Effects of content and particle size of TiH2 powders on the energy output rules of RDX composite explosives

Ho Wng , Yngfn Cheng ,,*, Shoujun Zhu ,b, Zihn Li ,b, Zhowu Shen

a School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, China

b Anhui Engineering Laboratory of Explosive Materials and Technology, Anhui University of Science and Technology, Huainan 232001, China

c School of Engineering Science, University of Science and Technology of China, Hefei 230027, China

Keywords:RDX Hydrogen storage alloy Air blast After-burn reaction Colorimetric thermometry

ABSTRACT In order to improve the detonation characteristics of RDX, a RDX-based composite explosive with TiH2 powders was prepared.The effects of content and particle size of TiH2 powders on thermal safety,shock wave parameters and thermal damage effects of RDX-based composite explosive were studied with the C80 microcalorimeter,air blast experiment system and colorimetric thermometry method.Experimental results showed that TiH2 powders could enhance the thermal stability of RDX-based composite explosive and increase its ultimate decomposition heat.The content and particle size of TiH2 powders also had significant effects on the thermal safety, detonation velocity, shock wave parameters, fireball temperature and duration of RDX-based composite explosives.Furthermore, the differences of TiH2 and Ti powders on the detonation energy output rules of RDX-based composite explosives were also compared,showing that TiH2 powders had better influences on improving the explosion power and thermal damage effect of RDX-based composite explosives than Ti powders, for the participation of free H2 released by TiH2 powders in the detonation process.TiH2 powders have important research values as a novel energetic additive in the field of military composite explosives.

1.Introduction

Military explosive RDX has the advantages of high detonation velocity,energy density and thermal safety,which is widely used as the main charge in weapons such as artillery shells, missiles, and torpedoes [1-3].To meet the demands of high power and high damage performance of modern weapons,researchers usually add high-energy metal powders to explosives [4-8].As a typical hydrogen storage material, TiH2is characterized by high combustion calorific value and high hydrogen storage capacity, and its stability is so remarkable that it can be stored with KClO4for more than 20 years without decomposition, showing promising application prospects as an energetic additive[9].Yue et al.[10]explored the effects of Mg(BH4)2on the thermal behaviors of RDX explosives,and the results indicated that the combustion energy of RDX-based composite explosives enhanced, while the combustion efficiency decreased as the content of Mg(BH4)2increased.Fang et al.[11]studied the effects of LiAlH4on the thermal decomposition behaviors of RDX, and the results showed that the addition of LiAlH4would decrease the decomposition peak temperature of RDX.Cheng et al.[12]studied the underwater explosion performance of emulsion explosives containing TiH2powders, and the results revealed that emulsion explosives with TiH2powders had larger shock wave specific impulse and total energy values than traditional emulsion explosives.Yu et al.[13] added TiH2powders into PTFE/Al mixture and discovered that the reaction heat of the mixture improved greatly with the maximum value to be 3.7 times than that of TNT.Xue et al.[14,15] concluded that TiH2powders could dramatically improve the air blast shock wave parameters of RDX-based composite explosives.

The detonation power of composite explosive was affected directly by the content and particle size of energetic additives[16],and researchers had carried out a series of related studies over the years.Zhang et al.[17] studied the effects of various Al powders content on the detonation performance of RDX-based explosive with the air blast experiment,and found that when the content of Al powders was 20%-40%, the shock wave characteristics of aluminized explosives decreased as the content of Al powders increased.Lin et al.[18] explored the effects of aluminum fiber content on the detonation performance of composite explosives by underwater explosion experiments,and found that the peak overpressure decreased with the increasing aluminum fiber content,whereas the bubble energy and explosion energy enhanced.Huang et al.[19] added Ti powders into RDX explosive to investigate the underwater explosion characteristics,and the results demonstrated that RDX-based composite explosives with 20%Ti powders had the best energy density.Zhou et al.[20] studied the detonation performance of TNT with Al powders of different particle sizes, and concluded that the detonation pressure of TNT with nano Al powders was substantially better than that of TNT with micron Al powders,and the attenuation rate of detonation pressure gradually slowed down with the decrease of aluminum particle size.Liu et al.[21]used a small-scale confined plate push test to study the effects of the content and particle size of Al powder on Cl-20 explosive,and the results showed that Al powders with smaller particle size would participate in the reaction earlier, and the reaction rate declined with the increasing mass fraction of Al powders.

