Shu-jie Liu ,Bi-dong Wu ,* ,Ji-ni Xie ,Zhi-min Li ,Chong-wei An ,Jing-yu Wng ,** ,Xio-dong Li

a School of Environment and Safety Engineering,North University of China,Shanxi,030051,China

b State Key Laboratory of Explosion Science and Technology,Beijing Institute of Technology,Beijing,100081,China

Keywords: DAAF RDX Insensitive nergetic microspheres Thermal stability

ABSTRACT Insensitive energetic materials are promising in the defense weapons field.However,energetic materials still suffer from great challenges and the concern about their safety limits their utilization.In this work,insensitive energetic explosive 3,3′-diamino-4,4′-azoxyfurazan/hexahydro-1,3,5-trinitro-1,3,5-triazine(DAAF/RDX) microspheres were fabricated by self-assembly method.Rod-like DAAF/RDX was prepared by mechanical ball milling for comparison.DAAF/RDX composites with different mass ratios (90:10,80:20,and 70:30)were obtained.The morphologies and structures of as-obtained DAAF/RDX composites were characterized by scanning electron microscopy(SEM),powder x-ray diffraction(PXRD)and fourier transform infrared spectroscopy (FT-IR).The results showed that DAAF/RDX microspheres exhibited

1.Introduction

Energetic materials have always been intense areas of research for countries to ensure safety and security which plays a very important role in the development of national defense.Nevertheless,high energy and safety is an inherent contradiction in its application [1].In addition to the significant advantage of high energy[2,3],physical properties,including crystal morphology and microstructure,crystal defects,and purity all play significant roles in the sensitivity of energetic materials [4,5].Therefore,great attention has been paid to insensitive energetic materials to enhance the power of weapon systems.

In recent years,3,3′-diamino-4,4′-azoxyfurazan (DAAF) has become the subject of significant investigation as an insensitive material [6,7].In particular,DAAF has received considerable interest for use in explosives [8,9].DAAF exhibits a favorable performance of insensitivity to initiation with a high standard enthalpy of formation (+443 kJ/mol),insensitivity to impact (＞320 cm),and moderate friction sensitivity (＞36 kg) [10,11].On the other hand,DAAF possesses a special structure consisting of the layered molecular stack,leading to low sensitivity and high thermal stability,thereby matching the energy with safety performance [12,13].For instance,Tappan et al.prepared DAAF with NTO (3-nitro-1,2,4-triazole-5-one) group to obtain energetic explosives with tailored shock sensitivity and critical diameter [14].Li et al.coated the surface of DAAF with VitonA to yield better thermal stability than crude DAAF[15].Wang et al.formulated nano-sized DAAF explosive particles with diameters ranging from 50 nm to 100 nm with improved initiation sensitivity than microparticles [16].Hence,doping of energetic composites and change in particle morphology might improve the performances of DAAF composites.As a result,finding less complex methods to prepare DAAF and other energetic composites with better properties is highly desirable.

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a sixmembered ring with higher molecular stiffness widely used in military explosives and propellants [17,18].However,RDX still suffers from high sensitivity,limiting its applications.For instance,Yao et al.[19]used electrostatic-spray method to prepare RDX/DOS composites.Compared to raw RDX,the impact sensitivity and friction sensitivity of RDX/DOS(10 wt%DOS)composites increased to reachH50of 125.9 cm combined with the lowest friction sensitivity of 8%.Jangid et al.[20]partially replaced RDX with spherical NTO to yield sheets based-explosives,as well as products with higher shock and friction insensitive.Jin et al.[21] employed the sol-gel method to prepare NC/RDX/AP nano-composite energetic materials with lower impact sensitivity and more energy during the explosion.Bian et al.[22] utilized a uniform size through solution impregnation to obtain smooth RDX/cellulose composite aerogel spheres.Compared with raw RDX,H50values of RDX/cellulose composite increasing by more than 65 cm.In conclusion,the improvement in the performance of explosives could be achieved by combining with other substances,such as energetic materials or coating by high polymer materials.In this respect,the emerging energetic composites technique would provide a potential direction to prepare DAAF/RDX explosives with the insensitivity and high energy.

