Changes in structure and thermal properties with phosphate content of ternary calcium sodium phosphate glasses

Phosphate glasses in the system P2O5-CaO-Na2O dissolve in aqueous solutions, making them of interest for use as controlled release materials, e.g. as degradable implants or fertilisers. However, glasses showing slow dissolution tend to be in the poly- or pyrophosphate range and thus have high tendencies to crystallise, making the processing of these glasses challenging. This study therefore aims at systematically investigating how structural changes in the glass affect thermal properties and crystallisation. P2O5-CaO-Na2O glasses with phosphate contents between 55 and 35 mol% were prepared by a melt-quench route. Glass density increased and molar volume decreased with increasing modifier content, owing to a more densely packed glass structure. Raman and 31P magic angle spinning nuclear magnetic resonance spectroscopy showed that the glass structure changed from a network (55 mol% P2O5) to short chains (35 mol%) with decreasing phosphate content. The increasing concentration of non-bridging oxygens with increasing modifier content resulted in a pronounced increase in glass transition temperature and a smaller processing window, making processing, such as sintering, challenging.