Previous studies of damage assessment of high energy explosives were mostly focused on the shock wave damage,but very few on the thermal damage effect.The colorimetric thermometry method developed in recent years has advantages of high measurement efficiency and excellent interference resistance [22],which could meet the measurement requirements of transient explosion temperature field.In our previous study [23-26], colorimetric thermometry had been applied to map the temperature field of explosion fireballs,and its accuracy had been verified with the explosion heat testing results.In the study, RDX-based composite explosives containing different content and particle sizes of TiH2powders were prepared, and the effects of TiH2powders on the thermal decomposition characteristics of RDX were studied using a C80 microcalorimeter, and the thermal decomposition mechanism was discussed.The fireball temperature distribution of RDX-based composite explosives was reconstructed using the colorimetric thermometry method,and the detonation velocity and shock wave parameters were also measured.The optimal content and particle size of TiH2powders of the RDX-based composite explosives were obtained, and the relationship between the thermal effect and shock wave effect was discussed.Furthermore, in order to explore the influence of H2on the detonation performance of RDX-based composite explosives, the detonation characteristics of composite explosive added with TiH2powders were compared with that added with Ti powders.The research results are of great significance in revealing the energy output rules and thermal damage mechanism of RDX-based composite explosives.

2.Experimental

2.1.Materials and characterization

The experimental materials were commercial-grade TiH2powders with hydrogen storage capacity of 3.85 wt% (Thermo Fisher Scientific,USA)and commercial-grade Ti powders(Baoji Quanxing Titanium Industry Co., Ltd.China).The explosive was paraffin passivated RDX, provided by Chinese Wanhuai Electromechanical Co., Ltd.The particle size distribution and micro-structure of TiH2and Ti powders were characterized by a laser particle size analyzer(Mastersizer 2000, Malvern, UK) and a scanning electron microscopy(SEM, VEGA3, TESCAN), respectively.

Fig.1.Particle size distribution of experimental powders.

Fig.1 shows the particle size distributions of TiH2and Ti powders, and the mean particle sizes (D50) of TiH2were 16.4 μm,33.7 μm,50.1 μm and 112.0 μm,respectively,and D50of Ti powders was 18.4 μm.Figs.2(a)-2(f) are the scanning electron microscopic images of passivated RDX,TiH2powders with four different particle sizes and Ti powders.RDX particles were coated with paraffin wax and had smooth surfaces,and Ti and TiH2powders had sharp edges and rough surfaces.

2.2.Preparation of composite explosive samples

To investigate the effects of content and particle size of TiH2powders on the energy output rules of RDX,RDX-based composite explosives containing TiH2powders with different content and particle sizes were prepared.Furthermore,a RDX-based composite explosive containing Ti powders was prepared as a comparison to study the effect of free H2on the energy output rules of RDX.The compositions of RDX-based composite explosive samples are listed in Table 1.Sample A was pure RDX, which was used as a blank sample.Samples A1-A4 were composite explosive samples containing 2.5, 5, 7.5 and 10 wt% TiH2powders (D50= 16.4 μm),respectively.Samples B1-B4 were composite explosive samples containing 5 wt% TiH2powders with the particle size of 16.4 μm,33.7 μm,50.1 μm,112.0 μm,respectively.Sample C was a composite explosive containing 5 wt% Ti powders with D50of 18.4 μm.The experimental explosive samples were all 10 g cylindrical charges(2 cm in diameter and height) with a density of 1.59 g/cm3.

2.3.Experimental facilities and methods

2.3.1.Thermal stability test

In order to investigate the effects of TiH2powders as an energetic additive on the thermal stability of RDX, calorimetric experiments were carried out on RDX-based composite explosives using a C80 microcalorimeter (CALVET, SETARAM, French).In the experiments, the composite explosive samples of 25 mg were heated in an 8.5 ml high-pressure standard chamber (high pressure to 100 bar).The heating rate of each sample was heated from 30 to 300°C with a heating rate of 1°C/min.

2.3.2.Air blast experiment

Fig.2.Micro-structures of (a) Passivated RDX; (b) 16.4 μm TiH2 powders; (c) 33.7 μm TiH2 powders; (d) 50.1 μm TiH2 powders; (e) 112.0 μm TiH2 powders and (f) 18.4 μm Ti powders.

