Study of Diffusion in Sodium Silicate Glass Using Molecular Dynamics Simulation

Using molecular dynamics simulation on sodium silicate glass we have investigated the sodium motion through Voronoi Si and O polyhedrons. The result shows that Na atoms are almost not present in Si polyhedrons, and sodium number density in non-bridging oxygen and free oxygen polyhedrons is larger by 2.5 – 10.5 times than in bridging oxygen polyhedrons. The volume of space occupied by non-bridging oxygen and free oxygen polyhedrons varies from 25 to 66% of total volume of system. The simulation reveals that Na atoms move frequently along non-bridging oxygen and free oxygen polyhedrons and rarely along bridging oxygen polyhedrons. Moreover, they often leave and comeback to starting polyhedron. Such movement is responsible for decreasing the correlation factor F. The system contains unconnected sodium mobile regions which consists of polyhedrons connected with each other by preferential moving paths. With decreasing SiO 2 content the system possesses long diffusion pathways. We have established the expression for sodium diffusion constant D via the rate of hops ξ, average square distance per visiting polyhedron d 2 and factor F. We find that as the temperature or SiO 2 content changes, the variation of F is significantly larger either than ξ or d 2 . Moreover, the dependence of D on F is found linear.