As an efficient approach to fabricate energetic materials,facile molecular self-assembly exerts a tremendous fascination on researchers.In this paper,DAAF and RDX were employed to prepare DAAF/RDX microspheres by self-assembly method.As a comparison,rod-like DAAF/RDX was prepared by mechanical ball milling method.The morphologies,structures,thermal behaviors and impact sensitivities were all evaluated.The results revealed that self-assembled DAAF/RDX microspheres had better safety features than rod-like DAAF/RDX prepared by ball milling technology and promising for use in explosives.

2.Experiment

2.1.Molecular dynamics (MD) simulations

All MD simulations were conducted on a forcite module consisting of Materials Studio software (MS).The original model of DAAF/RDX composites was built according to the number and proportion of molecules used in the model.The experimentally determined structures of DAAF and RDX crystals were used as input structures in calculations.To this end,DAAF and RDX molecules were placed in a cubic periodic box,and then subjected to COMPASS II force field.A stable structure among the equilibrium trajectory documents was selected to calculate the thermodynamic properties.The constructed models were first optimized by a smart algorithm,and then MD simulations were conducted on an ensemble made of NPT and NVT.The pressure and temperature(298.0 K) were kept constant during the whole process.Hybrid structures of DAAF/RDX prepared with different ratios are displayed in Fig.1.

2.2.Chemicals and reagents

RDX(the fineness was 99.7%)was provided by Gansu Yinguang Chemical Industry Group Co.,and DAAF (the fineness was 99.6%)was obtained according to our earlier published work by Li et al.[13].Polyvinyl pyrrolidone (PVP) was purchased from Gbcbio Technologles Inc.,and dimethyl sulfoxide (DMSO) was obtained from Tianjin Beichen Founder Chemical Co.,Ltd.Pure water was supplied from Taiyuan Iron and Steel Co.,Ltd.The balls used for the ball milling process (φ=0.1 mm,ZrO2) were provided by Zibo Sinoshine Industrial Ceramic Co.,Ltd.

2.3.Preparation of DAAF/RDX composites

2.3.1.Design of DAAF/RDX composites

DAAF may be suitable in formulations designed to replace PBXN-7 with many other intriguing properties.PBXN-7 is a kind of insensitive explosive,which contains 60% TATB,35% RDX and 5%Viton[23,24].The detonation velocity of PBXN-7 is about 7.68 km/s.In this study,Urizar method[25,26]was used to design DAAF/RDX composites with 0.5 km/s higher theoretical detonation velocity than that of PBXN-7.The mass ratios of DAAF and RDX were 90:10,80:20 and 70:30,respectively.All samples were prepared by the self-assembly method and ball milling method.In order to study the performance of different structures of DAAF/RDX composites,DAAF/RDX particles with microsphere structure were prepared by self-assembly method and short rod-shaped DAAF/RDX particles were prepared by mechanical ball milling method.

2.3.2.Preparation of DAAF/RDX microspheres by self-assembly technology

DAAF/RDX composite microspheres were prepared by selfassembly technology.The schematic illustration is shown in Fig.2.Briefly,raw DAAF and RDX were dissolved in DMSO at room temperature (26°C) until it was completely dissolved to form a DAAF/RDX mixture.Next,1 g PVP surfactant was added to 400 ml water to form a solution.The DAAF/RDX mixture was then slowly added to the PVP solution at the rate of 2.0 ml/min followed by stirring for 40 min to yield DAAF/RDX microspheres after freezing and drying processes.The corresponding samples with the mass ratios of 90:10,80:20 and 70:30 synthesized by self-assembly were abbreviated as D/R-M1,D/R-M2 and D/R-M3,respectively.

2.3.3.Preparation of rod-like DAAF/RDX by ball milling technology

For comparison,rod-like DAAF/RDX was obtained by mechanical ball milling method.In order to determine the appropriate ball milling time to obtain the composite particles with the best morphology,different ball milling times(3,5 and 7 h,respectively)were selected.The results showed that the composites prepared for 5 h had the best morphology with good uniformity and small particle size (see SI).Therefore,5 h was selected to be the ball milling time for the preparation of rod-shaped DAAF/RDX particles.To this end,raw DAAF and RDX were first placed in the ball milling pot followed by the addition of 60 ml water.Next,the mixture was stirred for 5 h at the speed of 340 r/min,and rod-like DAAF/RDX composites were formed after filtration and drying.The corresponding specimens with the mass ratios of 90:10,80:20 and 70:30 obtained by the ball milling method were abbreviated as D/R-R1,D/R-R2 and D/R-R3,respectively.

