The Prepation Of Hydroxyapatite Whisker By Homogeneous System And Step-growth Mechanism Of Hydroxyatite Whiskers

Hydroxyapatite (HAP) whiskers were synthesized via using urea as the precipitator by a homogeneous method, and their phase transformation processes and growth mechanism were investigated. The results from phase transformation process study showed that with the decomposition of urea and the corresponding increase of pH value of reaction system, dicalcium phosphate anhydrous (DCPA) and octacalcium phosphate(OCP) were precipitated when pH≈3.3–4.3, then Ca²⁺ and HPO₄^2- began to be ionized from DCPA when pH > 4.5, finally HAP whiskers heterogeneously nucleated and grew up into short column crystal along the surface of OCP flakes. In the absence of the addition of ionic resources, DCPA gradually dissolved, OCP flakes translated into HAP continuously meanwhile short column HAP whiskers grew up. Finally synthesized HAP whiskers had 20–100μm in length and 1–2μm in diameter, and exhibited a slenderness ratio of 25. The study about the growth mechanism indicated that HAP whiskers grew along the c-axis direction, and the (100) steps were clearly visible at their heads through FETEM and AFM, and also straight step lines, instead of helical Frank ones, were present on the side face of the (100) steps. The results from the interface energy calculation showed that the growth rate of the heads of HAP whiskers could be fastest which lead to the preferential growth of HAP along the c-axis direction, that of homogeneous twist sites could be relatively slow, and that of heterogeneous twist sites could be slowest. The formation of the growth steps was resulted from the different growth rates at the different sites in various growth layers.To study the role and mechanism of the pressure effecting on morphologies of HAP crystals, four polyethylene vessels were kept the temperature at 95oC for 24 hours in the autoclave which pressure were controlled at 0.11MPa, 0.5MPa, 0.9MPa, 1.1MPa. The HAP crystals were characterized using XRD, SEM, and HRTEM technologies. The XRD patterns indicated that increasing pressure is helpful to Improves the purity of the product and the starting pressure change inside the autoclave has a great effect on the growth of HAP crystal planes. The effect of pressure on the growth of various crystal planes was certified by the analysis of HRTEM micrographs. The SEM images indicated that HAP crystal morphology changed to long plate-like crystals,hexagonal prism-like crystals, and the crystal flakes from needle-like whiskers as increasing the starting pressure inside the autoclave to 0.5MPa, 0.9 MPa, 1.1 MPa from 0.11 MPa. The formula (â…£) about the growth rate of a-surface and c-surface was presented in the basis of the traditional formula about the growth rate of the polyhedral crystallization and the crystal and the growth unit model. According to the formula (â…£), a reasonable interpretation was introduced for pressure effecting on morphologies of HAP crystal.