Xing-ling Wu ,Sen Xu ,,Ai-min Png b,,Wei-guo Co ,D-bin Liu ,Xin-yu Zhu ,Fei-yng Xu ,Xu Wng

a School of Chemical Engineering,Nanjing University of Science and Technology,Nanjing,Jiangsu,210094,PR China

b Science and Technology on Aerospace Chemical Power Laboratory,Hubei Institute of Aerospace Chemotechnology,Xiangyang,441003,PR China

c School of Environmental and Safety Engineering,North University of China,Taiyuan,030051,Shanxi,PR China

Keywords: Ignition energy Explosion pressure Reaction activity Combustion heat

ABSTRACT MgH2,TiH2,and ZrH2 are three typical metal hydrides that have been gradually applied to composite explosives and propellants as additives in recent years.To evaluate ignition sensitivity and explosion severity,the Hartmann device and spherical pressure vessel were used to test ignition energy and explosion pressure,respectively.The results showed that the ignition sensitivity of ZrH2,TiH2 and MgH2 gradually increased.When the concentration of MgH2 is 83.0 g/m3 in Hartmann device,the ignition energy attained a minimum of 10.0 mJ.The explosion pressure of MgH2 were 1.44 times and 1.76 times that of TiH2 and ZrH2,respectively,and the explosion pressure rising rate were 3.97 times and 9.96 times that of TiH2 and ZrH2,respectively,through the spherical pressure vessel.It indicated that the reaction reactivity and reaction rate of MgH2 were higher than that of TiH2 and ZrH2.In addition,to conduct in-depth theoretical analysis of ignition sensitivity and explosion severity,gas production and combustion heat per unit mass of ZrH2,TiH2 and MgH2 were tested by mercury manometer and oxygen bomb calorimetry.The experimental results revealed that MgH2 had a relatively high gas production per unit mass(5.15 mL/g),while TiH2 and ZrH2 both had a gas production of less than 2.0 mL/g.Their thermal stability gradually increased,leading to a gradual increase in ignition energy.Furthermore,compared with theoretical combustion heat,the combustion ratio of MgH2,TiH2 and ZrH2 was more than 96.0%,with combustion heat value of 29.96,20.94 and 12.22 MJ/kg,respectively,which was consistent with the explosion pressure and explosion severity test results.

1.Introduction

Metal hydride is a new energetic material with advantages of high hydrogen storage capacity,high energy density,high combustion heat and fast energy release ef ficiency[1-3].Therefore,it is widely used in other energetic materials to improve or enhance the comprehensive performance.

Currently,the most widely used metal additive in energetic materials is aluminum,which can enhance air blast and raise reaction temperatures in explosives,and can increase the speci fic impulse and flame temperature in propellants[4].In recent decades,vast researches on the application of aluminum in energetic materials[5-10]revealed that the size,shape and content of aluminum powder had a great effect on the size and form of energy release of energetic materials.Jiang et al.[11]concluded that the particle gradation of aluminum could increase the energy output in the initial explosion reaction.Ahmed et al.[12]found that RDX-based PBX had the highest underwater explosion performance when the Al content was 25.0%and the NH4ClO4content was 30.0%.Furthermore,Hu et al.[13]obviously demonstrated that nano-aluminum powder had a signi ficant effect on explosion reactions and secondary reactions.Li et al.[14]completed a detailed investigation of RDX/Al formula explosives with 15.0%Al content had the best metal acceleration ability.