Table 1 Formulations of RDX-based composite explosives samples.

The air blast experiment was used to assess the shock wave characteristics of RDX-based composite explosives containing various content and particle sizes of TiH2powders, and the experimental facility is shown in Fig.3.The explosive charge was fixed on a steel frame and ignited by a detonator.The shock wave signal was obtained by a PCB pressure sensor(137B24B,PCB,USA),and after being processed by a constant current source(PCB,USA),the signal was recorded in an oscilloscope (LeCroy HDO4034, Teledyne LeCroy, USA).The detonation process of RDX-based composite explosives was captured using a high-speed camera(Memrecam HX-3,NAC,Japan)with a frame rate of 173,160 frames per second.Each composite explosive used in the experiment was 10 g,and the explosive charge was hung 60 cm vertically above the ground with a 70 cm horizontal distance between its center and the pressure sensor.In addition, the high-speed camera was placed 30 m away from the explosive charge.In order to study the effects of TiH2powders on the detonation velocity of RDX-based composite explosives,the ion probe method[24]was used to measure the detonation velocity of RDX-based composite explosives with different formulations(5 charges adhered together).To reduce the experimental error, each sample should be tested at least 3 times.

2.3.3.Colorimetric thermometry

Explosive detonation process is usually accompanied by gas products with high temperature and high pressure, and normal contact temperature measurement methods could not meet the requirements of environmental adaptability of explosion sites.Colorimetric thermometry method could process the high-speed images to calculate the temperatures, which has significant advantages in the measurement of explosion temperature field [23].When use the colorimetric thermometry to measure the explosion temperature, it should be calibrated at first.In the study, the tungsten lamp calibration system was used for temperature calibration, as shown in Fig.4, and the function relation between R/G ratio(the symbols of R and G represent the luminous intensities of red and green,respectively)and actual temperature of the tungsten lamp was calculated using a Python code,and then the temperature correction coefficient was obtained [25,26].After that, the temperature reconstruction Python code was used to interpolate the gray image and the R/G ratio of each pixel in the gray image of the explosive fireball was obtained.Finally, the temperature value of each pixel was calculated according to the mapping relations between R/G ratio and the temperature correction coefficient, and presented as a temperature distribution map.

Fig.3.Schematic diagram of air blast experiment system.

Fig.4.Calibration system of tungsten filament lamp of(a)Schematic diagram of calibration circuit and(b)Mapping relations between R/G ratio and actual temperature of tungsten filament lamp.

3.Results and discussion

3.1.Thermal decomposition characteristics of RDX composite explosives

To investigate the effects of TiH2powders on the thermal decomposition characteristics of RDX, thermal analysis experiments of RDX-based composite explosives with different formulations were studied by a C80 microcalorimeter.Fig.5 shows the heat flow curves of pure RDX and different RDX-based composite explosive at a heating rate of 1°C/min, and the pure RDX was served as a reference sample.In the process of dynamic heating,the decomposition of RDX had a phase transition, that is, melting to absorb heat at the beginning and then decomposing to release heat[19].However,as the heat flow curves of RDX shown in Fig.5,there was only an exothermic peak within the range of 200-230°C,which was due to the coupling phenomenon between fusion endothermic and decomposition exothermic of RDX[27].Under the condition of a low heating rate (1°C/min), the coupling phenomenon was more obvious, and the intensity of the decomposition exothermic peak exceeded that of the melting endothermic peak,so only one exothermic peak appeared in the heat flow curves.In addition, it could be seen from the heat flow curves that the decomposition curves of RDX-based composite explosives containing TiH2powders with different content and particle sizes were still had one exothermic peak,and their peak shape did not change significantly as compared with that of the pure RDX.

Fig.5.Heat flow curves of RDX-based composite explosives with (a) different content and (b) different particle sizes of TiH2 powders measured with C80 microcalorimeter.

Table 2 Thermodynamic characteristic parameters of C80 curves of RDX-based composite explosives.

