Showing papers by "Walter Kob published in 1996"
TL;DR: Using molecular-dynamics computer simulations, it is demonstrated that the cooling-rate dependence of microscopic quantities is significantly more pronounced than the one of macroscopic properties, thus demonstrating that the used potential is a good model for silica glass.
Abstract: 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 ${\mathit{T}}_{\mathit{b}}$(t), we cool the system linearly in time, ${\mathit{T}}_{\mathit{b}}$(t)=${\mathit{T}}_{\mathit{i}}$-\ensuremath{\gamma}t, where \ensuremath{\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 ${\mathit{T}}_{\mathit{g}}$ is in accordance with a logarithmic dependence on \ensuremath{\gamma}. The enthalpy, density, and thermal expansion coefficient for the glass at zero temperature decrease with decreasing \ensuremath{\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 \ensuremath{\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. \textcopyright{} 1996 The American Physical Society.
588 citations
TL;DR: In this article, a two component Lennard-Jones model was used to investigate how the glass transition and the properties of the resulting glass depend on the cooling rate with which the sample has been quenched.
Abstract: Using molecular dynamics computer simulations we investigate how the glass transition and the properties of the resulting glass depend on the cooling rate with which the sample has been quenched. The system we study is a two component Lennard‐Jones model which is coupled to a heat bath whose temperature is decreased from a high temperature, where the system is a liquid, to zero temperature, where the system is a glass. The temperature Tb of this heat bath is decreased linearly in time, i.e. Tb=Ti−γt, where γ is the cooling rate, and we study the cooling rate dependence by varying γ over several orders of magnitude. In accordance with simple theoretical arguments and with experimental observations we find that the glass transition, as observed in the specific heat and the thermal expansion coefficient, becomes sharper when γ is decreased. A decrease of the cooling rate also leads to a decrease of the glass transition temperature Tg and we show that the dependence of Tg on γ can be rationalized by assuming that the temperature dependence of the relaxation times of the system is given by either a Vogel–Fulcher law or a power law. By investigating the structural properties of the glass, such as the radial distribution functions, the coordination numbers and the angles between three neighbor‐sharing particles, we show how the local order of the glass increases with decreasing cooling rate. The enthalpy H and the density ρ of the glass decrease and increase, respectively, with decreasing γ. By investigating the γ dependence of clusters of nearest neighbors, we show how the cooling rate dependence of H and ρ can be understood from a microscopic point of view. Furthermore we demonstrate that the frequency of icosahedral‐like structures is decreasing with decreasing cooling rate. We also show that the spectrum of the glass, as computed from the dynamical matrix, shows a shift towards higher frequencies when γ is decreased. All these effects show that there is a significant dependence of the properties of glasses on the cooling rate with which the glass is produced.
141 citations
TL;DR: It is shown that in simulations of the dynamics of this strong glass former surprisingly large finite size effects are present, and it is demonstrated that the relaxation times of the incoherent intermediate scattering function and the time dependence of the mean squared displacement are affected.
Abstract: We present the results of a molecular dynamics computer simulation in which we investigate the dynamics of silica. By considering different system sizes, we show that in simulations of the dynamics of this strong glass former surprisingly large finite size effects are present. In particular, we demonstrate that the relaxation times of the incoherent intermediate scattering function and the time dependence of the mean squared displacement are affected by such finite size effects. By compressing the system to high densities, we transform it to a fragile glass former and find that for that system these types of finite size effects are much weaker.
141 citations
01 Sep 1996
TL;DR: In this paper, the dependence of the properties of amorphous silica on the cooling rate with which the glass has been produced was studied and it was shown that the density, the glass transition temperature, the radial distribution function and the distribution of the size of the rings depend on cooling rate.
Abstract: We use molecular dynamics computer simulations to study the dependence of the properties of amorphous silica on the cooling rate with which the glass has been produced. In particular we show that the density, the glass transition temperature, the radial distribution function and the distribution of the size of the rings depend on the cooling rate.
10 citations
TL;DR: In this article, a two component Lennard-Jones system is coupled to a heat bath whose temperature is decreased from a high temperature, where the system is a liquid, to zero temperature, and the resulting glass is a glass.
Abstract: Using molecular dynamics computer simulations we investigate how the glass transition and the properties of the resulting glass depend on the cooling rate with which the sample has been quenched. This is done by studying a two component Lennard-Jones system which is coupled to a heat bath whose temperature is decreased from a high temperature, where the system is a liquid, to zero temperature, where the system is a glass. The temperature $T_b$ of the heat bath is decreased linearly in time, i.e. $T_b=T_0-\gamma t$, where $\gamma$ is the cooling rate. In accordance with simple theoretical arguments and with experimental observations we find that the glass transition, as observed in the specific heat and the thermal expansion coefficient, becomes sharper when $\gamma$ is decreased. A decrease of the cooling rate also leads to a decrease of the glass transition temperature $T_g$ and we show that the dependence of $T_g$ on $\gamma$ can be rationalized by assuming that the temperature dependence of the relaxation times of the system is given by either a Vogel-Fulcher law or a power-law. By investigating the structural properties of the glass, such as the radial distribution functions, the coordination numbers and the angles between three neighbor-sharing particles, we show how the local order of the glass increases with decreasing cooling rate. The enthalpy and the density of the glass decrease and increase, respectively, with decreasing $\gamma$. By investigating the $\gamma$ dependence of clusters of nearest neighbors, we show how these observations can be understood from a microscopic point of view. We also show that the spectrum of the glass, as computed from the dynamical matrix, shows a shift towards higher frequencies when $\gamma$ is decreased. All these effects show that there is a significant
6 citations
TL;DR: In this paper, the authors present the results of a molecular dynamics computer simulation of a supercooled binary Lennard-Jones mixture, and show that the ideal version of the mode-coupling theory of the glass transition is able to give a good qualitative description of the dynamics of this system.
Abstract: We present the results of a molecular dynamics computer simulation of a supercooled binary Lennard-Jones mixture. By investigating the temperature dependence of the diffusion constant and of the intermediate scattering function, we show that the ideal version of the mode-coupling theory of the glass transition is able to give a good qualitative description of the dynamics of this system. Using the partial structure factors, as determined from the simulation, as input, we solve the mode-coupling equations in the long time limit. From the comparison of the prediction of the theory for the critical temperature, the exponent parameter, the wave-vector dependence of the nonergodicity parameters and the critical amplitudes with the results of the simulation, we conclude that the theory is also able to predict correctly the non-universal properties of the dynamics of a supercooled simple liquid.