The oxidized characteristic of aluminum and detonation products is significant to studies on characteristics of aluminized explosives in the field of energetic materials.^[1,2] When aluminum burns in a composite solid propellant, it typically reacts with gas products of the energetic oxidizer and binders, such as H O, CO, and CO .^[3–5] Under some conditions, the addition of aluminum powder can strongly reduce the explosion performance, similar to chemically inert substances such as NaCl, SiO , lithium fluoride, and talc.^[6] Thus, a displacement with lithium fluoride as an inert salt was studied by Trzcinski and Cookin.^[7,8] After that lithium fluoride was used as an inert surrogate for aluminum because it has very similar density and shock impedance and is not expected to participate in detonation and post-detonation expansion.^[9]

There are a great number of researchers devoted to finding out exactly how many aluminum powders contribute to the energy release of mixed explosives. Cowperthwaite^[10] proposed that approximately 70% of aluminum reacted in detonation reaction zone through a comparison between theoretical calculation and experimental phenomena. Ding^[11] studied Hexal PW30 in 1994 and concluded that aluminum powders produced heat release before and after detonation wave front. In 1999, Keicher^[12] concluded that up to 30% of the aluminum of non-ideal explosives was completely oxidized without an external oxygen source such as air or surrounding water. According to Waldemar, experimental explosion energy is higher than calculated by CHEETAH with inert aluminum, but it is much lower than the mechanical energy estimated for reactive aluminum.^[13] In addition, Ma^[14] found that when the Al/O ratio (the atomic ratio of Al and O in the mixed explosive molecular formula) is equal to 0.67, the total energy reached maximum. Ma's article gave no explanation for that phenomenon.

Although preliminary studies showed that the Al/O ratio, the size of aluminum powder, and the morphology of aluminum powders affect the energy release, these studies did not give accurate heat release data from the combustion of aluminum powder in an UNDEX. In the present work, we investigate the effect of the Al/O ratio on the Al energy release rate and the energy release of an underwater explosion (UNDEX). The explosion heat and UNDEX performance of RDX-based aluminized explosives were also measured.

In an actual process of explosion, aluminum do not completely react.^[15] Therefore, it is important to study relationship between energy release and aluminum reaction. In this study, explosion heat and main explosion products are calculated. Due to a large difference between the experimental results and the calculated results of the maximum heat release principle, the EXPLO5 program is used to calculate the explosion parameters. Two groups of explosive formulation are designed, as shown in Table 1. Sample A is used to get a large scale of the Al/O ratio, from 0 to 1.43. In reality, those proportions (the Al/O ratio ) is hard to achieve. Sample B is designed for experimental verification.

Table 1.

Table 1.

Table 1. Calculated explosion heat of explosives containing 45% and 52% RDX. . Sample A Formulation Al/O /MJ kg Sample B Formulation Al/O /MJ kg A1 RDX/binder/Al/LiF 45/8/47/0 1.43 6.97 B1 RDX/binder/Al/LiF 52/8/40/0 1.05 7.80 A2 RDX/binder/Al/LiF 45/8/42/5 1.28 6.93 B2 RDX/binder/Al/LiF 52/8/35/5 0.93 7.74 A3 RDX/binder/Al/LiF 45/8/35/12 1.05 6.88 B3 RDX/binder/Al/LiF 52/8/30/10 0.79} 7.70 A4 RDX/binder/Al/LiF 45/8/31/16 0.93 6.86 B4 RDX/binder/Al/LiF 52/8/25/15 0.67 7.62 A5 RDX/binder/Al/LiF 45/8/26/21 0.79 6.84 B5 RDX/binder/Al/LiF 52/8/20/20 0.54 6.67 A6 RDX/binder/Al/LiF 45/8/22/25 0.67 6.63 B6 RDX/binder/Al/LiF 52/8/15/25 0.40 5.81 A7 RDX/binder/Al/LiF 45/8/18/29 0.54 6.06 B7 RDX/binder/Al/LiF 52/8/10/30 0.26 4.80 A8 RDX/binder/Al/LiF 45/8/9/38 0.26 4.22 B8 RDX/binder/Al/LiF 52/8/5/35 0.13 3.88 A9 RDX/binder/Al/LiF 45/8/0/47 0 2.51 B9 RDX/binder/Al/LiF 52/8/0/40 0 2.82

Table 1. Calculated explosion heat of explosives containing 45% and 52% RDX. .