With the gradual improvement of the requirements for energetic materials,aluminum is not enough to meet the actual demand to a certain extent.Under the condition of continuous progress in ultra-re fining technology,metal hydrides were gradually applied to the explosives and industrial production due to excellent combustion performance.Considerable research efforts have been devoted to the application of metal hydrides in energetic materials.Xue et al.[15]added MgH2and TiH2to RDX-based mixed explosives,and the underwater explosion results showed that the shock wave energy and bubble energy were signi ficantly increased.Cao et al.[16]clari fied that adding MgH2to aluminized explosives could increase the explosive heat and enhance the afterburning potential.Xue et al.[17]con firmed that the addition of small particle size TiH2could improve the underwater explosion performance of TiH2/RDX-based composite explosives,and the particle size and content of TiH2had a signi ficant effect on its performance.Cudzi?o et al.[18]completed a detailed investigation of the detonation heats of RDX-based non-ideal explosives with additives of Al,(Al/ZrH2),TiH2and ZrH2and analyzed the solid post-detonation products.The energy of all metal-containing explosives was higher than RDX itself,and aluminum produced positive in fluence on the release of total energy,and TiH2was the least reactive additive among the three additives.Cheng et al.[19,20]studied the promoting effect of hydrogen on Ti/TiH2dust explosion intensity in a 20.0 L spherical container,and found that the presence of hydrogen transformed the explosion from the discrete phase to the continuous phase,and increased the explosion pressure of the system.Furthermore,Yang et al.[21]revealed the reaction mechanism of HTPB propellant containing ZrH2,which could dehydrogenate independently to produce H2and metal Zr,thus promoting gas-phase combustion reaction.In our previous study,Chen et al.[22,23]compared and analyzed the explosive properties of aluminum powder and metal hydride,characterized the properties of high-energy metals and alloy materials from the perspectives of thermodynamics and dynamics,and found that different assembly processes could improve the combustion ef ficiency.To summarize,the above researches show that metal hydrides have considerable advantages in the use of energetic materials,but the ignition sensitivity and explosion severity of metal hydrides have not been paid enough attention at present in present study.Therefore,how to reduce the harm caused by accidental explosion in industrial production and use of energetic materials is still a challenging research project.

In this paper,three typical metal hydrides of magnesium hydride(MgH2),titanium hydride(TiH2),and zirconium hydride(ZrH2)were selected as experimental samples to study ignition sensitivity and explosion severity.Hazard evaluation of the three metal hydrides was carried out from the perspective of the reaction mechanism by investigating the gas production and combustion heat of the three metal hydrides.This study aimed to enrich the current understanding of the hazard evaluation of ignition sensitivity and explosion severity for three typical metal hydrides by adopting experimental methods.

2.Test samples and devices

Three powders of MgH2,ZrH2,and TiH2were used in this experiment.MgH2,TiH2and ZrH2powders were self-made and provided by Institute of metal research,Chinese Academy of Sciences.The purity was 99.5%,99.7%and 97.9%,respectively,with particle size less than 44.0μm(325 mesh).The SEM(Scanning Electronic Micrograph)images were showcased in Fig.1.The particle sizes of the three metal hydrides are between 1.0 and 20.0μm ZrH2and TiH2show irregular flakes,while the main shape of MgH2is spherical and the surface of the sphere is uneven,which results in the speci fic surface area larger than ZrH2and TiH2.

2.1.Hartmann apparatus

The ignition energy of MgH2,ZrH2,and TiH2were tested by Hartmann tube apparatus.As listed in Fig.2,the experimental device consists of a vertical combustion tube with volume of 1.2 L,a high-pressure dispersion system,an ignition system and a control system.

As the hydride is relatively sensitive,and considering the safety of the test personnel and the protection of the experimental equipment,the quantity of experimental samples is controlled at 0.1-0.9 g.Before the experiment,sample was placed around the nozzle at the bottom of Hartmann tube,and then the sample was blown by high-pressure air to form dust cloud.An electric spark was released through the ignition electrode to ignite the dust cloud,and the dust cloud with the same concentration was ignited 10 times at each energy level.The ignition probability of dust could be calculated by the number of ignitions.The minimum ignition energy(Emin)of hydride lies between the maximum energy value(E1),which has not been ignited in 10 consecutive tests,and the minimum energy value(E2),that has been ignited at least once in 10 consecutive tests[24]:

The test was conducted at environment temperature of 20-30°C and humidity of 10-30%.

