Structural characterization and thermal behaviour of biological hydroxyapatite

Thermal study and structural characterization of biological hydroxyapatite (HA) samples were done as well as their comparison with commercial and synthetic samples in this study. The X-ray micro analyser shows that all three samples of human teeth (HT1–HT3) contain two types of HA structures with different crystallite sizes, unlike sample of bovine thigh-bone (BTB). The bone sample was composed only of one HA phase with varied porosity. The molar Ca/P ratio in biological samples was lower compared to theoretical ratio for pure HA; moreover, in the case of teeth, Ca/P ratio varyies between the centre and the periphery of the cross-sectional samples. Thermogravimetry of the biological samples showed mass decreases—three regions for the bone and four regions for the teeth. In comparison, commercial HA has only two-step weight loss and synthetic HA three-step weight loss. After the calcination up to 1280 °C all the samples of teeth transformed into whitlockite, β-(Ca,Mg)₃(PO₄)₂ (98 wt%) and 2 wt% HA. Besides, HT3 contained further trace amount of hilgenstockite (HIL, Ca₄P₂O₉). The sample BTB partly transited from natural HA into HIL (6 wt%) and lime, CaO (14 wt%). X-ray powder diffraction (XRD) proved occurrence of HIL (9 wt%) beside stability part HA (91 wt%) in the commercial HA after thermal treatment but the synthetic HA composed from Ca₃(PO₄)₂ (74 wt%) and HA (26 wt%).