Atomistic Insights into Structures and Dynamic Properties for Amorphous Aluminum/Lithium Alloys and Oxides

Aluminum/lithium (Al/Li) alloy is a promising energetic material for solid composite propellants. The bonding structure, topological shape, density, cohesive energy, and mechanical and diffusion properties of the Al/Li alloy bulks and oxidation shells are calculated systematically using the large-scale force-field molecular dynamics simulations together with the ab initio quantum chemistry calculations. Theoretical predicted structures and dynamic properties for various crystalline and amorphous reference compounds are compared with the available experimental data to validate the force-field simulations. The dependence of the structures and properties on the Li contents ranging from 2 to 50 wt% is clarified. It is revealed that both Al and Li atoms are resident in the same Al or Li environment in the Al/Li alloys. The presence of the crystalline δ’-Al3Li and β-AlLi phases in the Al/Li alloys is rationalized in terms of the coordination of Al/Li and the thermodynamic free energy of Li substitution. A homogenous six-coordinated Al/Li alloy could be generated with a Li content of 20 wt%. Young’s moduli of the alloys are improved via the low Li addition due to the anisotropic effect. The Al/Li/O oxidation shell is less dense than the amorphous alumina but the densities of oxides are generally higher than those of the corresponding Al/Li alloys. As the Li content increases, the Al/Li/O oxides form the ordered four-coordinated AlO4 passages together with the under-coordinated Li-O units, leading to considerably deteriorated mechanical performance and efficient Li diffusion with an activation energy of about 20 kJ/mol. The present work provides a deep understanding of the Al/Li alloys and Al/Li/O oxides in terms of performance and exposure stability.