Ag-Cu双金属团簇中Ag原子偏析行为的分子动力学研究

Molecular dynamics investigation on the Segregation behaviors of Ag atoms in Ag-Cu bimetallic clusters

采用恒温分子动力学方法系统模拟研究了不同尺寸不同组分的Ag-Cu双金属团簇的退火过程.分析低温退火结构可得团簇中Ag原子的偏析行为:在Ag原子所占比率较少时,Ag原子全部占据在团簇表面;随着Ag原子数的增多,直到Ag和Cu的比率接近时,绝大多数Ag原子仍占据在团簇表面;这与实验观测Ag-Cu混合团簇中Ag原子的偏析行为完全一致.通过细致研究Ag-Cu双金属团簇中Ag原子的偏析行为与团簇尺寸、组分和温度的关系发现:当Ag原子所占比重明显少于Cu原子时,在各不同尺寸下,Ag原子偏析温度点均高于熔点,即在熔点以上一定温度范围内仍会出现Ag原子的偏析现象;当Ag原子所占比重明显多于Cu原子时,在各不同尺寸下,体系偏析温度点均低于团簇熔点;对于较大尺寸团簇,在Ag原子所占比重接近于Cu原子时,体系偏析温度点与团簇熔点相同.

The annealing processes of the Ag - Cu bimetallic clusters with different sizes and compositions are systematically investigated by using the constant temperature molecular dynamics simulation method. Through an- alysing the annealing structures, the segregation behaviours of Ag atoms in the bimetallic clusters can be deduced as follows ： When the proportion of Ag atoms is greatly less than Cu, Ag atoms all occupy the sites on the surface of the cluster; With the increase of the number of Ag atoms until the proportion of Ag and Cu are close to each other, the vast majority of Ag atoms are still on the surface, and this is consistent with the recent experimental observations of the segregations of Ag atoms in the Ag - Cu clusters. Through analyzing the relationships between the segregation behaviors of Ag atoms in the bimetallic clusters and the sizes, compositions as well as the temper- atures of the bimetallic clusters, one can deduce that： When the proportion of Ag atoms is greatly less than Cu, the segregation temperature is always higher than the melting temperature of the cluster at all sizes, which means that the segregation behaviours of Ag atoms still occur at a temperature range higher than the melting point; When the proportion of Ag atoms is greatly larger than Cu, the segregation temperature is always lower than the melting temperature of the cluster at all sizes; When the proportion of Ag and Cu are close to each other for the cluster with comparatively large size, the segregation temperature is equal to the melting temperature of the cluster.