| Development of new high-energy materials is necessary for national security.The 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane(CL-20)has high energy density but limited applications due to its high sensitivity and high cost.In recent decade,the synthesized CL-20 cocrystals and host-guest energetic crystals show the potential for possible compromise of the inherent contradiction between energy and safety of pure CL-20 by cocrystallization or host-guest inclusion strategy.However,it is still a challenge to modulate the material structure and chemical properties properly.A deep understanding of the reaction mechanisms of thermal decomposition of CL-20 cocrystals/host-guest materials,particularly the interactions between components are fundamental for designing new CL-20 multicomponent crystals with high-energy density.With no prior knowledge of reaction pathways required,the reactive molecular dynamics using ReaxFF force field(ReaxFF MD)has been found useful and widely used to improve the understanding of microscopic chemical behaviors of energetic materials.Aimed at unraveling the similarities and differences of decomposition behaviors and reaction mechanisms during thermolysis of CL-20 multicomponent energetic materials,thermal decomposition of CL-20 cocrystals and CL-20 host-guest crystals is systematically investigated using ReaxFF MD simulations at different conditions with varied heating history.By taking advantage of the unique code VARxMD for reaction analysis of ReaxFF MD simulations,rich details about the species evolution and chemical reactions in thermolysis process of CL-20 multicomponent crystals are obtained,based on which the reaction mechanism on similarities and differences among different CL-20 multicomponent crystals,as well as the interactions between component molecules is achieved.The main results obtained in this work are as follows:(1)The thermal decomposition of condensed CL-20 is first investigated by ReaxFF MD simulations at temperatures of 800-3000 K.The ?-CL-20 whose molecular conformation is widely existing in many CL-20 cocrystals,is chosen as the baseline model for further evaluation the chemical behaviors of CL-20 cocrystals.Exposing to thermal stimuli,the unimolecular pathways of N-NO2 bonds cleavage to generate NO2 and C-N bonds scission leading to ring-opening dominate in the initial decomposition of condensed CL-20.The bimolecular pathway of oxygen-abstraction by NO2 to generate NO3 should be considered for CL-20 decomposition at low temperature.The imidazo[4,5-b]pyrazine intermediates of CL-20 skeleton destruction will further decompose into single pyrazine fragments.Meanwhile,lots of active intermediates generated from the secondary reactions of primary products NO2 participate into the chain-like reactions and gradually promote the generation of stable gases.The revealed destruction mechanism of CL-20 skeleton clearly supports the detection of single pyrazine in Py-GC/MS experiment.Moreover,both the yields of stable gas products and total amount obtained from ReaxFF MD simulations at 3000 K agree quantitatively with what obtained experimentally.The final product yields N2,H2O,CO2 and H2 obtained in the ReaxFF MD simulation agree quantitatively with the measurements of detonation experiment,and the total amount of gas products equals to that from manometric measurements of complete decomposition of CL-20.The agreement between ReaxFF MD simulations and experiments elucidates that 3000 K is a proper simulation temperature to obtain a reasonable distribution of stable gases of CL-20 complete decomposition by ReaxFF MD simulations.The comprehensive understanding of CL-20 thermolysis suggests that ReaxFF MD simulations combined with the reaction analysis capability of VARxMD is a practical computational approach for deep insight on the complex chemistry of energetic materials exposed to thermal stimuli.(2)With the decomposition behaviors of CL-20 as a baseline,the thermal decomposition of CL-20/HMX and CL-20/TNT,two typical CL-20 cocrystals,are systematically investigated using ReaxFF MD simulations under various thermal stimuli including adiabatic conditions,programmed heating,and isothermal conditions with wide temperature range and different heating history.To meet the challenge in understanding about the similarities and differences of the thermolysis behaviors of the two CL-20 cocrystals,three-stage classification strategy on the basis of the double peaks of the major intermediates NO2 amount as stage boundary is first proposed.Taking the advantage of the proposed analysis strategy,similar three-stage characteristic is found in thermal decomposition of CL-20/HMX and CL-20/TNT as that of pure CL-20 among the evolutions of active intermediates,fragments of varied ring structures,stable gases,and potential energy under programmed heating and adiabatic condition with self-heating.Stage I corresponds to the induction stage of the