Influence of synthesis and sintering parameters on the characteristics of carbonate apatite

A new method to synthesise carbonate-substituted hydroxyapatite (CHA) powder has been set up introducing a CO2 flux, as a source of carbonate, in the HA synthesis process based on the neutralisation reaction. The reactants are abundant and inexpensive. The yield is good compared to other CHA powder synthesis. The reaction may be performed at low temperature and without pH control and does not produce any by-products. The influence of the synthesis parameters (temperature, H3PO4 solution dropping rate, i.e. reaction time, CO2 flux, ageing time) has been tested to optimise the process conditions in order to obtain the highest carbonation degree and favour the B-type CHA precipitation with respect to A-type one. The prepared powder (5.8 wt% of total carbonate with an A/B ratio of 0.78) was thermally treated at various temperatures in the range 500-1400°C in different atmospheres (air, wet and dry carbon dioxide). The thermal treatments were performed with a double aim, to eliminate selectively the carbonate groups in A-position maintaining the B-type substitution, and to evaluate the thermal stability of the CHA and the total loss of carbonate as a function of temperature. The thermal treatment at 900°C in wet CO2 gave the best result in terms of a high carbonate residue and a low A/B ratio. We also investigate the use of different techniques (inductively coupled plasma, TGA, Fourier transformed infrared spectroscopy, X-ray diffraction) for characterising CHA and calculating sensitivity and accuracy in the quantification of carbonate ions for each molecular site.