Molecular Dynamics Study On Combustion Oxidation And Interface Passivation Of Al-Based Nano-Energetic Materials

Nanoparticles are widely used in research fields such as material design,material synthesis,catalysis,energy storage and conversion,and combustion applications.The study of their oxidation mechanism is crucial for understanding,predicting,and tuning material properties.The combustion of energetic nanomaterials involves complex and rapid physicochemical processes,the mechanisms of which have not been well elucidated.In this work,the morphological evolution,phase transition mechanism,elementary reaction and other thermodynamic and kinetic behaviors of Al-based nanoparticles during combustion are studied in the atomic scale,and the oxidation mechanism of Al-based nanoparticles and the internal mechanism of morphological evolution are revealed.Molecular dynamics simulation is used to study the reaction mechanism of nano-aluminum with oxygen-containing gas,the high-temperature oxidation of modified nano-aluminum（including nano-aluminum hydride and nano-aluminum-magnesium alloy）,the active protection of nano-carbon materials on aluminum and catalytic mechanism.The main contents are as follows:1.High-temperature reaction mechanism of nano-aluminum and oxygen-containing gasThe oxidation mechanism of nano-aluminum（ANP）in gaseous oxides（CO₂,CO,NO₂,and NO）was investigated to elucidate the detailed mechanism of ANP surface oxidation,chain-like products formation,and hollow evolution.The O atoms in gaseous oxides are adsorbed on the ANPs’surface followed by the cleavage of O–C/N bond.The gaseous oxides decompose on the surface of ANP through CO₂+Al→Al O+CO,CO+2Al→Al O+Al C,NO+Al→NAIO,NO₂+Al→Al O+NO,etc.Gaseous oxides in the gas phase can take part in direct addition reactions with Al atoms by the following reaction:Al+CO→COAl（13）,Al+CO₂→CO₂Al（10）,NO+Al→ONAl（26）,Al+NO₂→O₂NAl（32）.The ignition of ANP in various gases was found to depend on the combined effect of temperature and pressure,as well as its morphological changes.The higher the temperature,the greater the asymptotic value of the potential energy decay,indicating that the system accelerates the heat release rate.The higher the ambient gas concentration,the higher the energy released by the system.The oxide shell of ANPs formed and expanded rapidly in CO₂ gas,resulting in the formation of voids between the oxide shell and the Al core.Al atoms are transported from the core to the oxide shell through bridges composed of Al atoms.The Al core gradually diffuses outward and is eventually forms a central void.The nucleation and growth of chain-like products occurs in dense gaseous oxides.The four forms of chain products include tilted chain,twisted chain,branched chain,and cyclic chain were observed in CO gas.A similar chain structure is also formed in NO gas,but the chain length is significantly shortened.In addition,the final product has carbon deposits(C₄₈ and C₉₈)on the surface and core.2 Structural evolution mechanism of aluminum hydride in combustionThe dehydrogenation and oxidation mechanisms of aluminum hydride nanoparticles（AHNPs）were investigated.The morphological evolution of AHNPs in oxygen is induced by temperature and structure,and classified into four shapes four shapes:spherical,notched spherical,large branch and small string.The surface dehydrogenation and oxidation are almost co-occurring.The initial dehydrogenation inhibits the rapid oxidation of the surface layer of AHNP.The increase in temperature induces the instability of the oxide shell and provides sufficient kinetic energy for the hydrogen bubbles.As a result,the oxide shell ruptures and the hydrogen bubbles inside physically escape from the nanoparticles.At T=3000 K,some notches appeared in the shell,making its appearance to be branchy.At T=3500 K,AHNP micro-explodes and ejects small atomic chains.The oxidation of AHNPs in gaseous oxides（CO,CO₂,NO,and NO₂）went through four stages:dehydrogenation（<84 ps）,Al nucleation and growth（>25 ps）,micro-explosion（～31 ps）,and oxidation（>28 ps）.Only a small part of Al on the surface is oxidized to form a thin and uneven oxide film（0.18-0.54 nm）.In the core,the formed H₂ is hindered by the shell and gradually gathers into H₂bubbles.H₂ bubbles have great kinetic energy and become a micro-explosion promoter,eventually causing nanoparticles to burst at high temperature.The