Well-Formed One-Dimensional Hydroxyapatite Crystals Grown by an Environmentally Friendly Flux Method

Highly crystalline, well-developed hydroxyapatite (HAp) crystals were grown successfully for the first time by a flux cooling method at a temperature less than 500 degrees C under air pressure. Environmentally friendly growth was induced by heating a mixture of solute [Ca(NO(3))(2)center dot 4H(2)O, (NH(4))(2)HPO(4), and KOH] and flux (KNO(3) and LiNO(3)) at 400-500 degrees C and holding at this temperature for 10 h. After that, the mixture was cooled at various rates of 5, 200, and > 1000 degrees C.min(-1) (water quenching). The obtained HAp crystals, which had average sizes of up to 18.0 x 2.1 mu m and aspect ratios of up to 8.6, were colorless and transparent. They had hexagonal prismatic one-dimensional (1-D) forms with pyramidal or truncated pyramidal end faces. The 1-D crystal forms and sizes were obviously dependent on the growth conditions, such as the holding temperature and the cooling rate of the high-temperature solution. High-resolution transmission electron microscopy (HR-TEM) images indicated that the grown HAp crystals were of a very good crystallinity. From the HR-TEM and X-ray diffraction, the I-D HAp crystals, which were hexagonal cylinders ({10 (1) over bar0} faces), elongated in the < 0001 > directions.