Physical Properties of Multicomponent Borosilicate Glasses: a Study of Viscosity, Processing and Durability.

The physical and chemical properties of multicomponent borosilicate glasses, in particular viscosity and chemical durability, are sensitive functions of their composition. In order to understand and systematize the effects of various oxide components on the properties of the glass it is necessary to analyze their effect on the glass structure, which is based on a three-dimensional network of silica tetrahedra. The particular context in which these properties are explored in this dissertation is the development of glasses for nuclear waste immobilization. This requires the development of models of compositional dependence which would pave the way towards preparing glasses with high durability over long periods of time, and at the same time, sufficiently low viscosities for rapid production at moderate processing temperatures (around 1100 centigrade); furthermore, appropriate modelling can define the composition range and the amount of control which is necessary to ensure that the melt viscosity and the glass durability will remain within the required range. The viscosity model is based on a correlation between the viscosity and the amount of non-bridging oxygens in the glass structure. Non-bridging oxygens lead to a looser structure and thus a lower viscosity. Their number is related to the relative proportions of glass forming cations and network modifiers. Established glass melt viscosity theories such as the Arrhenius equation and the Fulcher equation to take account of the temperature dependence are employed to develop a quantitative, predictive model for the viscosities of these glass melts. The process model uses first order error analysis to determine the overall compositional errors in the continuous melter process for the production of nuclear waste glass. This approach evaluates the use of frit as the primary glass former and determines the overall variability in glass composition. The section on glass durability comprises a comparison between long term extrapolation based on laboratory tests on microtektites, a class of natural glasses, with the observed extent of their corrosion in nature. Based on these tests, one can validate the leach test methodology currently used in the durability testing of nuclear waste glasses.