The enhancement of hydroxyapatite thermal stability by Al doping

Al-substituted hydroxyapatite (Al-HA) powders with apatite structure and particle size of 30–70 nm were obtained via precipitation method. The effect of Al content on specific surface area and morphology of powders was studied, and a formation of highly anisotropic phase due to Al doping was observed. The influence of heat treatment in 300–1400 °C range on the phase composition, lattice parameters, Fourier-transform infrared spectroscopy (FTIR) spectra and mass loss of powders was investigated. Introduction of Al in the hydroxyapatite (HA) lattice in the range 0.5–1.0 mol.% resulted in the improvement in thermal stability, which had not been reported previously. Incorporation of 5–10 mol.% of Al resulted in the formation of biphasic materials based on HA and whitlockite-like structure at 900 °C and apatite and α- tricalcium phosphate (α-TCP) phases at 1200–1400 °C. Introduction of 20 mol.% of Al resulted in the formation of isomorphic Al-substituted whitlockite phase Ca9Al(PO4)7. We have estimated the onset of Ca9Al(PO4)7 transformation into α-TCP phase at 1400 °C.