The effects of phosphorus pentoxide additions on the thermal, rheological and structural properties of sodium borosilicate glass

This research has characterized the effects of P2O5 additions on the physical, chemical, thermal, rheological properties and the structure of SiO2-B2O3-Na2O glasses relevant to radioactive waste immobilization. Properties analysed include high-temperature viscosity, glass transition temperature (Tg) and structure. X-ray diffraction (XRD) confirmed a solubility limit of between 3.0 and 4.0 mol% P2O5 for the onset of crystallinity with formation of Na3PO4 and Na4P2O7 in annealed bulk glasses; and between 4.0 and 5.0 mol% P2O5 in unannealed (quenched) glasses. This difference in behaviour is attributed to the bulk glass annealing temperature being greater than Tg of phase-separated, phosphate-rich droplet phases present in the 4.0 mol% P2O5 glass, which promoted crystallization, where back scattered scanning electron microscopy (SEM) images suggested that phase separation had occurred as liquid-liquid phase separation, where there was evidence of a droplet like morphology in glasses containing between 4.0 and 6.0 molar % P2O5. However, the exact mechanism cannot be unequivocally confirmed as nucleation and growth based purely on phase morphology. Differential Thermal Analysis (DTA) and dilatometry showed modest increases in Tg with increasing P2O5 contents below the onset of crystallisation and a more substantial increase coinciding with the onset of crystallisation between 3.0 and 4.0 mol% P2O5. High-temperature viscosity measurements showed that increasing P2O5 contents led to increases in viscosity, with samples containing the highest P2O5 additions displaying non-Newtonian behaviour consistent with the presence of crystals in the melt at 950 °C. Raman difference spectra showed that the increase in P2O5 content resulted in characteristic peak intensities at 729 cm−1, 937 cm−1, 1000 cm−1, 1026 cm−1, 1109 cm−1 and 1445 cm−1 which were associated with phosphate species.