Isothermal dynamics simulations of spontaneous alloying in a microcluster

Abstract

Dynamics of surface atoms penetrating into microclusters is investigated in connection with the very rapid alloying phenomenon in metal microclusters first discovered by Yasuda, Mori M. Komatsu, K. Takeda, and H. Fujita, J. Electron Microsc. 41, 267 (1992). A new algorithm to simulate the cluster dynamics where isothermal condition is satisfied without adding any stochastic noise is developed in order to suppress gradual temperature rise due to the emission of reaction heat. The dependence of radial diffusion and alloying processes on temperature and negative heat of solution are elucidated separately. The diffusion process obeys an Arrhenius-like law, and unexpectedly, the radial diffusion rate is not very sensitive to the magnitude of negative heat of solution. This fact implies that the rapid diffusion is not a peculiar feature of binary clusters but a universal feature of small clusters. However, the magnitude of negative heat of solution still dominates the alloying process through a preexponential factor. The mechanism causing the rapid radial diffusion is quite different from the mechanism of diffusion in bulk solid, and very active motion of atoms along the surface of cluster plays a crucial role. Based upon our quantitative results, the mechanism with which the surface activity is converted into the rapid radial diffusion is discussed.