C,N,and O atoms dissolved in the nanoparticles compete with each other.The higher the temperature,the more Al-O bonds are formed.3.Core-shell evolution mechanism in oxidation of Al-Mg alloysThe core-shell evolution in the combustion of aluminum-magnesium alloy（AMNP）is divided into three stages:（i）AMNPs first undergo phase separation and aggregate into Mg and Al phases.Since Mg is volatile and its vapor pressure is much higher than that of Al,the Mg phase diffuses outward;（ii）the Mg near the surface volatilizes and is rapidly oxidized,and the oxidized Mg adheres to the outer surface of Al.As the inner Mg gradually overflows and is oxidized,the Al surface is covered by magnesium oxide;（iii）oxygen atoms further diffuse inward through the magnesium oxide layer and oxidize the Al core.The oxidation rate of Al is much lower than that of magnesium due to the hindrance of aluminum’s passivation layer and magnesium oxide layer.In addition,the formation of the outer magnesium oxide coating hinders the agglomeration of nanoparticles.At lower temperature and high oxygen concentration,AMNPs favor to form hollow structures.AMNPs expand under the influence of temperature,and the Al atoms in the outer layer are oxidized to form a rigid shell.The out-diffusion of Mg in the core gradually hollows out the nanocore.The escaping Mg is oxidized and adhered to the outer layer of Al.Finally,the structure of magnesium oxide（surface layer）-aluminum oxide（middle layer）-hollow（interior）is formed.4.Interaction between nano-aluminum and carbon nanotubes and its combustion mechanismTo improving the stability challenges of metal nanoparticles,the details of self-assembly and oxidation between carbon nanotubes（CNTs）and aluminum nanoparticles were investigated.ANPs can completely self-roll into CNTs to form a stable core-shell structure by van der Waals forces.Inside the tubes,the ANPs move toward the cap at a velocity of 2.27?/ps.However,it accelerates to 3.17?/ps when near the cap of CNTs.The initiation of the ANPs’oxidation and degradation can be effectively checked by coating CNTs.The diffusion of the Al atoms in the encapsulated ANPs occurred earlier than their oxidation in combustion,verified by using Reax FF molecular dynamics simulations.The morphological evolutions of the nanostructures in the initial combustion of the encapsulated ANPs are predicted.The interplay between the encapsulated ANPs’responses and external stimuli is classified into core–shell separation,shell damage,and core–shell burst.Two types of nanocapsules were further designed.The stability and controllability of capsules I-VI consisting of various lengths of CNTs were revealed by means of reactive molecular dynamics simulations.The driving force for the self-assembly of nanocapsules is the vd W interaction.The assembled capsule has excellent stability,and the interaction energy between the tube and ANPs and between the tubes is as high as?599.55 and?1014.78 kcal/mol,respectively.The opening of the nanocapsules during combustion is dependent on the length of the CNTs and temperature.Above 2000 K,the outermost CNT can be opened spontaneously when the length is greater than 31.73?.When used as propellants,pyrotechnics and explosives,these nanocapsules can be triggered remotely by visible/infrared lasers without the need of detonating wires.5.Research on the catalytic reaction of graphene on energetic materialsA computational strategy based on Reax FF reactive force field unravels details of the combustion of 1,3,5-trinitroperhydro-1,3,5-triazine/graphene（RDX/GR）.A comparative analysis of decay rate in RDX（pure RDX,RDX/GR,and RDX/porous GR）demonstrates that the catalytic activity of GR is dependent on the density and temperature.In the initial stage,the high-temperature reaction hardly damages the carbon nanosheets,and only a few defects exist in graphene at high density and high temperature.The wrinkled graphene formed by thermal fluctuation and extrusion in the initial stage has more significant catalytic activity.O,OH,and NO₂ tend to bond to the dangling sp² C atoms in the wrinkled graphene.In combustion catalysis,GR accelerates the interatomic exchanges between active groups on GR and RDX.The density functional theory method further verified these species are adsorbed on graphene.The adsorption energy of NO₂ attached to the peak of wrinkled GR is 2.79 kcal/mol higher than that on the flat GR.