| Energetic materials are important energy carriers to realize weapon launch,propulsion and damage effect, which play an important role in the development ofnational defense. At the same time, they are also widely used in coal mining,blasting demolition, seismic exploration and other industrial areas and scientificresearch. With the rapid development of science and technology, newrequirement is raised to improve the energetic materials—not only they should beof high energy density and high stability characteristics but also could satisfy theproperties of environment friendly. As a typical energetic material containingnitro explosive group, nitromethane (referred to as NM) is used as rocket fuel andhigh primary explosive material because of its simple structure and low cost.Research on such simple nitro compound properties and reaction mechanism canhelp people to explore and understand more complex energetic material so it ispopular in recent years. In addition, because the external conditions underdifferent temperatures and pressures could bring significant influence on theenergetic materials properties and reaction path, studies on various structure anddecomposition mechanism under high temperature and pressure of nitromethanecan help people to assess and forecast the application of energetic material inmilitary struggle and industrial development.Although it has been hundreds of years from the invention of the energeticmaterials to now and the materials have been widely used in military and industrial fields, but the research of the decomposition reaction develops slowly. This is notonly the most important basic problem in the field of energetic materials, but alsoone of the hottest topics in the science of high temperature andpressure. Decomposition reaction of energetic materials is a process in which itsfunction gets released and reflected and it involves a complex chemical reactionand numerous intermediate products, so it has attracted widespread attention inacademic circles in recent years. However, it is a great challenge to the currentexperimental techniques and instrumentation because of the decompositionprocess of energetic material is extremely small and complicated in time andspatial scales. Meanwhile, because of its high energy density and oxidationdecomposition, it is also a threat to the security of testers. Therefore, using thecomputer to carry on the research of decomposition process of energetic materialsis a more reasonable simulation method. At present, there are two methods tosimulate nitromethane decomposition reaction process. One is the classicalmolecular dynamics method. This method is fast and it can simulate the largerunit cell system. But the description of physical and chemical processes in thismethod must rely on the empirical force field based on ab initio method orexperimental value. And the decomposition process of energetic materialinvolves various formation and dissociation of chemical bond, so whether or notthe method of empirical force field can accurately simulate the process is worthdiscussing. The second kind of method is ab initio molecular dynamics. In thismethod, starting from the first principles, simulation system is processed by usingquantum mechanics. It can accurately describe the changes of chemical bondsbetween atoms, and then study the properties of materials and the reaction processunder different temperatures and pressures. It gives more accurate simulationresults. At the same time, with the rapid development of computer hardware, ithas been able to simulate the considerable supercell system. Therefore, itgradually becomes the effective means for the study of energetic materials. This paper mainly studies on typical energetic materials by using ab initiomolecular dynamics method—the nature and decomposition behavior ofnitromethane and its decomposition products in different temperature and pressureconditions, including the change of the potential energy and pressure withdifferent heating modes and compression conditions, the effects of simulated sizeon nitromethane decomposition reaction and population, nitromethane molecularstructure under different compression condition, especially the research of motionand change of methyl. According to the complexity of the process of energeticmaterial decomposition, three kinds of strategy searching decomposition productsand chemical reaction are suggested and programmed to analyze the changes inthe number of main products of the initial reaction mechanism. At the same time,the mechanical properties of nitromethane decomposition products of long chainand carbyne chain (pure carbon atom chains) are compared. In this thesis, theresearch contents and innovations mainly include the followings.1) Use the ab initio molecular dynamics method to simulate thedecomposition process of nitromethane in gradually heating and rapid heatingmode, constructing the unit cell and super cell of nitromethane model to validatethe effect of the size of simulation on decomposition. The calculation resultsshow that the temperature of nitromethane decomposition is stable at about2300K, hardly affected by the heating rate influence. The potential energy andpressure curve have great relationship with the target temperature and heating ratesetting, and has nothing to do with the size of simulation cell. Through the postprocessing program to analyze the initial reaction mechanism and population ofdecomposition product, it is observed that proton transfer involved with C-H bondrupture is more likely to cause decomposition than traditional C-N bonddisintegration in gradually heating mode, and several initial reaction mechanismsin gradually heating mode is introduced. In rapid heating mode, nitromethanefirstly reacts using C-H bond rupture under low temperature (T=2400K and3000K). And it changes to a C-N bond cleavage under high temperature (T=4000K). Compared