Preparation of orthophosphate glasses in the MgO-CaO-SiO²-Nb²O⁵-P²O⁵ system

BACKGROUND: Niobia/magnesia-containing orthophosphate invert glasses were successfully prepared in our earlier work. Orthophosphate groups in the glasses were cross-linked by tetrahedral niobia (NbO₄) and magnesia. OBJECTIVE: The aim of this work is to prepare calcium orthophosphate invert glasses containing magnesia and niobia, incorporating silica, and to evaluate their structures and releasing behaviors. METHOD: The glasses were prepared by melt-quenching, and their structures and ion-releasing behaviors were evaluated. RESULTS: 31P solid-state nuclear magnetic resonance (NMR) and Raman spectroscopies showed the glasses consist of orthophosphate (PO₄), orthosilicate (SiO₄), and NbO₄ tetrahedra. NbO₄ and MgO in the glasses act as network formers. By incorporating SiO₂ into the glasses, the chemical durability of the glasses was slightly improved. The glasses reheated at 800°C formed the orthophosphate crystalline phases, such as β-Ca₃ (PO4) ₂, Mg₃ (PO₄) ₂ and Mg₃Ca₃ (PO₄)₃ in the glasses. The chemical durability of the crystallized glasses was slightly improved. CONCLUSIONS: Orthosilicate groups and NbO4 in the glasses coordinated with each other to form Si-O-Nb bonds. The chemical durability of the glasses was slightly improved by addition of SiO₂, since the field strength of Si is larger than that of Ca or Mg.