化硅。在氧化分解过程中，CH3、O、H、OH等小自由基在工作流体的氧化分解中起了非常重要的作用，极大地促进了工作流体的分解和主要产物的形成。