2.4.Characterization

The morphologies of the materials were viewed by scanning electron microscopy (SEM,S4800,Hitachi).The crystal structures were examined by powder X-ray diffraction (XRD,DX-2700,Dandong Haoyuan Corporation,Liaoning,China)and Fourier transform infrared spectrometry (FT-IR,VERTEX 70).The thermal behaviors were investigated by differential scanning calorimetry (DSC,DSC-800,Shanghai Yingnuo Precision Instrument Corporation,China).TheH50values were measured by type 12 drop hammer apparatus at the drop hammer of 2.5 kg.

Fig.1.Hybrid structures of DAAF/RDX composites prepared with different ratios.(a):DAAF/RDX(90:10);(b):DAAF/RDX(80:20);(c):DAAF/RDX(70:30).

Fig.2.Experimental steps for DAAF/RDX microspheres.

3.Experimental results and analysis

3.1.Simulated binding energies of D/R composites

The binding energy (Ebind) can be used to evaluate the stability of materials [27],as well as judge the intermolecular interactions(Einter).The relationships and stability of explosives have been previously reported by Xu et al.[28],showing that higherEbindvalues should yield more stable explosives.Here,the goal was focused on evaluating the binding energies between DAAF and RDX molecules in composites consisting of DAAF and RDX and the simulation results are listed in Table 1.Note thatEinterreferred to the interaction energy andEtotalwas the total energy of the composites model under equilibrium state.

Table 1 Binding energies (in kJ/mol) of different D/R composites obtained by the proposed models.

Table 2 Thermal decomposition kinetic parameters of the samples.

The binding energy of materials prepared with different ratios decreased in the following order:(90:10) >(80:20) >(70:30).In other words,the material with mass ratio of 90:10 exhibited the greatest stability,while the material with 70:30 ratio displayed the worst stability.The reason for this had to do with a tighter combination of both DAAF and RDX molecules in the models at mass ratios around 90:10,as well as the existence of stronger interactionenergies between both components.In other words,the model possessed relatively higher stability for DAAF/RDX composites at the mass ratio of 90:10.

3.2.Morphology analysis

The morphologies of as-obtained D/R microspheres were characterized by SEM and the data are shown in Fig.3.Raw DAAF presented irregular surface topography and rod-like structure with a particle size of around 40 μm.The neat RDX crystals showed mean particle sizes about 3 μm-6 μm and smooth surface.The ballmilled D/R illustrated irregular rod-like shape with sizes ranging from 0.2 μm to 0.6 μm after modification.A comparison between samples revealed differences between D/R microspheres and ballmilled D/R composites.However,D/R microspheres exhibited relatively regular shaped microspheres with sizes from 0.5 μm to 1.2 μm after self-assembly.In particular,the surface topography of D/R changed after the addition of PVP due to the attachment of PVP to the hydrogen bonds in DAAF and RDX.Thus,molecular PVP may affect the morphology of D/R composites.As the reaction ratio of RDX increased,little change happened in the morphology of the particles,showing the same regular-shaped microspheres.In sum,self-assembly of energetic D/R composite was an efficient and controllable route to construct insensitive energetic materials.

3.3.Possible formation mechanism of D/R microspheres

Fig.3.SEM images of raw DAAF,RDX,and as-prepared D/R samples.

Fig.4.Schematic illustration of the formation mechanism of D/R microspheres.

