Fabrication of calcium phosphate nanoparticles in a continuous flow tube reactor

Calcium phosphates (CPs) were fabricated with using a continuous flow-type tube reactor under pH 6.4 and 9, within the temperature range from 0 to 100°C. The sample fabricated at the temperature range from 0 to 60°C involved not only dicalcium hydrogenphosphate dihydrate (DCPD) as the primary phase but also octacalcium phosphate (OCP). As the relative X-ray diffraction intensity of OCP increased with increase in the synthesis temperature, particles of a sheet-like shape appeared. The samples fabricated at 80 and 100°C have a crystalline phase of hydroxyapatite (HAp) and morphology of rod-like shape. Under pH 9, in contrast, the reactor fabricated HAp crystalline, irrespective of the synthesis temperature. In particular, at 80 and 100°C, only oval or spindle-shaped HAp was yielded. It was therefore confirmed that the present tube reactor system was versatile to control the size, shape, and crystalline phase of CPs. The morphology, particle size, and specific surface area dependent on the pH and temperature of synthesis were discussed the relative stability of the relevant calcium phosphate phases and equilibrium relations among the orthophosphate ions.