Reaction Path and Mechanisms of Li/SF6 Combustion
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摘要: 揭示Li/SF6燃烧反应机理是构建燃烧动力学模型、高效组织燃烧过程的关键基础, 但是现有实验测试以及宏观模拟方法难以揭示Li/SF6燃烧的分步反应机理。文中结合ReaxFF反应分子模拟和第一性原理计算方法研究了Li和SF6的微观反应过程, 分析了反应物和产物组分的动态演化特性, 获得了主要反应路径及其反应热。研究发现, 反应的起始过程为SF6分子S-F键的断裂, LiF是反应初始时刻的主要产物; 随着反应的进行, 多余的Li形成Li2并与S成键形成Li2S; 反应后期, 2个LiF分子结合反应形成Li2F2。基于反应物浓度变化获得了Li和SF6的总反应速率, 结果表明, 总反应速率与反应物浓度、反应物比例呈正相关关系, 这是因为反应物浓度或比例的增大均会导致反应物分子之间碰撞概率的增加; 初始温度对总反应速率的影响则相对较小。结合第一性原理得到的反应物焓值, 计算各个分步反应的放热量, 进而得到Li/SF6的反应热为−2216.7 kJ/mol, 与理论值及实验值均接近。研究结果为复杂燃烧反应的分步反应机理揭示和反应热计算提供了有效途径。
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关键词:
- Li/SF6燃烧反应 /
- 反应路径 /
- 分子动力学 /
- 第一性原理 /
- 反应热
Abstract: Revealing the combustion reaction mechanism of Li/SF6 fuel is the key basis for constructing the combustion kinetics model and efficiently organizing the combustion process. However, it is difficult to reveal the stepwise reaction mechanism of Li/SF6 combustion by existing experimental tests and macroscopic simulation methods. This paper studied the microscopic reaction processes of Li and SF6 by combining the ReaxFF molecular reaction simulation and first-principles calculation method. The dynamic evolution characteristics of reactants and product components were analyzed, and the main reaction paths and reaction heat were obtained. It is found that the breakage of the S-F bond in SF6 molecules is the initial stage of the reaction, and LiF is the main product of the initial reaction. As the reaction progresses, the excessive Li forms Li2 and bonds with S to form Li2S. At the later stage of the reaction, two LiF molecules combine with each other to produce Li2F2. Based on the variation of reactant concentration, the total reaction rates of Li and SF6 are obtained. The results show that the total reaction rate is positively correlated with the reactant concentration and reactant proportion because the rise of reactant concentration or proportion leads to a larger collision probability between reactant molecules. The impact of initial temperature on the total reaction rate is relatively small. According to the enthalpy value of the reactants obtained from the first-principles, the heat release of each stepwise reaction is calculated. The reaction heat for Li/SF6 combustion is −2 216.7 kJ/mol, which is close to the theoretical and experimental values. The research results provide an effective way to reveal the stepwise reaction mechanism and calculate the reaction heat of complex combustion reactions.-
Key words:
- Li/SF6 combustion reaction /
- reaction path /
- molecular dynamics /
- first-principles /
- reaction heat
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图 1 Li/SF6反应的分子动力学仿真模型
Figure 1. Molecular dynamics simulation model of Li/SF6 reaction
图 2 Li/SF6反应前后系统构象及体系中组分分子数随时间变化情况
Figure 2. Conformation of Li/SF6 system before and after reaction and variation of molecule number with time
图 3 不同初始温度下Li和LiF分子数随仿真时间变化情况
Figure 3. Variation of Li and LiF molecule number with simulation time under different initial temperatures
图 4 不同反应物浓度下Li、F和LiF分子数比例随时间变化情况
Figure 4. Variation of proportion of Li, F, and LiF molecule number with time under different reactant con- centrations
图 5 不同反应物比例下Li和LiF分子数占比随时间变化情况
Figure 5. Variation of proportion of Li and LiF molecule number with time under different reactant pro- portions
图 6 不同反应条件下反应速率随时间变化情况
Figure 6. Variation of reaction rate with time under different reaction conditions
图 7 反应过程中反应物的能量折线图及分子示意图
Figure 7. Energy line diagram and molecular diagram of reactants in reaction process
表 1 Li/SF6反应仿真工况
Table 1. Simulation conditions of Li/SF6 reaction
控制变量 变量值 Li/SF6分子数 64/8 128/16 256/32 Li/SF6比例
(SF6分子数为32)Li∶SF6=1∶1 Li∶SF6=2∶1 Li∶SF6=8∶1 Li∶SF6=16∶1 反应初始温度/K 500 600 700 800 900 
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表 2 Li/SF6反应过程中反应物/产物焓值计算结果
Table 2. Calculation results of reactant/product enthalpy value during Li/SF6 reaction
反应物
/产物Etotal
/(kcal/mol)Htotal/(kcal/mol) H=Etotal+Htotal
/(kcal/mol)T=500 K SF6 −625904.86 22.10 −625882.90 LiF −67435.93 4.96 −67435.93 2Li −9400.09 4.54 −9395.07 Li2S −259359.05 7.48 −259352.77 2LiF −134872.48 9.89 −134863.07 Li2F2 −134931.46 11.00 −134920.80
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