Theoretical calculation results were analyzed and the explosion products were compared one by one. In explosion products, substances with a yield greater than 0.1% are analyzed. As shown in Fig. 1, calculation explosion heat and reaction production of Al O increases with the Al/O ratio. The growth rate changes slowly after the Al/O ratio is more than 0.67. The yield of main oxygen-containing products (Al O , H O, CO, and CO ), reduces monotonously. All of oxygen-containing gases have an inflection point at 0.67. CO runs out and CO has surplus with the amount of aluminum increase. The elemental oxygen in explosion products keeps decreasing when the Al/O ratio increases to 1.43. Elemental oxygen in H O, CO , and CO is not completely combined with aluminum powder. This is the reason for increasing explosion heat. It can be seen from the comparison between Fig. 1(a) and Fig. 1(b), the growth rate of explosion heat is less than 3% when the Al/O ratio is greater than 0.67. The decreases of H O, are less than 4%. Content of carbon bonded with oxygen decreases by more than 10% The results showed that explosion heat and content of Al O and H O have the same turning point at Al/O=0.67. The incomplete reaction of aluminum and explosion products leads to an incomplete energy release of RDX/Al explosives.

Fig. 1.

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	Fig. 1. (color online) Explosion parameters versus Al/O ratio for (a) % and (b) %.

To determine whether there is a fixed Al/O ratio that makes all aluminum powders in mixed explosives completely oxidized, the explosion heats of explosives containing 45% RDX were calculated. Mixtures containing 0, 20%, 25%, 35%, and 40% aluminum content were prepared. The main explosive formulations were two typical RDX-based explosives of aluminum enriched RDX compositions and lithium fluoride enriched RDX compositions. Table 2 shows the formulation and properties of the pouring charges.

Table 2.

Table 2.

Table 2. Formulation and properties of sample charges. . No. Formulation Mass ratio Al/O g cm 1 RDX/binder/Al/LiF 52:8:40:0 1.05 1.69 2 RDX/binder/Al/LiF 52:8:35:5 0.93 1.68 3 RDX/binder/Al/LiF 52:8:25:15 0.67 1.66 4 RDX/binder/Al/LiF 52:8:20:20 0.54 1.64 5 RDX/binder/Al/LiF 52:8:0:40 0 1.56

Table 2. Formulation and properties of sample charges. .

A measuring device of explosion heat is shown in Fig. 2. The explosion heats were measured in nitrogen environments using a constant temperature calorimeter. An electric detonator with a copper-clad shell was hung below the lid of the calorimeter. The mixed explosive sample ( g) was put into the ceramic shell. It was suspended under the lid of the calorimeter with a silver-plated copper wire. The electric cap was inserted into the sample. Then, the lid was closed and the calorimeter was filled with nitrogen gas. With distilled water as the temperature measuring medium, the outer barrel temperature was adjusted by a temperature control instrument. The temperature of the inner and outer barrel was between 20 and 30 °C. The temperature fluctuation was not more than 0.02 °C in 15 min. At the same time, the temperature difference between the water in the barrel and the outer barrel was not more than 0.03 °C within 15 min. Afterwards, the water temperature in the barrel was recorded. Then, the explosive sample was ignited. About half an hour after the ignition, the water temperature in the barrel was recorded when the change in temperature was less than 0.003 °C in 15 min. Explosion heat was calculated by

At least two measurements were taken of each formula. The density difference in the two measurements was not greater than 0.02 g/cm and measuring errors were less than 3%. The average values of explosion heat are presented in Table 3.

Table 3.

Table 3.

Table 3. Explosion heat and the energy release degree of each formulation. . No. Al/O Q /MJ kg Q /MJ kg 1 1.05 7.59 7.80 97.23 2 0.93 7.32 7.74 94.26 3 0.67 6.53 7.62 85.70 4 0.54 5.40 6.67 80.96 5 0 2.94 2.98 –

Table 3. Explosion heat and the energy release degree of each formulation. .

To compare with measured explosion heat, calculation explosion heats are modified. Assuming that the explosion heat is dominated by the energy released by the main explosive component and the energy released by the combustion of aluminum powder and is the value of the modified calculation heat for Al/O = 0. The energy release degree of aluminum in mixed explosives can be expressed as

Fig. 2.

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	Fig. 2. Diagram of constant temperature calorimeter.

