Melting Of Confined Metallic Nanowires

During the past decades, one-dimensional materials, which are represented by carbon nanotubes and nanowires, have gained great attention. Especially, owing to the specific physical and chemical properties, nanowires have tremendous potential applications in the field of photoelectric devices, catalysis, biotechnology, defense technology, energy storage, composite materials and so on. During the development of nanowires, how to maintain the stability of their structure and properties is the most important problem to solve. In recent years, researchers have found that encapsulating nanowires into confined space can not only improve the stability of their structures and protect them from the oxygen and water vapor in the air but also gain composites with superior properties. The interaction from the confined space along with their size and shape can exert great influence on the inner nanowires. To study the melting of confined nanowires is of great significance in enriching and improving the study focusing on the properties of nanomaterials, expanding the application of nanowire-related composites and developing advanced materials processing technology.In this paper, molecular dynamics simulations are performed to study the thermal stability and the melting of two kinds of representative metllic nanowires, Al and Fe nanowires, in confined space.It has been found for the first time that three intriguing phenomena will appear during the whole heating process of Al nanowires in carbon nanotubes:absence of visible diffusion of inner layers, complete disappearance of inner layers and the recovery of inner layers, all of which depend on the diameters of their own and the lengths of the nanotubes. The van der Waals potential well in the carbon nanotubes along with the difference between interactions of the carbon walls and the inner atoms plays the most important role in the abnormal melting behavior of the confined Al nanowires.Huge differences exist in the thermal stability of three kinds of Fe nanowires along [100], [110] and [111] crystallographic orientations, respectively. It is found that [111] Fe nanowires have the best structural and thermal stability because of their lowest surface energy and largest number of neighbor atoms of the surface atoms. This results are in good agreement with the experimental observation that spontaneous growing Fe nanowires are [111] orientational favorable. When we encapsulate the nanowires into confined walls, the axial pressure will enhance the confined effect of the walls on the inner nanowires, resulting in the increasing melting temperatures with the increment of the axial pressures. However, there exits a threshold value of the axial pressure, about 3 GPa from the simulation results, below which the confined Fe nanowires can maintain their structural and thermal stabilities.We compare the influence of two kinds of confined space with the same shape and size, confined walls and carbon nanotubes, on thermal stability and the melting behavior of the inner nanowires for the first time in this paper. The results show that Fe nanowires in carbon nanotubes have the best structural stability and the carbon nanotubes have the strongest confined effect compared to the confined walls. These findings vividly illustrate that the influence of the carbon nanotubes on the melting of the inner nanowires is not only due to the size limitation but the arrangement of the carbon atoms and the van der Waals potential well in the carbon nanotubes.