Synthesis and microstructural characterization of nano-size calcium phosphates with different stoichiometry

Calcium phosphates with Ca/P molar ratios of 0.5, 0.75, 1.33, 1.5, 1.55, 1.67, 2.0, and 2.5 were synthesized by a wet chemistry precipitation method and sintered at 500 °C, 700 °C, 900 °C, 1100 °C and 1300 °C for 2 h. Presence of phases and microstructures of calcium phosphates were determined by X-ray diffraction and scanning electron microscopy. In all different Ca/P ratios, the precipitated phase was always hydroxyapatite with very small size and/or partial disorderness regardless of the Ca/P ratios in the starting precipitating medium. For samples with 0.5 and 0.75 Ca/P ratios in starting solution, tricalcium phosphate and calcium pyrophosphate phases were observed. In contrast, for samples with 1.0 and 1.33 Ca/P ratios, the only stable phase was tricalcium phosphate. For the samples with Ca/P ratio of 1.5, the tricalcium phosphate phase was dominant. However, small amounts of hydroxyapatite started to appear. For samples with Ca/P ratio of 1.67, the hydroxyapatite phase was dominant. Lastly, for samples with the Ca/P ratios of 2.0 and 2.5, the CaO phase started to appear in addition to the hydroxyapatite phase which was the dominant phase. Moreover, the average grain size, porosity (%) and the average pore size decreased with increasing the Ca/P ratios.