Al原子在Ni基衬底表面的扩散及团簇的形成

NiAl纳米颗粒具有较高的能量密度和良好的高温力学性能,铝吸附原子在不同镍基表面上的扩散行为与不同扩散机制对铝在镍基表面沉积生长的影响有待进一步阐明.本文通过采用肘弹性带和分子动力学结合嵌入原子势的方法,系统地研究了单个铝吸附原子在镍基表面的扩散行为和纳米颗粒团簇在十面体(DEC)、立方八面体(CUB)和二十面体(ICO)结构上的生长.研究结果表明:Al吸附原子在三种Ni基底上表面扩散的交换与跳跃两种机制,最低的Ehrlich-Schwoebel(ES)势垒为0.38 eV(交换CUB{111}→100})、0.52 eV(交换DEC{111}→100})和0.52 e V(跳跃ICO{111}→111}),从{111}面扩散到{100}面主要以交换机制为主,而相邻两个{111}面之间的扩散则以跳跃机制为主.沉积的铝原子首先倾向于扩散到台阶边缘和顶点附近.随着Al原子数量的增加,沉积的Al原子开始聚集.对于Ni团簇上的Al原子,在较低温度下在镍基底表面沉积Al原子,可以获得良好的Ni核/Al壳结构.对于二十面体结构基底,其对应的核壳团簇的缺陷数最小,随后为十面体结构和八面体结构.随着生长温度的增加NiAl纳米粒子的表面逐渐开始合金化.

Diffusion of Al atoms and growth of Al nanoparticle clusters on surface of Ni substrate

NiAl nanoparticles possess high-energy density and good mechanical properties at elevated temperatures,and are considered as an important material.However,the differences in the diffusion behavior of Al adsorbed atoms on different Ni substrate surfaces and the effects of different diffusion mechanisms on the deposition growth of Al atoms on the Ni substrate surface are highly desired to be clarified.Therefore,in the present work,the diffusion behavior of single Al adsorbed atoms and nanoparticle cluster growth on the Ni substrate surface of decahedral(DEC),cuboctahedral(CUB)and icosahedral(ICO)structures are systematically studied by molecular dynamics(MD)throuh analyzing the embedded atom potentialand using the nudged elastic band method.The diffusion barriers of Al adsorbed atoms on three different Ni substrates are calculated by nudged elastic band methodand analyzed,showing that the diffusion barrier is greatly affected by the smoothness of the step edge and the atomic coordination number of substrate as well.The diffusions of Al adsorption atoms on the surfaces of three Ni substrates are realized by two mechanisms,namely exchanging or hoping,and the lowest Ehrlich-Schwoebel(ES)barrier is 0.38 eV for exchange CUB{111}→{100},0.52 eV for exchange DEC{111}→{100},and 0.52 eV for hoping ICO{111}→{111}.The exchanging mechanismsupports Al adatoms diffusing from{111}to{100}facet on the three Ni substrates,while the diffusion between two adjacent{111}facets is mainly driven by the hoping mechanism.On this basis,atom-by-atom growth MD simulation is used to study the structure of the Ni-Al cluster.The deposited Al atoms first tend to diffuse near the edges of the steps and the vertices.The deposited Al atoms begin to aggregate into islands with the increase of their number.For Al atoms on the Ni cluster,a good Ni-core/Al-shell structure can be obtained by depositing Al atoms on the surface of Ni substrate at lower temperatures.In this core-shell structure,Al atoms have a larger surface energy and atom radius compared with Ni atoms.For the ICO substrate,the corresponding defect number of core-shell clusters is smaller than for the CUB and the DEC substrate,which is in good agreement with the diffusion behavior of Al adsorbed atoms on the Ni substrate cluster surface.The surface of Ni-Al bimetal is gradually alloyed with the increase of growth temperature.This study provides a good insight into the diffusion and growth of Al adsorbed atoms on Ni substrates surface on an atomic scale.