2.2.Spherical pressure vessel test system

The explosion pressure of the sample was measured by spherical pressure vessel test device,and its principle is depicted in Fig.3.Before the experiment,vacuum the spherical pressure vessel to-0.6 MPa(gauge pressure).During the test,a 2.4 g chemical igniter(Zr:BaO2:BaNO3=4:3:3)was used to detonate the sample blown into the spherical pressure vessel at the center of the vessel by high-pressure gas,and obtain the pressure-time curve during the explosion.By analyzing the explosion pressure-time curve,the rising rate of explosion pressure(dP/dt),maximum explosion pressure(Pm)and de flagration index(Kst)[25,29]of the hydride were obtained.Kstis defined as the product of the maximum explosion pressure rise rate(dP/dt)mgenerated by the dust explosion and the cubic root of the explosion vessel volume(V),the equation is as follow:

2.3.Gas production experimental device

The mercury manometer method was used to study the gas production of hydride,and the test device is displayed in Fig.4.The metal hydride samples were dried in vacuum oven at temperature of 55.0°C and vacuum of 9.0-12.0 kPa for 2.0 h,weighed 5.0 g,and placed in the test tube to start the experiment.The experimental conditions were a constant temperature of 100.0°C for 48.0 h.As a result of the metal hydride is heated to produce gas,the mercury manometer scale changes,and the gas production is calculated.

2.4.Combustion heat

The parr6300 oxygen bomb calorimeter was used to measure the combustion heat value of the sample(weight 0.5 g)through the resistance wire ignition method,and the combustion heat ratio was calculated.The equation is as follow:

Fig.1.SEM images of(a)MgH2,(b)TiH2,(c)ZrH2.

Fig.2.Hartmann tube apparatus.

Fig.3.Schematic diagram of the spherical pressure vessel test system.

whereqis the combustion heat value per unit mass,tested by system(J/g),m1is the mass of samples(g),Eis the thermal capacity of the calorimetric system(2414.29 J/°C),tnandt0are the pre-combustion and post-combustion temperatures(°C)of the oxygen bomb system respectively,C is the cooling correction value(°C).

As the resistance wire was added in the experiment,the heat value generated by the resistance wire should be deducted.Therefore,Equation(3)was amended as follows:

Fig.4.Mercury manometer.

whereq1is the combustion heat value of samples,q2is the combustion heat value per unit length of the nickel-cadmium alloy ignition wire(6.28 J/cm),Lis the length of the nickel-cadmium alloy ignition wire(cm).

The experiment was conducted at the environment temperature of 20-30°C,and the oxygen pressure of 3.0 MPa.

3.Results and discussion

3.1.Ignition energy of MgH2,TiH2 and ZrH2

The mass of the metal hydride was selected from six types:0.1,0.2,0.3,0.5,0.7 and 0.9 g,and the test results of ignition energy were listed in Fig.5.

Fig.5.The results of ignition energy of(a)MgH2,(b)TiH2,and(c)ZrH2.

Fig.5 delineates the ignition energy corresponding to the maximum(100.0%)ignition probability of MgH2,TiH2,and ZrH2which are 20.0,200.0 and 300.0 mJ,respectively.And the order of the ignition energy is MgH2

The in fluence factors of ignition of metal hydride dust cloud come from metal particles and stored hydrogen.Therefore,to study the ignition mechanism of three hydrides(MgH2,TiH2and ZrH2),the gas production in vacuum was measured.

3.1.1.Ignition mechanism in the perspective of gas production

Table 1 presents the results of measuring the gas production of hydride by the mercury manometer method.As can be seen from Table 1,the average unit gas production of MgH2among the three metal hydrides is the largest,reaching 5.15 mL/g after a constant temperature of 100.0°C for 48.0 h.The unit gas production of the three metal hydrides is in the order of MgH2>TiH2>ZrH2.Since the test was performed under vacuum,a large amount of gas generated by MgH2was caused by the self-reaction,indicating that a decomposed dehydrogenation reaction occurs in MgH2and a larger amount of hydrogen was released under this condition.However,TiH2and ZrH2with an average gas production of 0.73 and 0.54 mL/g was less than MgH2,illustrating that ZrH2and TiH2were more stable and have higher thermal stability.According to the gas production results of the three hydrides,MgH2was more likely to release the stored hydrogen,when the metal hydride was heated.In the 3.1 ignition experiment,we analyzed and speculated that when the MgH2dust cloud was stimulated by electric sparks,a small amount of metal hydride would be ignited first,and then the MgH2would be partly decomposed by the heat to liberate hydrogen.The hydrogen reacted with oxygen in the air and generated a large amount of heat,and the released heat further promotes the decomposition of MgH2at the same time.As the high thermal stability of ZrH2and TiH2,it was dif ficult to decompose to produce hydrogen when stimulated by electric sparks.

