DEVITRIFICATION OF BIOACTIVE INVERT PHOSPHATE-GLASSES

Na2O/CaO/Al2O3/P2O5 glasses with low phosphorous content were crystall ized to produce bioactive glass-ceramics. To investigate the nucleatio n 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 co ntent, whereas others show only surface crystallization. The same is v alid, if 3-4 wt% of fluorine are added. The crystalline phases Ca2P2O7 , AlPO4 (tridymite modification) and Na27Ca3Al5(P2O7)(12) were identif ied by powder-diffraction in the glasses with low alkali content. In t hose containing 18-22 wt% Na2O, Na5Ca2Al(PO4)(4), AlPO4, Na27Ca3Al5(P2 O7)(12) and a structurally unknown pyrophosphate precipitated. The add ition of fluorine supports the crystallization of apatite (Ca-5(PO4)(3 )(OH/F)) and another pyrophosphate with unknown structure.