Hydrothermal Sintering under Mild Temperature Conditions: Preparation of Calcium-deficient Hydroxyapatite Compacts

Calcium-deficient hydroxyapatite (CDHA) prepared by the coprecipitation method was solidified by the hydrothermal hot-pressing technique, and compacts of CDHA with high bulk density beyond 80% were obtained at 200 ℃. Each reaction parameter, viz. reaction temperature, pressure, and time, was systematically changed from the standard conditions to investigate its effects on density, Vickers hardness, and Ca/P ratio of the compacts obtained. The reaction temperature and pressure had a large effect on densification, but not the reaction time because the densification proceeds in a short time. The densification by hydrothermal hot-pressing involved dissolution and precipitation of the starting CDHA powder, so that the Ca/P ratio changed from 1.52 of the starting powders to 1.61 of the compact obtained by hydrothermal hot-pressing at 200 ℃ and 35 MPa for 24 h with the addition of 10 wt.-% water