Molecular Dynamic Study On Sensitivity And Performance Of Energetic Materials

The physical and chemical behavior of condensed energetic materials?EM?under extremely conditions?such as ignition,initiation,formation and propagation of detonation?contains abundant scientific connotation,spanning over multiple scales of time and space.The period from initiation to detonation,fleet processes with massive chemical reactions and complex state changes,are the hottest and most difficult problems in EM field.No matter by experimental means,or from the theory of impact dynamics or detonation,we can hardly get detailed information about chemical kinetics,physic-chemical changes,or heat-mechanics-chemistry coupling mechanisms.The systematic investigations by first principle based molecular dynamic methods and developing new multiscale models based on MD means are great helpful to intensive study initiation mechanisms and state changes in detonation process,further promoting the combination of new methods from micro and macro perspective.Meanwhile,the computational study on EM materials is of great fundamental significance to design,manufacture,production and application for industry development.Based on the First-Principle Molecular Dynamic method,this thesis,for one thing,focuses on the physico-chemical mechanisms of cocrystal material under thermal and mechanical lodading;for another thing,develops new models to explore the changing state of macro properties and evolution of micro behaviors both during the detonation propogation process and at the specific CJ condition;in addition,applies these models to predict the influence of silicon element to detonation performance of energetic material.The results and the main contributions of the thesis are briefly described below:Based on ReaxFF reactive force field,the thermal decomposition mechanisms of TNT/CL-20 cocrystal,a new low sensitivity and high performance condensed energetic material,was studied.The cook-off?NVT?and thermal expansion?NVE?simulations are carried out to study the influences of molecular species and molecule arrangement structures for reaction dynamics by comparing different energy release history and the products distribution evolution of TNT/CL-20 cocrystal,with TNT and CL-20 pure crystals,and TNT-CL-20 physical mixtures.These simulations indicate that the beginning of thermal decomposition of cocrystal is CL-20 molecule dissociation.In addition,the sandwich arrangement of cocrystal has great impact on the decomposition rate:the initial decomposition rate is decreased because active CL-20 molecule layers are separated by stable TNT ones;however,the faster energy release rate was observed in later process because TNT molecules were activated by the CL-20 fragments.Furthermore,TNT molecules attract intermediates around to form carbon rich clusters,making a slower energy release rate and fewer gases products.Secondly,a new method named“Compressive Shear Reactive Dynamics”is proposed based on the principle of frictiongraph and drop-weight equipment.According to the temperature increasing rate and the dominant fragments amounts of the easiest slip system,the friction and impact sensitivity of cocrystal,CL-20 and TNT are compared.The results indicate that TNT/CL-20 cocrystal is harder to initiate than CL-20,but easier than TNT.A first-principle based ReaxFF Reactvie Dynamics to achieve CJ state protocol is proposed to search the fully detonated Hugoniot curve and the critical characteristic Chapman-Jouguet state without ad hoc hypothesis of reaction products.By applicated for RDX,HMX and PETN,it is shown that this model is an effective way to prodict parameters and detailed products distributions of CJ state.The simulation results indicate that most of the masses of three systems are in simple gases forms,but small number of carbon and oxygen atoms is existed in clusters.In the expansion zone with pressure decreases rapidly,these clusters decompose quickly,releasing large amount of CO and CO₂.Based on the CJ calculation method above,the influence of Si element to detonation performance was studied.The results show that adding Si element change energetic material's properties.Firstly,large amount of heat are produced while Si bonds forming;secondly,more silicon clusters were formed with C,H,and O.As a result,low detonation pressure and detonation velocity as well as high temperature were observed.A first-principle based ReaxFF Reactvie Dynamics achieves Hugoniot state in detonation reaction zone to understant the propagation process of detonation wave and the transformation evolution from initial material to detonation products.According to the results,active particles appeared in detonation intrafront.Along with the chemical reactions continue and energy release,masses leave condensed matters quickly and become complex intermediate fragments,then transfer to stable gas products after various kinds of reactions.Pressure of detonation wave and material types have significant impact on the state during detonation.