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 °C under air pressure. Environmentally friendly growth was induced by heating a mixture of solute [Ca(NO3)2·4H2O, (NH4)2HPO4, and KOH] and flux (KNO3 and LiNO3) at 400−500 °C and holding at this temperature for 10 h. After that, the mixture was cooled at various rates of 5, 200, and >1000 °C·min−1 (water quenching). The obtained HAp crystals, which had average sizes of up to 18.0 × 2.1 μm 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 1-D HAp crystals, which were hexagonal cylinders ({101̅0} faces), elongated in the ⟨0001⟩ directions.