Lithium fluoride does not take part in the secondary reaction, where the reaction of aluminum powder and detonation products occurs, so the energy release of aluminum can be calculated by subtracting the total energy released by the explosive without aluminum from the total energy provided by all explosive formulations. Because detonation products and aluminum powders are not fully reacted, the energy released in secondary reaction does not reach the theoretical exothermic value in these two cases. As shown in Table 3, a plot of the change in the aluminum energy release rate with the Al/O ratio is shown in Fig. 3. The Al energy release rate increased with increasing Al/O ratio. reaches 97.81% when the Al/O ratio is equal to 1.05. The explosion heat from theoretical density and actual density are calculated respectively. The calculated data are compared with the measured data. The comparison results are shown in Fig. 4. Thermal heats are basically the same trend at different densities. The measured explosion heat monotonically increases without an inflection point, which is different from calculated data. This is due to the formation of thick oxide layer on the surface of aluminum nucleus in the process of aluminium oxidation. The active aluminum is prevented from participating in oxidation reaction. Without considering incomplete reaction of active aluminum in interior particle, the main constraint is contents of detonation gas products when the Al/O ratio is more than 0.67 in theoretical calculation. Besides, five aluminized explosives have different initial temperatures. The higher the initial temperature is, the more aluminum powders release energy. Theoretical calculations do not take into account the effect of initial temperature on the energy release rate of aluminum powders.

Fig. 3.

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	Fig. 3. (color online) Dependence of thermal explosion heat and aluminum energy release rate on the Al/O ratio.}

Fig. 4.

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	Fig. 4. (color online) Comparison of experimental explosion heat and calculated explosion heat.

Kicinski and Trzcinski^[17] used lithium fluoride instead of aluminum and concluded that the aluminum powders are inert in detonation reaction zone. Accordingly, the detonation reaction zone and the secondary reaction zone are completely separated. After RDX detonation, gas products of RDX and metallic substances in mixed explosives are heated to the same temperature. We called the initial temperature . is calculated under three basic assumptions: non-equilibrium reaction process in explosion divided into different equilibrium states. Specific heats of compounds in explosive reaction have constant pressure in the aluminum powder combustion stage. The calculation formula of initial temperature is given by

Equation (4) is the relationship on specific volume and temperature. Thermodynamic parameters and mass of detonation products of pure RDX are shown in Table 4. It also shows the parameters a, b, and c in Eq. (4). Among detonation products only matters with the contents greater than 0.1% in products are listed in Table 4. In the detonation zone, specific heat is usually considered to be constant-volume specific heat due to little volume change. In secondary reaction zone behind detonation wave front, volumetric specific heat capacity is not useful for the gas expansion. Sobaric specific heat capacity is more nearing to reality.

Table 4.

Table 4.

Table 4. The products of RDX and some parameters in Eq. (3) and Eq. (4). . m/kg a b c LiF(s) – 4.18× 1.87× 6.94× LiF(l) – 6.42× 4.20× −2.30× C(s,gr) 9.58× 5.39× 9.11× 0.00× H O 1.39× 7.76× 1.55× −1.14× N 1.91× 6.77× 8.15× −1.15× CO 1.00× 9.00× 3.59× −8.25× CO 2.79× 6.79× 9.20× −1.53× CH 1.49× 7.96× 7.81× −1.14× NH 7.19× 2.02× 1.98× −3.61× CH O 1.83× 1.17× 1.36× −8.41× H 7.00× 6.92× 1.09× −9.30× C H 3.57× 1.64× 4.50× −1.55× HF 3.00× 2.46× 6.89× −1.24× Li CO 1.85× 1.85× −5.10× 3.00× Al(s) − 2.81× -5.41× 8.56× Al(l) – 3.18× 3.94× −1.79×

Table 4. The products of RDX and some parameters in Eq. (3) and Eq. (4). .

Since heat absorption of aluminum powder and lithium fluoride are taken into account, the initial temperature better explains why aluminum energy release rate increased. To analyze the phenomenon of increasing aluminum energy release rate, and the categories and molar mass of final explosion products were discussed. Among detonation products only the amount of CO , CO, and H O change with the increase of Al/O ratio. As displayed in Table 4, the initial temperature increased and the amount of CO , CO, and H O decreased with increasing Al/O ratio. This illustrates that aluminum powder is more fully combined with elemental oxygen in the product. Meanwhile, the explosion heat and Al energy release rate increase with increasing Al/O ratio.

