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Effect of Nano-Magnesium Hydride on the Thermal Decomposition Behaviors of RDX
In order to improve the detonation performance of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) explosive, addictives with high heat values were used, and magnesium hydride (MgH2) is one of the candidates. However, it is important to see whether MgH2 is a safe addictive. In this paper, the thermal and kinetic properties of RDX and mixture of RDX/MgH2 were investigated by differential scanning calorimeter (DSC) and accelerating rate calorimeter (ARC), respectively. The apparent activation energy (E) and frequency factor (A) of thermal explosion were calculated based on the data of DSC experiments using the Kissinger and Ozawa approaches. The results show that the addition of MgH2 decreases both E and A of RDX, which means that the mixture of RDX/MgH2 has a lower thermal stability than RDX, and the calculation results obtained from the ARC experiments data support this too. Besides, the most probable mechanism functions about the decomposition of RDX and RDX/MgH2 were given in this paper which confirmed the change of the decomposition mechanism.
Metal hydrides belong to the hydrogen storage material, which have been widely applied for energy storage carriers in recent years. However, metal hydrides are also attracting significant attention as one of the potential fuel components of energetic materials due to their peculiar properties. Metal hydrides have high activity and they can provide high heat values in the process of explosion.
In middle of 1960s, the former Soviet Union conducted the research on the application of metal hydrides for energetic materials. It was found that the metal hydrides gave off a large amount of heat when they were burning and caused relatively lower flame temperature due to the generation of light molecular weight gases. As a result, adding metal hydrides to propellants is likely to lead higher energetic density. For example, propellants containing aluminum hydride (AlH3) have 9.8 to 39.2 N·s/kg higher specific impulses than propellants containing Be. Recently, Russia has applied it to solid propellants and fuel air explosive weapons .
Nitrocellulose propellant composition containing AlH3 was proposed in 1970s in USA . It was also reported that adding magnesium hydride (MgH2) in some explosives such as TNT, tetryl, and C-4 will increase the total work capacity generated by organic, noninitiating explosives . Selezenev et al.  considered the influence of aluminum, magnesium and their hydride powders on the detonation characteristics of the compositions based on ammonium nitrate, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and cyclotetramethylenetetranitramine (HMX). It concluded that the detonation velocity in the compositions with hydride powders were higher than in the composition with metal powders. Besides, MgH2 was also applied as a pyrotechnic composition which provides improved ignition rates and increased burning time without reduction in luminous intensity .
From the above-mentioned researches, it can be found that metal hydrides have great potential as additive components for energetic materials. Among all the metal hydrides, MgH2 has a high hydrogen storage capacity up to 7.6 wt%. However, as an additive, MgH2 should be of a good compatibility with the original explosives or propellants and possess a satisfactory thermal stability. In this paper, thermal behaviors of RDX and the mixture of RDX/MgH2 were investigated by differential scanning calorimeter (DSC) and accelerating rate calorimeter (ARC). In addition, the hazard characterizations of the mixtures were analyzed in order to ensure the safe application of MgH2 in RDX.
Nano-MgH2 used in this paper was obtained from Shanghai Engineering Research Center of Magnesium Materials and Applications . It was prepared through a DC Arc Plasma method followed by hydrogenation under high hydrogen pressure at 400°C. The details about the preparation and microstructures of the MgH2 can be found in the references . RDX was dried at 60°C to constant weight.
2.2. Powder X-ray Diffraction
X-ray diffraction (XRD) was utilized to analyze the phase components of the hydrides. The XRD apparatus used was made by Bruker (type: D8 ADVANCE, Cu Kα radiation source).
2.3. DSC Analysis
DSC is a routine tool to study the thermal stability, heat generation caused by phase transition and chemical reaction, kinetic parameters, decomposition of reactive substances, and so forth. In this paper, the DSC apparatus used was made by Mettler Toledo (type: DSC1). MgH2 powder was tested at 10°C/min and the mixtures of RDX/MgH2 were heated at a constant rate (1, 2, 4, 8°C/min) in nitrogen atmosphere from 25 to 500°C. The stain steel crucible was used.
2.4. ARC Analysis
ARC is an effective tool for hazards evaluation of reactive substance. Compared with DSC, ARC possesses the characteristic of larger sample quality and adiabatic experimental environment. A lightweight spherical titanium bomb was used in the experiment. The instrument used was made by Thermal Hazard Technology (type: esARC). The ARC experiments were started at ambient pressure of air. The standard ARC procedure of heat-wait-search was used .
