EFFECT OF NUCLEATING-AGENTS ON THE CRYSTALLIZATION OF CALCIUM-PHOSPHATE GLASSES

Phase evolution in calcium phosphate-based glass ceramics has been exa mined, Pure CaO:P2O5 readily formed a glass which surface nucleated up on annealing, but volume nucleation at 680 degrees C was observed only after the addition of the nucleating agents, TiO2 and Al2O3. Phase se paration of Ti and Al occurred along with the nucleation and growth of a calcium phosphate phase, similar to beta-Ca2P2O7. Heat treatments a t higher temperatures and/or for longer times resulted in crystallizat ion of Al- and Ti-rich, phase-separated regions, A glass with a higher CaO:P2O5 ratio (approximately 2:1) could be prepared only when a tota l of 25-35 mol% of TiO2, Al2O3, and SiO2 were present in the batch. Th e glass phase-separated into respective SiO2- and CaO/P2O5-rich region s 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 readil y occurred followed by the heterogeneous nucleation and growth on thes e nuclei of a calcium phosphate phase, Although this phase mas macrosc opically composed of spherulites, TEM revealed that they consisted of intertwined nanodendrites whose individual arms mere approximately 20 nm wide and 50 nm long.