Long Term Crystallization Behavior of Glasses at Temperatures T < Tg

ABSTRACTBoulos et al. [1], derived a mathematical expression predicting the minimum time needed for a glassy system to experience fractional crystallization, not exceeding a few volume percent at temperatures lower than the glass transition temperature, T. The range of applicability of this expression is extended to include crystallization processes governed by either homogeneous or heterogeneous nucleation and by diffusion controlled one-, two-, and three-dimensional crystal growth. This study demonstrates that the Boulos et al. expression is fairly independent of the nucleation, growth habit and dimensionality of the crystallization process. Moreover, the inherent theoretical as well as experimental limitations of the technique are reviewed and discussed.The calculation for evaluating this minimum time limit has been applied for an alkali ferrisilicate alpha-waste reference glass. It is found that, for this system, a barely observable crystallized volume fraction (less than 5 % vol) will be reached at 25°C only after storage for more than 10 years. This time period is certainly beyond the times to be considered for evaluations of geological disposal. In addition some preliminary calculations of the minimum time limit for two types of borosilicate high level waste glass forms (HLW) are presented. These indicate safe time limits of the order of 10–100 years at 450°C, suggesting that crystallization effects cannot be ignored if the assumptions made in the calculations are valid.