Influence of calcination conditions on dispersibility and phase composition of hydroxyapatite crystals calcined with anti-sintering agents

We investigated the influence of calcination conditions to fabricate dispersible hydroxyapatite (HAp) nanocrystals via calcination with an anti-sintering agent, poly(acrylic acid, calcium salt) (PAA-Ca), surrounding the nanocrystals. First, two kinds of low crystallinity HAp nanoparticles with spherical morphology (58 nm in diameter) and rod-shaped morphology (around 350 nm in length) were synthesized by wet chemical processes, and then coated with PAA-Ca in aqueous media. The HAp/PAA-Ca mixture was dried and calcined at 800 or 1,000 °C for 1 h. The resultant mixture was finally washed with aqueous media to remove CaO—the thermally decomposed product of PAA-Ca—surrounding the nanocrystals. The calcination-induced sintering between the nanocrystals was efficiently prevented by increasing the amount of PAA-Ca, but the increased amount of PAA-Ca also led to the formation of CaCO₃ due to the incomplete thermal decomposition of PAA-Ca at 800 °C for 1 h. Highly dispersible and phase pure HAp nanocrystals could be obtained by calcination at higher temperature (1,000 °C). We also found that calcium ions were migrated from the anti-sintering agent into the HAp nanocrystals, and the degree of calcium ion migration varied by the nature of the low crystallinity HAp nanoparticles and the calcination temperature.