Cluster Evolution at Early Stages of 1,3,5-Triamino-2,4,6

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Clusters Evolution at Early Stages of 1,3,5-Triamino-2,4,6-trinitrobenzene under Various Heating Conditions: A Molecular Reactive Force Field Study Yushi Wen, Xianggui Xue, Xinping Long, and Chaoyang Zhang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b03795 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 18, 2016

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The Journal of Physical Chemistry

Clusters Evolution at Early Stages of 1,3,5-Triamino-2,4,6-trinitrobenzene under Various Heating Conditions: A Molecular Reactive Force Field Study Yushi Wen, Xianggui Xue, Xinping Long, and Chaoyang Zhang* Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P. O. Box 919-311, Mianyang, Sichuan 621900, China.

Abstract: We carried out reactive molecular dynamics simulations by ReaxFF to study the initial events of an insensitive high explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) against various thermal stimuli including constant temperature heating, programmed heating, and adiabatic heating to simulate TATB suffering from accidental heating in reality. The cluster evolution at the early stage of the thermal decomposition of condensed TATB was the main focus, as cluster formation primarily occurs when TATB is heated. The results show that the cluster formation is the balance of the competition of intermolecular collision and molecular decomposition of TATB, i.e., an appropriate temperature and certain duration are required for cluster formation and preservation. The temperature in the range 2000–3000K was found to be the optimum for the fast formation and a period of preservation. Besides, the intra- and intermolecular H transfers are always favorable, whereas the C-NO2 partition was favorable at high temperature. The simulation results are helpful to deepen the insight into the thermal properties of condensed TATB.

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1. INTRODUCTION Many deflagration and detonation mechanisms of explosives remain unclear and require deeper insight owing to their significance in both civilian and military purposes. Clusters have been found experimentally and theoretically as the important intermediates and/or final products of the deflagration and detonation of some explosives. The experiments indicated that the clusters dominantly comprised C with a smaller component of N, O, and H. Various carbon polymorphs such as diamond, graphite, nanotubes, and other amorphous forms were observed in these clusters. Moreover, C-rich explosives and explosive mixtures such as 1,3,5-trinitrotoluene (TNT)1-3, explosive/graphite4, 1,3,5-trinitro-1,3,5-triazinane (RDX)/TNT4, trinitrobenzene/RDX5, and picric acid with addition of paraffin or benzene6 favor the cluster formation. Moreover, clusters have also been reported in molecular dynamics (MD) simulations of heated nitromethane (NM)7-10, 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)11,β-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)11and TNT12. Cluster evolution can affect the duration of the deflagration and detonation reactions, one of the most crucial indexes of detonation properties. For example, Shaw et al.13studied the C-rich clusters in a detonation region with a premise of a diffusion-limited clustering process. They showed that it takes 1,000 times as long for the C clusters to release 90% of the latent energy, leading to a very low reaction rate in the reaction zone. Manaa et al.14determined that the high concentrations of N-rich heterocyclic clusters generated from shocked TATB could play a role as a reactivity retardant. In this case, the highly dense clusters prolong the decay time of TATB. Our recent simulations15 on heated TATB, HMX, and pentaerythritol tetranitrate (PETN) showed that the cluster distribution (size and amount) and the cluster lifetime are variable and depend on the chemical components and reaction temperatures. Moreover, the apparent oxygen balance-sensitivity 2


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relationship could be well interpreted by the delayed effect of clustering, i.e., it retards the secondary and further decomposition of explosives to final small molecule products, as well as the energy release.

Figure 1. Molecular and crystal structure for TATB. The C, H, O, and N atoms are indicated in grey, green, red, and blue, respectively. These representations are considered in the following figures.

TATB is the so called wood explosive with low sensitivity to heat, impact, shock and electric sparks, and therefore has been applied as an insensitive explosive and a desensitizer in mixed explosives. Its molecular and crystal structures are shown in Figure 1. It is a typical explosive favorable for the cluster formation during heating, which should strongly dependent on its highly superfluous C atoms or highly negative oxygen balance. However, in practice, the accidental heating style can be varied. Constant temperature heating, programmed heating, and adiabatic heating are three representatives. For example, a small amount of samples heated in a constant temperature bath can be regarded as constant temperature heating; the condition of a DSC examination or a big cook-off test16 can be regarded as programmed heating; when a big sample is restricted in a shell and preheated and thereafter has no heat exchange with circumstance, it is thought to be adiabatically heated17. Practical applications of TATB should be among these three representative conditions. Furthermore, to a certain extent, the results of explosives suffering from low-temperature thermal decomposition are helpful to understand the mechanism of explosives undergoing shock, as there has been a relationship between the kinetics of the low-temperature thermal decomposition and reaction rates in the reaction zone of their detonation18,19. Nevertheless, 3