3. Results and Discussion
3.1. XRD Result
The XRD pattern of MgH2 is shown in Figure 1. Besides MgH2, the existence of Mg and MgO was also detected in the powder.
3.2. Thermal Behaviors of MgH2 Powder
The DSC curve of MgH2 powder is shown in Figure 2. An endothermic peak of MgH2 appears at 481°C, which is different from the values reported in the literature [9, 10]. It is because metal hydrides release hydrogen at different temperatures due to the various surface conditions from different preparation methods. For instance, Huot et al.  found that MgH2 made by ball milling began to decompose at 396°C, while MgH2 synthesized through a direct-hydriding method decomposed at about 287°C .
3.3. DSC Analyses on RDX and RDX/MgH2 Mixture
The DSC curves of RDX and RDX containing 10 wt% MgH2 at different heating rates of 1, 2, 4, and 8°C/min are shown in Figures 3(a)–3(d). The decomposition temperatures of these samples are shown in Table 1 also with the quantities of the heat release. It can be seen from the curves that adding nano-MgH2 powder has no significant effect on the melt point of RDX. As for the decomposition process of RDX, it has at least two stages which can be observed obviously from Figures 3(a)–3(c). This result confirms to the literature research . However, there is only one exothermic peak in the DSC curve of RDX at 8°C/min.
(a) DSC curve at heating rate of 1°C/min
(b) DSC curve at heating rate of 2°C/min
(c) DSC curve at heating rate of 4°C/min
(d) DSC curve at heating rate of 8°C/min
Brill and Brush  investigated the mechanism of thermal decomposition of RDX, HMX, and some other cyclic nitramine explosives based on the T-jump/FTIR and SMATCH/FTIR spectroscopy. The mechanism of thermal decomposition was presented that the C–N and N–N bonds break at the same time, as shown in formula of (1) and (2). It is considered that the low temperature and slow heating rate are benefit to the breakdown of C–N bonds and make the RDX go through a long decomposition in molten state which is favorable for the generation of CH2O. Formula (1) is an exothermic reaction (−212.3 kJ/mol). In addition, high temperature and rapid heating are helpful to the breakdown of N–N bonds, and it is an endothermic reaction (+117.2 kJ/mol). At last, the following reaction shown in formula (3) releases a lot of heat:
Based on the above theory, different peak shapes of RDX in Figures 3(a)–3(c) can be explained. When the heating rate is slow, the breakdown of C–N bonds is the dominant reaction and this causes a gentle and slow heat release in the early stage of decomposition. Then, the reaction shown in formula (3) occurs and forms a short spike in DSC curve. On the contrary, when the heating rate increases to 8°C/min, the reaction shown in formula (2) becomes dominant and absorbs part of heat which may be due to the slight concave between 210°C to 230°C in curve (d) of RDX. Therefore, the two stages of RDX’s decomposition are not observed in Figure 3(d).
As shown in the graphs, adding 10 wt% of MgH2 has some effect on the peak shape of RDX’s decomposition. Adding MgH2 makes the shoulder peak become sharper especially at the heating rate of 1°C/min. However, the hydrogen releasing temperatures of MgH2 used in this paper are far below the decomposition temperature of RDX. So, it can be considered that the effect on the peak shape is mainly due to the metal hydride itself but not the resolved products.
MgH2 used in this paper was prepared through hydrogenation of Mg ultrafine powders and the high transformation-induced lattice distortion will cause intense cracks in the powder particles. As a result, lots of hydrogen atoms exist in the gaps on the surface of crystal cells . Palopoli and Brill  proposed a opinion about the breakdown of N–N bonds of cyclic nitramine that the H atoms in the –CH2– group transfer to nearby –NO2 group firstly, which results in N–N bonds’ breakdown. Therefore, lots of H atoms in the gaps of MgH2 may increase the opportunity for the interaction between H atoms and –NO2 group and promote the generation of NO2. As a strong oxidizer, NO2 can react with MgH2 and provide extra heat which makes the second exothermic peak of RDX become obvious .
