Aging effect on the phase evolution of water-based sol–gel hydroxyapatite

In a number of recent reports on the synthesis of sol–gel hydroxyapatite, aging of the precursor solution has been found to be critical in developing an apatitic phase. Critical aging time is required to complete reaction between Ca and P molecular precursors to form a desired intermediate complex that permits a further transformation to apatite phase under appropriate thermal treatment. In this investigation, we employed a water-based sol–gel process recently developed to fabricate hydroxyapatite at relatively low temperatures. The aging effect on apatite formation was systematically studied in terms of aging time and temperature. Experimental results show that the aging time is considerably reduced as aging temperature rises. Long-term thermal aging was unfavorable for apatite formation. The optimal aging parameters for apatite formation were experimentally determined, which was further consolidated into a phase evolution map. Aging kinetic was investigated by monitoring the variation of solution pH, following the determination of an apparent activation energy, which has a value as high as 10.35 kcal/mol, for the chemical reaction occurring upon aging. Optimal solution chemistry was elucidated based on the corresponding phase evolution map.