Phase transformation process and step growth mechanism of hydroxyapatite whiskers under constant impulsion system

Hydroxyapatite (HAP) whiskers were synthesized using urea as the precipitator by a phase transformation method, and their phase transformation process and growth mechanism were investigated. The results showed that with the decomposition of urea and the corresponding increase of pH value of the reaction system, dicalcium phosphate anhydrous (DCPA) and octacalcium phosphate (OCP) were precipitated at pH of 3.3–4.3; then Ca2+ and HPO42− ions began to be released from DCPA at pH values greater than 4.5. Finally HAP whiskers heterogeneously nucleated and grew up into short column crystals along the surface of the OCP flakes. In the absence of the ionic resources, DCPA gradually dissolved and the OCP flakes transformed into HAP continuously and the short columnar HAP whiskers grew up. The aspect ratio of the HAP whiskers with length of 20–100 μm and diameter of 1–2 μm was about 25. The HRTEM and AFM images showed that HAP whiskers grew along the c-axis direction, the (1 0 0) steps were clearly observed at their heads and the straight step lines instead of helical Frank ones were present on the side face of the (1 0 0) steps. The calculation on the basis of the surface energy of the HAP crystal showed that the growth rate of the (0 0 1) plane was the fastest, the growth rate at the homogeneous twist sites was the second and that at heterogeneous twist sites could be the slowest, which were the main factors finally leading to the preferential growth of HAP whiskers along the c-axis direction as well as the formation of the growth steps.

► Hydroxyapatite crystals with different morphologies were prepared under constant impulsion system.
► Phase transformation process and mechanism from DCPA and OCP to HAP were studied on the basis of the pH value change with different holding times of reaction system.
► Growth of HAP whiskers was explained by the step growth mechanism of HAP whisker.