Molecular dynamic calculations of glass structure and diffusion in glass

Abstract

During the past few years several studies have been made of the structure of network forming glasses using the method of molecular dynamics (MD). In general, the MD structures closely resemble the random network structures first proposed by Zachariasen. For example, silica glass has a cage-like structure resembling a randomly distorted high-cristobalite. In simulated sodium silicate glasses, Na ions create non-bridging oxygen atoms and occupy interstitial sites with no well-defined coordination but a most probable coordination number of five. Vitreous B 2O 3 contains planar regular triangles linked at apices to form a random network of ribbons and sheets. The MD structure of vitreous B 2O 3 does not contain boroxyl rings. In sodium borosilicate glasses the trigonal to tetrahedral conversion of boron with the addition of sodium was observed to agree with the NMR results. In sodium alumino-silicate glass simulations, aluminum prefers four coordination, regardless of the concentration of Na 2O. The mechanism is discussed. The MD structure can be used to interpret many physical properties. All of the simulated glasses undergo a kinetic glass transition as they are cooled. Glass formation occurs when the system reaches a temperature at which the network forming cations and anions cease to diffuse on the time scale of the MD runs. This occurs at temperatures corresponding to viscosities of 1 P or less diffusion constants of around 10 −5 cm 2/s. Hence, the MD glasses have very high fictive temperatures. Sodium and other alkali ions, however, can be observed to diffuse at much lower temperatures. “Experiments” can be performed on the computer simulated network glasses and liquids.