RDX及其衍生物高温热解的反应分子动力学模拟

Pyrolysis of RDX and Its Derivatives via Reactive Molecular Dynamics Simulations

本文采用ReaxFF反应分子动力学方法,研究了RDX及其衍生物晶体在高温条件下(2000、2500和3000 K)的热分解机理以及主要产物随时间的变化情况。结果表明:RDX及其衍生物晶体热解的第一步反应均为N―NO_2键断裂生成NO_2分子,随后反应由于六元环上和侧链基团的不同导致侧链脱除或开环反应的顺序不同。在这四种单胞体系热解中,NO_2和NO分子为共同的中间产物,形成之后迅速发生次级反应并最终生成N_2。各体系热解终产物一致,均为N_2、H_2O和CO_2,其中N_2分子数最多,大于20个。由于原始分子结构和组成中C/N比、H/O比的不同,各体系热解后H_2O和CO_2分子数目相差较大。不同温度下,各单胞体系热分解生成的最大含碳团簇中C原子数均较小。在进一步超胞体系的模拟中,RDX和RDX-D2体系生成的含碳团簇中C原子数分别达到约30和16个,远高于单胞模拟,且受温度影响较大;而RDX-D1和RDX-D3单胞或超胞模拟结果相近,均未生成含碳团簇,仅存在小分子含碳碎片。因此,初始分子的结构和元素比对含碳团簇的生成有明显影响。

The thermal decomposition mechanisms of cyclotrimethylene trinitramine （RDX） and its derivatives are investigated using ReaxFF reactive molecular dynamics simulation at high temperatures （2000, 2500, and 3000 K）. It is shown that the first pyrolysis step of RDX and its derivatives is the N--NO2 homolytic cleavage to produce NO2; however, the subsequent reaction mechanism is completely different due to the different six- membered rings and side chain groups. In these four model systems, NO2 and NO molecules are common intermediates in the thermal decomposition process, which eventually transform to N2 in the following reactions. The most stable products are N2, H20, and CO2 in the thermal process, among which N2 has the maximum molecular number （more than 20）. The numbers of cracked H20 and CO, molecules are very different in the four model systems due to the different C/N and H/O ratios. At different temperatures, the maximum numbers of carbon atoms in the carbon clusters are all small in the unit cell simulations of the four systems. In the further super cell simulation for RDX and RDX-D2 the numbers of carbon atoms reach about 30 and 16, respectively, which are higher than those from the unit cell simulation. In both the unit and super cell simulations for RDX-D1 and RDX-D3, the carbon cluster cannot be formed and there exist only small carbon molecular fragments. Therefore, the initial molecular structure and elemental ratio have a large effect on the production of carbon clusters.