Chinese Journal of Chemical Physics  2016, Vol. 29 Issue (5): 557-563

The article information

Yang Zhen, He Yuan-hang
杨镇, 何远航
Pyrolysis of CL20-BTF Co-crystal via ReaxFF-lg Reactive Force Field Molecular Dynamics Simulations
CL20/BTF共晶高温热分解ReaxFF/lg分子动力学模拟
Chinese Journal of Chemical Physics, 2016, 29(5): 557-563
化学物理学报, 2016, 29(5): 557-563
http://dx.doi.org/10.1063/1674-0068/29/cjcp1603054

Article history

Received on: March 22, 2016
Accepted on: July 4, 2016
Pyrolysis of CL20-BTF Co-crystal via ReaxFF-lg Reactive Force Field Molecular Dynamics Simulations
Yang Zhen, He Yuan-hang     
Dated: Received on March 22, 2016; Accepted on July 4, 2016
State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China
Author: E-mail:heyuanhang@bit.edu.cn
Abstract: To obtain detailed information on the potential energy, the evolution of species, the initial reaction paths, and thermal decomposition products, we conducted simulations on pyrolysis process of CL20/BTF co-crystal using the ReaxFF/lg reaction force field, with temperature set at 2000 K to 3000 K. With the analysis of evolution curves of potential energy based on exponential function, we obtain the overall characteristic time. Via a description of the total package reaction with classical Arrhenius law, we obtain the activation energy of CL20/BTF co-crystal: Ea=60.8 kcal/mol. Based on the initial path of CL20/BTF co-crystal thermal decomposition we studied, we conclude that N-NO2 bond of CL20 molecules breaks first, working as a dominant role in the initial stage of thermal decomposition under the condition of different temperatures, and that all CL20 molecules completely decompose before BTF molecular regardless of different temperatures. We also find that the main products of CL20/BTF co-crystal are NO2, NO, NO3, HNO, O2, N2, H2O, CO2, N2O, and HONO, etc., on which the temperature forms certain influence.
Key words: ReaxFF/lg     Molecular dynamics     CL20/BTF co-crystal     Reaction mechanism     Pyrolysis    
I. INTRODUCTION

With continuously increasing demands of weapon system on the energy materials, the materials not only need have properties such as high density, high heat, and high pressure, but also retain very high security performance, thus great importance has been attached to the study of new energy materials in the world. Energy materials constructed by multi-nitro cage compound have become a hot topic recently and 2, 4, 6, 8, 10, 12-hexanitro-2, 4, 6, 8, 10, 12-hexaazaisowurtzitane (hereinafter referred to as HNIW, commonly known as CL20) first prepared by Nielsen et al. [1] is a typical representative. CL20 retains properties such as high density ($\sim$2. 0 g/cm$^3$), high oxygen balance (-10. 95%), high heat production and high energy output (14% above octahydro-1, 3, 5, 7-tetranitro-1, 3, 5, 7-tetrazocine (HMX)) [2, 3], making it one of the most promising energy materials.

Co-crystal technology has been put into application to improve the performance of the single crystal powder in energy material field in recent years [4, 5]. Onas et al. [6] and Yang et al. [7] have synthesized CL20 and TNT successively by experiments, with a molar ratio of 1:1, a density of 1. 92 g/cm$^3$ (approximate to that of CL20), a detonation volocity ratio slightly lower than that of CL20, and the sensitivity much lower than that of CL20. Onas et al. [8] then synthesized a 2:1 molar ratio co-crystal of CL20 and HMX, displaying a similar co-crystal explosive sensitivity and higher explosive ratio compared with that of HMX. Wang et al. [9] synthesized a 1:1 molar ratio co-crystal of CL20 and 1, 3-dinitrobenzene (DNB). In 2012, Yang et al. [10] synthesized a 1:1 molar ratio co-crystal CL20 and BTF by the solvent evaporation method, claiming that the co-crystal possessed superior performance than BFT crystal and displayed density and detonation properties higher than CL20/TNT co-crystal explosive as shown in Table I, but studies on the properties of co-crystal of CL20 and BTF were not enough.

Under the experimental conditions, it is extremely challenging to analyze the reaction steps and products, especially at the initial stage, due to the complexity of the reaction under extreme conditions, the large number of intermediate products, and short reaction time. Abundant achievements [11-18] have been made by the simulation of reaction process of energy materials under extreme conditions via ReaxFF/lg reaction force field, to the extent demonstrating from the scale of atoms and molecules the initial reaction process and mechanism within one picosecond, which have provided information that experiments and quantum mechanics can not. In this work, we conducted simulation of CL20/BTF co-crystal initial thermal decomposition process with ReaxFF/lg reaction force field from LAMMPS molecular simulator [19] program package and displayed the co-crystal thermal decomposition mechanism from atomic scale, providing vital information for damage assessment and the safety storage, and guiding significance for the further synthesis of co-crystal of excellent properties in the future.

