Crystallization of a niobium phosphate glass

Niobium phosphate glasses with composition 37P2O5 · 23K2O · 40Nb2O5 are stable in relation to crystallization during the heating process, exhibit a low critical cooling rate, and are potentially good for nuclear wasteforms. The crystallization of these glasses was evaluated by optical microscopy after proper heat treatments, showing that surface crystallization is the main process occurring during the heat treatment. Two main crystalline phases were observed. These crystalline phases were KNb3O8 and K3NbP2O9. Surface crystal growth rates were measured in the temperature range of 806–972 °C (Tg = 683 °C) for both crystalline phases. Apparent crystallization enthalpies were determined through the Arrhenius plots of lnU vs. 1/T. The enthalpies are 496 kJ/mol and 513 kJ/mol for each crystalline phase, respectively. The surface density of nucleation sites (Ns) on 3 μm diamond paste polished surfaces is (2.4 ± 0.7) × 108 nuclei/m2 for one crystalline phase and (9.8 ± 0.8) × 109 nuclei/m2 for the other crystalline phase, when revealed at 838 °C/17.5 h, and these values show a slight variation depending on the time and the temperature. At the tested temperatures, only one crystal phase appeared inside the volume, and a volume density of nucleation sites Nv = 5 × 106 nuclei/m3 was measured.