Thermochemical behavior of nano-sized aluminum-coated nickel particles

Molecular dynamics simulations are performed to investigate the thermochemical behavior of aluminum-coated nickel particles in the size range of 4-13 nm, beyond which asymptotic behavior is observed. The atomic interactions are captured using an embedded atom model. Emphasis is placed on the particle melting behavior, diffusion characteristics, and inter-metallic reactions. Results are compared with the corresponding properties of nickel-coated nano-aluminum particles. Melting of the shell, which is a heterogeneous process beginning at the outer surface of the particle, is followed by diffusion of aluminum and nickel atoms and inter-metallic reactions. The ensuing chemical energy heats up the particle under adiabatic conditions. The alloying reactions progressively transform the core-shell structured particle into a homogeneous alloy. The melting temperature of the shell is weakly dependent on the core size, but increases significantly with increasing shell thickness, from 750 K at 1 nm to 1,000 K at 3 nm. The core melts at a temperature comparable to the melting point of a nascent particle, contrary to the phenomenon of superheating observed for nickel-coated aluminum particles. The melting temperature of the core decreases from 1,730 to 1,500 K, when its diameter decreases from 10 to 7 nm. For smaller cores, the majority of nickel atoms participate in reactions before melting. The diffusion coefficient of nickel atoms in aluminum shell exhibits a temperature dependence of the form D = D (0) exp(-E (A)/RT), with an activation energy of 43.65 kJ/mol and a pre-exponential factor of 1.77 x 10(-7) m(2)/s. The adiabatic reaction temperature, also a size-dependent quantity, increases with increasing core diameter, attains a maximum value of 2,050 K at 5 nm, and decreases with further increase in the core diameter. The calculated values agree reasonably with those obtained via chemical equilibrium analysis. The burning time exhibits strong dependence of particle core size and shell thickness of the form tau (b) = a d (c) (n) delta (s) (m) , where the exponents n and m are 1.70 and 1.38, respectively. The finding further corroborates the fact that the reaction rate is controlled by diffusion process.