Hydroxyapatite (HAp) whiskers, which are elongated along the crystallographic c-axis, were syn-thesised under hydrothermal method using citric acid. Starting HAp slurries with two different concentrations of 1.7 and 4.4 wt% were prepared by a precipitation method, and became clear solutions by adding suitable amounts of citric acid (Cit.) to dissolve HAp particles. The solutions of 1.0 dm3 were hydrothermally treated in an autoclave at a temperature from 180 to 220 °C for 0,1,2, 3, 5 and 10 h. The crystalline phases of the products detected by powder X-ray diffractometry depended on both the concentration of HAp and the temperature and the duration of hydrothermal treatments. No HAp whiskers could be obtained from the solutions containing 4.4 wt% of HAp at any Cit./HAp ratio. They could be obtained, however, only from those of 1.7 wt% above a limited Cit./HAp molar ratio such as 14.8. In addition, hydrothermal treatment for more than 3 h at 200 °C was necessary to synthesise HAp whiskers. The whiskers were from 10 to 30 μm in length and about 0.5 μm in diameter. However the weight proportion of HAp in the products rapidly decreased with the increasing that of plate-like calcium hydrogenphosphate anhydrate (DCPA: monetite) below 200 °C within the first 3 h.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)