II. METHODOLOGY AND CALCULATION DETAILS A. ReaxFF/lg reactive force field

ReaxFF force field allows for accurate description of bond breaking and bond formation because it is based on a bond order/bond distance relationship. It is used to determine the connectivity between any two atoms through bond order. Energy of the system can be expressed as:

(1)

where $E_{\rm{bond}}$ is bond order and bond energy, $E_{\rm{lp}}$ is the lone pair energy. $E_{\rm{over}}$ is energy penalty for atom under-/over coordination. $E_{\rm{under}}$ is the energy contribution for the resonance of the $\pi$-electron between attached undercoordinated atomic centers. $E_{\rm{val}}$, $E_{\rm{pen}}$, and $E_{\rm{coa}}$ are valence angle terms. $E_{\rm{C2}}$ is energy contribution that captures the stability of C2. $E_{\rm{triple}}$ is triple bond correct term. $E_{\rm{tors}}$ is energy of torsion angle. $E_{\rm{conj}}$ is the contribution of conjugation effects to the molecular. $E_{\textrm{H-bond}}$ is hydrogen bond interaction term. $E_{\rm{vdW}}$ is nonbonded van der Waals interations. $E_{\rm{Coulomb}}$ is Coulomb interactions. In ReaxFF/lg the total energy of the system can be expressed as the following:

(2)

where $E_{\rm{lg}}$ is the long-range-correction terms can be determined using the low-gradient model:

(3)

Here $r_{ij}$ is the distance between atom i and atom j, $R_{eij}$ is the equilibrium vdW distance between atoms i and j, and $c_{\textrm{lg}, ij}$ is the dispersion energy correction parameter.

Table I Detonation properties for CL20, CL20/TNT co-crystal, BTF, and CL20/BTF co-crystal[10].
B. Simulation of models and details

We studied CL20/BTF co-crystal with parameters from experiments on 1:1 molar ratio CL20/BTF co-crystal cell [10]. Figure 1 shows the structure of CL20/BTF co-crystal supercell in periodic boundary conditions. With the uniform velocity distribution, we obtain the initial speed of all the atoms generated at the temperature of 300 K and minimum energy structure system after optimizing the location of atoms. We then relaxed the system for 1 ps using isothermal-isobaric system (NPT. In NPT, thermostat is Nose-Hoover thermostat is Nose-Hoover thermostat barostat is Nose-Hoover barostat) to set the pressure and temperature at 0 Pa and 300 K respectively, obtaining the contrast result between parameters of the co-crystal cell and experimented results, as shown in Table II. After relaxation, With canonical (NVT. In NVT, thermostat, Nose-Hoover thermostat), through berenden thermostat rapid temperature heating systems, we then performed 60 ps simulation with the temperature set respectively at 2000, 2250, 2500, 2750, and 3000 K. Time step is 0. 1 fs. In NPT, $T_{\rm{damp}}$ and $P_{\rm{damp}}$ are 1 and 1000, respectively; in NVT, $T_{\rm{damp}}$ is 10. Time step is 0. 1 fs. Bond cutoff is 0. 3. The bond value determines whether new bond forms between atoms. Any fragments that are connected by a bond order larger than 0. 3 are taken as new molecules.

FIG. 1 Structures of CL20/BTF supercell (3×2×1) and single molecule.
Table II The lattice parameters of CL20/BTF co-crystal.
III. RESULTS AND DISCUSSION A. Potential energy and total species

Figure 2 shows the time evolution curve of CL20/BTF co-crystal supercell system at different temperatures. We find at different temperatures, the evolution trend of the potential energy is similar. Because endothermic reaction occurs in an extremely short time in the system, potential energy rapidly increases to the maximum, which is the induction period (induction time $t_{T-1}$, see Table III). The higher the temperature is, the smaller the $t_{T-1}$ will be, and then the system potential energy decays quickly. The higher the temperature is, the faster the potential energy decays.

FIG. 2 Potential energy for CL20/BTF system.

Figure 3 shows the evolution curve of the total species over time in the supercell system at different temperatures. At the initial moment, only CL20 and BTF are in the system, and with the continuous decomposition of CL20 and BTF, species gradually increase. During 0-10 ps, species increases rapidly, and chemical reactions reach dynamic equilibrium, so does the species (about 70). As can be seen from the Fig. 3, the higher the temperature is, the faster the reaction is, and the more species will be, at the initial stage of thermal decomposition.

