Size Dependence of Melting and Crystallization in Ni Nanoclusters

Yue Qi,# Tahir Cagin,# William L. Johnson,@ and William A. Goddard III#*

#Materials and Process Simulation Center, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA

@Keck Laboratory of Engineering Materials, California Institute of Technology, Pasadena, California 91125, USA

Abstract

We studied melting and freezing of Ni nanoclusters, containing up to 8007 atoms (5.74 nm diameter), using molecular dynamics (MD) with the quantum Sutten-Chen (Q-SC) many-body force field. In particular, we examine how the melting temperature and heat of fusion depend on cluster size, including premelting prior to the transition. Then we examine the local structures formed upon solidification. The nature of the premelted and supercooled regions are analyzed by using local cluster analysis.

We find that the nanoclusters melt via surface processes, leading to Tm,N = Tm,bulk - a N-1/3, which drops from Tm,bulk =1760K to Tm,336 = 980K. Cooling from the melt leads first to supercooled clusters with icosahedral local structure. For N > 400 the supercooled clusters transform to FCC grains, but for smaller N we find a glassy structure with substantial icosahedral character.

 

  1. Average displacements for the atoms in various spherical shells, as a function of the distance from the center of the cluster. This cluster contains 1004 atoms, and the melting temperature is 1160K.

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  2. Dependence of melting temperature on cluster size, showing a linear function of N(-1/3).