Formation Mechanisms of Sol-Gel-Derived Hydroxyapatite Using Different Thermal Processings

Sol-gel-derived hydroxyapatite (HA) precursors were calcined by two thermal processings, a rapid-thermal-calcine (RTC) heating at 100–600°C/min and a conventional-furnace-calcine (CFC) heating at 1.67°C/min, respectively. X-ray diffraction patterns revealed that the onset temperature of HA crystallization is lower in RTC, 350°C, as compared to 600°C in CFC. Pyrolytic-GC/MS programs showed that the evolved gases out of a sample subjected to RTC are mainly H2O, N2O and ethylamine, while those of a sample subjected to CFC are CO2 and small organic molecules. The results leads to models that RTC can quickly remove organic portion of the gel networks, leading to a porous surface morphology and a collapse of gel networks at local areas, so that HA crystallite nucleation is facilitated due to intimate contacts among inorganic species at lower temperatures. On the other hand, slow evolution of organics during CFC leads to carbonaceous residues that isolate the inorganic species and inhibit nucleation of HA crystallites until at a higher temperature.