Cooling-rate effects in amorphous silica: A computer-simulation study

Using molecular-dynamics computer simulations we investigate how in silica the glass transition and the properties of the resulting glass depend on the cooling rate with which the sample is cooled. By coupling the system to a heat bath with temperature T-b(t), we cool the system linearly in time, T-b(t) = T-i-gamma t, where gamma is the cooling rate. In qualitative accordance with experiments, the temperature dependence of the density shows a local maximum, which becomes more pronounced with decreasing cooling rate. We find that the glass transition temperature T-g is in accordance with a logarithmic dependence on gamma. The enthalpy, density, and thermal expansion coefficient for the glass at zero temperature decrease with decreasing gamma. We show that also microscopic quantities, such as the radial distribution function, the bond-bond angle distribution function, the coordination numbers, and the distribution function for the size of the rings, depend significantly on gamma. We demonstrate that the cooling-rate dependence of these microscopic quantities is significantly more pronounced than the one of macroscopic properties. Furthermore, we show that these microscopic quantities, as determined from our simulation, are in good agreement with the ones measured in real experiments, thus demonstrating that the used potential is a good model for silica glass. The vibrational spectrum of the system also shows a significant dependence on the cooling rate and is in qualitative accordance with the one found in experiments. Finally we investigate the properties of the system at finite temperatures in order to understand the microscopic mechanism for the density anomaly. We show that the anomaly is related to a densification and subsequent opening of the tetrahedral network when the temperature is decreased, whereas the distance between nearest neighbors, i.e., the size of the tetrahedra, does not change significantly.