Vitrons as Flow Units in Alkali Silicate Binary Glasses

Two volume-changing processes found useful in studying the mechanism of viscous flow in glasses are: (a) stress-induced variation in fissuring near vitrons at their peripheries where the Si–O bonds are tensed and weakened, and (b) distention and modification of all silica network by added oxides. These idealized processes, readily derivable from the vitron concept of pentagonal ring structure in glass [1], are sensitive to both temperature and composition and have previously been found useful [1, 2, 3, 4, 5] for understanding other properties of glasses. Viscous flow of vitrons as units in annealing ranges, and at higher processing temperatures for silica-rich glasses, is here viewed as permitted primarily by the breaking of tensed and weakened Si–O bonds in shell-like stressed tissues surrounding vitrons. For modifier-rich glasses at high temperatures, where the activation energy of flow is known to be low for alkali silicates but slightly higher for alkaline earths, the shearing of cation-to-oxygen bonds may also be involved. The fissures between vitrons and matrix network may provide free volume for cooperative maneuvers among the vitrons. At high processing temperatures, both viscosity and activation energy of flow are known to decrease monotonically and at first with extreme rapidity as the content of modifiers increases. This is understandable if even slight modification materially decreases the local pliability of the matrix network and oxide-volume expansion progressively widens the fissures that must be created at vitron peripheries where the radial contraction of vitron growth potential is checked by tangential Si–O bond stresses. On cooling toward annealing temperatures, the observed rapid increases in viscosity and in activation energy of flow are here ascribed to a general tendency toward narrowing of peripheral fissures, with stronger bonds therein, as the distorted dodecahedral cages return toward symmetry in response to the increases in 0–0 repulsions. Such repulsions increase with extreme rapidity if ionic separations become smaller than equilibrium distances. Near the annealing ranges where it becomes necessary to hold glasses at constant temperatures for appreciable times in order to observe flow of the high-temperature type, it is noticeable that the activation energy of flow increases as the content of modifiers increases. This suggests that the closure of fissures at these very low processing temperatures may be more effective in preventing cooperative maneuvers than in resealing the weak and broken Si–O bonds. Below annealing temperatures, where the readily observed viscosities become somewhat higher while the activation energies decrease very rapidly, it is suggested that the character of flow is changing until finally there are possible only volume relaxations with such small relative movements as are permitted by the bending rather than breaking of bonds.