initial decomposition of CL-20 molecules to produce radicals.Stage II is characterized by the development of chain-like reactions involving more generation of radicals and intermediates in secondary decomposition.Stage ? is dominated by the rapid exhaustion of all active intermediates to fast formation of stable gases.Such a similarity indicates that the dominant chemical reactions in two CL-20 cocrystals following the same pattern as that of pure CL-20.The decomposition pathways of CL-20 molecules are not influenced by cocrystallization with HMX and TNT.The initial dissociation of CL-20 was still dominated by N-NO2 and C-N cleavage.The five-membered imidazole rings are the major reaction sites of cage skeleton destruction,and the six-membered pyrazine ring are more stable with a long lifetime.The similarities between CL-20 cocrystals and pure CL-20 revealed by ReaxFF MD provides theoretical explanation for their similar ignition manners observed in laser ignition experiment.Despite of these similar characteristics,obvious kinetic differences between decomposition of pure CL-20 and its cocrystals are observed in ReaxFF MD simulations.The decomposition rate of CL-20 molecules is decreased due to the greatly retarded cleavage of N-NO2 and C-N bonds,and the generation of stable gases is also slowed down.The slowed energy release of adiabatic thermolysis leads to a prolonged reaction zone and moderate self-heating.Therefore,the sensitivity to thermal stimuli of two CL-20 cocrystals is decreased when compared with pure CL-20.The greatly decreased sensitivity and improved thermal stability of CL-20/TNT can be explained by the interplay between CL-20 and TNT.The decrease of thermal sensitivity is largely attributed to the capture and confinement of early formed active intermediates and radicals from CL-20 decomposition by their surrounding TNT or ring intermediates from TNT,retarding the development of chain-like reactions during early stage.(3)The approach combining ReaxFF MD simulations with the same analysis strategy proposed for that of CL-20 cocrystals is further extended to explore the thermolysis mechanism of CL-20 host-guest systems.The CL-20/H2O,CL-2O/H2O2 and CL-20/N2O with similar crystal structure but different components are chosen to explore the thermolysis of the host-guest energetic materials and the interplay between host and guest molecules therein.Similar three-stage characteristic to that of pure CL-20 and its cocrystals is found during programmed heating thermolysis of three CL-20 host-guest energetic materials among the major species evolution and dominating reaction pathways.The dominant pathways of host CL-20 decomposition are not altered by the existing of guest molecules.But the ratios of varied pathways of CL-20 decomposition and the final product distribution are greatly changed due to the interplay between guest and host molecules.The energy evolution of thermolysis indicates that the critical temperature for energy-releasing of CL-20/H2O2 and CL-20/N2O is increased,while the amount of energy-releasing retain almost the same,comparing with ?-CL-20.The incorporation of oxidizing guest molecules into the host CL-20 can achieve well-balanced high energy and thermal safety.In conclusion,the similarities of chemical behaviors and product distribution in thermolysis of CL-20 cocrystals and host-guest energetic materials of CL-20 to that of pure CL-20 under heating-up conditions are rooted in the CL-20 chemistry dominating over the whole process from the very initial response to thermal stimuli and primary decomposition to the secondary reactions,which accounts for the retained highly energetic characteristic of varied CL-20 cocrystals and other CL-20 multicompount energetic crystals.The chemical interactions between different components contribute significantly to the kinetic property differences among varied CL-20-involved reactive systems in energy-releasing process,decomposition of CL-20 molecules and stable product generation.Facilitated by the three-stage classification,the application of the proposed analysis scheme from CL-20 corystals to host-guest materials is critical in successfully unraveling the similarities and differences of thermolysis among CL-20 multicomponent systems with ReaxFF MD.The deep insight obtained indicates that ReaxFF MD is a convenient method to capture and depict the common chemical characteristics of thermolysis of CL-20 multicomponent systems.Combined with the proposed analysis scheme permitted by function of VARxMD,ReaxFF MD is a practical computational approach for fast evaluation of the reactive properties of CL-20 multicomponet crystals.The comprehension on the chemical structures and reactivity on CL-20 multicomponet crystals obtained should be useful in supporting the design of new CL-20 cocrystals or host-guest materials with desirable structure and chemical reactivity. |
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