To better explain the formation of D/R microspheres,a possible formation mechanism was proposed according to the recrystallization and self-assembly processes,as shown in Fig.4.The nucleation of composites was affected by super saturation and surface free energy during the self-assembling process.The slow addition of DMSO solution caused the DMSO solution to become supersaturated and resulted in the primary nucleation and subsequent growth of the DAAF and RDX particles.Simultaneously,the addition of PVP resulted in modification of the grain boundary energies and hydrogen bonding,which led to oriented and controlled growth of particles.The stirring process made the mixture homogeneous and accelerated the aggregation of DAAF and RDX particles.When the particles reached an appropriate length and level of conglomeration,they tended to reduce the surface energy by self-assembling into spherical shapes.The small DAAF and RDX particles further aggregated to form the cores by the grain boundary energies.The primary nuclei were aggregated and grew on the crystal surfaces.The crystal grows up gradually with the increment of the crystallization time.During the process,the small particles were deposited and induced an aggregation from the core to form sheaf-like structures.Furthermore,many cluster-like particles were randomly located on the crystal surface,possibly attributed to the DAAF and RDX particles’ complete self-assembling process.

3.4.Crystal structure

The crystal structures of raw DAAF,raw RDX,and as-prepared samples were characterized by XRD and the results are gathered in Fig.5 (a).D/R samples showed broader and weaker diffraction peaks than those of raw DAAF and RDX,which may be caused by the reduction in sample size.Also,the characteristic diffraction peak of RDX was absent in the D/R composite due to the small incorporated amount of RDX.Compared to pure DAAF and RDX,the peak intensity of (0 2 3) in D/R composites increased,and peak intensity enhanced slightly with RDX content.D/R exhibited principal crystal planes of (0 2 0),(0 0 2),(1 1 0),(0 0 1) and (2 1 0),consistent with raw DAAF and RDX.Hence,the XRD patterns of D/R agreed well with those of DAAF and RDX,showing the nondefectiveness of crystal modification.Besides,peaks of ball-milled D/R samples at (1 1 0) and (0 2 3) planes were still present in patterns of D/R microspheres.Both composites possessed characteristic peaks of DAAF and RDX,suggesting no crystal transition during the modification process.

Fig.5.XRD spectra and FT-IR spectra of raw DAAF,raw RDX,and prepared composite samples:(a):XRD spectra of samples;(b) FT-IR spectra of samples.

The structures of the as-prepared samples were further studied by FT-IR and the data are provided in Fig.5(b).Note that the FT-IR spectra showed little differences between the pristine DAAF,RDX,and the as-prepared samples.In Fig.5(b),the peaks corresponding to D/R displayed no shift when compared to those of raw DAAF and RDX.The FT-IR spectrum of DAAF showed many functional groups on DAAF surface.The peaks located at 3429 cm-1and 3337 cm-1were related to the stretching vibration of -NH2.The stretching vibration of C-N can be identified by the peak around 1017 cm-1.For D/R composite,the peak positions were consistent with those of RDX,indicating the successful introduction and combination of RDX in D/R composites.For example,the peak in raw RDX at 921 cm-1appeared in D/R composites,confirming the preservation of DAAF and RDX structures during the preparation process.Also,no other peaks were noticed in the FT-IR spectrum of D/R composites,ruling out any atmosphere formation of elemental and transition during the modification process.Hence,FT-IR and XRD data confirmed the physical mixing between DAAF and RDX.

3.5.Thermal analysis

The thermal properties of the as-prepared samples were evaluated by DSC analysis and the results are depicted in Fig.6.The rod-like D/R and D/R microspheres displayed a big exothermic peak temperature.For rod-like D/R-R2,one endothermic melting peak was also noticed at 253°C,which was close to the exothermic peak of D/R-M2 microspheres at a heating rate of 5°C/min.Similar characteristics also appeared in the patterns of as-prepared D/R-R1,D/R-M1 and D/R-M3 samples.The exothermic peak temperature increased with the heating rate.Under the same experimental conditions,most exothermic peaks of D/R(90:10)were higher than those D/R(80:20)and D/R(70:30).Therefore,RDX as a dopant can change the thermostability of composites.

Fig.6.DSC curves of raw DAAF,raw RDX,and D/R composites.

The equations reported by Kissinger et al.[29],Ozawa et al.[30],and Starink et al.[31] were used to calculate the decomposition kinetic parameters of samples (the calculation process is in SI).EKdescribes the thermal stability of the substance.EKof D/R-M microspheres are higher than those of other D/R-R samples,indicating D/R-M microspheres were more thermally stable than rod-like D/R samples.

