Molecular dynamics simulation of the alloying reaction in Al-coated Ni nanoparticle

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Abstract

Using molecular dynamics simulation in combination with the embedded atom method we analyze the alloying reaction in Al-coated Ni nanoparticle with equi-atomic fractions and diameter of ∼4.5 nm. The alloying reaction in the nanoparticle 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 much easier compared with the crystalline state. Though, a fraction of Al atoms is retained to be segregated to the surface of the nanoparticle because Al has a lower surface energy than Ni. Then, the crystallization of the Ni–Al amorphous alloy into the B2-NiAl ordered crystal structure is observed. The energy release from 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 the reaction self-heating rate ∼1 K/ps occurs when the reaction is ignited before the formation of thin Ni–Al layer will take place at the interface. In this case, the ignition temperature can be as low as ∼100 K. The alloying reaction will occur much more slowly if thermal explosion is not ignited at the stage of Ni–Al layer formation at the interface, and the reaction time will be at least one-two orders of magnitude greater. The formation of the thin Ni–Al layer at the interface results in the appearance of a stronger interfacial diffusion barrier. The barrier slows down the alloying reaction in the nanoparticle.

Introduction

Reaction synthesis (combustion synthesis) has received considerable interest as an economic route for the production of nickel aluminide intermetallic compounds (in particular B2-NiAl) known as the next generation of high-temperature structural and oxidation-resistant materials [1], [2], [3], [4], [5]. This technique utilizes exothermic reaction processing, which reduces the time and energy associated with conventional synthesis methods – e.g. intensive sintering processing [4]. The advantages of reaction synthesis also include the potential for rapid near-net shape processing and phase pure products. Furthermore, the reaction-synthesized products have been reported to possess better mechanical and physical properties [6].

Reaction synthesis is a process wherein once the reactants are ignited they spontaneously transform to products in an exothermic reaction in a very short processing (reaction) time [3]. Experimentally, the self-sustaining reaction can be started either by igniting the sample at one end (combustion wave propagation mode) or by heating the whole volume of the sample to the ignition temperature Tig (thermal explosion or simultaneous combustion mode). An important parameter in an experimental reaction synthesis process is the adiabatic temperature Tad. This temperature represents the upper limit of the temperatures achieved by self-heating during a particular exothermic reaction whose rate can be high enough to assume adiabatic conditions [3]. Therefore, it is assumed that all of the heat of formation raises the temperature of the reacting system to the adiabatic temperature with zero heat losses. The ignition and adiabatic temperatures and the reaction time are fundamental processing parameters which help to clarify and classify reactions mechanisms; they can show the expected physical state of the reactants and product during a particular reaction [1], [2], [3], [4], [5].

In practice, the synthesis of bulk nickel aluminides is usually achieved by using conventional coarse-grained powders whose grain sizes are of the order of micrometres [1], [2], [3], [4], [5]. However, advantages have been reported of using nanometre-sized particles to synthesize the intermetallic NiAl [7]. It has been shown [7] that due to the physical and chemical characteristics of nanoparticles (especially their high stored energy and high chemical activity) the reaction mode and mechanism are distinctly different from those when conventional coarse-grained powders are used. In particular, it was demonstrated [7] that using nanoparticles can dramatically decrease the ignition temperature of the reaction process. Furthermore, due to the significant brittleness of intermetallic NiAl, it is considered that refinement of the grain size to the nanometre level could be a promising way to overcome a limitation of applying the material industrially, because enhancement of hardness and strength can be expected with nano-structures according to the Hall–Petch relation [8]. Moreover, mixtures of nanosized reactant nanoparticles of Ni and Al that undergo an exothermic reaction can be considered as promising nanoenergetic materials for a wide range of advanced applications, including localized heat sources for chemical and bio neutralization and disease treatment, environmentally clean primers and detonators, welding, ultrafast fuses, and smart thermal barriers [9].

Nowadays, the combination of first principles-based many body potentials (developed, for example, by making use of the embedded atom method (EAM) [10]) and atomistic simulation techniques such as molecular dynamics (MD) allow the capturing of the complex physical and chemical processes occurring in nanoenergetic materials with the necessary accuracy [9]. This powerful tool can provide physical insight into understanding phenomena on an atomic scale which is crucial for the development of next generation nanoenergetic materials with functional properties. Therefore, it is not surprisingly that MD efforts have recently started to focus on Ni–Al nanomaterials [9], [11], [12], [13], [14]. In [9], [11], shock-induced melting and exothermic alloying reaction in Ni/Al nanolaminates with atomic fractions close to the stoichiometry of Ni3Al were simulated by MD with the EAM potential proposed in [15]. It was observed [9], [11] that the final reaction product is very similar to compressed Ni3Al liquid. In MD simulations [12], [13] with a tight-binding potential based on the second-moment approximation to the electronic density of states [16], demixing phenomena in NiAl nanoparticles after melting [12] and the mechanical response of Al-core/Ni-shell nanoparticles on melting of the Al-core [13] were investigated. In [14], MD simulations with an EAM potential [17] were used to investigate the kinetic sintering of initially liquid Al and solid Ni nanoparticles with an atomic ratio of unity. It was shown [14] that under adiabatic conditions, a liquid Al nanoparticle first coats the solid Ni nanoparticle and alloying is only completed after the Ni nanoparticle has melted. The effect of nanoparticle size on sintering time, adiabatic temperature and liquid solution formation were analysed [14].

In the present work, we use MD simulations in combination with EAM potential to study the alloying reaction in an Al-coated Ni nanoparticle with equi-atomic fractions and a diameter of ∼4.5 nm. This system can be considered as a useful model 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 [2].

Section snippets

Details of the simulation

The interatomic interactions are described by an EAM potential developed by Mishin et al. [15]. The EAM potential was constructed using experimental data and a large set of ab initio structural energies. This potential provides an accurate description of basic lattice properties, thermal expansion, diffusion, and equations of state for Ni, Al and different phases of the Ni–Al system, especially for the intermetallic compound B2-NiAl [9], [11], [15].

An initial model of the spherical Al-coated Ni

Results and discussion

As can be seen in Fig. 1b, just after static relaxation of the initial Al-coated Ni nanoparticle model, the position of the interface between the Ni-core and Al-shell can be approximated by a spherical layer with radius ∼16.5 Å. Actually, because of the large lattice parameter mismatch, the interface here is strongly distorted with quite a wide range of radii with the middle at a radius ∼16.5 Å. This method results in a quite thin Al-shell that exists in what might be considered a very strongly

Conclusions

The alloying reaction in Al-coated Ni nanoparticle with equi-atomic fractions and diameter of ∼4.5 nm has been studied by using molecular dynamics simulation in combination with the embedded atom method.

We found that the alloying reaction in the nanoparticle is accompanied by solid state amorphization of the Al-shell and Ni-core in the vicinity of the interface region. The solid state amorphization of the nanoparticle, due to the large alloying driving force of Ni and Al, makes intermixing of

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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