Effect of Nucleating Agents on the Crystallization of Calcium Phosphate Glasses

Abstract

Phase evolution in calcium phosphate‐based glass ceramics has been examined. Pure CaO:P2O5 readily formed a glass which surface nucleated upon annealing, but volume nucleation at 680°C was observed only after the addition of the nucleating agents, TiO2 and A12O3. Phase separation of Ti and Al occurred along with the nucleation and growth of a calcium phosphate phase, similar to β‐Ca2P2O7. Heat treatments at higher temperatures and/or for longer times resulted in crystallization of A1‐ and Ti‐rich, phase‐separated regions. A glass with a higher CaO:P2O5 ratio (approximately 2:1) could be prepared only when a total of 25‐35 mol% of TiO2, A12O3, and SiO2 were present in the batch. The glass phase‐separated into respective SiO2‐ and CaO/P2O5‐rich regions on cooling. The SiO2‐rich regions did not influence crystallization and remained amorphous throughout the heat treatments. In the CaO/P2O5‐rich regions, homogeneous volume nucleation of a Ti‐rich phase readily occurred followed by the heterogeneous nucleation and growth on these nuclei of a calcium phosphate phase. Although this phase was macroscopically composed of spherulites, TEM revealed that they consisted of intertwined nanodendrites whose individual arms were approximately 20 nm wide and 50 nm long.