Exploration Of Aluminum And Aluminum Hydride As An Important High Energetic Component

It is an effective way to increase energy release by adding active metal and hydride powder into high-energy materials.Highly active metals and their hydrides have been widely used in propulsion,aerospace,explosives,fuel and other energy fields.Aluminum and aluminum hydride are the most widely used active metal components.In this paper,reactive molecular dynamics and density functional theory are used to study the effect of aluminum or aluminum hydride on a variety of typical high-energy materials including 2,2’,4,4’,6,6’-hex-anitrostilbene（HNS）,1,3,5,7-Tetranitro-1,3,5,7-tetraazacyclooctane（HMX）,1,3,5-trinitro-1,3,5-triazacyclohexane（RDX）,pentaerythritol tetranitrate（PETN）,nitroglycerin（NG）,propylene oxide and organic fuel methanol.The influence mechanism of aluminum or aluminum hydride on the combustion process of high-energy materials was studied from atomic point of view.The reactive dynamic revealed the combustion mechanism of nano-aluminum hydride in oxygen atmosphere and molten nano-aluminum droplets with water vapor.These results provide some guidance for their application in high energy field.The main research contents of this paper are as follows:（1）Based on the reactive dynamic simulation,the interaction between aluminum nanoparticles and typical energetic materials PETN,RDX,HNS,and NG was studied.The nano-aluminum accelerates the decomposition of these energetic materials and makes these energetic molecules decompose in advance in a way different from that of the pure system,because nano-aluminum has a strong ability to capture C/H/N/O atoms in the energetic molecules.Nano aluminum reduces the activation energy of decomposition of these energetic materials and improves the energy release efficiency.Nano aluminum can break the N,H,and O atoms out of the carbon skeleton to produce more N₂ and H₂O,which increases the total amount of gas produced.Because aluminum readily sticks to the carbon skeleton,nano-aluminum inhibits CO₂ production to a certain extent.To some extent,the oxidation layer on the surface of nano aluminum reduces the attraction of aluminum to oxygen and nitrogen atoms in energetic molecules,leading to passivation effect.The oxide layer tends to cause the carbon skeleton of energetic molecules to accumulate on the surface.Moreover,the passivated aluminum nanoparticles are more likely to agglomerate at the later stage of the reaction and are not easy to form small aluminum clusters.The oxidation of nano aluminum in energetic materials follows the diffusion oxidation theory,which depends on the mutual diffusion of oxidizer atoms and aluminum atoms.Particle size and passivation layer affect the oxidation of nano aluminum.The nanoaluminum with smaller particle size even exhibits a microexplosion oxidation behavior dominated by the diffusion of aluminum atoms,and ejects tiny aluminum clusters.The oxidation of aluminum particles with larger size goes through three stages:surface oxygen atom diffusion,intermediate bidirectional diffusion and aluminum diffusion after melting.The presence of oxide layer delay the time when aluminum particles reach the melting point.O/Al and C/Al clusters are the main agglomerations of aluminum particles at the later stage of reaction.The temperature distribution shows that the hot spots are concentrated in the area where aluminum particles exist.（2）The Reax FF-lg reactive field was used to simulate the interaction between Al H₃ and HMX,NG,HNS.The nano-aluminum hydride accelerates the thermal decomposition of these energetic molecules,reduces the activation energy required for the decomposition of energetic molecules,increases the energy released by decomposition,and changes the initial decomposition mode of energetic molecules,which is mainly due to the strong attraction of aluminum atoms to oxygen.The aluminum hydride increased the production of stable gaseous products,especially the formation of H₂O and N₂.The particle size and surface passivation layer affect the hydrogen release rate of aluminum hydride.The passivation layer on the surface tends to accumulate the carbon skeleton of energetic molecules.The smaller particle size aluminum hydride has a microexplosion reaction,which greatly increases the reaction rate and hydrogen release rate.Aluminum hydride with larger particle size and passivated surface layer is easy to form hydrogen cavity agglomeration in the reaction process,and hydrogen is difficult to release quickly.The oxidation of nano aluminum hydride in energetic molecules depends on the mutual diffusion of oxidizer atoms and aluminum,and the formation of H₂ accelerates the evolution of intermediates.（3）The reactive molecular dynamic was used to reveal the reaction mechanism of molten nanometer aluminum droplet（ANDP）with water vapor at high temperature from the atomic point of view.The effects of temperature,particle size and water vapor concentration on ANDP combustion were investigated.In general,the combustion process of ANDP expands outward with oxidation,and the oxidation phase is dendritic and microexplosive.The reaction of ANDP with H₂O can be divided into:（1）H₂O+Al→O-Al+H₂;（2）2H₂O+2Al→2h-Al+H₂;（3）H₂O+Al→OH-Al+H or（h-Al）;（4）H₂O+H-Al→OH-Al+H₂.The branching oxidation of ANDP is mainly a heterogeneous reaction of active aluminum to O-Al phase.The microexplosion of ANDP leads to the combustion of Al in the gas phase,which rapidly catapults the active Al into the water environment.Microexplosions cause the decomposition of H₂O molecules in pathway（1）,increasing the consumption of H₂O and the generation of H₂.The microexplosion of ANDP can be promoted by increasing temperature,decreasing particle size or increasing water vapor concentration.The charge and temperature distributions indicate that the ANDP microexplosion leads to the rapid formation of hot spots,voids and large electrostatic potential differences.（4）The oxidation behavior of pure aluminum hydroxide and core-shell aluminum hydroxide under different oxide layer thickness,particle size and oxygen concentration were studied by molecular reactive dynamic method.The initial oxidation of AHNP presents uneven surface distribution in tributaries.The Al diffusion coefficient of 40 ps（1.42×10^⁴ cm²/s）is much larger than that of shell O atom（4.90×10^⁵cm²/s）,which indicates that the oxidation of core-shell AHNP is mainly caused by the heterogeneous diffusion of nuclear aluminum to the oxide layer driven by electrostatic force,and hydrogen cavity of different sizes are formed during the oxidation process.The mean squared displacement indicates that the nuclear aluminum has a tendency to diffuse to the thinner oxide layer.The oxidation layer inhibits the diffusion of Al and H in the nucleus,resulting in slow oxidation rate.The smaller AHNP exhibits microexplosion oxidation accompanied by the formation of large aluminum clusters,and the reaction process is dominated by aluminum diffusion.The initial reaction of larger AHNP is mainly concentrated on the surface,and the subsequent oxidation process depends on the heterogeneous reaction between the oxidation phase and Al H₃.At lower oxygen concentration,AHNP exhibits lower oxidation rate and reaction exothermic rate,and the core Al atoms and the O atoms in the environment diffuse to form a uniform O-Al phase.At higher oxygen concentrations,the active aluminum atoms diffuse outward to form hollow spherical structures.（5）Molecular reactive dynamic was used to simulate the cloud explosion of epoxy-propane/aluminum and combustion processes methanol/aluminum nanodroplets respectively.In general,the nano aluminum reduces the ignition delay of propylene oxide and methanol and the activation energy required for the combustion decomposition of methanol and propylen.Nano aluminum has an ability to extract hydrogen and oxygen atoms from methanol and propylene oxide.In addition,aluminum accelerates the decomposition of O₂ and increases the number of O free radicals in the system.The ability of the system to generate active fragments such as H,H2,OH and O was enhanced.During the reaction process,aluminum particles undergo a process from H-Al phase to C-Al phase,and finally to O/Al phase.The surface aluminum particles explode in the middle stage of combustion.In the later stage of composite droplet combustion,a gas combustion mode is presented.