科研之友
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摘要
Molecular dynamics simulation is used to study the behavior of a cubic cavity of 14 atoms in the center of a fcc crystal. Periodic boundary conditions are imposed to eliminate free surface effects. Parameters pertaining to solid argon are presented for illustration. The technique of repeated quenching and relaxation is employed to reveal the equilibrium structure of a superheated solid without the complication of atomic vibrations. The results show that at temperatures below 50 K, the cavity remains empty all the time during simulation. At temperatures above 50 K, atoms move into the cavity. The average equilibrium number of atoms inside the cavity increases with increasing temperature corresponding to a heat of sublimation of 0.01 eV for Ar. This energy is comparable to that of the latent heat of fusion for Ar, 0.0123 eV. The cavity itself can move and change its shape. The system with a nanocavity has a melting temperature depending on the ratio of cavity size to system size. The higher this ratio, the lower the melting point. At all temperatures below the melting point, a cavity with some gas-like atoms is stable and will not dissociate into individual vacancies. On the other hand, isolated vacancies will condense into a cavity unless the temperature is so low that vacancies do not move. These results support Gleiter's contention that the grain boundary in the nanocrystalline materials may be gas-like with a density less than that of the glass-like structure usually assumed for high-angle grain boundaries.
- 出版日期1992-7
