Molecular Dynamics Study On Interfacial Interaction And Thermal Performance Of The Composite System Of Nano-Aluminum And Nitrocellulose

Nitrocellulose/Aluminum nanoparticles（NC/ANPs）composite energetic materials retain the high reactivity of ANPs and have good chemical stability,which is of great significance to the storage and energetic application of ANPs.Due to the limitation of traditional experimental methods in studying the mechanism of microscopic interface and complex transient process,the molecular dynamics（MD）simulation based on ReaxFF-lg force field is applied on the atomic scale to investigate the coating,thermal decomposition,and combustion process of NC/ANPs systems.Firstly,the geometric optimization of ethanol/diethyl ether/NC molecular and annealing optimization of ANPs are carried out.Then,to verify the accuracy and reliability of the ReaxFF-lg field used in this paper,the melting process of annealed ANPs and the combustion process of diethyl ether are studied,in which the simulation results are consistent with previous studies.In this paper,ethanol-ether solution（volume ratio 1:1）was selected as the organic solution of NC,and then the adsorption mechanisms of ethanol-ether solution and NC molecular chain on the Al substrate surface are studied.The charge distribution,potential energy change,and radial function distribution analysis reveal that ethanol,ether,NC molecules are chemically adsorbed on the Al substrate surface.Note that the lower the atomic density of the crystal plane,the easier the adsorption occurs.On this basis,the coating process of ethanol-ether solution and NC-ethanol-ether solution on the annealed/passivated ANPs are studied.The results show that both the ethanol-ether binary coating and the NC-ethanol-ether ternary coating are a combination of chemical adsorption and physical adsorption.Compared with annealed ANPs,the chemisorption on the surface of passivated ANPs is weaker.The ethanol composition ratio in the binary coating is much higher than that of ether.The addition of NC molecular chains could reduce the ethanol/ether ratio in the ternary coating layer.Most of the chain segments of the NC extended under the traction of chemisorption chain segments to form a network covering the surface of ANPs.The atomic displacement and charge distribution analysis prove that the internal aluminum atoms of the coated annealed ANPs have good stability and high activity.The oxidation resistance test shows that the binary coating layer and,especially,the ternary coating layer have good stability and oxidation resistance at room temperature.Next,the thermal decomposition of the NC system and NC/ANPs system are studied.The simulation explains the product evolution and reaction mechanism of the NC thermal decomposition from the atomic scale.High temperature could accelerate the thermal decomposition of NC,and the main mechanism is to promote the decomposition of 444 and the transformation of H₂O,N₂,and CO₂.As for NC/ANPs system,the first stage occurs the chemical adsorption of the NC molecules on the surface of ANPs.A large amount of Al-O bond formation promotes the thermal decomposition of NC molecules but also restricts the generation of CO₂and H₂O.In the middle and late stages,C atoms gained the ability to combine with Al atoms in large quantities,releasing part of O atoms.In addition,it is confirmed that the addition of ANPs can eliminate the energy barriers for NC decomposition,increasing the energy release efficiency and quantity.Finally,the ignition and combustion processes of NC-ethanol-ether ternary coated ANPs are studied.At low temperatures,the ternary coating could block the contact between the active Al core inside ANPs and the external O₂molecules,while having a good capacity to adsorb O₂molecules.With the increase of ignition temperature and the accumulation of NC decomposition energy,the surface temperature of the ternary coated ANPs is higher,and the Al atoms inside ANPs could diffuse to the surface faster.Meanwhile,the desorption and diffusion of the ternary coating expose more reaction sites,and O₂molecules quickly adsorb to the surface to react with Al atoms.As a result,ternary-coated ANPs exhibit shorter ignition delays and lower ignition temperatures.During the combustion process,passivated ANPs undergo surface oxidation/melting diffusion,flame-spreading,and homogeneous combustion.The ternary coating rapidly desorbs and diffuses to the gas phase area during combustion,while a fraction of coating molecules are involved in the surface oxidation of ANPs.The energy released by NC decomposition and gas-phase reaction also promoted the atomic diffusion of ANPs,so that accelerate the combustion propagation rate.In addition,the organic carbon source in the coating is not only a high-quality additional fuel source for gas-phase combustion,but also directly participates in the solid-liquid phase combustion reaction with ANPs,increasing the combustion time of ANPs.