Hydrothermal Synthesis of Hydroxyapatite Whisker from Amorphous Calcium Phosphate and the Effect of Carboxylic Acid.

Hydroxyapatite (Ca10(PO4)6(OH)2, HAp) whisker was synthesized by hydrothermal crystallization of amorphous calcium phosphate (Ca3(PO4)2⋅nH2O, ACP) with 0-5.0mol·dm-3 of different carboxylic acid solutions such as acetic acid, malic acid, and citric acid at 140-220°C, in autoclave. The products were characterized by X-ray diffractometry, Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The shape of the products changed by kinds of carboxylic acid used, and the largest HAp whisker was obtained under conditions such as acetic acid of concentration 1.0mol·dm-3, hydrothermal temperature of 200°C, and holding time of 3h; the resulting whisker was hexagonal prismatic with 100μm in length and 4μm in diameter, and had Ca/P atomic ratio of 1.56±0.02. The formation of HAp whisker is ascribed to adsorption of dimer-carboxylic acid on the a-plane of HAp. Accordingly, carboxylic acid promoted growth toward the c-axis. The existence of dimer-carboxylic acid was also influenced by the acid dissociation constant of carboxylic acid. Synthesis conditions for HAp whisker was calculated from the dissociation constant.