It is concluded that no matter how much aluminum added, oxygen in detonation products cannot be completely react with aluminum. However, the Al/O ratio increase can continuously improve the energy release degree of explosive by analyzing Figs. 3 and 4 and Table 4.

Table 5.

Table 5.

Table 5. Explosion products in secondary reaction and system initial temperature. . No. 1 2 3 4 5 Al/mol 3.62 1.78 0 0 0 Al O /mol 4.67 4.67 4.63 3.71 0 CO/mol 5.01× 5.93× 1.27× 1.32 1.00 CO /mol – – – 0.05 2.27 H O/mol 3.37× 3.75× 7.98× 1.37 7.70 /K 1255.4 1140.3 1109.9 1086.2 1006.4

Table 5. Explosion products in secondary reaction and system initial temperature. .

In underwater explosion, heat energy and kinetic energy of detonation product gas make the surrounding water vaporization and push the air bubbles expand outwards.^[16] Shock wave energy and bubble energy are released in this process. Underwater explosion experiment is carried out on RDX/Al/LiF explosives to study the energy distribution. The main charges were initiated by a nonel blasting cap and a cylindrical pressed JH-14 booster (RDX/ fluororubber and graphite 96.5%:3.5%). In this study, we used two sections of expansion booster. The diameter of the first section booster was 40 mm with a mass of 50 g, and the diameter of the second section booster was 80 mm with a mass of 200 g. The mass of explosive sample is 1 kg. The diameter of explosive columns is 10.0 cm. To investigate the initial reaction time and the energy release degree of metal with the surrounding water medium.

As shown in Fig. 5, charges of several formulations were performed in water pool and the pressure time curve was tested and analyzed by pressure transducers (702-QG-W01), and a data collecting instrument (GENESIS 986A0151). For each formulation two identical charges were carried out. Figure 5 shows the scheme of the experimental arrangement. The explosion center and the pressure transducers were at the same level as the horizon and 6 m below the surface of the water. The distances between the explosion center and pressure transducers were 2, 3, 4, and 5 m, respectively. The UNDEX experiments were carried out to discuss the effect of aluminum on energy release. The shock wave histories of composite explosives with different lithium fluoride content were measured and are shown in Fig. 6. It can be seen that the bubble pulsation period and shock wave pressure keep increasing with rising Al/O ratio. However, the change of peak pressure is very small when the Al/O ratio is 0.93. The peak pressure difference between formulation 1 and 2 are very small at the distances of 3, 4, and 5 m. The shock wave energy, bubble energy, total energy, and the volume work provided per unit mass of the UNDEX are calculated by^[18]

Table 6.

Table 6.

Table 6. Energy release of UNDEX. . No. Al/O /MJ kg /MJ kg /MJ kg /MJ kg 1 1.05 1.24 4.82 6.06 7.47 2 0.93 1.21 4.45 5.66 7.02 3 0.67 1.07 3.56 4.63 5.82 4 0.54 0.93 2.83 3.76 4.78 5 0 0.55 1.50 2.05 2.64

Table 6. Energy release of UNDEX. .

Table 6 shows energy parameters of UNDEX for different formulations. The calculated data are average values of results of two duplicate tests at distances of 3, 4, and 5 m, and the difference between duplicate test results is less than 2%. The total energy does not include heat loss which was used to heat the surrounding water. Figure 7 demonstrates explosion heat, volume work, total energy and ratio of bubble energy and total energy. The changing trend of volume work and total energy are consistent with calculated and measured explosion heat. Bubble energy proportion increases with the increase of Al/O ratio in the total energy. The secondary reaction heat produced by aluminum powder combustion mainly increases the bubble energy in UNDEX.

Fig. 5.

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	Fig. 5. The UNDEX experiment.

Fig. 6.

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	Fig. 6. (color online) Pressure histories with different LiF contents.

Fig. 7.

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	Fig. 7. (color online) Comparison of explosion heat, volume work, and the energy ratio.

To explore the Al reaction and effect of aluminum powder on explosion heat and energy output of UNDEXs, the experimental explosion heats were measured using Al/LiF-containing explosives. The most significant conclusions in the reported experiments are as follows. (i)

Theoretical calculation shows that explosion heat increases monotonously with the Al/O ratio increase. Alumina in explosion products increases. The content of oxygen bonded with carbon and H O decreases. The increases of explosion heat and the alumina output begin to slow down when Al/O ratio is greater than 0.67. Contents of H O, CO, CO continues to slowly decrease but is still remaining.