Pressure effecting on morphology of hydroxyapatite crystals in homogeneous system

Abstract

Using Ca(NO₃)₂·4H₂O, (NH₄)₂HPO₄ and urea as the starting raw materials, hydroxyapatite (HAP) crystals were successfully prepared at low temperature (95 °C) by changing the starting pressure inside the autoclave. The HAP crystals were characterized by XRD, SEM, HRTEM, and FTIR technologies. The XRD patterns indicated that increasing the pressure is helpful to improve 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 illustrated that HAP crystal morphology changed to long plate-like crystals, hexagonal prism-like crystals, and the crystal flakes from needle-like whiskers when increasing the starting pressure inside the autoclave to 0.5 MPa, 0.9 MPa, 1.1 MPa from 0.11 MPa. The formula (IV) about the growth rate of a-surface and c-surface was presented on 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 (IV), a reasonable interpretation was introduced for pressure effecting on morphologies of HAP crystal.