FIG. 3 Time evolution of total species for the system.
Table III Time evolution of total species for the system.
B. Total reaction analysis of the Arrhenius law

The system potential energy begins to decay after induction period, and temperature conditions have significant influence on the system's potential energy attenuation. This process can be fitted with the simple exponential function [15].

(4)

where $E_{\rm{p0}}$ is the asymptotic energy of the products, $\Delta E_{\rm{p}}$ is the exothermicity of the reaction, and $\tau$ is overall characteristic time of reaction. Table III shows the parameters obtained under different temperature conditions. Classic Arrhenius law reveals the relationship between the reaction rate and temperature. The logarithmic relation is:

(5)

is rate constant, $A$ is exponential prefactor, $R$ is general gas constant, $E_{\rm{a}}$ is activation energy. According to $k$=1/$\tau$, we obtain the following equation:

(6)

where $\ln\tau$ is proportional to the 1/$T$. The slope obtained by fitting is shown in Fig. 4. Figure 4 is relationship between $\ln\tau$ and 1/$T$, $\tau$ is the average of 4 samples, seen in Table IV, $\ln\tau$ and 1/$T$ present linear relationship. Through calculation, we get $E_{\rm{a}}$=65. 68 kcal/mol, close to 62. 18 kcal/mol presented in Ref. [23].

FIG. 4 Relationship of ln τ and 1/T. The fitting line is y=-27. 2 +7. 9x.
Table IV τ obtained from fitting potential energy of samples.
C. CL20/BTF co-crystal pyrolysis trigger reaction path

First of all, we analyzed CL20 and BTF decomposition on the whole. Figure 5 shows the evolution curve of CL20/BTF co-crystal molecule thermal decomposition. At different temperatures, CL20 decomposes faster than BTF. As the temperature rises, decomposition velocity of CL20 and BTF increases significantly. At five kinds of temperatures, the CL20 decomposes completely within 2 ps, and BTF decomposes fast within 5 ps with 1-2 molecules left.

FIG. 5 Evolution of (a) CL20 and (b) BTF at various temperature.

Through the analysis of the products, we can get CL20/BTF co-crystal initial reaction path.

During the initial reaction of CL20/BTF co-crystal, reaction path 3 is the main one. NO$_2$ free radical isolates from CL20 first, and then the cage structure damage occurs, which is consistent with the result got from the theories and experiments [24, 25, 26]: CL20 decomposition plays a dominant role in the early stage. BTF is also involved in the reaction by forming C$_{12}$H$_6$N$_{18}$O$_{18}$ combined with CL20 and forming more stable products such as C$_6$N$_6$O$_7$ and C$_6$N$_6$O$_5$ with O atoms from CL20 or BTF molecules, and BTF molecular N-O is the easiest to break, which conforms to the results resulted from theory [27] that N-O in BTF molecules is the longest bond length. Then, N$_2$O$_2$ occurs, hexahydric carbon ring molecule is relatively stable.

D. Main products distribution of CL20/BTF co-crystal thermal decomposition

The evolution curve of main products of CL20/BTF co-crystal thermal decomposition over time at different temperatures is presented in Fig. 6. In the process of simulation, the main products are NO$_2$, NO, HNO, N$_2$, O$_2$, N$_2$, H$_2$O, CO$_2$, N$_2$O, and HONO, etc. NO$_2$ is a small molecule that appears first, being the main product during the initial stage (0-5 ps). After a sharp increase in the number of NO$_2$ in the beginning, the number gradually declines. NO$_2$ participates in secondary reaction, producing NO$_3$, NO, N$_2$; moreover, NO$_2$ react with C cluster, producing hydroxide radical. Compared with Fig. 5, NO$_2$ molecular number always peaks (about 60) after CL20 decomposes completely, because NO$_2$ mainly comes from the N-NO$_2$ bond rupture of CL20, and a CL20 molecular can separate multiple NO$_2$.

FIG. 6 Evolution of main intermediate products at various temperature.

As can be seen from the Fig. 6, CO$_2$ and N$_2$ accumulate gradually in limited simulation time and have similar distribution: the higher the temperature is, the sooner they appear, and the larger the producing rate is. At the same temperature, production time of CO$_2$ obviously lags behind that of N$_2$. For example, at temperature 2000 K, N$_2$ has been produced at the initial stage, but CO$_2$ appears only from 20 ps and producing the rate of CO$_2$ is smaller than that of N$_2$. At different temperatures, the distributions of CO$_2$ are similar, but the temperature has an obvious influence on the output of CO$_2$. H$_2$O is another product of high yield. H$_2$O gradually increases at first, and then falls into balance; the higher the temperature is, the shorter time it takes to reach the balance. This may be caused by the oxygen imbalance of CL20 and BTF, and when CO occurs, the number of H$_2$O becomes stable, suggesting possible existence of CO+H$_2$O$\rightarrow$CO$_2$+H$_2$ dynamic balance.