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previous simulations only focused on the constant temperature heating of TATB, and other heating programs have not been explored. Therefore, we were motivated to perform a series of simulations considering these three representative heating regimes to obtain comprehensive insight into the heat-induced cluster formation under various conditions. A series of MD simulations were carried using the molecular reactive force field to gain the knowledge of the earlier cluster evolution of TATB heated under various conditions including constant temperature heating at 3000 K for 50 ps, heating from 300 to 3000 K at the rates of 20, 100, and 200 K/ps, and adiabatic heating for 50 ps with preheating at 3,000 K for 0.5 ps. The related processes of TATB against various accidental heating are important to investigate. The cluster evolution of TATB was found to be strongly dependent on heating temperature and duration. At the time scale of simulations of 50 ps, the TATB cluster formation and existence are favorable within the temperature range 2000–3000 K at a fixed density of 1.937 g/cm3. The higher temperature will cause fast cluster decomposition, and the lower temperature will lead to very slow cluster formation. Overall, the cluster formation and decomposition are a balance of the collision of TATB molecules; clusters will not form under too slow or too fast collision. During the practical complicated defragment and detonation of TATB, the clusters are formed and aggregated to soot particles, because of the suitable conditions such as fit temperature, pressure, and duration. 2. METHODOLOGIES A recently developed molecular reactive force field, ReaxFF20, has been confirmed to be effective to describe the reactions of heated and shocked explosives, fuel combustion, and pyrolysis9,21-26. The improved version ReaxFF_lg27 has been used to simulate the thermal decay of TATB and the cluster evolution, showing its feasibility to TATB already11,15, and no verification is required. The simulations were performed on a supercell of TATB, enlarged from the original unit 4


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cell 6 ascertained experimentally at room temperature with 6×6×6times along their a, b, and c axes. The cell includes 432 TATB molecules 10368 atoms. The information of the supercell is detailed in Figure S1of the Supporting Information (SI). As mentioned above, three heating programs including constant temperature heating, programmed heating, and adiabatic heating were considered in our simulations. After relaxing the TATB supercell using energy minimization and the conjugation gradient algorithms, a MD simulation was started with a canonical ensemble (NVT) and the Berendsen thermostat method at 300 K to further relax the supercell for 2 ps. The relaxed supercell was used as the initial structure for subsequent five separate MD simulations at a fixed volume: (1) constant temperature heating at 3000 K for 50 ps (i.e., an NVT MD simulation); (2)–(4) programmed heating from 300 K to 3000 at the rates of 20, 100, and 200 K/ps, for 135, 27, and 13.5 ps, respectively, and (5) adiabatic heating for 50 ps with a preheating at 3000 K for 0.5 ps (i.e., an NVE MD simulation). A temperature of 3,000 K was selected for the simulation and attributing to the TATB cluster formation, and decomposition occurred within the simulation time limit according to our previous study15. For each simulation, the timestep was set to 0.05 fs, and the dynamic trajectory including atomic positions and velocities was recorded every 100 fs. These data were collected to analyze the molecular species to provide information on chemical reactions based on the idea of Strachanet et al.21 Because this study focuses on clusters, their confirmation becomes crucial. Herein, we defined a fragment as a cluster provided its molecular weight (MW) is above the MW of the TATB molecule, with a lifetime of >100 fs, and the largest cluster is defined for the cluster with the highest MW. Thereby, the cluster evolution analysis was implemented by a series of FORTRAN scripts of reported by us9,15,28. The bond order minimum values were applied to determine the chemical species of heated TATB and are detailed in Figure S2 of the SI. 3. RESULTS AND DISCUSSION 5



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This study focused on the effect of heating programs on the cluster evolution of TATB. Therefore, the heating histories under five conditions were examined as shown in Figure 2. As demonstrated in the figure, the temperature was constant at 3000 K for 50 ps for the NVT MD simulation; the temperature increases linearly from 300 to 3000 K under the programmed heating conditions and the higher heating rate rapidly increase the temperature; and the temperature first decreases slightly and thereafter increases rapidly during the adiabatic condition probably attributed to the self-heating caused by the first endothermic and the subsequent exothermic TATB decomposition29. Overall, the heating histories regarding temperature and duration can be distinguished. The temperature of the NVE simulation is significantly above 3000 K when time exceeds 5 ps. The temperature of the NVT simulation is fixed at 3000 K, and those of the programmed heating are always

Cluster Evolution at Early Stages of 1,3,5-Triamino-2,4,6

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