Sintering and deposition of homo- and heteronanoparticles of aluminum and nickel on aluminum (100) substrate

Abstract

Nanoparticles are very attractive materials owing to their high chemical reactivity compared to conventional micron-sized particles due to their high surface area to volume ratio. In light of this, nanoparticles may provide enhanced energy release rates for explosive and propellant reactions. However, their performance depends strongly on their surface structure. Thereby, nanoparticle coalescence plays an important role as it determines the resultant structure of the active sites where for example catalytic reactions actually take place, i.e. facets, edges, vertices or protrusions. With this in mind, we conduct molecular dynamics simulations to investigate coalescence of two identical nanoparticles of Al and Ni (homo spheres) and two different nanoparticles Al-Ni (hetero spheres) and their deposition on an Al (1 0 0) substrate at various temperatures from 200 to 800 K using the embedded atom method. Radial distribution function, x-y plane projection, collapsing and spreading indexes are calculated to characterize the coalescence and surface deposition process. Our simulation results show that the degree of coalescence and deposition are strongly temperature dependent. The deposition rate increases with the temperature while the sintering and deposition of hetero nanoparticles is significant at lower temperatures than melting. This trend is important to develop a lead-free metal composite materials for the electrical interconnect material due to their low processing temperature.

Introduction

Nanoparticles are defined as an agglomeration of atoms or molecules with at least one dimension less than 100 nm [1], [2]. At this scale, attractive physical phenomena are observed, such as quantum confinement in semiconductor particles or surface plasmon resonance in some metal particles. This size effect is explained by the high surface area per weight than larger particles which causes them to be more reactive to some other molecules. In nano-scale materials, the percentage of atoms on the surface of a material becomes important and it depends on the size, shape, and composition of nanoparticles, which increasing their surface energy. So they are often able to react very quickly. This makes them useful for many practical applications; in biomedical, optical, and electronic fields. For example, nanoparticles are being considered for use as drug delivery vehicles, as radiation resisting materials, to produce patterned lines and as catalysts in chemical reactions [3], [4], [5], [6], [7], [8], [9], [10], [11]. However, during processing or usage, nanoparticles have a strong tendency to agglomerate, coalesce and sinter even below the melting temperature, which leads to significant changes in behavior and performance. Prediction of sintering and coalescing behavior at various temperatures is of great importance for practical engineering. However, in catalysis, the coalescence is detrimental as it is responsible for a reduction in the electrochemically active surface area that reduces cell performance in the proton exchange membrane fuel cells [12], [13]. In other situations, as for example in the fabrication of thick film conductors, the ability to enhance sintering is critical to producing high conductivity lines at low temperatures and would, therefore open the door to the development of many novel devices.

As mentioned above, nanoparticles are widely used in various applications such as solar cells [14], photodetectors [15], disease recognition [16], drug delivery [17], and sensing [18]. In these applications, some nanoparticles are easily heated which induce them a strong tendency to coalesce and sinter, even at much lower temperature than microparticles [19], which leads to significant changes in behavior and performance of the nanoparticles and might be driven by more than one mechanism such as surface diffusion, grain boundary diffusion, lattice diffusion, mechanical rotation, plastic deformation, and evaporation–condensation [20], [21], [22]. However, making good use of the approach and coalescence of nanoparticles is a great method of bottom-up fabrication of micro- and nano-sized electrical devices [23], [24], [25].

The coalescence of nanoparticles has mainly attracted considerable attention in recent years both theoretically and experimentally [21], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]. Above melting temperatures, the situations of the coalescence can be classified into two categories, called welding and sintering [26], [27], [28], [29], [30]. Nanoparticle sintering have been reported before numerically for a number of materials, such as Au [40], [41], Cu [41], [42] and Ni [43] and experimentally for ZrO₂ [44]. The authors found that sintering is characterized by three stages. The first is neck formation between two particles followed by a rapid increase in the neck radius. The second is gradual increase in the neck radius and the third is densification of the structure. In addition, they exhibited enhanced sintering rates due to higher diffusion coefficients for nano-sized materials and a large driving force of sintering, which is caused by high curvature of the nanoparticles surfaces.

On the other hand, Groza et al. [45] and Yeadon et al. [46] found that neck sizes were well below those were predicted from conventional scaling models. They concluded the reason was the increased curvature that occurred as the particle size decreased. Schwesig et al. [26] studied the evolution of density during the sintering process of nanoparticles and observed that density fluctuated on the micrometer scale. Similarly, Yeadon et al. [47] demonstrated that the nanoparticles reoriented upon heating. Asoro et al. [27] summarized the possible reasons for the differences in sintering of nanoparticles between models and experimental results: (1) unique defect structures in nanoparticles (twins and facets); (2) enhanced diffusivity due to size effects; and (3) enhanced, localized agglomeration present in nanoparticles. It is obvious that the nanoparticles show quite different characteristics from macroparticles when melt.

The current study is a systematic atomistic investigation suggesting intuitive mechanisms that govern fcc metallic nanoparticles coalescence. Aluminum and Nickel nanoparticles are widely used and investigated, primarily because of their increased reactivity as compared with conventional micron-sized particle. Their performance depends strongly on surface structure; therefore, nanoparticle coalescence can play an important role, as it determines the resultant structure of the active sites where reactions (e.g. catalysis) actually take place, i.e. facets, edges, vertices or protrusions. Furthermore, NiAl aluminate systems are used in reactive welding applications. With this in mind, we investigate the coalescence of two identical (homo) nanoparticles of Al and Ni and two different (hetero) nanoparticles of Al-Ni and their deposition on an Al substrate at various temperatures from 200 to 800 K. Our studies are achieved using molecular dynamics (MD) simulations in conjunction with embedded atom method (EAM) which is discussed in the Computational Methodology section. Coalescence index, pair-correlation distribution function and spreading index are calculated to characterize the coalescence and surface deposition process. A comprehensive description of the underlying processes involved are presented and discussed in the Results section. We end this paper with a summary of our conclusions.