Atomistic Simulation of Self-Heating Reactions in Al-Coated Ni Nanoparticles
1. Introduction
In practice, the synthesis of bulk nickel aluminides is usually achieved by using conventional coarse-grained powders where the grain sizes are of the order of micrometers [1]. However, distinct advantages have been reported by using nanometer size particles to synthesize NiAl [2]. It has been shown [2] that due to the physical and chemical characteristics of nanoparticles (especially their high stored energy and therefore high chemical activity) the reaction mode and mechanism are distinctly different from those where conventional coarse-grained powders are used. In particular, it was demonstrated [2] that using nanoparticles can dramatically decrease the ignition temperature of the reaction process. Furthermore, mixtures of nano-sized reactant nanoparticles of Ni and Al that undergo an exothermic reaction can be considered as promising nano-energetic materials for a wide range of advanced applications such as localized heat sources for chemical and bio neutralization and disease treatment, environmentally clean primers and detonators, welding, ultrafast fuses, and smart thermal barriers [3].
In this work we focus on molecular dynamics simulation with an embedded-atom method potential [4] of formation by alloying reaction of nanoparticle of B2-NiAl of diameters of ~ 4.5 nm and ~ 9.5 nm with goal to analyze in details phase and structure transformations accompanying of alloying reactions that start from Al-coated Ni precursor nanoparticles with equi-atomic fractions and then to explore the effect of nanoparticle size on the alloying reaction. These systems can be considered as useful models for a highly compacted mixture of Ni and Al nanoparticles or a powder blend which can be approximated by an ensemble of identical Ni spherical nanoparticles surrounded by a continuous Al matrix [5].
2. Discussion and Conclusions
It is found that the alloying reaction in the nanoparticle of diameter of ~ 4.5 nm is accompanied by solid state amorphization of the Al-shell and Ni-core in the vicinity of the interface region. The large driving force for alloying of Ni and Al promotes the solid state amorphization of the nanoparticle because it makes intermixing of the components more probable compared with the crystalline state. A fraction of Al atoms remain segregated to the surface of the nanoparticle since Al has a lower surface energy than Ni. This is followed by the crystallization of the Ni-Al amorphous alloy into the B2-NiAl ordered crystal structure at ~ 900 K. The heat of the transformation of the initial Al-coated Ni nanoparticle into the B2-NiAl ordered nanoparticle can be estimated as ~ - 0.46 eV/at. The B2-NiAl ordered nanoparticle melts at a temperature of ~ 1500 K. The adiabatic temperature for the alloying reaction in the initial Al-coated Ni nanoparticle can be estimated to be below the melting temperature of the B2-NiAl ordered nanoparticle. It is shown that very rapid intermixing and Ni-Al amorphous alloy formation with a reaction self-heating rate ~ 1 K/ps may occur when the reaction is ignited. It is proposed that this takes a place before any formation of the Ni-Al interfacial layer. In this case, the ignition temperature can be as low as ~ 100 K. The alloying reaction will be limited by the degree of pre-heating within the system, which if insufficient, will reduce the interdiffusion rate and hence promote interfacial intermixing. The formation of a thin Ni-Al layer at the interface will produce a strong interfacial diffusion barrier, slowing the alloying reaction within the nanoparticle.
The alloying reaction in the nanoparticle of diameter of ~ 9.5 nm demonstrates the possibility of the formation of a hollow B2-NiAl nanoparticle. In this case, the following stages of the transformation can be observed: intermixing between the f.c.c. Al-shell and f.c.c. Ni-core; amorphization of the shell and then intermixing between the amorphous Al-rich shell and f.c.c. Ni-core; crystallization (in the shell) of the Al-rich amorphous alloy into an Al-rich B2-NiAl and then intermixing between Al-rich B2-NiAl shell and f.c.c. Ni-core. Taking into account that B2-NiAl is a triple-defect compound [6], it is verified that deviation from the exact stoichiometric composition toward the Al-rich composition in the B2-NiAl shell of the nanoparticle is predominantly accommodated by vacancies on the Ni sublattice. Therefore, under certain conditions, interdiffusion between an Al-rich B2-NiAl shell and a f.c.c. Ni-core in such a nanoparticle may result in a flux of the vacancies from the shell into the core and then eventually to form a hollow B2-NiAl nanoparticle.
Acknowledgements
This research was supported by the Australian Research Council through its Discovery Project Grants Scheme. One of us (E.V.L.) wishes to thank the University of Newcastle for the award of a University Fellowship.
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