3.1 Evolution of potential energy and total number of RDX
Potential energy (EP) is a reflection of the relative position of the molecules and changes with temperature, which can be used to judge whether the chemical reaction reaches equilibrium [9]. The total number of RDX (nRDX) of RDX/HTPB/Al and RDX supercell can be related to the degree of thermal decomposition. Therefore, EP and nRDX of RDX/HTPB/Al and RDX supercell are analyzed after ReaxFF MD simulations. EP and nRDX of RDX/HTPB/Al and RDX varies with time under 3000 Kare displayed in Fig. 2.
It can be concluded from EP curves that with the increasing of temperature the EP of RDX/HTPB/Al and RDX supercell increase initially, because RDX/HTPB/Al and RDX supercell gradually absorbs heat to reach primary decomposition energy. With the rising temperature, RDX/HTPB/Al and RDX supercell begin to decompose and generate a series of products and releases a large amount of heat, so the potential energy decreases significantly. When the chemical reactions between intermediate products reach equilibrium, the EP of RDX/HTPB/Al and RDX supercell tend to a constant value. It can be concluded from the simulated EP curves that at low temperature RDX/HTPB/Al and RDX molecules absorb energy to reach its thermal decomposition activation energy initially, and then RDX/HTPB/Al and RDX molecules start to decompose and release energy until the reaction reaches equilibrium. Compared with RDX, the decrease rate of EP of RDX/HTPB/Al is faster, illustrating that the decomposition rate of RDX/HTPB/Al is faster. In addition, nRDX decrease rate of RDX/HTPB/Al is faster than that of RDX supercell, which further illustrates that the decomposition rate of RDX/HTPB/Al is faster than pure RDX. Generally, the decomposition of energetic materials can be divided into two steps, i.e., the decomposition of explosive molecules to generate intermediate products and the reaction between intermediate products to form final products [27, 28]. Figure 2 illustrates that the EP decrement of RDX/HTPB/Al and RDX is higher when the total number of RDX tend to zero, illustrating that the second step releases more energy than the first step during the decomposition process.
3.2 Evolution of intermediate and final products
During the thermal decomposition process of RDX/HTPB/Al and RDX, a large number of intermediate and final products are produced. The simulated final products of RDX decomposition at 3000 K are displayed in Fig. 3a. Final products of RDX/HTPB/Al decomposition at 3000 K are displayed in Fig. 3b, aluminides of final products of RDX/HTPB/Al decomposition at 3000 K are displayed in Fig. 3c, final products of RDX/HTPB/Al decomposition at 3000 K without aluminides are displayed in Fig. 3d.
It can be seen from Fig. 3a and Fig. 3b that there are no RDX molecules existence after ReaxFF MD simulation, and many small products are generated. For RDX, the final products are mainly small molecular products such as N2, H2O, CO2 etc. For RDX/HTPB/Al, the final products contain small molecular products and aluminides. Compared with RDX, there are few H2O in the final products of RDX/HTPB/Al decomposition, in that H2O can react with Al to produce H2 in high temperature. Figure 3 displays the final products of RDX and RDX/HTPB/Al decomposition, while the evolution of intermediate and final products can’t be obtained from this figure. In order to obtain more details about the major intermediate and final products, Fig. 4 summarizes the abundance evolution of main unstable intermediate products and final stable products generated by RDX and RDX/HTPB/Al during thermal decomposition.
Figure 4a illustrates that the amounts of C3H6N5O4, NO and NO2 climb to a peak and then decrease to zero for their further reactions with some other intermedia products. The amounts of H2O and CO2 are increasing throughout the simulation time, indicating that they are generating all the time at current conditions. The amounts of N2 are increasing initially and then decrease at high temperature, indicating that N2 can react with some intermediate products or Al when the temperature is high. The amounts of C3H6N5O4 and NO2 begin to increase when RDX start to decompose, illustrating that the first step of RDX decomposition is RDX → C3H6N5O4 + NO2. The abundance of NO2 is much higher than that of C3H6N5O4, illustrating that other reaction can produce NO2 as well. In the initial decomposition stage of RDX, the main intermediate products are C3H6N5O4 and NO2. With the decomposition going on, the main intermediate products change to NO that are generated by the reactions between intermediate products. With the reaction going on, intermediate products decrease to zero and final products tend to constant, illustrating that decomposition process of RDX has completed. For RDX/HTPB/Al, the amounts of C3H6N5O4, NO and NO2 increase initially and then decrease to zero, which are similar with RDX. However, the abundance of C3H6N5O4, NO and NO2 detected from the decomposition of RDX/HTPB/Al is lower than that of RDX, in that the reactions between the intermediate products produced by RDX/HTPB/Al are much higher, resulting in low accumulated C3H6N5O4, NO and NO2. In addition, CH2O is detected in the decomposition of RDX/HTPB/Al, illustrating that the decomposition of HTPB or intermediate products reacted with HTPB will produce CH2O. With the reaction going on, CH2O will react with other products, resulting in the content of CH2O tend to zero. Compare with RDX, there are few H2O detected in the decomposition of RDX/HTPB/Al while many H2 are detected, illustrating that H2O will react with Al to produce H2 during the decomposition. The generated Al(OH)3 will react with other products to form the complex aluminides.
