Thermochemical behavior of nickel-coated nanoaluminum particles

Thermochemical behavior of nickel-coated aluminum particles in the size range of 4-18 nm is studied using molecular dynamics simulations. The analysis is carried out in isothermal-isobaric and isochoric-isoenergetic ensembles using an embedded atom method. Emphasis is placed on analyzing the melting points of the core and shell, diffusion of atoms, and intermetallic reactions. The aluminum core melts at a temperature greater than the melting point of a nascent aluminum particle due to the cage-like mechanical constraint imposed by the nickel shell. The melting point of the aluminum core increases from 775 to 1000 K when the core diameter increases from 3 to 12 nm. The melting point of the core is not significantly affected by variations in the shell thickness in the range of 1-3 nm, although the melting point of the shell increases with increasing thickness from a value as low as 1100 K at 1 nm to 1580 K at 3 nm. Melting is followed by diffusion of atoms and energy release due to intermetallic reactions, which result in ignition of the particle in vacuum. For a core diameter of 3 nm, the ignition temperature increases from 800 to 1600 K when the shell thickness increases from 0.5 to 3.0 nm. The diffusion coefficient of aluminum atoms in the nickel shell exhibits an exponential dependence on temperature, with activation energy of 34.7 kJ/mol. The adiabatic reaction temperature of the particle increases from 1650 to 2338 K when the core diameter increases from 3 to 8 nm. The calculated values agree reasonably well with those obtained via thermodynamic energy balance analysis.