Isothermal Crystallisation of a Glass from the 3CaO.P₂O₅–SiO₂–MgO–K₂O System Studied by Impedance Spectroscopy

Impedance spectroscopy was used to measure the changes of electrical properties during the isothermal crystallisation of the phosphate phase in bulk samples of a glass with nominal molar composition (3CaO.P2O5)20(SiO2)35(MgO)38.354(K2O)6.646, developed for biomedical applications. Crystallization studies were performed in isothermal conditions at 800ºC and 860ºC, and were monitored by impedance spectroscopy. XRD shows that a stable orthophosphate phase, Ca9MgK(PO4)7 crystallizes at about 860ºC, after formation of earlier phosphate precursors formed at lower temperatures, namely oxyapatite (Ca5(PO4)3O) and tricalcium phosphate (Ca3(PO4)2). The conductivity of bulk glass samples decreases at 800ºC, as the phosphate precursors phases crystallize. The opposite trend is observed on crystallizing the orthophosphate phase, at 860ºC.