Biomaterials 25 (2004) 1763–1770 Influence of synthesis and sintering parameters on the characteristics of carbonate apatite Elena Landi*, Anna Tampieri, Giancarlo Celotti, Lucia Vichi, Monica Sandri Institute of Science and Technology for Ceramics ISTEC-CNR, via Granarolo 64, 48018 Faenza (RA), Italy Received 18 February 2003; accepted 11 August 2003 Abstract A new method to synthesise carbonate-substituted hydroxyapatite (CHA) powder has been set up introducing a CO 2 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, H 3 PO 4 solution dropping rate, i.e. reaction time, CO 2 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 CO 2 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. r 2003 Elsevier Ltd. All rights reserved. Keywords: Carbonate-substituted hydroxyapatite (CHA); CHA synthesis; CHA characterisation; Carbonate site evaluation 1. Introduction In the last decades, the target research was the production of highly crystalline and stoichiometric hydro- xyapatite. More recently, the interest has focused on the preparation of apatites more similar to bone in terms of both crystallinity degree and deviation from stoichiometry. Non-stoichiometric apatites are quite easy to synthe- sise. In fact, they can be obtained by any precipitation method over a wide range of pH, temperature and concentration. Their composition and crystal character- istics are, however, more difficult to control, mainly because of the ability of the lattice to accept substituents and vacancies [1]. The most interesting of the non- stoichiometric apatites are HPO 4 2 and CO 3 2 containing apatites which are close to bone mineral. Although bone mineral has a variable composition, diverse apatites mimicking the evolution of bone mineral in young and old animals can be prepared and their composition can be represented by a chemical formula analogous to that of bone mineral. The trivalent anionic phosphate sites may be occupied by both the bivalent ions hydrogen phosphate and carbonate, while the monovalent anionic site substitution of hydroxyl is possible only by carbonate. The phosphate site cannot accept vacancies, probably because the trivalent anions are quite large and vacancies would destabilise the lattice. Calcium sites can also be occupied by other cations and can also accept vacancies up to a maximum of two sites out of the 10 existing in stoichiometric apatites. At present, prepara- tion of the carbonate apatite is a must. To study the effect of carbonate as a unique substituent, it is important to avoid the hydrogen phosphate and the cationic substitutions: otherwise, the identification of ARTICLE IN PRESS *Corresponding author. Tel.: +39-546-699-711; fax: +39-546- 46381. E-mail address: elena@istec.cnr.it (E. Landi). 0142-9612/$-see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.biomaterials.2003.08.026