Formation of calcium-deficient hydroxyapatite from α-tricalcium phosphate

This study investigated the factors influencing the kinetics of Ca9HPO4(PO4)5OH (calcium deficient hydroxyapatite or CDHAp) formation from α-Ca3(PO4)2 (α-TCP). The kinetics of CDHAp formation were investigated by isothermal calorimetry at constant temperatures ranging between 30 and 75°C and by changes in pH at 37.4 and 70°C. The calorimetric curves were characterized by two reaction peaks. Activation energies were calculated for the events resulting in these peaks. Values obtained were 48.4 and 67.7 kJ mol-1, respectively, indicating nucleation and growth mechanisms for both events. Temperature had a significant effect on the growth rate as indicated by a decrease in surface area (26.5–15.0 m2 g-1) of the CDHAp with increasing temperature (30–75°C). A linear relationship between hydrolysis temperature and CDHAp surface area was observed. The morphology of the CDHAp was plate-like and the crystallites became more regular as the reaction temperature was increased. A rapid elevation in pH upon mixing with water indicated the synthesis method initially used did not entirely eliminate slight compositional variations within the α-Ca3(PO4)2. Rapid elevation in pH retarded subsequent reaction. This effect was eliminated by increasing the duration of high-temperature firing during α-TCP synthesizing.