Preparation and characterization of nano-sized hydroxyapatite powders produced in a radio frequency (rf) thermal plasma

Nano-sized hydroxyapatite (HA) powders were successfully produced in radio frequency (rf) induction plasma. Fine spray dried (SD) HA powders (average size ∼15 μm) were employed as feedstock. The microstructure, phase composition and thermal stability of SDHA feedstock and the nano-sized HA powders were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), and differential scanning calorimetry (DSC). Decomposition of powders into other calcium phosphate phases occurred in the high temperature plasma flame. XRD results indicated that an increase in the working power level from 15 to 21 kW led to different trends for SDHA decomposition in the rf plasma flame. At 21 kW, the extreme rf plasma temperature was dominantly responsible for the decomposition of SDHA in the flame. The decomposition at the working plate power of 15 kW was attributed mainly to the relatively lower velocity of the plasma flame. The moderate decomposition of HA feedstock was obtained at a working power level of 20 kW due to mitigating conditions through combined influence of moderate flame temperature and flame velocity. DSC results indicated that amorphous phase content has a positive relation with the recrystallization enthalpy of phase transformation in the nano-sized powders.