E. In uence of temperature on the main intermediate products

Figure 7 shows the evolution curve of main intermediate products of CL20/BTF co-crystal thermal decomposition process at different temperatures. It shows the evolution and distribution of NO$_2$: the higher the temperature is, the shorter time it takes to reach the peak value and the sooner it begins to decay, mainly because the NO$_2$ in the reaction generates NO$_3$, NO, N$_2$, and NO$_3$. NO and N$_2$O are important intermediate products, and these products have similar evolution trend to that of NO$_2$, which rapidly increases to the maximum and then begins to decay.

FIG. 7 Evolution of main intermediate products at various temperature.

NO$_3$ occurs later than NO$_2$, and the production rate of NO$_3$ is smaller than that of NO$_2$. NO$_3$ mainly comes from the combination of NO$_2$ and O, the break of N-NO$_3$ which is produced by the combination N-NO$_2$ and O. The NO$_3$ molecule maximum and the time NO$_3$ takes to reach maximum decrease as the temperature increases. The production of NO and N$_2$O is significantly lower than the production of NO$_2$. The N$_2$O mainly comes from the CL20 cage structure damage. Because the N$_2$O has strong oxidation, the consumption of N$_2$O is very fast, and the higher the temperature is, the faster the consumption is. On the whole, products such as N$_2$O$_2$, HNO, and HONO during thermal decomposition first increase and then decrease; because of instability of these products, their numbers have great volatility in the process of thermal decomposition.

IV. CONCLUSION

Through studies on the thermal decomposition of CL20/BTF co-crystal using ReaxFF/lg reaction force field simulation at temperature as high as 2000 K to 3000 K, we obtain valuable details such as potential energy, evolution curves of products, initial reaction path and decomposition products. With the analysis of evolution curves of potential energy based on exponential function, we obtain the overall characteristic time; then we get the activation energy $E_{\rm{a}}$=60. 8 kcal/mol of CL20/BTF co-crystal: via a description of the total package reaction with classical Arrhenius law. Based on the initial path of CL20/BTF co-crystal thermal decomposition we studied, we conclude that N-NO$_2$ of CL20 molecules breaks first, working as a dominant role in the starting stage of thermal decomposition under the condition of different temperatures, and that CL20 molecules all completely decompose before BTF molecular do regardless of different temperature. We also find that the main products of CL20/BTF co-crystal are NO$_2$, NO, NO$_3$, HNO, O$_2$, N$_2$, H$_2$O, CO$_2$, N$_2$O, and HONO, etc. , among which N$_2$, H$_2$O and CO$_2$ are the main terminal products; N$_2$ and CO$_2$ accumulate gradually during the simulation process; CO$_2$ forms later than N$_2$ and more susceptible to temperature. NO$_2$, NO, NO$_3$ and N$_2$O are important intermediate products, and they present similar evolution curve at different temperatures, with NO$_2$ outnumbering the other three. The temperature has certain influence on the products. This work provides atomic level detailed information on thermal decomposition processes of CL20/BTF co-crystal such as reaction path, contributing to a better understanding of detonation process and its reaction process, also providing part of the characteristic parameters for further establishment of chemical reaction kinetics model.

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CL20/BTF共晶高温热分解ReaxFF/lg分子动力学模拟
杨镇, 何远航     
北京理工大学爆炸与科学国家重点实验室, 北京 100081
摘要: 用ReaxFF/lg反应力场模拟CL20/BTF共晶在2000~3000 K高温条件下的热分解过程,获得了势能和物种数的演化、初始反应路径及热分解产物等详细信息。通过指数函数对势能的演化曲线进行拟合得到反映特征时间等参数,采用经典的Arrhenius反应速率方程描述总包反应,获得CL20/BTF共晶的活化能Ea=60.8 kcal/mol。研究得到CL20/BTF共晶热分解的初始路径,CL20分子中N-NO2首先断裂,在热分解起始阶段占主导作用。在不同温度条件下,CL20分子均在BTF分子前完全分解。CL20/BTF共晶的主要产物为NO2、NO、NO3、HNO、N2、H2O、CO2、O2、N2O、HONO 等。温度对产物均产生一定程度的影响.
关键词: ReaxFF/lg     分子动力学     CL20/BTF共晶     反应机理     高温热分解