Table 2 shows the thermodynamic parameters including initial decomposition temperature(Ts), decomposition peak temperature(Tp), decomposition termination temperature (Te) and decomposition heat (ΔH) of RDX-based composite explosives.According to Table 2, TiH2powders increased the initial decomposition temperature of RDX-based composite explosive, which could effectively improve its thermal stability in storage and usage.Furthermore, with the addition of TiH2powders, the decomposition heat(ΔH)of the composite explosives also increased.There are two reasons might be account for the above phenomena, On the one hand,TiH2powders did not decompose at 300°C[6],and when RDX began to undergo thermal decomposition, TiH2powders would absorb part of the heat released by decomposition, and the CaF2crystal structure of TiH2powders would adsorb the decomposing gas products,promoting the thermal decomposition of RDX[19,28].On the other hand,the thermal decomposition of RDX had a competition between the breaking of C-N bond and N-N bond[29],and C-N bond breaking mainly occurred at a low heating rate.Palopoli et al.[30]indicated that the H atoms in the-CH2-groups would transfer to the -NO2groups, which led to the breaking of N-N bonds.In addition, the bond dissociation energy of the N-O bond in the nitro group is far larger than that of the N-N bond,resulting in the N-N bond more easily ruptured than the N-O bond[31].Therefore,when the TiH2powders were introduced into RDX,an abundance of free H atoms on the surface of TiH2crystal cells might increase the chance for H atoms to interact with -NO2groups, promoting the breaking of N-N bond to produce strong oxidant NO2, which would react with TiH2powders to release additional heat[32,33].When the amount of TiH2powders ranged from 0 to 10 wt%, the ΔH of RDX-based composite explosive increased at the beginning and then gradually dropped with the increasing mass ratio of TiH2powders, reaching the maximum value of 6393.38 J/g at 5 wt%content.When the mass ratio of TiH2powders in the RDX-based composite explosive exceeded 5 wt%,the promotion effect of TiH2powders was weaker than the weakening effect of decreasing RDX content on heat release.In addition,TiH2powders with smaller particle sizes had larger contact areas and better heat transfer performance with RDX explosives, which would reach the activation temperature earlier for the reaction,so ΔH increased gradually with the decreasing particle size of TiH2powders.

3.2.Detonation velocity of RDX-based composite explosives

Detonation velocity is one of the important parameters to assess the detonation energy of explosives.The detonation velocities of RDX-based composite explosives containing various content and particle sizes of TiH2powders were measured.As shown in Fig.6,as the mass ratio of TiH2powders in RDX-based composite explosives increased from 0 to 5 wt%, the detonation velocity values of RDXbased composite explosives were 7736 m/s, 7707 m/s, 7509 m/s,7374 m/s and 7143 m/s, respectively, and the detonation velocity values showed an approximately linear negative correlation with the content of TiH2powders.There are three reasons may account for this phenomenon.Firstly,the physical and chemical properties of TiH2powders were stable, so they needed to absorb additional energy generated by RDX detonation before participating in the secondary reaction.Secondly, TiH2powders added to RDX explosive as an energetic material, and the whole reaction process included two stages of RDX detonation and TiH2secondary reaction,for the detonation velocity of RDX was much higher than that of micron TiH2powders,so the main contribution to the detonation velocity is RDX.Thirdly,TiH2powders had little or no participation in the reaction zone and its thermal decomposition reaction would absorb part of the energy, which played a dilution effect in the detonation kinetics [12,17].Therefore, as the content of TiH2powders in RDX-based composite explosives increased, the detonation velocity decreased.Furthermore, Fig.6 also demonstrates that when the particle size of TiH2powders increased from 16.4 to 112.0 μm, the detonation velocity values of RDX-based composite explosives were 7736 m/s, 7509 m/s, 7428 m/s, 7401 m/s, and 7244 m/s, respectively, that is, the detonation velocity values dropped with the growing particle size of TiH2powders.The reason for that was compared to smaller TiH2powders, larger TiH2powders had lower reactivity and slower reaction rate [34], which would absorb more detonation energy in the detonation process to reduce the detonation velocity of composite explosives.

Fig.6.Detonation velocity values of RDX-based composite explosives.

3.3.Shock wave parameters of RDX composite explosives

Shock wave parameters such as peak overpressure, positive duration and positive impulse formed on the surrounding medium after the explosion are important parameters to assess the shock wave damage effect of ammunition [24].The detonation characteristics of RDX-based composite explosives with different content and particle sizes of TiH2powders were investigated through air blast experiments, and the pressure data was denoised using the Modified-Friedlander equation [23] as shown in Eq.(1).

where, Δpmaxis the peak overpressure, kPa.T is time, s.T+is the positive duration, s.α is the attenuation coefficient of shock wave.

