Effect of chemical composition on viscosity of oxide glasses

The dependence of viscosity on composition of silicate melts is described by means of a structure based model. It distinguishes between network forming oxides, such as SiO2, and network modifying ones, such as Na2O. The structural assumption is that there are two different types of bonds in the system: rigid ones like Si–O–Si and floppy bonds like Si–O−Na+. The activation energy for shear viscosity is reduced in the presence of floppy bonds. Therefore, even low concentrations of modifying oxides reduce considerably the viscosity of the melt, a lubricant effect.