Phase separation and crystallization effects on the structure and durability of molybdenum borosilicate glass

The effects of phase separation by nucleation and growth as well as those of crystallization on the structure and the chemical durability of sodium borosilicate glasses enriched in molybdenum oxide were investigated. Increasing concentrations of MoO₃ (0 to 10% molar) were added to glasses which were homogeneous (MoO₃ = 1.4% molar) or with crystallizations and separated phases (MoO₃ N 1.4% molar). Their structure was determined by ¹¹B, ²³Na, ²⁹Si and ⁹⁵Mo MAS NMR, and ²³Na MQMAS NMR, coupled with DFT calculations of NMR parameters. Their microstructure and morphology were characterized by X-ray diffraction and by electronic microscopy. These complementary approaches pointed out a decrease in the number of non-bridging oxygens (NBOs) within the vitreous matrix, first initiated by an increase in polymerization for glasses with a low molybdenum oxide content (≤2 % molar), and combined with the decrease in the number of tetrahedral boron for higher contents. Correlations between the chemical durability, the structure and the microstructure of the glasses were then obtained at low and high dissolution reaction progress. The modifications to the chemical composition and to the structure of the surrounding glass induced by increasing MoO₃ strongly influenced the matrix durability. Moreover, the nature and the morphology of the separated phases influence the dissolution kinetics by modifying the solution chemistry. (authors)