PREPARATION OF HYDROXYAPATITE NANOPARTICLES USING A MODIFIED PRECIPITATION METHOD

摘要

Hydroxyapatite (HAp) is the primary constituent of the human hard tissues. The preparation of fine and well sinterable HAp powder is the most important step in fabrication of bioceramics. HAp nanoparticles were prepared using rapid increase of the pH value, by adding an amount of concentrated NH_4OH solution, of a well-mixed solution of Ca(H_2PO_4)_2·H_2O and CaCl_2 with stoichiometric atomic ratio of Ca/P. This modified precipitation method is called "pH shock wave method". The XRD and FTIR analyses indicated that the precipitated powder was crystallized HAp. The HAp was stable during calcination at 550°C, while a significant amount of β-Ca_3(PO_4)_2 appear at 800°C. The TEM micrograph showed arbitrary morphology of nanoparticles, with average grain size of ～80-200 nm in length, and of ～10-30 nm in width and thickness. The adsorption isotherm of HAp was typical for nonporous particles. The specific surface area of powder was 74 m~2 · g~(-1). The chemical analysis indicates the formation of nonstoichiometric hydrated HAp with empirical formula Ca_(8.86)(HPO_4)_(1.28)(PO_4)_(4.72)(OH)·7.50H_2O and atomic ratio of Ca/P equal to 1.477. The lattice parameters were different from standard ones, due to the presence of HPO_4~(2-) groups in the powder. The sizes of the crystallites responsible for the Bragg reflection of (002) and (300) planes were 31.1 and 14.9 nm, respectively, indicating anisotropic growth with maximum along the <001> direction. The HAp particles form aggregates, which average diameter is 1-5 μm. In the interspaces between the aggregated particles, the water remained entrapped after drying. The thermogravimetric analysis confirmed the above mentioned conclusions. The formation of β-Ca_3(PO_4)_2 took place at about 780°C. Sintering of a unixially compressed cylindrical specimen, took place mainly in the temperature range from 800 to 1150 °C.