Ultrastructural observation of hydroxyapatite ceramics with preferred orientation to a-plane using high-resolution transmission electron microscopy

The dense hydroxyapatite (HAp) ceramics with preferred orientation to a-plane were fabricated using composite powders of single-crystal apatite fibers (AF) and wet-synthesized apatite gels (AG). AF was synthesized by homogeneous precipitation method. The composites of AF and AG were prepared as the following process. Aqueous (NH₄)₂HPO₄ solutions were added into the AF slurry with the amount of 1 mass%. In addition, aqueous Ca(NO₃)₂ solutions were added dropwise to the slurry to precipitate the AG on the AF at room temperature, and then aged at room temperature for 24 h with stirring. The dried powders were uniaxially pressed at 200 MPa of compaction pressure and then fired at 1300 °C for 5 h. All the resulting ceramics were identified with HAp single phases. The (100), (200) and (300) reflections which correspond to a-plane were more intense than those of a standard HAp ceramics. With increasing the additive amount of AG, the orientation degree of a-plane decreased, and the relative density increased. Furthermore, the ultrastructure of the resulting ceramics was examined by highresolution transmission electron microscopy. The selected area electron diffraction patterns showed clear spots corresponding to [110] zone axis direction of HAp crystal with high crystallinity. The lattice spacings of crystal grains were determined to be 0.34 nm or 0.68 nm which corresponds to the lattice constant of (002) or (001) plane of HAp crystal. Therefore, the resulting ceramics were HAp single phases with preferred orientation to a-plane.