with supercell structure simulation, we can see the initial reaction withsimilar results. In the population analysis, the rate of decomposition processguided by C-N bond rupture is slow in gradually heating mode. While in therapid heating mode, the nitromethane decomposition speeds up as well as the rateof water molecule generated with the increasing of target temperature. At thesame time, as the stable decomposition products, nitrogen will not participate inthe subsequent reaction once it generates.2) Based on ab initio molecular dynamics, we simulate molecule structure ofnitromethane of different compression and decomposition mechanism of initialstate in different temperature and pressure conditions. Before simulation, bymeans of the analysis of previous experimental and computational data, we get therules of nitromethane lattice constant which is that, it changes proportional in pacewith the coefficient of compressibility and build a model using this rule. And bycalculating the bulk modulus and its first derivative with press, we find that resultsagree with the previous numerical values fairly well, which proves the rationalityof constructing model. Research shows that, with the increase of externalpressure, C-N bond can be relatively more easily compressed than C-H bond andN-O bond. At the same time, the difference of bond order between C-N bond andC-H bond is reducing, that is, C-N bond is in reinforced while C-H bondweakened, and they are basically the same when the unit cell volume compressedto70%of the initial volume. Through counting the cumulative distribution curveof dihedral angle Hi-C1-N2-O3in different compression coefficient and the curvewith the change of time, it can be seen that with the continuous compression,cumulative distribution curve begins to get to a peak value and increasecontinuously, which suggests that methyl rotation is inhibited. Same results canalso be obtained when observing the curves of dihedral angle versus time. Thenwe analyze the hydrogen bond between H and O elements and observe that alongwith the pressure increasing, the average bond length of intramolecular hydrogen,and the shortest bond length and number change little, but the bond length of intermolecular hydrogen decreases obviously while the number increasessignificantly, which indicates that the increasing of the external pressure makes thestrength and number of intermolecular hydrogen bonds continuouslyenhance. Through simulating initial decomposition process of nitromethaneunder different temperatures and pressures, it is indicated that C-H bond rupturehappens at an earlier time when volume compression is larger. And when theexternal pressure is reduced, the carbon nitrogen bond fracture earlier, whichagrees with the result of bond order analysis. The potential energy functionanalysis shows that increase of volume compression makes the initial value ofpotential energy continuously improve and with the simulation process, potentialenergy in higher pressure first decreases. By observing the initial reaction processof nitromethane before water molecule forms at3000K, it is found that even thefirst reaction fractures in the same way, the initial decomposition paths have manydifferences, which need to be discussed in detail. Through the analysis of thechange of populations of product, it can be seen that as the external pressure isincreasing, the time of water molecule generates and and the decomposition rate ofnitromethane accelerates in advance, which suggests that as external pressureincreases, the rate of nitromethane decomposition and water molecule formationcan be improved.3) According to the chain products of the nitromethane decomposition—C3N2,we carry on tensile test based on ab initio molecular dynamics. Before thesimulation, the Young’s modulus of SWCNT are calculated using the same method,and the results with the experimental values and the previous calculated results arein good agreement, which suggests that ab initio molecular dynamics method canbe applied to the tensile test of one-dimensional material. Then we calculate onedimensional C-N chain and the stress-strain curve of different length of thecarbyne chain. Also, we use two methods—the formula and numerical fitting toobtain the Young’s modulus of their. By means of the analysis of the Young’smodulus, ultimate strength and other mechanical properties, the fact that the mechanical properties of one-dimensional C-N chain is much better than the samelength of carbyne chain is obtained. Then we calculate the curve of the weakestbond of these one-dimensional monatomic material with strain and from thechemical point of view, we analyze the fracture mechanism of them and obtain therelationship between the size of the weakest bond, ultimate stress and Young’smodulus. Finally, we calculate the influence of temperature on the mechanicalproperties of one-dimensional C-N chain and obtain the result that with theincrease of temperature, the mechanical properties of carbon and nitrogen chainweakened.4) Aiming at decomposition products of nitromethane and other energeticmaterials and the numerous chemical reactions, we programme to solve some postprocessing problems. We propose three kinds of methods for the reactionproducts based on the bond length, bond order and potential energyfunction. Considering that the models in ab initio molecular dynamics simulationis small, the cutoff at boundary conditions are set to increase confirmation ofmolecular fragments over neighbor cells and thus to count statistics of smallmolecular number and identify information of large clusters moreaccurately. And based on that, we connect molecular fragments and countdecomposition products category information so as to obtain a complete list of thedecomposition products, then analysis the chemical reactions and changes ofpopulation further. |
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