| As a typical metal fuel agent,aluminum(Al)nanoparticle has excellent combustion rate and energy density.However,its energetic material application has long been limited by the deactivation of particle caused by high surface activity and specific surface area.The activity of Al nanoparticle can be maintained by preparing Al based core/shell composite nanoparticle.In this article,molecular dynamic(MD)simulations,as the major method,is applied to investigate the adsorption mechanism of nitrocellulose(NC),ethanol and ether on Al nanoparticle surface,as well as the coating interface of two phases.In particular,we are focused on the analysis of the motion state of three adsorbates and their transient characteristics in the formation of coating layer,as it is difficult to be captured by experimental methods.Firstly,the modeling and optimization of Al nanoparticle,NC,ethanol,and ether molecules are carried out.The optimization of Al nanoparticle applies the recrystallization and annealing method basing on EAM force field.The melted Al phase is diffused from particle surface to core.In this process,the transformation laws of its atomic potential energy and mean square displacement are determined.Then,the accuracy of the EAM force field is verified by the melting point.During the cooling process of liquid Al clusters,Al atoms will be frozen if the cooling rate is too fast.This may weaken the relaxation effect of Al nanoparticle,and bring up the percentage of amorphous Al.When the cooling rate is between 10-20 K/ns,the value of potential energy and the extent of relaxation are both higher than other conditions.The percentage of FCC phase is stabilized around 80%.The optimization of NC decamer homopolymer model with two substitution degrees applies the COMPASS force field based molecular mechanics method.This research obtained the structural model of the pyran ring and glycosidic bond molecular chain in NC.The atomic charge value optimized by the QEQ algorithm.In the GGA-PBE density functional theory(DFT)structure optimization of ethanol and ether molecules,the ground state energy,charge and electrostatic potential of ethanol and ether molecules are obtained through self-consistent calculations.Their adsorption postures predicted by the electrostatic potential are in good agreement with MD results.The setting of“K value of the Brillouin zone:4×4×4;cut-off energy:800 e V”shows better convergence performances than other tested conditions.Secondly,the study on the adsorption of NC,ethanol,and ether monomolecules on Al surface are carried out.The adsorption force of ethanol is mainly from the bonding force.The ethanol molecules on the Al surface form a new Al-O bond with the surface Al atoms.Compared with the chemical adsorption mechanism of ethanol under the single-molecule adsorption model,ether belongs to a weaker physical adsorption mechanism.Since the ether molecules on the Al surface keep the intactness of group and do not bond with Al,the adsorption force of ether mainly comes from van der Waals force.At this moment,the ether molecule is not fixed on an adsorbing site,so it still remains the surface migration.On the Al(001)crystal plane,both ethanol and ether tend to adsorb to the H site;on the Al(110)crystal plane,both molecules tend to BS site;on the Al(111)crystal plane,the adsorption tendency of ethanol-ether has the largest difference,including fcc and T positions respectively.Because NC is a linear chain structure,its horizontal adsorption state is derived from the mechanism of extended adsorption.In which,part of the NC chain produces the chemical adsorption through the Al-O and Al-N bonds,then their drag the adjacent chain links into the adsorption zone to extend the entire adsorption state.On Al crystal plane,the critical adsorption angle of NC is 30°,and its critical adsorption distance is about 4-6(?).Thirdly,the binary coating structure composed of ethanol and ether is discussed.In the molecular mixture system,parts of the ether are transformed from the original physical adsorption to the chemical adsorption.In other words,the-C2H5 and-OC2H5 groups are adsorbed independently.However,due to the low adsorption stability of ether,they are adsorbed less than ethanol.The binary coating structure after"competitive adsorption"can be divided into three layers.Among them,the middle layer is equivalent to the media layer,which maintains the adsorption state of-CH3 and-C2H5 groups with the intramolecular bonding force.It also separates H~+into the inner surface layer and generates the Al-O bonds to keep the entire core/shell structure.In double and trible particle models,the agglomeration gap caused the unavailability of the adsorption site,the limitation of space,the reduce of coating layer density.Meanwhile,the surface properties of the gap shell are different from the non-interstitial adsorption surface,resulting in a non-uniform core/shell structure.Fourthly,the formation of the NC-ethanol-ether ternary coating layer is studied.Because the adsorption of NC is later than ethanol and ether molecules,the ternary coating layer on the Al nanoparticle is formed on the basis of the ethanol-ether binary coating layer.In the liquid phase environment,adsorption positions are mainly occupied by ethanol and ether,so NC has horizontal and vertical adsorption positions at the same time.In the near-surface coating layer(within 10(?)thick),the proportion of NC component is lower than that of ethanol and ether molecules.The presence of the ternary near-surface coating can reduce the adsorption rate of O2 and H2O molecules.It can also block the hydrogen bond chain of H2O molecules in the Grotthus mechanism.So,the influence of the deactivation reactions(Al-O2 and Al-H2O)on Al particles are weakened.In other words,the deactivation rate of Al nanoparticle is reduced.In summary,the NC-ethanol-ether ternary molecular system can not only stably adsorb on the surface of Al particle and form a coating layer,but also bring the passivation effect on the Al nanoparticle surface.So,they are beneficial to maintain the activity of the Al particle. |
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