The influence of Fe2O3 and B2O3 additions on the thermal properties, crystallization kinetics and durability of a sodium aluminum phosphate glass

A sodium aluminum phosphate glass, identified as a potential encapsulating phase for radioactive wastes, has been noted to be thermally unstable, crystallizing upon heat treatment to yield a variety of phases which may be detrimental to the longer term properties of the final wasteform. In an attempt to improve the thermal stability and overall resistance of this glass to crystallization, the influence of various oxide additions including Fe2O3 and B2O3 has been investigated. It has been shown that quite small additions of B2O3 are extremely effective in improving the stability, with ⩾2 mol% B2O3 almost completely suppressing crystallization whilst only marginally affecting the excellent chemical durability of the ternary glass, at least at the lower B2O3 concentrations. In contrast, addition of Fe2O3 or B2O3/Fe2O3 mixtures does not lead to significant improvements in the thermal stability, although durability relative to that of the ternary composition is improved significantly with increasing Fe2O3 content.