Kinetics of thermal decomposition of hydroxyapatite bioceramics

The mechanism and kinetics of thermal decomposition of injection moulded hy droxyapatite ceramics were studied over the temperature range of 1473-1758 K. At temperatures below 1473 K the sintering and transformation of hydroxy apatite to hydroxyoxyapatite proceeded to a conversion degree of 0.4 to 0.5 . At temperatures between 1473 and 1758 K the hydroxyapatite was decomposed to alpha-TCP, H2O and CaO. The decomposition of HOA started on the surface of the HOA ceramics. The rate of increase in the thickness of the reaction products (alpha-TCP) was described by the parabolic law. The kinetic analy sis of the time dependence of HOA conversion to TCP by means of the J-M-A-J -K equation also showed that the thermal decomposition of HOA ceramics was controlled by diffusion of water from the reaction zone to the surface of t he ceramic sample. The activation energy of the thermal decomposition of HO A ceramics amounted to 283.5 kJ/mol. (C) 1999 Kluwer Academic Publishers.