Formation of nanodimensional structures in precipitated hydroxyapatite by fluorine substitution

Hydroxyapatite Ca-10(PO4)(6)(OH)(2) is a bioactive compound, which is of great interest for medical application as a component of artificial bones and implants. The modification of its composition via anion and cation substitution can strongly affect its properties due to the formation of nanosized particles in the basic structure. In this work the influence of fluorine substitution of hydroxyl groups is examined. The powders of fluorine-substituted hydroxyapatite Ca-10(PO4)(6)(OH)(2-x)F-x (x=1; 1.5; 2) were precipitated by chemical condensation from solutions. Such characteristics of powders as phase composition, morphology, particle size distribution, density, specific surface area have been studied. It was established that partial replacement of hydroxyl groups by fluorine leads to a decrease in the particle size to 10 nm. The degree of substitution affects the mechanical properties of synthesized powders and ceramics after annealing. A comparison of the particle size distribution in the initial samples and the distribution of grains in the sintered ceramics confirmed that the ceramics inherits the structure of the powders, which has a positive effect on its strength characteristics.