Table 1 Results of metal hydrides gas production.

The ignition energy and gas production showed that the activity of MgH2was the highest among the three hydrides,and its spark and thermal sensitivity were highest.

3.2.Explosion pressure of MgH2,ZrH2,and TiH2

The explosion severity of metal hydrides was measured by spherical pressure vessel.The three metal hydrides were detonated by chemical igniters inside the vessel and then produced explosive pressure.Fig.6 showcases the explosion pressure test results of MgH2,TiH2,and ZrH2at concentration of 750.0 g/m3.

Fig.6.Explosion pressure of MgH2,TiH2,and ZrH2(750.0 g·m-3).

The maximum explosion pressure(Pm),the rising rate of explosion pressure(dP/dt)and de flagration index(Kst)are important parameters to characterize the explosion severity.Fig.7 shows the explosion pressure rise rate of MgH2,ZrH2,and TiH2at concentration of 750.0 g/m3.In this experiment,five concentration speci fications of 125.0,250.0,500.0,750.0 and 1000.0 g/m3were selected.The maximum explosion pressure of three hydrides was shown in Fig.8.As can be seen from Figs.6 and 7,when the concentration was 750.0 g/m3,the maximum explosion pressure of MgH2,TiH2and ZrH2were 1.17,0.81,0.66 MPa,respectively,and the explosion pressure rise rate were 481.13,120.90,48.19 MPa/s,respectively.As can be seen from Fig.8,with the increased of mass concentration,the explosion pressure of hydride first increased and then decreased.This observation indicated that 750.0 g/m3was the optimal concentration,and the explosion pressure reached the maximum value.With the increased of mass concentration,the number of combustion particles per unit volume increased when the metal hydride occurred de flagration reaction through the ignition of chemical igniter in the spherical pressure vessel.Meanwhile,the pressure and temperature increased inside.When the mass concentration exceeds 750.0 g/m3,oxygen in the spherical pressure vessel was limited,and the metal hydride particles per unit volume could not be completely burned,resulting in the reduction of the explosion pressure.The order of explosion pressure of the three hydrides was MgH2>TiH2>ZrH2.During the reaction,some metal hydrides reacted directly,while some metal hydrides were decomposed by heat to form metal particles and hydrogen[26].The reactions are as follows:

Fig.7.Explosion pressure rising rate of MgH2,TiH2,and ZrH2(750.0 g·m-3).

The activity of hydrogen in the product was higher,which made the reaction more intense[27,28,30].When the mass concentration was the same,the hydrogen storage content and hydrogen release ef ficiency of MgH2were higher than that of TiH2and ZrH2.

Fig.8.The maximum explosion pressure of MgH2,TiH2,and ZrH2.

The de flagration indexes of MgH2,ZrH2,and TiH2were calculated on the basis of explosion pressure results,as listed in Fig.9.The de flagration index of the three metal hydrides increased first and then decreased with the increased of concentration,reaching the maximum value at 750.0 g/m3.When the concentration was 750.0 g/m3,the de flagration index of MgH2was 134.37 MPa m/s1,which was 3.94 and 9.81 times that of TiH2and ZrH2.The order of de flagration index of the three hydrides was MgH2>ZrH2>TiH2.Those observations revealed that the explosion pressure rise rate of MgH2was higher than that of TiH2and ZrH2,and its explosion reaction speed was faster and activity was higher.When the mass concentration of MgH2was 1000.0 g/m3,the de flagration index was greatly reduced.Because the mass concentration in the sphere was too high,the heat transmission after metal hydrides dust combustion was hindered,resulting in most metal hydrides dust not participating in the reaction.