The positive duration was obtained by calculating the pressuretime curves after noise reduction,and Eq.(2)was used to calculate the positive impulse.

3.3.1.Effects of TiH2powders content

Fig.7.Typical pressure-time curves of RDX-based composite explosives with different contents of TiH2 powders.

Fig.7 demonstrates the typical pressure-time curves of RDXbased composite explosives with various content of TiH2powders,and the peak overpressure of RDX-based composite explosives presented a trend of first rising and gradually dropping with the increasing content of TiH2powders.For the duration and width of chemical reaction zone were too short for TiH2powders to participate in the detonation reaction before the C-J surface [15], they mainly participated in the after-burn reaction of detonation products area behind detonation wave, which promoted the propagation of the shock wave and slowed down its attenuation.Since the mass of RDX-based composite explosive sample was fixed at 10 g,the amount of RDX decreased as the content of TiH2powders increased.When the content of TiH2powders reached an appropriate level, the promoting effect of TiH2powders on the shock wave surpassed the weakening effect due to the reduction of RDX content,which indicated that the peak overpressure enhanced with the increasing content of TiH2powders.When the content of TiH2powders was excessive, the negative oxygen balance of composite explosives was intensified, and the promoting effect of increased TiH2powders on the shock wave was less than the weakening effect caused by the reduced RDX content, resulting in a diminution of shock wave peak overpressure.In addition,because of the large difference in the physical properties of RDX and TiH2powders,their detonation followed the mixed reaction mechanism [14,15].Excessive TiH2powders would reduce the uniformity among components of the composite explosive, and also lead to the continuous decline of peak overpressure of the composite explosives.

As shown in Table 3, the peak overpressure (ΔPmax), positive duration (t+) and positive impulse (I+) of different RDX-based composite explosive samples were calculated using Eqs.(1) and(2).Table 3 shows that with the increase of TiH2powders content, the positive duration and positive impulse of shock wave of RDX-based composite explosives containing various content of TiH2powders represented the same change laws as the peak overpressure,and both of them showed a trend of firstly rising and then decreasing.In the case of 5 wt% TiH2powders in the RDX-based composite explosive (Sample A2), the peak overpressure, positive duration and positive impulse reached their maximum values of 62.3 kPa,542.4 μs and 12.38 Pa s,respectively,which increased by 4.7%,10.4%and 9.1%when compared with the pure RDX.That was mainly because an appropriate amount of TiH2powders would be involved in the detonation reaction and after-burn process of RDXbased composite explosives, which enhanced the strength and delayed the attenuation of the shock wave.Table 2 displays that the decomposition heat (ΔH) of RDX-based composite explosives increased in the beginning and then gradually dropped with the growth of TiH2powders, reaching its maximum value when the content of TiH2powders was 5 wt%,which was consistent with the air blast experimental results shown in Table 3.

3.3.2.Effects of particle size of TiH2powders

The effects of particle size of TiH2powders on the shock wave parameters of RDX-based composite explosives were alsoinvestigated using the air blast experiment, and the typical pressure-time curves were shown in Fig.8.The shock wave peak overpressures of RDX-based composite explosives with various particle sizes of TiH2powders were all higher than that of the pure RDX explosive(Sample A),which were 62.3 kPa,61.9 kPa,59.8 kPa and 57.2 kPa, respectively.The shock wave peak overpressure decreased continuously with the increasing particle size of TiH2powders, reaching its maximum value when the particle size of TiH2powders was 16.4 μm(Sample B1).The results of shock wave parameters were shown in Table 4, and the peak overpressure,positive duration and positive impulse of composite explosives all decreased with the increasing particle size of TiH2powders.This phenomenon could be explained as that TiH2powders with smaller particle size had higher reaction activity and faster reaction rate,which would participate in the reaction earlier and more sufficiently, thus increasing the output energy of detonation reaction and improving the shock wave peak overpressure, positive duration and positive impulse more obviously.However, TiH2powders with larger particle sizes had a lower reaction rate and mainly participated in the after-burn reaction, which would absorb heat and then dilute the kinetic effect of detonation reaction [14],resulting in the peak overpressure, positive duration and positive impulse of RDX-based composite explosives with TiH2powders(D50= 112 μm) even lower than that of the pure RDX explosive.Table 2 shows that the decomposition heat (ΔH) of composite explosives decreased with the increased particle size of TiH2, which was consistent with the air blast experimental results shown in

Table 3 Shock wave parameters of RDX-based composite explosives with different content of TiH2 powders.

