Hindawi Publishing Corporation Bioinorganic Chemistry and Applications Volume 2011, Article ID 453759, 10 pages doi:10.1155/2011/453759 Research Article Sintering Behavior of Magnesium-Substituted Fluorapatite Powders Prepared by Hydrothermal Method S. Nasr and K. Bouzouita U.R. Mat´ eriaux Inorganiques, Institut Pr´ eparatoire aux Etudes d’Ing´ enieur, University of Monastir, Rue Eben El Jazar 5019, Monastir 5000, Tunisia Correspondence should be addressed to S. Nasr, nasrsamia@yahoo.fr Received 13 December 2010; Accepted 19 January 2011 Academic Editor: Lorenzo Pellerito Copyright © 2011 S. Nasr and K. Bouzouita. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Magnesium-substituted fluorapatite powders were synthesized by hydrothermal method, and their sintering behavior was investigated by dilatometry in the temperature range 25–1100 ◦ C. Analysis of the obtained powders by X-ray diffraction and 31 P NMR spectroscopy showed that the powders consisted of a single apatite phase and no amorphous phase has been formed. Compared to pure fluorapatite, the shrinkage of the substituted samples occurred in two steps and the temperature at which the sintering rate was maximum is lower. In addition, the shrinkage was interrupted by an expansion of the samples due to the formation of a liquid phase. The latter phase led to the crystallization of needle-crystals by a dissolution-diffusion-reprecipitation process. 1. Introduction Owing to their physicochemical and biological properties, the hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 , HA) and to a lesser extent the fluorapatite (Ca 10 (PO 4 ) 6 F 2 , FA) have been exten- sively investigated over the last three decades. They offer important opportunities for applications in a diversity of areas particularly in medicine and dentistry [1–7]. Moreover, many applications have become possible thanks to their notable stability and their ability to accommodate a great number of substitutions; both cationic and anionic substi- tutions are possible [8–10]. For example, Ca 2+ ion can be substituted by various divalent cations such as Mg 2+ , Sr 2+ , Pb 2+ , Cd 2+ , and so forth. Among these ions, some of them (Sr 2+ , Cd 2+ or Pb 2+ ) lead to a solid solution in the whole range of composition [11–13], while the incorporation of other ions such as Mg 2+ or Mn 2+ , into the apatite structure remains limited [14–19]. Magnesium, whose concentration varies from 0.44 to 1.23 wt% [20], is one of the most abundant elements that are present in hard tissues. It has a prominent effect on the osteoporosis [21] and mineralization process [22, 23]. Thus, the synthesis of hydroxyapatite and fluorapatite powders in controlled Magnesium content is of a practical importance to develop biomaterials whose chemical composition is as close as possible to that of bone. Despite numerous studies that have been devoted to the incorporation of Mg into the hydroxyapatite structure [14–16], its substitution limit is a subject of controversy and the process of its incorporation remains unclear. Indeed, according to the used synthesis method, the substitution limit of this element in the hydroxyapatite varies from ∼0.3 to 28.4 wt% [16, 24]. Furthermore, it is reported that all of the Mg used was not really incorporated into the apatite structure, but a sig- nificant proportion is adsorbed on the surface of the particles in an amorphous phase and/or in another form [14, 25]. Different methods can be used for synthesis the apatite such as solid-state reaction [26], coprecipitation [15, 17], sol-gel [27] and hydrothermal synthesis [18]. Hydrothermal synthesis offers a relatively simple and effective way to