Fig.9.The de flagration indexes of MgH2,ZrH2,and TiH2.

3.2.1.Explosion mechanism in the perspective of combustion heat value and gas production

The theoretical combustion heat values of MgH2,TiH2,and ZrH2can be calculated by Hess law.The calculation formula is as follows:

whereΔcH0is the combustion heat value(kJ/kg),is the heat of formation for product(kJ/kg),is the heat of formation for reactant(kJ/kg).

The calculation results and experimental results are depicted in Table 2 and Fig.10.The combustion heat test values of MgH2,TiH2,and ZrH2were 29.96,20.94,12.22 MJ/kg,and the order of the combustion heat was MgH2>TiH2>ZrH2.It presented that the chemical potential of MgH2in the three metal hydrides was the largest.The combustion ratio of metal hydrides all exceeded 96.0%,implying that the combustion reactions of MgH2,TiH2,and ZrH2were very complete.This becomes a potential condition for metal hydrides to be able to generate high temperature and high pressure when de flagration occurs.The theoretical volumetric combustion heat of MgH2,TiH2and ZrH2can be converted by the density(ρ(MgH2)=1.45 g/cm3,ρ(TiH2)=3.91 g/cm3,ρ(ZrH2)=5.60 g/cm3)and theoretical combustion heat value of metal hydrides,and their values are 43.24,83.16 and 70.67 MJ/L respectively.This is an implication the volumetric heat of combustion of TiH2and ZrH2is much greater than that of MgH2.

Table 2 Results of metal hydrides combustion heat.

Fig.10.The combustion heat of MgH2,ZrH2,and TiH2.

In the data analysis results from Table 2 presented that the combustion heat value of MgH2was 1.43 times and 2.45 times that of TiH2and ZrH2.In a spherical pressure vessel,the MgH2dust cloud surrounding the chemical igniter was ignited to generate a large amount of heat.High temperature and high heat promote MgH2to accelerate the reaction on the one hand,and increase the pressure inside the vessel on the other hand.According to the experimental results in 3.1.1,the metal hydrides generate metal particles and gaseous hydrogen during combustion and explosion.Metal particles were distributed in the air in solid form,while gaseous hydrogen was dispersed on the surface of the metal particles.The hydrogen was more likely to catch fire and explode than metal particles during the reaction.The highly active hydrogen promoted the reaction of metal particles while generating heat.When the three metal hydrides were heated,MgH2had the highest activity and was the easiest to liberate hydrogen.Therefore,the explosion pressure and explosion rise rate of MgH2were higher than those of TiH2and ZrH2.

4.Conclusion

The ignition sensitivity and explosion severity of three typical hydrides were evaluated by Hartmann apparatus and spherical pressure vessel.The main conclusions are as follows：

(1)The order of the ignition energy for three metal hydrides is MgH2?TiH2ZrH2。

(2)The ignition mechanism and explosion mechanism of the metal hydride were investigated from the mechanism level through the gas production experiment and the combustion heat experiment,respectively.And the two experimental results were consistent with the experimental results of ignition energy and explosion pressure.The relationship between the activity of the three metal hydrides was MgH2>TiH2>ZrH2.Ignition energy was negatively correlated with gas production.The higher the gas production,the smaller the ignition energy and the higher the sensitivity to electric sparks.The explosion pressure of metal hydride dust cloud first increased and then decreased with increasing mass concentration.The metal hydride dust cloud would be decomposed to generate hydrogen after being ignited,and the highly active hydrogen would further promote the reaction.The higher the combustion heat value and gas production,the greater the explosion pressure and the explosion pressure rising rate.

(3)The MgH2is more sensitive and explosive than the other two substances.Therefore,it is necessary to selectively add different metal hydrides for different energetic materials and different practical applications.

Declaration of competing interest

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

Acknowledgements

This work was greatly supported by the Natural Science Foundation of China(11802272),and the Open Research Fund Program of Science and Technology on Aerospace Chemical Power Laboratory(STACPL220181B01).