薄膜の溶融・凝固過程における表面形状と微視構造変化の分子動力学シミュレーション

概要

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Changes in surface shape and microstructure of a thin film induced by heating and cooling are simulated by means of molecular dynamics method. A simple three-dimensional model of iron atoms as an example of bcc metal with periodic boundary conditions is employed to identify the temperature for melting and crystallization and the rate by use of Finnis-Sinclair potential function. Simulations during heating and cooling processes of a thin film are carried out with a model which has free surface and fixed boundary in one direction and periodic boundaries in the other two directions being a model of optical disk of phase-change type used in DVD system. A rippled shape of the surface is observed when a concentrated part of the surface is heated to be molten state, while planer surface is achieved by flat heat source. Such behavior depends on the difference in the surface tension and surface energy. The surface tension decreases as increasing temperature, and the tension of solidified crystal reveals to be larger than that of liquid. The potential energy near the molten surface gives higher value than that of the bulk, and it may minimize the surface area. Amorphous structure is obtained by rapid cooling of the molten region while it becomes crystallized by moderate cooling, which causes the small amorphous spot on the surface corresponding to a memory digit, and the digit vanishes by the subsequent reheating.
1998-10-25