Effects of additives on sintering of nanocrystalline hydroxyapatite

Nanocrystalline hydroxyapatite (HAp) powders were successfully synthesized by a simple method using polymerized complex method. To obtain HAp nanopowders, the prepared precursor was calcined in air at 750-1,000°C for 12 h. The phase composition of the calcined samples was studied by X-ray diffraction (XRD) technique. The XRD results confirmed the formation of HAp phase with a small trace of ß-TCP phase. With increasing calcination temperature the crystallinity of the HAp increased, showing the hexagonal structure of HAp with the lattice parameter a in a range of 0.9351-0.9446 nm and c of 0.6843-0.6906 nm. The particle sizes of the powder were found to be 16-125 nm and as evaluated by the XRD line broadening technique. The morphology of the powders was spherical of size less than 200 nm, as revealed by TEM. Increasing the calcination temperature resulted in the transition from polycrystalline to single crystalline phase of the HAp, as clearly confirmed by the analysis of TEM diffraction patterns. The study of sintering behavior was carried out by pressureless-sintering of the calcined HAp powders in air at 1250 °C for 2 h. The densities of the sintered HAp ceramics measured by Archimedes method were found to be 88 % of theoretical density, whereas those of the ceramics with sintering additives of La₂O₃, and CaO 2.5 wt.% were ~93 % of theoretical density. The grain sizes of the sintered HAp ceramics were 1.35-1.76 ¿m for the pure HAp ceramics and HAp ceramics with sintering additives of La₂O₃ or CaO 2.5 wt.% respectively.