Fig.8.Typical pressure-time curves of RDX-based composite explosives with different particle sizes of TiH2 powders.

Table 4 Shock wave parameters of RDX-based composite explosives with different particle sizes of TiH2 powders.

3.4.Transient explosion temperature field of RDX-based composite explosives

3.4.1.Effects of TiH2powders content

Explosion temperature is another important parameter for evaluating the thermal damage effect of explosives [23].To study the effect of TiH2powders content on the fireball temperature of RDX-based composite explosives,the transient fireball temperature distribution of pure RDX and composite explosives were reconstructed by colorimetric thermometry.Figs.9 and 10 are the fireball temperature filed of the pure RDX (Sample A) and RDX-based composite explosive (Sample A2) with 5 wt% TiH2powders,respectively.For the convenience of description, the moment that the first picture taken by the high-speed camera was recorded as t = 0 μs.After the explosive was ignited, the explosion products spread to the ambient and exchanged heat with the external environment, and the explosion fireball continued to expand and do work externally through compressing the air.In the whole detonation process, the explosion energy was dissipated into the surrounding environment in the form of heat,light and sound until the explosive fireball extinguished[23,28].Fig.9 demonstrates that the detonation products of RDX explosive diffused rapidly after being ignited, and the explosion fireball grew up in the shape of“mushroom”in the time range of 0 and 58 μs.The explosion fireball started to split and finally went out from 58 to 110.2 μs.As shown in Fig.11, the average fireball temperature of the pure RDX declined gradually in the whole detonation process, and the maximum fireball temperature was 2436 K when t=0 μs,which was close to the explosion temperature of 2423 K obtained by the theoretical calculation of passivated RDX in Ref.[35], further verifying the precision and reliability of the colorimetric thermometry.

Fig.10 presents the fireball temperature distribution of RDXbased composite explosive containing 5 wt% TiH2powders (Sample A2).When the time was between 0 and 75.4 μs, the explosive fireball of RDX-based composite explosive with 5 wt%TiH2powders also expanded in the shape of "mushroom" until ruptured after being ignited while the explosion temperature gradually dropped.Different from Sample A,the explosive fireball began to break from 75.4 to 179.8 μs, but the average temperature kept rising.In the range of 179.8 μs and 295.8 μs,the explosive fireball gradually went out and the explosion temperature gradually dropped.As shown in Fig.11, the RDX-based composite explosive with 5 wt% TiH2powders reached the maximum average temperature of 3374 K at t=179.8 μs.Compared with the pure RDX,the average temperature of RDX-based composite explosive with 5 wt% TiH2powders showed a decreasing trend consisting with pure RDX explosive at the initial stage of reaction,but subsequently continued to increase until the after-burn reaction began to extinguish,and its maximum average temperature and fireball duration increased by 38.5% and 168.4%,respectively.The results indicated that TiH2powders would effectively improve the fireball temperature and fireball duration of composite explosives to enhance their thermal damage effect.When the composite explosives with TiH2powders were ignited,the explosive fireball began to spread outwards,and the explosion temperature decreased due to the effects of doing work and heat exchange.When the TiH2powders absorbed the energy released by the RDX-based composite explosives reached their activation threshold value, it would continue to react with detonation gas products and air in the after-burn reaction, which contributed to releasing a lot of heat and slowing down the attenuation of explosive fireball to make the temperature of explosive fireball rise continuously.Finally, with the effect of after-burn reaction of TiH2powders weakened, the fireball temperature began to decrease during its diffusion process.

Fig.9.Transient fireball temperature field of pure RDX explosive.

