Thermal stability of a modified sol-gel derived hydroxyapatite nanopowders

Hydroxyapatite Ca10(PO4)6(OH)2 (HA) powder was successfully synthesized by a modified sol-gel method using a solution of calcium nitrate in ethanol, along with a solution of diammonium hydrogen phosphate in water and NH4OH as starting materials. The Ca/P molar ratio was maintained at 1.67. The powder was subjected to furnace and microwave heating to compare the decomposition of HA and study the crystallite sizes. It was found that microwave heated powders were pure HAP up to 230degC with absence of secondary phases. However, XRD patterns show that furnace heated powders convert completely to b-TCP when treated at 750degC and 1000degC. This result was confirmed by the absence of hydroxyl bands in the FT-IR spectra for these temperatures.