The obtained kinetic parameters of exothermic decomposition reactions of the microspheres are listed in Table 2.The thermal stability of the D/R composites can be evaluated through the critical temperature of thermal explosion (Tb).Compared to ball-milled composites,the exothermic peak temperature (Tb) of D/R microspheres was postponed.Thus,the thermostability of D/R composites can be improved by doping the microspheres.Composites made of microspheres would decrease the probability of hot-spots formation.Compared with D/R-M2 and D/R-M3,D/R-M1 showed the highestTbreaching 260.89°C.Therefore,D/R-M1 showed better stability.The results can be explained in combination with MS calculation results.The binding energy decreases with the increased content of RDX,which makes the decrease of stability.D/R-M1 shows the higher binding energy than D/R-M2 and D/R-M3,indicating the better stability.

The chemical potential during the activation course can be expressed by the ΔG≠.The positive number of ΔG≠means D/R microspheres were stable and a nonspontaneous process.ΔH≠of D/R-M1 was 200.86 kJ/mol,which is higher than that of D/R-M2(192.07 kJ/mol) and D/R-M3 (186.73 kJ/mol),indicating more energy for activation and better thermal stability than others.

3.6.Mechanical sensitivity

The characteristic drop height of the as-prepared samples was evaluated by a Type 12 drop-hammer apparatus and the data are presented in Table 3.As a novel insensitive explosive,DAAF displayed higherH50than 100 cm,thereby more insensitive than raw RDX withH50of 25 cm.Both D/R microspheres and rod-like shaped D/R showed insensitivity to impact.This meant that the addition of DAAF reduced the number of RDX particles per unit area,thereby resulting in decreased impact sensitivity.The composites after modification showed obviously reduced sensitivities than raw RDX.Upon the occurrence of impact,the morphology of themicrospheres,as well as the larger surface areas could act as a buffer layer to dissipate the energy and inhibit the formation of hot spots,thereby leading to superior safety performances.The addition of DAAF reduced the number of RDX particles per unit area,resulting in decreased impact sensitivity.Meanwhile,the modification of RDX explosives led to significant improvement in impact sensitivity in the presence of DAAF due to the unique layered molecular stack of DAAF.This may explain why D/R composites showed reduced impact sensitivities and improved mechanical safety.

Table 3 Impact sensitivity of samples.

From the analysis of Urizar calculations,the detonation velocities of D/R at mass ratio of 90:10,80:20 and 70:30 are 8.09 km/s,8.18 km/s and 8.26 km/s (theoretical density were 1.70 g/cm3,1.71 g/cm3and 1.72 g/cm3,respectively).The detonation velocity of D/R composite was about 0.1 km/s to 0.2 km/s higher than that of DAAF,and 0.5 km/s higher than that of PBXN-7.The results showed that D/R microspheres have good safety performance and high energy.

4.Conclusions

Self-assembly technology was used to fabricate D/R microspheres and ball-milling method was used to prepare rod-like D/R composites.The microspheres obtained by self-assembly showed microspheres-like structures with smoother surfaces and diameters ranging from 0.5 μm to 1.2 μm.The structure and thermal analysis results suggested the self-assembly method to differ from the ball-milling method of both substances.D/R (90:10) microspheres showed the higherTb(260.89°C)and better stability than D/R (70:30) microspheres (259.72°C) and D/R (80:20) microspheres(248.86°C).Also,theTP0value of D/R(90:10)microspheres(249.5°C)was higher than those of rod-like D/R(90:10)composites(245.8°C).Therefore,D/R microspheres show better thermal stability.The characteristic drop height of the D/R microspheres that were prepared via self-assembly was higher than 100 cm,and indicates that the D/R microspheres have pleasing stability.In sum,the self-assembly method revealed potential and promising prospects for the fabrication of explosive composites with improved performances.

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 project was supported by the National Natural Science Foundation of China (No.22005275),Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi（STIP,No.2019L0584),Equipment Pre-research Weapons Industry Joint Fund (No.6141B012896),Equipment Pre-research Key Laboratory Fund (No.6142020305),and the Advantage Disciplines Climbing Plan of Shanxi Province.

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dt.2021.02.001.