Changes in crystalline properties of nanosized hydroxyapatite powders prepared by low-temperature reactive crystallization

In the reactive crystallization of hydroxyapatite (HAP) done with CaCl₂, KH₂PO₄ and NaOH solutions at the reaction temperature of 60°C in a semi-batch reactor, the relationship of the crystalline properties of nanosized HAP powders with the operational conditions was experimentally investigated. The crystallinity of needle-like HAP powders was enhanced with decreasing OH- ion composition and reactants concentration, which corresponded well with the increase in the crystal size, but had nothing to do with the aging of the product suspension. Even though the supersaturation level was predicted not to be influenced by the OH- ion composition, the crystal growth of the HAP powders was striking at low OH- ion composition, thereby resulting in larger HAP with high crystallinity. Comparing the growth orientation of the HAP powders before and after sintering, the crystal growth was preferentially promoted on the face of (002) in the direction of the crystallographic c-axis, especially at low OH- ion composition and reactants concentration during the crystallization, and it was transferred on the (300) face along the a-axis after sintering at 1000°C for 3 h, which seem to be caused by the unbalanced grain coalescing according to the crystallographic axes of HAP.