Formation of calcium-deficient hydroxyapatite from alpha-tricalcium phosphate.

This study investigated the factors influencing the kinetics of Ca9HPO4(PO4)5OH (calcium deficient hydroxyapatite or CDHAp) formation from alpha-Ca3(PO4)2 (alpha-TCP). The kinetics of CDHAp formation were investigated by isothermal calorimetry at constant temperatures ranging between 30 and 75 degrees C and by changes in pH at 37.4 and 70 degrees 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 degrees 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 alpha-Ca3(PO4)2. Rapid elevation in pH retarded subsequent reaction. This effect was eliminated by increasing the duration of high-temperature firing during alpha-TCP synthesizing.