Crystal and Phase Evolution of Sol-Gel Derived Hydroxyapatite Synthesis with Various Ca/P Ratios

Hydroxyapatite due to its biocompatibility and chemical and biological affinity with bone tissue is used for orthopedic implant, dental implant, oral and maxillofacial surgery and as a coating film on metal or alloy implants. In this study, hydroxyapatite powders were synthesized with three different ratios of Ca/P = 1.6, Ca/P = 1.5 and Ca/P = 1.67 using sol-gel method and utilizing P2O5 and Ca(NO3)2.4H2O as starting materials. The X-ray diffraction (XRD) results reveal that the highest peak of hydroxyapatite and -TCP, a biphasic bioceramic with high ability of ossification, are found for the hydroxyapatite powder synthesized with Ca/P=1.6. Microstructural studies at high magnifications reveal a uniform distribution of hydroxyapatite nanoparticles (10-400 nm) while at low magnifications only clusters of nano-particles could be detected. The peak intensity of the TCP phase is increased after applying the sintering heat treatment for all three ratios of Ca/P. By increasing the Ca/P ratio from 1.5 to 1.6, the grain size of the sintered samples increased and the morphology of the microstructure became coarser. The biocompatibility evaluation of the synthesized hydroxyapatite indicate that increasing the ratio of Ca/P will increase the volume and rate of apatite formation on the surface of the sintered samples.