The main intermediate product (NO2) and final products (N2, H2) produced by RDX/HTPB/Al decomposition at different temperature are summarized in Fig. 5. It can be concluded from Fig. 5 that the increasing temperature enhances the abundance of N2, demonstrating that high temperature is benefit for the production of N2. In addition, the abundance of H2 decreases with the increasing temperature, illustrating that H2O can not only react with Al but also react with other unstable fragments at higher temperature. In addition, the number of total chemical reactions increase with the increasing temperature, which demonstrates that the higher the temperature is, the more completely the thermal decomposition reaction is. Therefore, the abundance of NO2 decreases with the increasing temperature, as a result of the more complex reaction and higher reaction rate.
3.3 Reaction kinetic parameter analysis
The decomposition rate constant of RDX/HTPB/Al are fitted by Eq. 2 from the beginning of decomposition t0 to tmax. The fitting results are summarized in Fig. 6. The calculated k of RDX/HTPB/Al at 2000 K, 2500 K and 3000 K are 0.29, 0.87 and 1.36, respectively. The pre-exponential factor and activation energy can be obtained by linear fitting with Eq. 1. The pre-exponential factor and activation energy of RDX/HTPB/Al decomposition obtained by Eq. 1 are 33.48 fs− 1 and 78.21 kJ·mol− 1, respectively.
It can be concluded from Fig. 6 that there is a good agreement between the simulated and fitted amounts of nRDX. The decomposition rate constant of RDX/HTPB/Al increase with the increasing temperature, illustrating that the decomposition rate of RDX/HTPB/Al are increased at higher temperature.
3.4 Experimental verification
In this study, Accelerating rate calorimeter is used to gather the final gaseous decomposition products, and the gathered gaseous products are analyzed by mass spectrometer (MS) technique. Full scanning mode of MS technique with m/z from 1 to 400 is performed to analyze the m/z of the gathered gaseous products generated by RDX/HTPB/Al decomposition. The obtained MS spectrum of RDX/HTPB/Al thermal decomposition products is shown in Fig. 7. As no products with m/z greater than 60 are detected, Fig. 7 displays the products with m/z from 1 to 60.
It can be concluded from Fig. 7 that the main MS peaks are m/z = 2 and 28. As RDX/HTPB/Al only contains C, H, O, N and Al five kinds of atoms, the species with m/z of 28 can be assigned N2 or CO. The species with m/z of 2 can be assigned to H2, because there are no other species with m/z of 2. The experimental results agree with ReaxFF MD simulations well, illustrating that the simulation results are reliable. In addition, some other gaseous products with low concentration are also detected such as m/z = 16, 17, 18 and 44 and 46, which can be assigned to CH4 and NH3, respectively. Typical fragments corresponding to the possible species are list in Table 1. As the gaseous products were gathered and analyzed at room temperature rather than their generation time, resulting in that some gaseous products condense to condensed state and some unstable ionic products react with each other to form condensed or other products. In addition, some products may have the same m/z such as N2 and CO. These phenomena will result in MS experimentally measured gaseous products are less than the real situations.
Table 1
Typical fragments corresponding to the possible species.
m/z | Assignments | m/z | Assignments |
2 | H2+ | 29 | CHO+, C2H5+, CH3N+ |
16 | CH4+ | 30 | CH2O+, C2H6+, CH4N+ |
17 | NH3+ | 43 | C3H7+, C2H5N+, C2H3O+, CHON+ |
18 | H2O+ | 44 | CO2+ |
28 | N2+, CO+, C2H4+, CH2N+ | | |