Solution synthesis of hydroxyapatite designer particulates

This paper reviews our research program for intelligent synthesis of hydroxyapatite (HAp) designer particulates by low-temperature hydrothermal and mechanochemical-hydrothermal methods. Our common starting point for hydrothermal crystallization is the generation and validation of equilibrium diagrams to derive the relationship between initial reaction conditions and desired phase assemblage(s). Experimental conditions were planned based on calculated phase boundaries in the system CaO-P₂O₅-NH₄NO₃-H₂O at 25-200 °C. HAp powders were then hydrothermally synthesized in stirred autoclaves at 50-200 °C and by the mechanochemical-hydrothermal method in a multi-ring media mill at room temperature. The synthesized powders were characterized using X-ray diffraction, infrared spectroscopy, thermogravimetry, chemical analysis and electron microscopy. Hydrothermally synthesized HAp particle morphologies and sizes were controlled through thermodynamic and non-thermodynamic processing variables, e.g. synthesis temperature, additives and stirring speed. Hydrothermal synthesis yielded well-crystallized needle-like HAp powders (size range 20-300 nm) with minimal levels of aggregation. Conversely, room-temperature mechanochemical-hydrothermal synthesis resulted in agglomerated, nanosized (∼ 20 nm), mostly equiaxed particles regardless of whether the HAp was stoichiometric, carbonate-substituted, or contained both sodium and carbonate. The thermodynamic model appears to be applicable for both stoichiometric and nonstoichiometric compositions. The mechanochemical-hydrothermal technique was particularly well suited for controlling carbonate substitution in HAp powders in the range of 0.8-12 wt.%. The use of organic surfactants, pH or nonaqueous solvents facilitated the preparation of stable colloidal dispersions of these mechanochemical-hydrothermal-derived HAp nanopowders.