Molecular Dynamics Simulation Of Aluminum-water Reaction

The reaction of aluminum metal with water can generate a large amount of hydrogen while giving off heat.Aluminum/ice solid propellant（ALICE）is a new type of green high-energy propellant,which has many advantages such as low cost,high safety,environmental friendliness and simple production process.It is expected to be applied in space propulsion,underwater propulsion and rapid hydrogen production.In order to have a deeper understanding of the application of metallic aluminum in high-energy propellants,this article has simulated the combustion characteristics of aluminum/water mixtures,which is divided into the following three parts.（1）Molecular dynamics simulation on the reaction of nano aluminum particle and water were performed by ReaxFF force field.The different reaction rates of aluminum with H₂O and D₂O indicate that the effect of kinetic isotope effect（KIE）is obvious.The generation rate of H₂ is 20%higher than that of D₂.The diffusion coefficients of the different atoms show that the replacement of H₂O by D₂O reduces the diffusion rate of all the atoms（5%）in the system.The mechanism of formation and consumption of AlH₃ as well as the generation of Al₂O₃ are elucidated.Specifically,the most frequent reaction pathways throughout the full period are clarified based on the changes of numbers of intermediate and final products.（2）The effect of particle size and surface passivation of aluminum particle on the reaction mechanism was studied by using reactive molecular dynamics（RMD）simulation.The reduction of aluminum particle size can accelerate the reaction rate in the medium term（20-80 ps）due to the increase of activity,but it also bring agglomeration effect as the temperature increases.The presence of surface passivation reduces the proportion of active aluminum and the yield of hydrogen by 30%and 33%,respectively,as the particle size decreases from 2.5 nm to 1.6 nm.The addition of AlH₃ can overcome these drawbacks when some aluminum powders were replaced by AlH₃.The hydrogen release is increased by the reaction 2AlH₃+3H₂O→Al₂O₃+6H₂.In the reaction of surface passivated Al（1.6 nm in diameter）and H₂O,when the proportion of AlH₃ reaches 25%,the energy release and hydrogen yield increases from 59.47 k J/mol and 0.0042 mol/g to 142.56 k J/mol and 0.0076mol/g,respectively.This performance even approximates the reaction of pure aluminum with water:180.67 k J/mol and 0.0087 mol/g.In addition,the surface passivation affects the reaction mechanism.Before the passivation layer melted,the reaction 4Al+Al₂O₃→3Al₂O occurs inside the nanoparticles.（3）Molecular dynamics simulations on the reactions of nano-aluminum,water with different proportions of hydrogen peroxide were performed by ReaxFF.The reaction rate of the system increases by 27%as the molar fraction of H₂O₂ increases from 0 to 30%.The reaction paths in each period are elucidated by the change of the numbers of reactants and products.Particularly,the production of H₂ decreases by 28.2%,49.8%and 68.6%,as the molar fraction of H₂O₂ increases from 0 to 10%,20%and 30%,respectively.Meanwhile,the reaction of Al/H₂O/H₂O₂ releases much energy（271.40-440.53 k J/mol）than that of Al/H₂O（180.67 k J/mol）.Moreover,the ignition temperature decreases with the increase of H₂O₂concentration,but is proportional to the heating rate of ignition.When the malar ratio of H₂O₂increases from 0 to 30%,the adiabatic flame temperature of the mixture increases linearly from 2400 K to 3000 K according to the adiabatic simulation.