Theoretical Studies On The Structure And Properties Of Al- And Mg-based Nanothermites

In this dissertation,density functonal theory（DFT）and ab initio molecular dynamics（AIMD） methods were used to systematically study the adsorption and replacement behaviors of Mg and Al on the CuO surface during initial deposition.The intrinsic physical and chemical mechanisms of a thin ZnO film as an interfacial interlayer to regulate the performance of Mg/CuO nanothermite were analyzed and discussed.The interfacial combustion mechanisms of Al/NiO nanothermite and the effect of mass transport on its combustion were explored.The differences in the adhesion properties,thermal stability,and diffusion behaviors of Al/NiO interfaces constructed with different dominant surfaces were compared.The ideal tensile strength,fracture mechanisms,and interfacial stability of Al/NiO films under uniaxial tension were investigated.The main contents of the dissertation are as follows:1.Interactions of Mg and Al with CuO（111）and（（?）11）surfacesFour adsorption scenarios and 32 adsorption configurations of Mg and Al adatoms on CuO（111）and（（?）11）surfaces were investigated.All the configurations were optimized by the spin-polarized DFT+U method and corresponding adsorption energies were calculated.The adsorption at the 3-Bridge site is a stable adsorption configuration in all the four adsorption scenarios.Based on the Bader charge analysis,the direction of the electron transfer following adsorption was examined.It was discovered that Cu_3c on the CuO（111）surface was easier to be reduced than that on the（（?）11）surface.In addition,Mg reduced Cu_3c more effectively than Al.The adsorption of Mg and Al on CuO（（?）11）is relatively stable,and no obvious replacement behavior occurs.However,the fact that Mg and Al can replace Cu_3c without encountering an energy barrier after adhering to the CuO（111）surface shows that initial mixing can take place at the interface when the coverage is low during the early deposition stage.2.Using a thin ZnO film as an intermediate layer to tune the performance of Mg/CuO nanothermiteDFT calculations and AIMD simulations were carried out to study the behaviors of adsorption,penetration and segregation for Mg on the ZnO（000（?））polar surface during the early stage of interface formation.We discussed how the ZnO interlayer regulates the properties of Mg/CuO nanothermite.The Mg adatom tends to be adsorbed on the fcc and hcp sites of the surface.The formation of an initial mixed interface is spontaneous at room temperature.The subsurface Zn atom prefers to migrate above the surface,i.e.,the segregation of Zn on the ZnO surface,rather than penetrate deeper into the interior of the ZnO slab.The thin ZnO film can act as a barrier layer to avoid the diffusion contact of Mg and Zn atoms with CuO.Our work provides some theoretical insights for tuning the performance of the nanolaminates through interface engineering at the atomic and electronic levels.3.Combustion mechanisms and mass transport properties of Al/NiO nanothermiteThe initial excitation and combustion processes of Al/NiO nanothermite were studied by AIMD simulations using gradient heating method.The primary and secondary heat releases are mainly caused by the interfacial and bulk thermite reactions,respectively.The migration of interfacial oxygen and the breaking of Ni-O bonds cause the initial ignition.The destruction of the bulk NiO lattice is the precursor process for the secondary heat release and retards the rapid thermite reaction stage.The ¹⁸O isotope leads to a lower oxygen migration rate,having a great influence on the reaction’s progress.The metal-oxygen flip mechanism facilitates the progress and propagation of the thermite reaction.Our work may be helpful for understanding the reaction mechanisms of the metal/metal oxide thermites and optimizing the formulation design and performance of the nanothermites.4.Thermal stability and initial diffusion mechanisms of Al/NiO nanothermites with various dominant surfacesDFT calculations and AIMD simulations were performed to disclose the significant differences in the interfacial structure,bonding strength,interfacial energy,and thermal stability of the Al/NiO nanothermites with various dominant surfaces.For the Octo-Top configuration,the interdiffusion can occur at room temperature.Al diffuses into the NiO layer and O diffuses into the Al layer,resulting in the formation of a mixed interface.The kinetic stability of other two interfacial configurations（100）-O-Top and（111）-Top is higher than that of Octo-Top.The presence of vacancies or defects in the interface area is detrimental to the stability of the system.Our work may provide a theoretical insight for optimizing the interfacial structure of Al/NiO at low temperatures.5.Effects of uniaxial tension on the mechanical properties and stability of Al/NiO interface The tensile properties,fracture mechanism,and film stability of two Al/NiO interfaces under pre-stress were investigated using first-principles tensile tests and AIMD simulations.An analysis of electron localization function and bond overlap population shows that the weakest locations of the（100）-Top and（111）-Top interface systems are distinctly different,so leading to two different fracture mechanisms.When pre-stress and temperature are coupled,the reactivity of Al/NiO as an energetic material is enhanced,particularly for（111）-Top.The amorphous mixed region between the two phases replaces the original coherent interface.With the increase of the pre-stress,the evolution of cracks in the system is clearly visible ahead of the critical stress in the quasi-static tensile test,indicating that the temperature and the existence of pre-stress have an important influence on the stability and fracture strength of the Al/NiO film system.