Fig.11 demonstrates the average temperature-time curves of RDX-based composite explosives with various content of TiH2powders, and their maximum average temperature and fireball duration were shown in Table 5.The average temperature of pure RDX showed a continuous decreasing trend after the explosive was initiated, while the RDX-based composite explosives containing various content of TiH2powders showed a"drop-rise-drop"trend.The fireball temperature and duration of RDX-based composite explosive increased effectively with the addition TiH2powders.Furthermore, as the mass ration of TiH2powders increased from 0 to 10 wt%,the maximum average temperature(Tmax)and fireball duration were 2436 K,3212 K,3374 K,3221 K,2973 K and 110.2 μs,237.8 μs, 295.8 μs, 284.2 μs, 266.8 μs, respectively, all of which increased initially and then dropped gradually with the increasing mass proportion of TiH2powders in the RDX-based composite explosive.It is noteworthy that the maximum average temperature and fireball duration of RDX-based composite explosives reached a maximum of 3374 K and 295.8 μs, respectively, as the content of TiH2powders was 5 wt% (Sample A2), increasing by 38.5% and 168.4%in comparison to that of the pure RDX explosive,which was consistent with the optimum content of TiH2powders on detonation characteristics in the air blast experiments.As mentioned above, TiH2powders were involved in the after-burn reaction as energetic additives, which improved the fireball temperature and duration of the composite explosive.However, as the content of TiH2powders continued to increase, the amount of RDX in the composite explosive decreased and the negative oxygen balance was intensified, which led to the sharp decrease in the fireball temperature and duration.

Fig.10.Transient fireball temperature field of RDX-based composite explosives with 5 wt% TiH2 powders.

3.4.2.Effects of particle size of TiH2powders

The effects of particle size of TiH2powders on the fireball temperature distributions of RDX-based composite explosives were also studied using the colorimetric thermometry.Fig.12 shows the transient explosion temperature distribution of RDX-based composite explosive with 5 wt%TiH2powders(D50=112.0 μm).In the range of 0 μs and 52.2 μs, the fireball of RDX-based composite explosives expanded in the shape of“mushroom”and then gradually broke up while the average temperature kept dropping slowly.When the time ranged from 52.2 to 156.6 μs,the after-burn reaction occurred and the average temperature of the fireball increased continuously.When the time was between 156.6 μs and 203 μs,the detonation reaction and after-burn reaction were ended, and the average fireball temperature began to drop from that moment on,and the maximum fireball temperature was 2872 K at 156.6 μs during the detonation process.Compared with RDX-based composite explosives with 16.4 μm TiH2powders (Sample B1), the fireball temperature and duration of RDX-based composite explosives with 112.0 μm TiH2powders (Sample B4) were both smaller which was attributed to the incomplete after-burn reaction of larger particle size of TiH2powders that had smaller specific surface area and lower reactivity.

Fig.11.Average temperature-time curves of RDX-based composite explosives with different content of TiH2 powders.

Table 5 Explosion temperature parameters of RDX-based composite explosives with different content of TiH2 powders.

Fig.13 indicates the average temperature-time curves of RDXbased composite explosives containing various particle sizes of TiH2powders, and the corresponding maximum average temperature and fireball duration were shown in Table 6.The particle size of TiH2powders significantly affected the thermal damage effect of composite explosives, when the particle size of TiH2powders increased from 16.4 to 112.0 μm, the maximum average temperature and fireball duration of RDX-based composite explosives with 5 wt% TiH2powders were 2436 K, 3374 K, 3221 K, 2963 K, 2872 K and 110.2 μs, 295.8 μs, 255.2 μs, 232 μs, 208.8 μs, respectively,which declined with the growing particle size of TiH2powders.In the after-burn reaction,TiH2powders with larger particle sizes had lower reactivity and needed to absorb more energy to reach their activation threshold value [12,35].Secondly, the larger TiH2powders had smaller surface area and slower heat transfer,resulting in an insufficient after-burn reaction of TiH2powders.In addition,compared with smaller TiH2powders, larger TiH2powders had higher initial temperature and slower rate in the dehydrogenation process, which reduced the energy release rate and the amount of heat of the detonation reaction within a certain period of time[36].Therefore,the after-burn effect of TiH2powders could significantly enhance the fireball temperature,and the fireball temperature and duration would both decrease with the growing particle size of TiH2powders in the meantime.

