Molecular dynamics (MD) simulation is a computational method to understand features of a material, and is performed only by assuming forces between the atoms composing the material.
We are now developing an "MD stencil for parallel computation" in order to facilitate the use of MD simulation for analyses of various kinds of material. By utilizing subroutines developed for the equilibrium analysis group of the stencil we executed an MD simulation of cristobalite, a low density SiO2 polymorph. Though quartz, a typical silicon dioxide, is used in various application fields due to its excellent thermal characteristics, in the high temperature phase quartz suffers from a degradation of thermal characteristics caused by the phase transition of the cristobalite. As the cristobalite in the high temperature phase has scarcely been studied either experimentally or theoretically, we carried out a time-consuming MD simulation of the cristobalite from 300K to 1,800K, and determined values of the elastic moduli. By this study we succeeded in identifying the process of the phase transition (Fig. 7-3) and clarified the mechanism of the mysterious feature of cristobalite. The feature of this substance exists in the negative Poisson ratio ("Poisson ratio" is the shrinking ratio in the perpendicular direction to the direction of pull) (Fig. 7-4). When one pulls an elastic string its thickness is reduced. Therefore, the Poisson ratio of rubber is negative, but substances with negative Poisson ratio are not so common as those with positive Poisson ratio. Through this MD simulation we found that the mechanisms of the negative Poisson ratio of cristobalite observed over a wide range of temperature are not same in the low temperature phase and in the high temperature phase (Fig. 7-5). |