3.4. Kinetic Parameters Getting from DSC
Potential hazards always associate with the thermal behavior of energetic materials, so it is essential to evaluate the stability and carry out the decomposition kinetic. In this paper, kinetic parameters were determined using the Kissinger approach  and the Ozawa approach  in the meantime as shown in formulas (4) and (5), respectively, the results are shown in Table 2,
As shown in Table 2, the values of calculated by the Kissinger method are in good agreement with the values calculated by the Ozawa method. Adding 10 wt% MgH2 decreases the activation energy of RDX from 260.57 kJ/mol to 202.74 kJ/mol. The frequency factors are also following this order. Therefore, the thermal stability lowered to a certain extent after adding MgH2 to RDX.
3.5. ARC Results
The experimental conditions and results are listed in Table 3. The curves of temperature-time, pressure-time, self-heating-rate-temperature, and so forth are drawn in Figures 4 and 5.
(a) Temperature/pressure versus time
(b) Self-heating rate versus temperature
(c) Pressure rising rate versus temperature
(a) Temperature/pressure versus time
(b) Self-heating rate versus temperature
(c) Pressure rising rate versus temperature
The sample quantities for ARC test were smaller than usual, which was constrained by the fierce reaction of the explosive samples. As a result, the thermal inertia was big, and it may lead to higher ignited temperature and smaller pressure. The heat release should be correct because of the heat evolution that was used to heat the titanium ball. Whereas, the data from the ARC test still had some reference significance to analyze the thermal stability of these mixtures.
Since the amount of sample is fairly small, the influence caused by a different sample weight was great. In this case, it may be improper to draw a conclusion from the data in Table 3 directly. However, it is feasible to compare the thermal stability of these two samples by the kinetic parameters getting from the data.
Figures 4(b) and 5(b) show the different variation of self-heating rate between RDX and RDX/MgH2 mixture. Similar change of the variation of pressure rising rate can be observed in Figures 4(c) and 5(c). From the figures, the self-heating rate of RDX achieves its maximum at the middle of the decomposition process and drops at the high-temperature period. However, the self-heating rate of RDX/MgH2 mixture achieve maximum at the high-temperature period as well as the pressure rising rate. That is to say, adding MgH2 influences the decomposition process of RDX in the high temperature stage, and it is consistent with the conclusion of DSC experiments.
In this paper, the apparent activation energy of adiabatic decomposition process was calculated by a method using pressure data , and the following formula (6) was used: where is the pressure at ; ; is the pressure rise; symbolizes the kinetic model in form of differential equation. The plot of versus should give a straight line with a slope providing the kinetic model which is correctly chosen. Different kinetic models are used to try to find the best linear fitting, from which the activation energy can be calculated. The 29 kinetic models are listed in Table 4.
By applying the 29 kinetic models in formula (6) and plot versus , the best linear fitting result can be screened. The highest correlation coefficient and the corresponding kinetic model are shown in Table 5 as well as the activation energy and frequency factor. Figure 6 shows the simulating lines which have the highest correlation with original data.
(b) RDX + 5 wt% MgH2
Samples used here were not exactly the same as the samples used for DSC experiment, the metal hydride content was 5 wt%. Meanwhile, the values of and in Table 5 are quite different from the value mentioned above. It is also due to the high thermal inertia. However, the kinetic parameters calculated from the ARC’s data show the same rules as the kinetic parameters calculated from the DSC’s data. Adding 5 wt% MgH2 decreased the values of and obviously. Besides, the most probable mechanism function has also changed.
(1)The addition of 10 wt% MgH2 to RDX has no obvious or regular effect on the melt point and the initial decomposition temperature of RDX. The second exothermic peak grows dramatically after the addition of MgH2 to RDX, and the exothermic heat quantity increases at the low heating rate. (2)Kinetic parameters obtained from the DSC data show that the mixture of RDX/MgH2 has the lower values of and , which means that the stability of mixture RDX/MgH2 is worse than RDX. The reason of the reduction may be due to the catalytic effect of the hydrogen atoms existing in the gaps on the surface of crystal cells. (3)The conclusion from the ARC experiment is similar to the result obtained from DSC. The apparent activation energy of the mixture RDX/MgH2 is also lower than that of RDX. The most probable mechanism function changed too. Therefore, it is necessary to ensure the safety of the mixture RDX/MgH2 before applying it to large-scale experiment.
Professor Zou would like to thank the financial support from Research Funds for the Doctoral Program of Higher Education of China (no. 20100073120007) and from Shanghai Education Commission (no. 12ZZ017). This work is partly supported by projects from the Science and Technology Committee of Shanghai under nos. 10JC1407700, 11ZR1417600, and “Pujiang” project under no. 11PJ1406000.
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