3.5.Effects of free hydrogen on the detonation characteristics of RDX composite explosives

In order to study the effects of free hydrogen on the detonation characteristics of RDX-based composite explosives, the energy output rules of RDX-based composite explosives with TiH2powders(Sample B1) and Ti powders (Sample C) were compared, and the pure RDX explosive(Sample A)was taken as the reference sample,as shown in Table 1.In the experiments, each explosive sample weighted 10 g, and each sample was tested more than 3 times.As shown in Table 7, compared with the pure RDX sample, the peak overpressure of RDX-based composite explosive with Ti powders decreased by 5.9%, while the maximum average temperature,positive duration and positive impulse all increased by 14.5%,3.2%and 1.7%, respectively.The maximum average temperature, peak overpressure of shock wave,positive duration and positive impulse of RDX-based composite explosive containing TiH2powders were increased by 20.9%, 11.3%, 7.0% and 7.3%, respectively, compared with that of RDX-based composite explosive with Ti powders.The thermal damage effect and detonation power of RDX-based composite explosives with TiH2powders were obviously better than those of Ti powders.That was because Ti powders were mainly involved in the reaction behind the C-J surface of RDX detonation,and its early endothermic effect played a dilutive role in the detonation dynamics to reduce the peak overpressure, but its subsequent after-burn reaction would delay the attenuation of shock wave and raise the explosion temperature [19,26].Unlike Ti powders,TiH2is a type of the hydrogen storage materials,and hydrogen in it mainly exists in the form of solid solution, and its thermal decomposition obeyed the "shrinking core" models [37].The high temperature provided by the explosive detonation made the TiH2rapidly to form elemental Ti and free H2, and then H2quickly participated in the chemical reaction zone to increase the detonation pressure[38].Furthermore,the elemental Ti would participate in the after-burn reaction to delay the shock wave attenuation and increase the explosion temperature.That was why the addition of TiH2powders in the RDX-based composite explosives showed a better detonation performance than that of the Ti powders.

4.Conclusions

The effects of content and particle size of TiH2powders on the thermal safety,detonation velocity,shock wave characteristics and fireball temperature distribution of RDX-based composite explosives were investigated experimentally.The experimental results of C80 microcalorimeter revealed that TiH2powders would enhance the initial decomposition temperature of composite explosives,which effectively improved its thermal stability in storage and usage.The decomposition heat of RDX-based composite explosives increased at first and then dropped with the growing content of TiH2powders, reaching its maximum values of 6393.38 J/g when the content of TiH2powders was 5 wt%,but decreased continually with the increase of particle size of TiH2powders.The air blast experiments results showed that the detonation velocity of RDXbased composite explosive was negatively correlated with the content and particle size of TiH2powders.The peak overpressure,positive duration and positive impulse of RDX-based composite explosive all increased at first and then dropped with the increasing content of TiH2powders, reaching its maximum values of 62.3 kPa,542.4 μs and 12.38 Pa•s,respectively,when the content of TiH2powders was 5 wt%, which increased by 4.7%, 10.4% and 9.1%, respectively, when compared with the pure RDX, but decreased with the increasing particle size of TiH2powders.The colorimetric thermometry results showed that the effect law of the content and particle size of TiH2powders on the explosion temperature was consistent with that of shock wave parameters, and the fireball temperature and duration reached their maximum values of 3374 K and 295.8 μs,respectively,when the mass ratio of TiH2powders was 5%.Compared to the RDX-based composite explosives with Ti powders, the maximum average temperature,peak overpressure, positive duration and positive impulse of RDXbased composite explosives with TiH2powders increased by 20.9%,11.3%, 7.0% and 7.3%, respectively, which indicated that TiH2powders were superior to Ti powders in improving the thermal damage effect and detonation power of RDX for the participation of free H2in its detonation process.

Fig.12.Transient fireball temperature field of RDX-based composite explosives with 5 wt% of 112.0 μm TiH2 powders.

Fig.13.Average temperature-time curves of RDX-based composite explosives with different particle sizes of TiH2 powders.

Table 6 Explosion temperature parameters of RDX-based composite explosives with different particle sizes of TiH2 powders.

Table 7 Detonation parameters of different RDX-based composite explosives.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The work was supported by the National Natural Science Foundation of China (Grant Nos.11972046, 12272001), the Outstanding Youth Project of Natural Science Foundation of Anhui Province (Grant No.2108085Y02) and Anhui University of Science and Technology Postgraduate Innovation Fund (Grant No.2022CX2108), and the authors would like to thank these foundations for the financial supports.