Molecular Dynamics Study On Coating And Combustion Of Aluminum Nanoparticles Based On ReaxFF Force Field

Aluminum nanoparticles are characterized as high specific combustion enthalpy,stability and its combustion products are environmentally friendly.At present,they have been widely applied in the field of energetic materials,such as solid rocket motor,composite propellant development and underwater high-speed propulsion.Because the bond and electronic states of the surface atoms are different from the internal atoms,and the low coordination number of the surface atoms,the number of surface active sites increase making aluminum nanoparticles have high chemical reactivity.These characteristics lead to the oxidation and deactivation of aluminum nanoparticles before ignition,and the compatibility between aluminum nanoparticles and other components in solid propellant is poor.The effective way to solve above problems is to coat the particles to form composite particles with shell-core structure.In addition,due to the complexity and instantaneity of the reaction in solid propellant,it is difficult to track the evolution of particle combustion by experimental equipment.The research on ignition and combustion mechanism of aluminum nanoparticles is still insufficient.Therefore,it is of great significance to study the combustion mechanism of aluminum nanoparticles from the microscopic point of view for the development of new energetic materials.In this paper,the coating and combustion process of aluminum nanoparticles are studied in nanoscale and femtosecond scale by reactive molecular dynamics simulation based on Reax FF force field.The melting process of aluminum nanoparticles is firstly studied by molecular dynamics simulation.The melting point of particles is determined by potential energy temperature curve,specific heat capacity and Lindemann factor with temperature.The results show that the Reax FF force field can identify the solid-liquid coexistence state of aluminum nanoparticles,and the linear relationship between the melting point of aluminum nanoparticles and the particle size in a certain range is obtained.The reliability of Reax FF force field is verified by comparing with previous experimental and simulation results.On this basis,the relationship between the melting point and the additional energy storage of aluminum nanoparticles with different defect concentrations and the temperature is analyzed.The critical defect concentration that affects the melting point and the additional energy storage of aluminum nanoparticles is given.Then,the aggregation process of aluminum nanoparticles is studied.By changing the particle size and sintering temperature,the general rules of aggregation of aluminum nanoparticles under different conditions are discussed.According to the study on the change of shrinkage ratio,inertia radius,potential energy and surface area,it is confirmed that the formation of neck region is the key process of aggregation.Ethanol molecule is selected as the adsorption material to study the process of organic coating aluminum nanoparticles.The adsorption mechanism of ethanol on aluminum surface is studied by combining molecular dynamics simulation and first principles calculation.In the study of the adsorption of hydroxyl group and ethyl group,it is confirmed that hydroxyl group plays a leading role in the adsorption process of ethanol molecule on aluminum surface by checking the changes of adsorption distance,time and potential energy.Obtain fully coated aluminum nanoparticles by cycle-coating method.Through the radial function distribution analysis,the structural changes of the coating layer at different temperatures are studied,and it is verified that the organic coating can effectively inhibit the oxidation of aluminum nanoparticles at room temperature.The results show that the atoms in the organic coating layer,except for hydrogen atoms,do not diffuse to the inside of the particles,but form the neck region and stay on the surface of the particles,which inhibits the aggregation.The research on ignition and combustion mechanism of nano aluminum particles is the focus of this paper.The ignition process of Al nanoparticles with different shell thickness is analyzed from the radial stress distribution of shell and the mean square displacement of internal and external atoms.The results show that the oxide shell does not break up at the moment of melting of aluminum core,and the ignition stage is dominated by the diffusion of internal and external components;the nano aluminum particles with different thickness of oxide shell follow different stress propagation rules,and the thick oxide shell can significantly inhibit the construction of electric field inside and outside the shell,thus inhibiting the outward diffusion of aluminum atoms in the core.The ratio of Al to O in the oxide layer is the key factor affecting the adsorption rate of oxygen molecules on the particle surface.The combustion process of aluminum nanoparticles can be divided into three stages according to the rate of temperature change.Finally,the ignition and combustion process of aluminum nanoparticles coated with ethanol and ether is studied.In the ignition stage,through the analysis of radial distribution function and reduced diffusion coefficient of Al-O atom pair,it is found that due to the different chemical environment of surface coating layer,ether coating layer has better ability to adsorb oxygen.By charge distribution analysis,it is found that the protective ability of the ethanol and ether coating layer on the active aluminum atoms is similar before combustion,and no particles are ignited in advance.It is found that the ignition delay of particles with ethanol coating is higher than that of particles with ethanol coating,which is related to the adsorption capacity of oxygen during ignition stage.The results show that no matter what kind of organic coating can significantly increase the combustion temperature,the presence of the coating can maintain a gas phase combustion,while the uncoated aluminum nanoparticles can only maintain a liquid phase combustion.