Devitrification of bioactive invert phosphate glasses

Na2O/CaO/Al2O3/P2O5 glasses with low phosphorous content were crystallized to produce bioactive glass-ceramics. To investigate the nucleation mechanism, glasses with increasing sodium content were devitrified. Depending on the amount of alkali oxide present, some of these glasses devitrified homogeneously without the addition of a nucleation agent. This behaviour is limited to compositions with high and low sodium content, whereas others show only surface crystallization. The same is valid, if 3-4 wt% of fluorine are added. The crystalline phases Ca2P2O7, AlPO4 (tridymite modification) and Na27Ca3Al5(P2O7)(12) were identified by powder-diffraction in the glasses with low alkali content. In those containing 18-22 wt% Na2O, Na5Ca2Al(PO4)(4), AlPO4, Na27Ca3Al5(P2O7)(12) and a structurally unknown pyrophosphate precipitated. The addition of fluorine supports the crystallization of apatite (Ca-5(PO4)(3)(OH/F)) and another pyrophosphate with unknown structure.