Atomic insights into combustion mechanism of nano‐aluminum hydride

Abstract

As an important hydrogen storage material, aluminum hydride is widely used in the combustion field. However, the oxidation behavior and potential mechanism of aluminum hydride nanoparticle (AHNP) in the combustion process are not clear. Molecular reactive dynamics was used to explore the oxidation behavior and mechanism of pure and core-shell AHNP with different oxide layer thickness, particle size and oxygen concentration. The simulation results show that uneven oxidation of AHNP surface presents branched distribution, and the oxidation process is carried out by the interdiffusion of O and Al. The core Al diffusion coefficient of 40 ps (1.42 × 10−4 cm2/s) is much larger than that of shell O atoms (4.90 × 10−5 cm2/s), indicating that oxidation of core-shell AHNP is dominated by the uneven diffusion of core Al into the oxide layer driven by an electrostatic force. This leads to the formation of hydrogen gas chamber of different sizes. Mean square displacement shows that core Al tends to diffuse into a thinner oxide layer. The oxide layer inhibits the diffusion of core Al and H, resulting in a slower oxidation rate. The smaller AHNP exhibits a micro-explosion oxidation accompanied by the formation of small Al clusters, and the reaction process is dominated by aluminum diffusion. The initial reaction of the larger AHNP is mainly concentrated on the surface, the subsequent oxidation processes rely on heterogeneous reactions between the oxidation phase and AlH3. Under lower oxygen concentration, AHNP shows a lower oxidation rate and heat release, where core Al and environment O atoms diffuse toward each other to form a homogeneous OAl phase. Under higher oxygen concentration, the active Al atoms diffuse outward to form a hollow spherical structure. This work provides fundamental insight into the storage and application of AHNP that serve as a high energy density fuel.