CAS OpenIR
反应分子动力学模拟揭示CL-20共晶热分解全景式反应机理
任春醒
Thesis Advisor李晓霞
2020-07-01
Degree Grantor中国科学院大学
Degree Name博士
Degree Discipline应用化学
Keyword反应分子动力学reaxff Md,Cl-20共晶,Cl-20主客体含能材料,热分解反应机理,组分间相互作用
Abstract

新型高能量密度含能材料的研发是国家安全的重要保障。极具前景的高能炸药六硝基六氮杂异伍兹烷(CL-20)因低安全性和高成本影响了其广泛的军事应用。近年来合成的CL-20共晶和主客体体系展示了利用共晶技术和主客体思想调控CL-20固有的能量和安全性矛盾的可能性和前景,但要实现精准调控含能材料的结构与反应性仍然充满挑战。从微观尺度深入认识CL-20共晶等双组分含能晶体热分解反应机理和组分分子间相互作用机制是合成新型CL-20共晶等高能含能材料的重要基础工作。基于反应分子力场ReaxFF的反应分子动力学(ReaxFF MD)方法具有无须预设反应路径、可连续描述化学键的断裂与生成、以较低的计算代价获得接近密度泛函DFT方法的准确度等特性,使其成为研究含能材料微观反应行为的重要分子模拟方法。本论文以揭示CL-20共晶等双组分含能体系热分解微观反应行为和机理的共性特征和差异为目标,采用ReaxFF MD反应分子动力学方法对多种热刺激下不同CL-20共晶和CL-20主客体含能体系的化学响应行为进行了较系统的研究。借助反应分析可视化程序VARxMD在复杂反应体系ReaxFF MD模拟结果机理分析方面的独特优势,获得了CL-20共晶等体系热分解过程的物种演化和微观反应细节,深入挖掘了CL-20双组分含能体系热分解行为的异同特征和组分分子间相互作用机制。本论文的主要结论如下:采用ReaxFF MD模拟系统揭示了宽温度范围内(800–3000 K)β-CL-20单组分晶体恒温热分解的全景式产物分布和反应机理。热刺激引发CL-20分子分解的主导反应是N―NO2断裂生成NO2和五元咪唑环上C―N键断裂引起骨架开环;低温条件下初始分解产物NO2攫取CL-20硝基氧生成NO3的双分子反应值得关注。CL-20笼型骨架初始开环生成的两元稠环中间物会进一步分解生成仅含吡嗪单环的中间物;初始分解产物NO2和其二次反应生成的活泼小分子或自由基参与了错综复杂的类链式反应,逐步促进体系分解生成大量稳定气体。模拟揭示的CL-20笼形骨架分解机理为Py-GC/MS实验检测到吡嗪结构提供了明确的理论解释。模拟预测3000 K分解产物N2、H2O、CO2及H2的产量与实验测得的CL-20分解完全的气体产量定量吻合,表明3000 K是得到CL-20完全分解合理产物分布的适宜模拟条件。所获得的CL-20单组分晶体热分解机理的新认识表明:结合VARxMD深度分析能力的ReaxFF MD模拟方法是一种深入研究含能材料热分解复杂反应机理的有效途径。以CL-20单组分晶体热分解反应机理为参考基准,采用ReaxFF MD对比研究了两种典型CL-20共晶CL-20/HMX和CL-20/TNT在绝热(自加热升温)条件、程序升温和宽温度范围的恒温条件等多种热刺激下的热分解过程。首次提出依据升温热分解重要中间物NO2数量演化的双峰特征划分CL-20双组分共晶的反应历程的策略,将划分的阶段作为反应分析的主线系统揭示了不同CL-20共晶热分解行为的共性特征和差异。结果表明:CL-20/HMX和CL-20/TNT两种共晶在绝热条件和程序升温条件的热分解过程中活泼中间物、复杂环结构碎片、稳定气体产物以及势能演化均呈现与CL-20单组分晶体相似的三阶段特征。第一阶段是CL-20分子初始分解生成自由基的引发诱导阶段、第二阶段为初始分解产物二次反应生成更多自由基和中间物的类链式反应发展阶段、第三阶段则是自由基和中间物快速消耗生成最终产物。模拟得到的CL-20/HMX、CL-20/TNT共晶与CL-20单组分晶体热分解主要反应行为和产物分布的相似性为激光点火实验观察到的三者相似的点火现象提供了理论解释。模拟发现形成共晶并未改变共晶中CL-20分解主要反应类型,单分子C―N键和N―NO2键断裂仍是CL-20初始分解优势路径。CL-20骨架中的咪唑环是开环反应的主要位点,而吡嗪环难以直接分解。模拟同时发现不同CL-20共晶热分解的动力学性质具有显著差异。CL-20/HMX和CL-20/TNT对热刺激响应的敏感度低于CL-20单组分晶体,绝热分解能量释放过程被减缓、分解反应区更长,共晶中的CL-20分子分解因N―NO2和C―N键断裂反应速率显著降低而变慢。深入研究CL-20/TNT降感机制的化学本质发现:CL-20早期分解形成的活泼小分子和自由基会被周围的TNT分子或TNT转化形成的环结构衍生物通过相互反应捕获,阻碍、减缓类链式反应的发展,从而提高了热安定性和安全性。进一步将所提出的深度分析CL-20共晶热分解行为异同特征的策略扩展用于快速评估CL-20/H2O、CL-20/H2O2和CL-20/N2O三种晶胞结构十分相似但组分不同的CL-20主客体体系的热分解反应性质。以升温热分解的三阶段特征为分析主线,发现CL-20主客体体系分解生成的主要物种演化及优势反应路径与CL-20单组分晶体及共晶的主要特征一致。不同客体分子并未改变体系中CL-20分子初始分解的优势路径,但主客体分子间的相互反应会改变CL-20分解路径的分支比和稳定产物分布。考察热分解能量释放过程所发现的氧化性客体分子H2O2和N2O使CL-20主客体材料在提高热稳定性的同时较好保留了CL-20高能量和高爆轰性能,这表明利用模拟分析主客体材料热分解能量释放的方法可作为筛选客体小分子、制备新型高能低感CL-20主客体材料的参考。总之,本论文工作发现CL-20共晶、CL-20主客体含能体系与CL-20单组分晶体升温热分解过程中具有相似的反应行为和产物分布,其共性的微观本质是CL-20分子的分解反应主导了CL-20共晶等体系受到外部刺激后的反应引发、初始分解及后续的二次分解过程,为CL-20共晶等双组分体系保有CL-20高能量特性的提供了清晰的微观解释。而不同CL-20共晶等多组分含能材料热分解过程的差异体现在能量释放、分解反应动力学方面,其差异主要源于CL-20共晶等体系中两组分分子间的相互反应。本论文提出依据升温热分解NO2演化的双峰特征将CL-20共晶等体系分解反应历程划分为三个阶段、并作为机理分析主线的深度分析策略,成功揭示了不同CL-20共晶等双组分体系热分解的共性特征和差异。结合这一分析策略,ReaxFF MD反应分子动力学模拟可便捷地捕捉和刻画热分解过程的主要特征,有望作为快速评估CL-20多组分含能体系反应性质的计算方法,进而为调控新型CL-20多组分含能晶体结构与化学反应性提供理论支持。;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 thermal decomposition of condensed CL-20 is first investigated by ReaxFF MD simulations at temperatures of 800–3000 K. The β-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. 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 III 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.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-20/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 ε-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. 

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/49683
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
任春醒. 反应分子动力学模拟揭示CL-20共晶热分解全景式反应机理[D]. 中国科学院大学,2020.
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